REGULAR ARTICLE

Research on potassium in agriculture: needs and prospects

Volker Römheld & Ernest A. Kirkby

Received: 1 April 2010 /Accepted: 30 July 2010 /Published online: 27 August 2010# Springer Science+Business Media B.V. 2010

Abstract This review highlights future needs forresearch on potassium (K) in agriculture. Currentbasic knowledge of K in soils and plant physiologyand nutrition is discussed which is followed bysections dealing specifically with future needs forbasic and applied research on K in soils, plants, cropnutrition and human and animal nutrition. The sectionon soils is devoted mainly to the concept of Kavailability. The current almost universal use ofexchangeable K measurements obtained by chemicalextraction of dried soil for making fertilizer recom-mendations is questioned in view of other dominantcontrolling factors which influence K acquisitionfrom soils by plants. The need to take account ofthe living root which determines spatial K availabilityis emphasized. Modelling of K acquisition by fieldcrops is discussed. The part played by K in most plantphysiological processes is now well understoodincluding the important role of K in retranslocationof photoassimilates needed for good crop quality.However, basic research is still needed to establish the

role of K from molecular level to field management inplant stress situations in which K either acts alone orin combination with specific micronutrients. Theemerging role of K in a number of biotic and abioticstress situations is discussed including those ofdiseases and pests, frost, heat/drought, and salinity.Breeding crops which are highly efficient in uptakeand internal use of K can be counterproductivebecause of the high demand for K needed to mitigatestress situations in farmers’ fields. The same is truefor the need of high K contents in human and animaldiets where a high K/Na ratio is desirable. Theapplication of these research findings to practicalagriculture is of great importance. The very rapidprogress which is being made in elucidating the roleof K particularly in relation to stress signalling by useof modern molecular biological approaches is indic-ative of the need for more interaction betweenmolecular biologists and agronomists for the benefitof agricultural practice. The huge existing body ofscientific knowledge of practical value of K in soilsand plants presents a major challenge to improvingthe dissemination of this information on a global scalefor use of farmers. To meet this challenge closercooperation between scientists, the agrochemicalindustry, extension services and farmers is essential.

Keywords Potassium availability . Potassiummicronutrient interaction . Spatial availability ofpotassium . K/Mg ratio . Abiotic stress . Biotic stress .

Frost resistance . Food quality . K/Cd relations

Plant Soil (2010) 335:155–180DOI 10.1007/s11104-010-0520-1

Responsible Editor: Hans Lambers.

V. Römheld (*)Institute of Plant Nutrition, University Hohenheim,70593 Stuttgart, Germanye-mail: v.roemheld@uni-hohenheim.de

E. A. KirkbyInstitute of Integrative and Comparative Biology,Faculty of Biological Sciences, University of Leeds,LS2 9JT Leeds, UK

Introduction

Large areas of the agricultural land of the world aredeficient in potassium which include 3/4 of thepaddy soils of China and 2/3 of the wheat belt ofSouthern Australia. Additionally export of agricul-tural products and leaching of K particularly insandy soils contributes to lowering soil K content(Rengel and Damon 2008). Soils on which potassiumdeficiency occurs vary widely and include acid sandysoils, waterlogged soils and saline soils (Mengel andKirkby 2001).

At two recent conferences held in India, the first inLudhiana (Punjab) (IPI-PAU Intern. Symposium2006), and the second in Bhubanaswar (Orissa) (IPI-OUAT-IPNI Intern. Symposium 2009) at both ofwhich crop nutrition was discussed, attention wasdrawn to the stagnation and even progressive declinein crop yields in the Indian sub continent as aconsequence of interruption of soil recycling oforganic matter and mineral nutrients, especiallypotassium (K). In India animal dung (as manurecakes) and crop residues are used as a source ofbioenergy for cooking and heating without recyclingthe K rich ash or sludge back to farming land whichreceives only low, if any, input of K fertilizers (Hasan2002). As a consequence, a progressive decline in soilfertility including organic matter and K status is to beexpected as an important factor in restricting cropyields.

This problem is not only restricted to India, it is aworldwide one. According to Smil (1999), more thanhalf the dry matter in the global harvest is in the strawof cereal and legume crops and in the tops, stalks,leaves and shoots of tuber, oil, sugar and vegetablecrops. This global bulk of dry matter which containsnutrients and is taken away at harvest and utilized forother purposes (e.g. heating, animal feed, biofuels),means that large amounts of nutrients are removedfrom the soil. Globally, the annual above ground partsof crops (phytomass), contains 75, 14 and 60 milliontonnes of nitrogen (N), phosphorus (P) and potassium(K), respectively. However, whereas nutrient applica-tions of N and P are at similar levels to total nutrientcontent in crop phytomass removal, (80 and 14million tonnes, respectively), K is applied at a muchlower level, to replenish only 35% of the K removed(Smil 1999), a figure which is likely to be much lowerin developing countries.

The consequence of a lack of adequate nutrientrecycling leading to a loss of soil structure and declinein soil fertility was appreciated long ago by therenowned German agricultural chemist Justus vonLiebig and discussed in the 9th edition (1876) of hiswell - known text book “Chemistry in its application toagriculture and physiology”. Justus von Liebig recog-nised that K as one of the major plant nutrientsplayed a key role in soil fertility and he developed Kmineral fertilizers, so called “patent fertilizers”, toincrease crop yields. These findings are as relevanttoday as they were then. The current move towardsusing crop residues or even entire crops as biofuels, inorder to place less dependence on fossil fuels indeveloped and developing countries such as the USAor China, will also in the long-term lead to a declinein soil fertility.

Taking a more holistic view, there is a need toconsider progressive crop yield decline not only interms of inadequate recycling of organic matter andmineral nutrients, but also in relation to annualflooding problems in India. The benefits of organicmatter in soil acting as a physical barrier to a run-offof rain water can not be ignored. Also the lowerinfiltration of rain water on agriculturally degradedland poor in soil structure promotes the regularflooding of river deltas during the Monsoon period(Hermann et al. 1994). It also brings about topsoilrunoff and erosion which is evident not only in Indiabut occurs worldwide. Yoshida (2001) estimated aneconomic value of 68.8 billion USD for the multi-functional detrimental role of agriculture in Japan onthe landscape and the environment including runoffand erosion.

Another consequence of decline in soil fertility inagricultural land is the greater prevalence of sustainedperiods of drought resulting from poorer water storagethroughout the soil profile. These increasing events ofdrought and other abiotic stresses (e.g. heat) arisingfrom loss of soil fertility as well as from globalwarming will necessitate a specifically high supply ofK for stress mitigation (see below). The inadequaterecycling of K in Indian agriculture thus puts thesesoils and the crops they carry at risk. In Germany too,farmers often respond irrationally to drought eventsby decreasing rates of K fertilization (Joachim Rauch2007 pers. comm.).

In this paper we consider various aspects relatingto K use in crop production. This includes not only

156 Plant Soil (2010) 335:155–180

the supply of K from the soil to crop plants but alsothe role of K in animal and human nutrition. Attentionis drawn to the great need for more effective transferof information to the benefit of the farmer from thevast amount of knowledge which has already beenaccumulated on K in soils and plants. We criticallydiscuss some of the more recent research work on Kin soils and plants included in various paperspresented at the Potassium Conference at Orissa(IPI-OUAT-IPNI Intern Symposium 2009). We alsoaddress areas of new and developing interest in K insoils and plants and discuss current interests in theimportant role of K in protecting crops from abioticand biotic stresses and consider areas in which basicand applied research might be carried out suggestingmeans by which farmers might benefit more fromresearch findings.

Need for knowledge dissemination

On a global scale there is an enormous gap betweenagricultural scientific knowledge and its dissemina-tion and application to farming practice, particularlyin developing countries. This point was made veryforcibly by Krauss (2003a), the then Director of theInternational Potash Institute, when in discussing thework of the Institute in retrospect and prospect, hewrote, “Much of the immediate future challenge is forknowledge transfer, particularly to poor farmers andtheir advisors and extension workers. Balanced Kfertilization and avoidance of K mining, (K appliedby fertilizers less than that K removed by cropharvest), will prevent farmers from falling into thepoverty trap and will help them leave the viciouscircle of declining soil fertility”.

This urgent need for dissemination of scientificknowledge was made very clear during two recenthorticultural visits, one to China and the other to Italy.

In the intensive tomato production area of Shandongprovince in China, severe Mg deficiency symptomswere visible on many of the plants. These symptomswere typical of what might be expected from toohigh a K supply (high K-induced Mg deficiency,Römheld and Kirkby 2007), which was confirmedlater by soil and plant analyses. The extremely highK content in the soils of these Chinese glasshousesnear Shouguang in Shandong had depressed Mguptake, inducing low Mg leaf concentrations(Heenan and Campbell 1981; Seggewiss and Jungk1988). Neither the local farmers nor even thescientific advisors were aware that these symptomsof intercostal chlorosis of the leaves adjacent to thefruit trusses, were caused by lack of Mg. In westernEurope and the USA too, Mg deficiency symptomsin some horticultural and agricultural crops can bewidely observed during reproductive growth stages(Römheld and Kirkby 2007).

Another example of an inappropriate recommen-dation for K fertilizer use, again arising because oflack of understanding of interactions between Mgand K in plant nutrition and the practical benefits ofsoil and plant analysis, was recently demonstrated intwo nearby kiwi (Actinidia deliciosa) orchards nearBologna in Italy. In one of the orchards (“Gurini”),Mg deficiency symptoms similar to those describedabove for tomato were clearly recognisable, whereasin the other (“Dalle”), the plants were showingsymptoms of necrosis of the leaf margins clearlyindicative of K deficiency. The visual diagnoses inboth orchards (Francesco Penazzi 2009 pers. comm.)were confirmed by soil and leaf analyses which areshown in Table 1. The inappropriate use of Kfertilizer recommendations in these kiwi orchardsstrongly supports Krauss`s view that “ignorance ofsoil tests prevents the application of balancedfertilization in the adequate use of potash” (Krauss2003a).

Table 1 Visual symptoms and results of analysis of soil (0–60 cm) and corresponding leaf samples of two kiwi (Actinidia deliciosa)orchards near Bologna, Italy

Kiwi orchard Visual symptoms (June 2009) Classification by soil analysis Leaf analysis (% in leaf DM)

“Gurini” (sandy soil) Mg deficiency K: high-very high K: 1.49 (adequate)

Mg: low-very low Mg: 0.43 (low)

“Dalle” (clay loam soil) K deficiency K: very low K: 0.80 (low)

Mg: very high Mg: 0.67 (high)

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Ensuring that appropriate scientific knowledge ispassed on to farmers and growers is still a greatchallenge in agriculture and horticulture. The meansof achieving this challenge for the benefit offarmers and growers are considered in more detailin relation to research developments in “Needs forfuture research on potassium” and “Summary andprospects”.

Potassium in soils: present knowledge

The present conceptual understanding of soil Kavailability is the existence of four distinct K poolsdiffering in accessibility to plant roots with reversibletransfer of K between the pools (Syers 2003). Thisconcept is illustrated in Fig. 1 which presents an up todate version of the K cycle in soils (see also Öborn etal. 2005). Soil solution plays a pivotal role inproviding the pathway for K uptake from the soil toplant roots. Although this pool is very low in Kcontent, representing only about 5% of total cropdemand at any given time (McLean and Watson1985), or 0.1–0.2% of the total soil K, it isimmediately available and replenished by both theexchangeable K (EK, readily plant available K) andthe slowly or non-exchangeable K (SEK, slowly plantavailable K) pools. These two pools, EK and SEK

make up about 1–2% and 1–10% of the total Krespectively and are the main contributors to K uptakeby plants. The exchangeable fraction (EK) i.e. the Kheld on negatively charged sites of clay minerals andsoil organic matter, is in rapid equilibrium with soilsolution K and is considered to be readily available toplants. Its measurement, as discussed below, canoften, but not always, provide a useful indicator ofK soil status in relation to plant supply. Potassium isreleased from the slowly or non-exchangeable K pool(SEK) from lattice wedge sites of weathered mica-ceous clay minerals which are selective for K ions(see Mengel and Kirkby 2001). The remaining poolwhich holds the bulk of K (90–98% of the total soilK), is held in structure of the primary K bearingminerals, such as micas and feldspars being releasedvery slowly by weathering to replenish the EK andSEK pools as indicated in the figure. Most of the totalsoil K available to plants is usually located in thetopsoil.

These different K pools are not only of relevanceto K acquisition by plants but also to K leachingthrough the soil profile as evident in Fig. 1. In sandysoils as well as in acid lateritic soils containingkaolinitic clay minerals low in CEC, rates of Kleaching can be very high so that considerableamounts of K can be lost (Table 2, Wulff et al.1998) (Sharpley 1990). On such soils where high

Plant K

K in fertilizers manure

Soil solution Kimmediately available

Exchangeablereadily available

Slowly-exchangeable K(fixed, non-exchangeable K)

slowly available

Structural K(lattice K)

very slowly available

Crop removal

External Input

Losses by leaching

release

relea

se

release

exchange

release

fixation

rele

ase

(rec

yclin

g)

Upt

ake

Fig. 1 The potassium cyclein soils (after Syers 2003)

158 Plant Soil (2010) 335:155–180

rainfall conditions prevail, split application of Kfertilizers during the growth period has provedbeneficial, simultaneously lowering loss of K byleaching and raising efficiency of use of the Kfertilizers applied (Kolar and Grewal 1994).

The most usual method used worldwide to assessthe K status of a soil for the likelihood of obtaining aresponse in crop yield to fertilizer additions is themeasurement of exchangeable K. This determinationis made by extracting EK from air dried topsoil byone of a number of various well accepted chemicalextractants which include NH4OAc, NH4Cl, CaCl2,Mehlich No 1 and 2, the choice depending mainly onlocal usage and tradition. Differences between theextractants are only marginal in sensitivity (McLeanand Watson 1985). The relationship between theamounts of exchangeable K and crop yield can beextremely close as reported for example by Johnstonet al. (1998) for grain yields of winter wheat andyields of field beans (Vicia faba L.) grown on a siltyclay loam soil at Rothamsted in the UK. However, asdiscussed by Syers (2003) much of the reportedinformation in the UK relates to single soils or anarrow range of soils which may have led to anoveremphasis on the usefulness of EK.

There is abundant evidence of the importance ofSEK in soils and its availability to plants (Syers2003). For example Mengel et al. (1998) were able toshow that silt in loess derived soils which is high in2:1 layer silicates interlayer K is able to provide largequantities of SEK to ryegrass. It is for this reason thatKuhlmann and Wehrmann (1984) found no responseto K in grain yield of cereals growing on these loesssoils even at very high levels of K application. Alsodifferent methods of soil analysis for available Kshowed no relationship to K fertilizer requirement. InIndia, Prasad (2009) has recently suggested that EKvalues are inadequate for fertilizer recommendationsbecause of the contributions of non-exchangeable(SEK) and subsoil K to uptake. Kuhlmann andBarraclough (1987) reported that winter wheat couldacquire 50% of its K from the subsoil. Certainlyalthough EK is used widely as a measure to determinesoil K availability and predict K fertilization needs ofcrops, its suitability and reliability is unsatisfactory insoils that contain 2:1 layer silicates and have theability to retain K as is the case of flooded soils usedfor rice production (Dobermann et al. 1996).

Plant available K can be affected by long-termchanges in total K in the soil. A simple calculationshows that in soils with a low total K content as insandy soils, rapid K depletion can occur overrelatively short periods if K removal is not balancedby regular K fertilization with mineral fertilizers or byadequate recycling of crop residues and organicmanures or both (Table 3). On this sandy soil, lowin K, with an annual negative balance of 40 kgKha−1,only 44 years are required to remove 25% of the stocksoil K. This so-called “potassium mining” is common.According to Hasan (2002), 72% of India`s agricul-tural area representing 266 districts are in immediateneed of K fertilization. Such imbalances in K are

Table 2 Average rate of K leaching in a sandy soil during thewinter seasons 1989/1990 until 1994/1995 as affected by theannual rate of K fertilization (Wulff et al. 1998)

K fertilization rate(kg K ha−1a−1)

K leaching(kg K ha−1a−1)

0 22

60 42

120 79

180 133

K content in topsoils: 0.1–3.3%=7 000–228 000 kg K ha−1

Required years for assumed depletion of 25%

Normal scenario: Topsoil 3.3%K

Balance: −5 kg K ha−1 a−1 228 800�255�100 ¼ 11 400 years ðe:g: clay soilÞ

Worst case scenario: Topsoil 0.1%K

Balance: −40 kg K ha−1 a−1 7 000�2540�100 ¼ 44 years ðe:g: sandy soilÞ

Table 3 Length of timerequired for 25% depletionof K from a topsoil with ahigh or low total K contentand a low or high negativeK balance sheet (the twomodel calculations shownormal and worst casescenarios)

Plant Soil (2010) 335:155–180 159

widespread in agriculture and can also be found inwestern states of Canada (Table 4). In contrast to K,the ratio of fertilizer use to that removed by harvestfor N and P is usually much higher. Imbalancebetween K and N is often exacerbated by the soleapplication or overuse of N fertilizer, a fact whichneeds to be stressed in agricultural practice.

On organic farms where the use of mineralfertilizers is strongly restricted, soil K status shouldbe carefully monitored. Not only has the immediate Krequirement for crop growth, including its beneficialeffects on biotic and abiotic stresses to be taken intoaccount (see below, “Plant aspects”) but additionallythe long-term K balance in the soil. As shown byMayer (1997), (Table 5), for an organically managedfarm in south Germany, a loss of 7 kgK/ha yard gatebalance between input and output over a 1 year periodensued which appears quite reasonable. In the fieldbalance over the same year of investigation, however,there was an extremely high internal loss which wasmore than five-fold greater at 36 kgK/ha representingan annual loss of almost 1 metric ton of K from thefarm i.e., (1195 – 239=956) kg K. This internal lossof K was traced back to K leaching during rainfallfrom manure heaps which had been temporarilydeposited on the field margins (Mayer 1997). Inter-estingly, drought-induced K deficiency symptoms innon-grasses such as legumes were observed on thisorganic farm.

Potassium in plants: present knowledge

Potassium (K) is the most abundant inorganic cationin plant tissues. In adequately supplied plants it maymake up about 6% of the dry matter or concentrationsof about 200 mM (Leigh and Wyn Jones 1984). K is

unique as a plant nutrient as it occurs exclusively inthe form of the free ion. Under K deficiencycytosolic K activity is maintained at the expenseof vacuolar K activity (Leigh 2001). Highestconcentrations of K are found in young developingtissues and reproductive organs indicative of its highactivity in cell metabolism and growth. K activatesnumerous enzymes including those involving energymetabolism, protein synthesis, and solute transport(Mengel and Kirkby 2001; Amtmann et al. 2008). Incells K is needed in the maintenance of transmem-brane voltage gradients for cytoplasmic pH homeo-stasis and in the transport of inorganic anions andmetabolites (see White and Karley 2010). In longdistance transport, K is the dominant cation withinthe xylem and phloem saps neutralizing inorganicand organic anions, conferring high K mobilitythroughout the entire plant (Jeschke et al. 1997).Uptake and accumulation of K by plant cells is theprimary driving force for their osmotic expansion(Mengel and Kirkby 2001).

The basic biochemical and physiological functionsof K have been are described in detail in the maintextbooks in plant nutrition (Marschner 1995; Mengeland Kirkby 2001; Epstein and Bloom 2005). Processesdescribed considered include osmoregulation andcell extension, stomatal movement, activation ofenzymes, protein synthesis, photosynthesis, phloemloading and transport and uptake. Uptake of K byroot cells from soil solution is a highly efficient processand not usually limiting to K uptake. Even when Kis in short supply the expression of genes encodinghigh-affinity K+ influx systems increases (Shin andSchachtman 2004). More recent findings and researchdevelopments concerning the role of K in biotic andabiotic stress mitigation in plants in relation to

Table 4 Potassium removal by crop harvest and application byfertilizer (M kg per Province) and the ratio (fertilizer use:removal by harvest) compiled for 3 provinces of West Canadain 1996

Province Potassium

Removal by harvest Fertilizer use Ratio (%)

Manitoba 331 92 28

Saskatchewan 640 59 9

Alberta 601 128 21

Table 5 Potassium balance sheets for an organically managedfarm (33.5 ha) at Stuttgart –Ruit, Germany measured at a farmlevel (yard gate balance) and at a field level (field balance) for1993/1994 (Mayer 1997)

Yard gate balance Field balance

kg a−1 kg ha−1 a−1 kg a−1 kg ha−1 a−1

Inputs 233 7 3910 117

Outputs 472 14 5105 152

Balance −239* −7 −1195* −36

• Internal farm K losses: 1195 – 239=956 kg K a−1

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agricultural practice are discussed in “Needs for futureresearch on potassium”.

Mild K deficiency in crops does not immediatelyresult in visible symptoms because of the high rate ofredistribution between mature and developing tissues.At first there is only a reduction in growth rate(hidden hunger) and only later do chlorosis andnecrosis begin in the more mature leaves. In manycrop species including maize and fruit trees thesesymptoms begin in the margins and tips of the leavesbut in others including some legumes irregularlydistributed spots occur on the leaves (Mengel andKirkby 2001). Plants suffering from K deficiencyshow decrease in turgor and become flaccid underwater stress particularly during the midday period.Plant roots sense or signal changes that occur after theonset of K deficiency but no major changes take placein biomass partitioning or root architecture as occursunder N and P deficiency. Arabidopsis roots respondto K deficiency by upregulation of high affinity Kinflux systems as mentioned above, and the produc-tion of reactive oxygen species (ROS) and ethylene,ROS being accumulated in discrete regions of the rootthat have been active in K+ uptake and translocation(Shin and Schachtman 2004).

In field crop nutrition, two of the most recognizedroles of K are in photosynthesis and the maintenanceof cell turgor in plants. Applying drought stress towheat plants at three levels of K supply at sub-optimal, optimal and supra-optimal rates, photosyn-thesis was shown to decrease under drought stress butthe effect was alleviated by the increased rate of Ksupply (Table 6) (Sen Gupta et al. 1989). Supply at2 mMK supported maximal photosynthesis in wellwatered plants but not under drought stress whereas at

the supra-optimal level of 6 mMK the effect of thedrought stress was much less severe. The practicalsignificance of this finding is the well-known greaterneed for K by crops that are subjected to droughtstress (see also “Role of K in drought and heatstress”).The primary effect of the higher K treatmentwas in maintaining the stromal K concentration of thechloroplasts to allow CO2 fixation. Under K deficientconditions photosynthesis is depressed as a conse-quence of sucrose accumulation in the leaves and itseffect on gene expression (Hermans et al. 2006).Depression of photosynthesis causes an excessiveaccumulation of light energy and photoreductants inthe chloroplasts which in turn leads to activationof molecular oxygen, the formation of reactiveoxygen species (ROS) and chloroplast damage(Cakmak 2005).

The importance of K as an osmoticum in main-taining turgor in crops is particularly evident fromN/K interaction studies in field grown crops (Milfordand Johnston 2007). A major determinant of growthand prerequisite for high yields in most arable crops isthe rapid expansion of the leaf canopy in the springfor the efficient capture of CO2 by photosynthesis andits conversion to sugars and dry matter. Nitrogen isthe major driver of leaf canopy expansion which isachieved by increase in cell division and cellexpansion i.e. cell number and cell volume, whichalso necessitates a corresponding uptake of K in theleaf tissues to maintain turgor. In field experiments asmuch as 10–15 t/ha more water was present in theshoots of cereals well supplied with N as comparedwith those that were not. For sugar beet the amountwas even greater with crops well supplied with Nhaving 30–35 t/ha more water than those with limitedN supply. This increase in hydration was expressed bythe presence of enhanced quantities of osmotic solutesin the cell vacuoles, particularly K to maintainadequate turgor for continued cell expansion andgrowth, in accordance with a higher K uptake.

In agreement with the classic experiments of Leighand Johnston (1983) with field grown spring barley,the concentration of K in the shoot tissue water ofsugar beet was fairly constant throughout the vegeta-tive growth. The value of about 230 mmol K per kgtissue water for sugar beet was also similar to thatfound for beets growing over a range of sites withdifferent soil types and under a wide range of growingconditions and N supply (Kirkby et al. 1987; Milford

Table 6 Effect of K+ supply (mM) to wheat plants grown in asand/peat mixture on photosynthesis of leaves at declining leafwater potentials (increasing drought stress) (Sen Gupta et al.1989)

K+ supply (mM) Photosynthesis (μmol CO2 m−2 s−1)

Leaf water potential (-MPa)

1.1 1.5 2.0 2.5

6.0 35 35 32 24

2.0 35 33 23 15

0.2 29 15 6 n.d.

Plant Soil (2010) 335:155–180 161

et al. 2008). The physiological basis for the interac-tion between N and K only becomes clear when K isexpressed on a water tissue basis. Expressed in termsof dry matter, K concentration declines during growthand is affected by N supply. The need for K uptake tomaintain the concentration of K in the cytoplasm andvacuole during growth also explains the erroneouslyused term in the older agronomic literature “luxuryconsumption of K”, which expressed increase in Kuptake without a corresponding increase in dry matteryield.

The important role of K in phloem loading andtransport needs special mention in relation to cropproduction. The stimulatory effect of both K and Mgon the activity of the plasma membrane boundATPase of the sieve tube cells is of crucial importance(Marschner 1995). The proton pumping ATPaselocated in the plasma membrane of the sieve tubecells creates a steep transmembrane potential gradientas well as a pH gradient between the lumen of thesieve tube (symplasm and apoplasm), the gradientacting as a driving force for the transport of sucrosefrom the apoplasm into the sieve tubes. Potassium,Mg, amino acids and sucrose are quantitatively themain constituents of the phloem sap which aretransported from the mature source leaf to sink sites(Jeschke et al. 1997). An adequate supply of K andMg in the leaves is thus essential in supplying sucroseto the roots to cover the energy requirement for rootgrowth and development as well as ion uptake(Cakmak et al. 1994). During the reproductive stagesof crop plants, K and Mg in source leaves play acritical role not only in ensuring an adequate supplyof sucrose but also in supplying K, Mg, N, S and P tothe filling of grains fruits and tubers. Recent findingsthat micronutrient demand (Zn, B, Cu) can also beparticularly high during the early reproductive growth(Kirkby and Römheld 2004) means that the transportof these nutrients into storage tissues also dependsclosely on the K (and Mg) status of source leaves.

Needs for future research on potassium

Lack of transfer of knowledge, between scientist andfarmer, of already well established research findingsconcerning soil and plant K, can present a majorlimiting step in agricultural production as referred toabove. A further limiting step, however, pointed out

by Cakmak and Schjoerring (2008) is that despite thekey roles of K in biochemical and physiologicalprocesses in plants which affect crop growth, therehas been surprisingly little published research on theimportance of K on crop production and nutritionalquality. From various discussions during the recentInternational Potassium Symposium at Bhubaneswar,Orissa, India (IPI-OUAT-IPNI Intern Symposium2009), it was also obvious that there are still futureneeds for fundamental work including study of K inmitigating various abiotic and biotic stresses as wellas the application of this work to basic field researchon K in soils and crops. Potassium also plays a veryimportant role in human and animal health particu-larly in relation dietary contents of Na and Mg. Belowwe discuss some of the current research developmentson K in soils, plants, and human and animal nutritionand discuss needs and prospects for future research.

Soil aspects

K fertilization recommendations

Exchangeable K (EK) and slowly exchangeable K(SEK) Soil extraction methods, particularly for ex-changeable K (EK) are widely used as the basis for Kfertilization recommendations for crops. These haveproved to be quite successful for many soils notcontaining 2:1 clay minerals where adequate calibra-tion has been carried out (Mengel and Kirkby 2001).However, when the contribution of SEK in soils israised by the presence of 2:1 layer silicates that havethe ability to retain K, the “power of prediction” usingEK soil extraction methods is lost.

On some such soils the SEK pool (i.e. K ininterlayer sites) can make a considerable contribution(80–100%) to available K to plants (Hinsinger 2002).As discussed above (“Potassium in soils: presentknowledge”), this pool plays a particularly importantrole under K-mining conditions when the EK is low.The significance of the contribution of SEK has beenunderestimated perhaps for two main reasons. In thefirst place unlike EK there is no easy routinelaboratory method for its determination and secondlythe means by which K release takes place frominterlayer sites is not well understood and in need offurther research. Reports in the older literatureindicate that cereals and grasses are more effectivethan dicotyledonous plants in exploiting interlayer

162 Plant Soil (2010) 335:155–180

sites for K (see Mengel and Kirkby 2001) whichmight relate to the higher root length density of thegrasses. Rengel and Damon (2008) in their recentreview deal with this different contribution of SEK ofcrop plants as a long-term process dependent ongenotype. There are observations that in general forexample sugar beet is more efficient than wheat andpotatoes in the use of SEK (Steingrobe and Claassen2000). Trehan and Sharma (2002) and Trehan et al.(2005) suggest that in contrast to K inefficient potatocultivars, efficient ones appear to bring about chem-ical mobilization of SEK most probably via secretedroot exudates. On the other hand Springob andRichter (1998) found that a drop of K concentrationbelow 4 μM in the rhizosphere solution triggers therelease of K from SEK.

Current research by Barré et al. (2007) investigatinginterlayer K in 2:1 clay minerals uses X-ray diffractiontechniques over short term periods to study the effectof plant roots on K depletion, aims to attempt to relateclay mineral modifications to plant uptake of interlayerK. Quantification of interlayer K dynamics is ofimportance in understanding the soil K cycle andcritical for modelling K acquisition by crops on soilscontaining 2:1 clay minerals. From a practical view-point, further work in this area might be useful inallowing a prediction of long-term K release capacityin field balance calculations (Öborn et al. 2005).Furthermore as also discussed by these authors, thepotential for particular crops to extract K from the SEKfraction should also be explored for possible introduc-tion as a green manure in the crop rotation.

Plant root—soil interactions and K availability Animportant reason to question EK and indeed also SEKas defined measures of K availability in soil, is theunderlying assumption often made that the supply ofK to plant roots is solely dependent on K availabilitywhich can be assessed by chemical extraction of airdried soil without taking into account the interactionof the living plant root. There is a general miscon-ception amongst some soil scientists that the predic-tion of suitable K fertilization rates is simply a matterof refining soil testing using chemical extractionprocedures. This approach, however, takes no accountof the importance of limitation of spatial availabilityof K as a consequence of variable root characteristics.Root morphology differs enormously between cropspecies especially between monocots and dicots and

between genotypes which may differ as for examplein root length and density and frequency of root hairs.Root hairs play a significant role in the acquisition ofK which is mainly transported from the bulk soil tothe root surface by diffusion in accordance with thelow K concentration in soil solution (usually less than1 mM). The presence of root hairs considerablyincreases the surface area of the root cylinder whichin turn steepens the K concentration gradient betweenthe bulk soil and root surface which drives K influx.In many plants, root hairs may contribute up to 70%of the total surface thus increasing the root cylindersurface area 27 fold (Jungk 2001). Root hairformation has energetic implications in relation toplant growth in that of all the possible ways ofincreasing root surface area, it is least metabolicallydemanding (Lynch and Ho 2005). The root length canalso vary considerably, that of winter wheat forexample being 6 times greater than that of the rootsof the potato crop which are relatively poor in roothairs (Johnston et al. 1998). Likewise vegetable cropswith shorter growth periods have smaller rootsystems. The high importance of root length densityin determining spatial availability of K for maizegrowing in a sandy soil has been demonstrated bymodelling work with data from a field experiment.A root length density greater than 2 cmcm−3

allowed delivery of K from 50% of the topsoilvolume which was reduced to only 10% when theroot length density fell below 1 cmcm−3 (Fussederand Kraus 1986).

Acquisition of K from soil is also dependent onnumerous physical and chemical soil factors whichto a large extent determine the development andspatial distribution of roots in the soil and thus theirability to acquire mineral nutrients. Soil factorsinhibiting root growth such as acute B deficiency,Al toxicity in acid soils, soil compaction, salinityand drought all depress K acquisition from the soilbecause of their effect in lowering spatial availabil-ity of K (Römheld and Neumann 2006). Forexample, Batey and McKenzie (2006) reported poorgrowth and low K content in reseeded grass as aconsequence of drought stress caused by surfacecompaction by over-cultivation of a moist fine sandyloam. Although the soil was adequately suppliedwith nutrients, the grass K content from thecompacted soil was only 1.3% as compared with4% of grass grown on a seedbed from the same soil

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which had not been compacted. Our own observa-tions of visual K deficiency symptoms occurring inlegumes and other dicotyledonous crops underdrought spells even on K rich soils, further empha-sise the importance of considering root growth andweather data in future applied research to improveevaluation of plant K availability. Under suchconditions of transient drought-induced K deficien-cy, investigation of foliar application of K might wellbe worthwhile. This lack of understanding inneglecting spatial K availability was apparent indiscussion at the recent International PotassiumSymposium at Bhubaneswa, Orissa (IPI-OUAT-IPNIIntern Symposium 2009), in relation to the effects oflow pH and possible Al toxicity in depressing rootgrowth in the soils of Orissa as the cause of inducedK deficiency.

The importance of root growth in relation to Kavailability is stressed from the results of themechanistic modelling experiment of Barber andMackay (1985) which separated the influences of soilmoisture on K uptake by corn (Zea mays L.) betweeneffects on root growth and the rate of K diffusion inthe soil. Lowering the volumetric soil moisture levelfrom 0.27 to 0.22 (i.e. from −33 to −7.5 kPa droughtconditions), induced low K acquisition which was duemainly to decrease in root length density as aconsequence of inhibited root growth in the dry soil(46–69%) and to a lesser extent to the lower effectivediffusion coefficient of K (11–27%).

An innovative approach for recommendation ofK fertilizer to soils of direct use to farmers hasbeen tested and partially applied in Germany asthe KALIPROG® system (Andres 1988). Use ofdata from a soil extraction method involving therelease of SEK in this information system is linkedwith site-specific factors such as amount and qualityof K-bearing minerals as well as weather factorsaffecting root growth. Extensive field trials overmany years for site-specific optimal K fertilizationallow the required recommendation (Andres andOrlovius 1989). For general application of thisKALIPROG® system further GIS data includingdata on mineralogy and long term weather forecast-ing for particular agricultural areas is needed as wellas calibration with field experiments. This appears tobe a topic for urgent research which if correctlyapplied could be of direct benefit to farmers. Theestablishment of this system for K recommendation

from these various parameters should bring to an endthe futile debate on the benefits of improved soilextraction methods in relation to K fertilizer use.

Modelling K acquisition during plant growth Thepioneering work of Barber in the USA and Nye in theUK , has been described in the publications of Tinkerand Nye (2000); Barber (1995) and Jungk andClaassen (1997), in which various mechanistic math-ematical models have been described to predictnutrient acquisition (usually P and K) by plants fromthe soil. These models take into account physiochem-ical processes in the soil as they influence thetransport of nutrients through the soil to the rhizo-sphere plasma membrane interface and uptake acrossthe plasma membrane. By and large, the models havebeen successful in their prediction of nutrient uptakeunder conditions of adequate nutrient supply but haveunder-predicted in nutrient restricted conditions main-ly because of morphological and physiological plantadaptations which increase nutrient acquisition nottaken into account by the model. These adaptationsinclude increase in effective root surface area as in thedevelopment of root hairs, upregulation of nutrienttransporters in the plasma membrane and the releaseof root exudates into the rhizosphere to increasenutrient concentration in soil solution by their reactionwith the soil. Although adaptive root responses are oflesser importance for K as compared with N and Pdeficiency, when K supply is restricted, root hairproliferation is increased and K transport across theplasma membrane is upregulated (see White andKarley 2010).

An example of a model to predict K uptake duringgrowth is that of Claassen (1994) which takes intoaccount nutrient uptake by both roots and root hairs,and has been used recently in pot experiments tostudy K uptake efficiency and dynamics in therhizosphere of maize, wheat and sugar beet (Samalet al. 2010). The model is based on three basicprocesses: (i) release of K from the solid phase intothe solution phase, which is governed by sorption anddesorption processes, (ii) transport of K by mass flowand diffusion, mainly diffusion, and (iii) K uptakeinto the root which depends on the nutrient concen-tration in the soil solution and is measured by amodified Michaelis-Menten equation. The radialdistribution of root hairs around the root is alsoaccounted for and an influx established. In the

164 Plant Soil (2010) 335:155–180

experiment plants were grown on a low K soil withand without K. Soil parameters used in the modelcalculation included: mean root radius, water influx,relative shoot growth rate, relative root growth rate,root hair distribution around the root, plant parametersrelated to uptake kinetics and net K influx.

The model demonstrated major differences in Kuptake efficiency for the three crop species. Sugarbeet and wheat maintained a higher shoot K concen-tration as compared with maize and therefore had ahigher K uptake efficiency. Wheat acquired more Kfrom the soil because of its higher root length to shootdry weight ratio whereas sugar beet accumulatedmore K in the shoot because of a 3- to 4-fold higher Kinflux in comparison with wheat and maize. At thehigher K supply, the model closely predicted K influxbut under-predicted it at low K supply and particu-larly so for sugar beet most probably because of anincrease in K concentration in the rhizosphereinduced by chemical mobilization of K by rootexudates. Likewise, a simulated mechanistic modelof K uptake at low K supply by field grown sugarbeet throughout the growing season accounted foronly 34% of the K uptake (Dessougi et al. 2002).These finding confirm the earlier work of Steingrobeand Claassen (2000) and further research is needed toelucidate the underlying mechanisms between sugarbeet roots and soil at low K supply by which Krelease takes place into the soil solution.

One easy-to-calibrate mechanistic model forcalculating arable crop response to K fertilizer in thefield was described by Greenwood and Karpinets(1997a). The model calculates for each day theincrease in crop K-uptake and growth and changesin K activity ratio of the soil solution, exchangeablesoil K and fixed soil K. The validity of the model wastested against the results of single year multi level Kfield experiments (Greenwood and Karpinets 1997b).Measurements of plant mass, % K of the plant and Kactivity ratio in the soil were made at intervals duringthe growing season and at harvest on spring wheat,summer cabbage and turnips. The degree of agree-ment between simulation and measurement wassubstantial. Some discrepancies did occur, however,interestingly enough in context of the above discus-sion, on root growth, probably because of uneven rootdistribution. One of the assumptions in the model wasthat the roots were evenly distributed throughout therooting layer and that K was not taken up from the

subsoil. Nevertheless the model provides an excellentapproach and is of direct value to the farmer.Simulations of the model indicate that in centralEngland, no response of 10 crops to K fertilizerwould be likely on soils containing more than 170 mgof 1 M ammonium nitrate extractable K /kg soil andhaving clay contents between 15 and 45% (withoutany major contribution of K from interlayer sites). Asimplified version of the model runs on the Internetat: www.qpais.co.uk/moda-djg/potash.htm.

Plant aspects

Plant breeding for K efficiency

Plant breeding of crops has for generations beencarried out in non limiting environments which hasled to the selection of highly productive genotypesthat are also highly demanding of plant nutrientsincluding K. Interest is now focusing on improvingefficiency of fertilizer application and timing fornutrient uptake as well as the introduction of nutrientefficient cultivars capable of yielding on poorer soilswith low fertilizer regimes as often occurs in thedeveloping countries (Lynch 2007). Genotypic differ-ences in efficiency of K uptake and utilization havebeen reported for all major economically importantcrop plants and the underlying physiological mecha-nisms for these differences have been reviewed indetail by Rengel and Damon (2008). These authorsdefine K efficiency as the capacity of a genotype togrow and yield well in soils of low K availability.Both efficiency in K uptake and utilization of Kwithin the plant are involved.

Efficiency in uptake particularly of the lessmobile nutrients like K and P is much dependenton root architecture i.e. the configuration of the rootsystem in time and space (Lynch 1995). Root traitsdetermining genetic differences in P acquisition bybean (Phaseolus vulgaris L.) have been identified indetail (Bates and Lynch 2001) and the findings ofLynch and his colleagues have been successfullyapplied to breeding P efficient genotypes used in thefield as for example, P efficient soybean lines whichhave yielded 15–50% more than existing genotypesin P deficient soils in south China (Yan et al. 2006).Comparative studies for K should be worthwhilebecause the acquisition of both nutrients requires a

Plant Soil (2010) 335:155–180 165

large surface contact area between roots and topsoiland exploration of the subsoil where water may bemore available. Root hair formation differs betweencrop genotypes for K (Jungk 2001) but there appearsto be no literature assessing the formation of roothairs as a mechanism for intraspecific differences inK uptake (Rengel and Damon 2008).

Uptake of K across the plasma membrane is ahighly efficient process and not considered as alimiting step in acquisition under adequate K supply.Under K deficient conditions however, plant speciesand genotypes differ in capacity in the high affinityuptake mechanism. In potato grown under K defi-ciency, a K efficient genotype had about a two-foldhigher K uptake rate than a K inefficient one (Trehanand Sharma 2002). Under K deficiency genotypesmay enhance K uptake either by morphological orphysiological response. In comparing two strains oftomato (Chen and Gabelman 1995) showed that onestrain responded morphologically by proliferatingroot length thereby producing greater root absorbingsurface areas to capture K. The other, a physiologicalresponse was demonstrated by high net K-influxcoupled with low pH around root surfaces, presum-ably a K+/H+ exchange with high accumulation of Kin the apoplast.

Genotypic differences in capacity to utilize K havebeen attributed to (1) differences in partitioning andredistribution of K at cellular and whole plant levels,(2) the substitution of K by other ions e g Na in thevacuole particularly important under salinity (3) thepartitioning of resources into the economic product(Rengel and Damon 2008). Differences in K distri-bution between genotypes can influence capacity toproduce high economic yield per unit K uptake. Forexample, Yang et al. (2004) reported that K- efficientrice genotypes grown under conditions of low supplyof K, had a two fold higher concentration of K in thelower leaves and a 30% higher concentration in theupper leaves as compared with inefficient –K ricecultivars at the booting stage. These higher Kconcentrations in the leaves (especially the lowerleaves) of the K-efficient genotypes were associatedwith higher RuBP carboxylase activities and netphotosynthetic rates allowing the leaves to maintaina higher photosynthetic capacity during grain filling.Damon and Rengel (2007) showed that in terms ofgrain yield of field and glasshouse grown wheatgenotypes, the main factors determining tolerance to

K deficiency were a high harvest index at Kdeficiency and the high ratio of harvest index atdeficient to adequate K supply.

In general from an agronomic viewpoint, high Kcrop use efficiency is beneficial particularly on soilslow in K availability. In contrast, however, asdiscussed by Cakmak (2005), crops of high Knutritional status are required to provide resistanceto the various common stress events which areconsidered below. The same is valid for the highK/Na ratio required in food products in the humandiet. It also has to be remembered in plant breedingprogrammes aimed at raising K use efficiency, thatunlike lack of efficiency in N and P use by cropswhich can be detrimental to the environment, this isnot so for K which is completely benign, posing nothreat to human health or the quality of natural waters.

Role of potassium in stress mitigation

Crops exposed to various environmental stress factorssuch as drought, heat, high light, chilling or salt allshow increased formation of reactive oxygen species(ROS), Cakmak (2005). This formation of ROS takesplace particularly during photosynthetic electrontransport as well as by activation of membrane-bound NAD(P)H oxidases (Jones et al. 2000). Thereis increasing evidence from the literature that opti-mizing the K nutritional status of plants can reducethis detrimental build up of ROS either by enhancingphotosynthetic electron transport or inhibiting themembrane-bound NAD(P)H oxidases.

It is well documented that K deficient plants aremore susceptible to high light intensity with associ-ated occurrence of photooxidative damage such aschlorosis and necrosis (Marschner and Cakmak1989). One reason for this enhanced ROS formationunder high light is the inhibition of photosynthesisand photoassimilate export from the leaf under Kdeficiency. Inhibition of sugar export via phloemprevents root morphological adaptation of crop plantsto K deficient stress conditions, in marked contrast toN and P stresses where sugar translocation to theroots is not restricted. Inhibited sugar export under Kdeficiency also restricts shoot growth and the forma-tion of reproductive organs such as grains.

There is ample evidence that ROS production israised in plants low in K exposed to various environ-mental stresses as for example to low temperature,

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drought and salinity (Cakmak and Engels 1999;Cakmak 2005), From these above examples it can beconcluded that optimizing plant K nutritional status isneeded to raise stress tolerance of crop plants asdiscussed in more detail below. This conclusion isfurther underlined by the involvement of K in stresssignalling

Role of potassium in stress signalling

Evidence is emerging from studies in molecularbiology that K might play a specific regulatory rolein plant stress responses (Ashley et al. 2006; Wangand Wu 2010). These authors review links betweenlow K plant status and activation of signallingcascades. Low K status not only triggers an up-regulation of K transporters, but also involves thesynthesis of molecules including reactive oxygenspecies (ROS) and the phytohormones jasmonic acid(JA), ethylene and auxin. In addition to up-regulation of transport proteins and adjustment ofmetabolic processes, K deprivation triggers develop-mental responses in roots, all these strategies en-abling plants to survive and compete in nutrientenvironments in which the availability of K mayvary. Evidence of changes in expressions of tran-scripts encoding K+ transporters and channels inresponse to ROS and phytohormones are alsosuggestive that K may play a specific regulatoryrole in plant stress responses which is very much inaccord with field observations as discussed in thesections below. The hypothetical model shown inFig. 2 is derived from the recent findings of Cheonget al. (2007) and Jung et al. (2009) of molecularchanges in response to K deficiency in Arabidopsisthaliana. The work of Cheong and his colleaguesindicates that in K deprived plants, drought—induced ABA may produce ROS which in conse-quence may trigger Ca flows as second messengerand subsequently the uptake of K by roots and theregulation of stomatal guard cells. This Ca signallingwhich regulates leaf transpiration and root K uptakeinvolves membrane localized Ca sensor interactingproteins. Jung and co-workers reported ethyleneproduction in K deprived plants. This phytohormonesignals stimulated production of (ROS) and isimportant for changes in root morphology and wholeplant tolerance to low K supply. Our scientificunderstanding of the role of K in stress mitigation

will—without doubt—improve in the near futurewhich will be of major importance for agriculture. Inthe following subsections below these needs forresearch are discussed in relation to well-knownspecific stress situations.

Role of K in disease and pest resistance

It is widely accepted that in general, high K status incrops decreases the incidence of diseases and pests(Perrenoud 1990; Prabhu et al. 2007; Bergmann1992). This benefit of K has been explained by itseffect on primary metabolism by favouring thesynthesis of high molecular weight compounds(proteins, starch and cellulose) thereby depressingthe concentrations of soluble sugars, organic acids,amino acids and amides in plant tissues. These lowmolecular weight compounds necessary for feedingpathogens and insects are thus more prevalent in K

Drought(ABA)

Low K status

ROS(Reactive oxygen species)

Root morphology

(Root hair length)

Transporterfor K uptake

K uptakeefficiency

Stomatalopening

Ethylene

Ca sensingproteins

Enhancedstress tolerance

Fig. 2 Schematic model for a proposed common signallingpathway induced by drought and low K nutritional status ofplants regulating K uptake and drought stress tolerance. (fromfindings of Jung et al. 2009 and Cheong et al. 2007)

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deficient plants which are thus more vulnerable todisease and pest attack (Marschner 1995). Forexample on K deficient soils, cotton and other cropscan be susceptible to Fusarium wilt and root rot,caused by Fusarium oxysporum sp.; application of Keither before or after planting has been shown to beequally effective in reducing this incidence (Prabhu etal. 2007). As pointed out by Amtmann et al. (2008),however, because of the variability of both diseasesusceptibility and metabolic profiles in K deficientplants, it is impossible at this stage to prove theircausal relationship and there is a great need for suchbasic and applied studies to be undertaken inagricultural crops.

This often observed variability in the effect of K onincidence of diseases and pests certainly relates to thedifferences in K nutritional status of plants or to theamounts and the forms of applied K or to both thesefactors (Amtmann et al. 2008; Perrenoud 1990). Inmost cases of compiled observations of experimentaltrials with increasing supply of K, the K nutritionalstatus of plants has either not been analysed or notgiven (Huber and Arny 1985; Kiraly 1976; Prabhu etal. 2007). Some recent studies on the incidence ofblack spot in potatoes as affected by K nutritionalstatus as reported by Ebert at the IPI-OUAT-IPNISymposium, held in Orissa (2009), are exceptions tothis. The frequent lack of data on the K nutritionalstatus of plants in many investigations, however,means that it is often not possible to relate the effectsof K treatment adequately to disease incidence. Inpractice this is required for a cost-benefit calculationfor the farmer (Amtmann et al. 2008) which alsoneeds to take into account other aspects of stressmitigation by K supply as discussed below. There isthus a real need for more detailed and comprehensivedata from applied research and field experimentsrelating K supply to plant disease.

The plasma membrane is not only a barrier toions and water transport but is also a recognitionsite for potential pathogenic invaders of plant cells.As a consequence of such possible attack, changesin the membrane potential with concurrent rise incytoplasmic Ca occur within seconds, which inturn acts as a second messenger triggering anumber of downstream events (Yang et al. 1997).Calcium transporting proteins can respond to otherearly defence signals such as H2O2 (Foreman et al.2003; Scheel 1998) and K is likely to be involved in

all these signals. The observations of Shin andSchachtman (2004) indicate that K deficiency resultsin early defence signalling including phytohormonessuch as ethylene in Arabidopsis roots. In addition,genes related to jasmonic acid are also induced atlow K status (Lorenzo et al. 2003; Armengaud et al.2004; Schachtman and Shin 2006).

Following the observations of Amtmann et al.(2008) and others that K deficiency results in earlydefence signalling, there is need to consider howthese nutrition- and pathogen-induced responseswithin general signalling networks may be applicableto agricultural practice. Basic research is necessary toconfirm that the K deficiency-induced changes intranscripts, metabolites and hormones in the defencemechanisms of the model plant Arabidopsis thalianais similar to those in crop plants. Amtmann et al.(2008) conclude that even without genetic engineer-ing, available data could be useful for improvingtiming of K fertilizer applications. They suggest alimited but essential supply of K early in the growthseason followed by K depletion at a later growth stagecould be a means to strengthen the inherent defencepotential of the plants to pathogens. This suggestedfertilization strategy is far from that of currentthinking of farmers and their consultants. This veryinteresting and new aspect on K-disease interactionsemphasises the urgent need for further collaborativeresearch between molecular biologists, plant nutri-tionists and agronomists.

Role of K in frost resistance

Both chilling and frost stress events result inphotooxidative damage to chloroplasts as a conse-quence of high light energy absorbance in excess ofthe capacity of chloroplasts to use it for CO2 fixationat low temperature. This excess energy is used forROS formation (Huner et al. 1998; Foyer et al. 2002)which impairs the photosynthetic electron transportchain, stomatal conductance and rubisco activity(Allen and Ort 2001).

The role of K in protecting crops against frostdamage has been recognised for many years anddiscussed in plant nutrition textbooks (Bergmann1992; Marschner 1995). This alleviating effect of Kis shown in Table 7 from results of a field experimenton potato growing on alluvial soils varying in K statusin Punjab, India (Grewal and Singh 1980). Potassium

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fertilization increased frost resistance on all three soilsand particularly so on the soil of lowest K status. Themarked effect of increasing K fertilizer application inmitigating frost damage on the soil of medium Kstatus but without effect on tuber yield is indicative ofthe requirement of the higher K supply to raise frostresistance at low temperature In this experimentwhich included 14 alluvial soils varying in availableK, frost damage was inversely related to the availableK content of the soils and the K concentration in thepotato leaves, damage being significantly reduced byK fertilization. Similar effects have been reported bySharma and Sud (2001). In various non glasshousegrown vegetable crops (tomato, pepper, egg plants) attemperature ranging from 4°C to 16°C, (Hakerlerler etal. 1997) have observed that increasing K fertilizeruse raised low-temperature-stress tolerance whichresulted in as much as 2 fold increases in yield.

In agreement with these findings, the benefit ofhigher K tissue concentrations on yield and chillingdamage on white carnation has been reported byYerminyahu and Kafkafi 1990 (cited by Kant andKafkafi 2002). Their results showed that plants withwhat might be regarded as high K tissue concentrationsunder non-stress situations can be economically ofadvantage to the farmers by acting as an insurancestrategy against unexpected climatic events. At lowerbut normally acceptable K tissue concentrations, onlyone night of chilling temperature can cause severeenough damage to the crop to be equivalent to thefertilizer cost for the entire season. Interestingly theeffects of this damage on the stem of the carnation isnot obvious until several weeks after the lowtemperature stress event.

Nowadays in farming practice frost resistance canbe a critical factor in the early (late spring frosts) aswell as the late season (early autumn frosts) particu-larly in regions with short vegetation periods. Manyfarmers are thus at the mercy of increasing frostdamage. Various observations have been made whichindicate alleviating effects or even the prevention offrost damage by the application of various cocktailscontaining K together with other mineral nutrientsincluding Ca, P and micronutrients. The beneficialeffects of these cocktails have been obtained by bothpre- and interestingly also, post-frost applications offoliar sprays as well as by seed dressing (RandySaskiw, Omex Inc., 2009, pers. comm.).

Experiments in East Germany, Ukraine and Russiaon winter rape (canola) have demonstrated themitigating effect of Cu on frost damage when appliedas a foliar spray, particularly under conditions ofadequate K fertilizer supply. In wheat this beneficialeffect of Cu could be further enhanced by supple-ments of B (Bernhard Bauer 2009, pers. comm.). Allthese field observations are in accordance withreports by Bergmann (1992) and Bunje (1979) andstrongly suggest that K should not be considered inisolation in relation to its effect in raising frostresistance.

A major function of K as an osmoticum is themaintenance of a high concentration of K in the cellsap thus lowering its freezing point. Additionally theactivities of numerous enzymes which might play apart in frost resistance are also dependent on adequateK cytoplasmic concentration (Kant and Kafkafi2002). The higher ratio of unsaturated to saturatedfatty acids in phospholipid rich cell membranes in

K status of the site K fertilization rate (kg ha−1)

0 42 84

low Frost damagea 65 26 12

K content 1.64 1.96 2.85

Tuber yield 18.0 22.9 29.6

medium Frost damage 52 30 4

K content 2.28 2.80 2.80

Tuber yield 19.8 26.0 26.3

high Frost damage 12 12 0

K content 2.61 2.79 2.82

Tuber yield 20.7 22.4 23.4

Table 7 Effect of increas-ing K supply on frostdamage (%) and K contentof leaves (mg g−1 DW)and tuber yield (t ha−1) ofpotato on sites with differentsoil K status (Grewal andSingh 1980)

a Percentage of foliage dam-aged by frost in the field

Plant Soil (2010) 335:155–180 169

plants of high K status, can also partially explainraised frost resistance as a consequence of enhancedmembrane fluidity. Other mineral nutrients than Kalso possess distinct physiological properties of directrelevance to frost resistance. Boron, Zn and inexceptional cases also Ca may raise frost resistanceby stabilizing cell membranes. Additionally coldinduced stomatal closure induced by apoplasticcalcium uptake by guard cells (Wilkinson et al.2001), can contribute decisively to chilling toleranceand protection of leaves from dehydration. Copper,Mn and Zn may also increase frost resistance in theirrole in superoxide dismutase enzymes which detoxifyoxygen radicals thereby preventing damage to mem-branes and other cellular constituents (Cakmak 2000,Kant and Kafkafi 2002). The functions of thesemineral nutrients in plant metabolism justifies theiruse in nutrient cocktails in mitigating frost resistanceboth on scientific grounds and in accord with fieldobservations as discussed above. However, much isstill to be learned as for example the interrelationshipsbetween the micronutrients and K to enable adequaterecommendations to farmers. Basic as well as appliedresearch is urgently needed in this area.

Role of K in drought and heat stress

Worldwide, crops are increasingly being exposed todrought and high temperature stresses with enhancedformation of ROS and corresponding leaf damage(Foyer et al. 2002; Cakmak 2005). The hypotheticalmodel in Fig. 2 tentatively indicates the interplay ofdrought-induced ABA and low K nutritional status ofplants in stress signalling. Drought and heat stressesare often considered together as a drought-heatsyndrome because they often occur simultaneously,but this is not always so. As discussed by Halford(2009), plants also have to cope with hot conditionswhere water is not limiting which is of particularrelevance to crops growing under irrigation. In manyparts of the world, especially in temperate conditions,however, drought and heat stresses act together torestrict agricultural production. In the “high maizeyield” model by Yang et al. (2006) therefore bothaspects—drought and heat—are treated separately sothat farmers can make appropriate allowance for thesetwo stress factors.

During crop production various plant physiologicaland soil aspects have to be considered in mitigating or

preventing damage by drought or heat stresses or bothas illustrated in Fig. 3. The numbers in circles on thisfigure refer to processes discussed in the subsectionsbelow in all of which K is directly or indirectlyinvolved. In general, maintaining adequate K plantnutritional status is vital in adaptation to drought (SenGupta et al. 1989; Kant and Kafkafi 2002; Cakmak2005). When drought impedes K acquisition byrestricting root growth a vicious circle comes intoplay in which the resulting lower plant K nutritionalstatus further depresses physiological resistance todrought and the acquisition of K. The particularrequirement for additional K fertilization underdrought conditions is often not appreciated by farmers(see also “Introduction”).

Forced deep rooting In suitable soils a worthwhileapproach drought resistance of crop plants is toinduce deeper rooting to allow access to availablewater at lower depths in the soil profile. This can beachieved by deep placement of K fertilizer togetherwith small supplements of mineral nutrients with root-signalling functions such as P or N or both thesenutrients to encourage root growth, because K itselfdoes not have a root-signalling function (Drew 1975;Kirkby et al. 2009). Ensuring adequate supply of Kduring drought events is essential in supporting therole of K in translocation of photoassimilates to feed

light energy

rain infiltration

Detoxification oftoxic oxygen radicals

Improved translocation of photoassimilates

5

6

2

1

4

3

Water storage

Deep water acquisition

Optimized stomata opening and closure (better water use efficiency)

Protection against dehydration of shoot tissue

Fig. 3 Possible factors for an improved drought and/or heatresistance of plants (numbers in circles refer to subsections ofsection “Role of K in drought and heat stress”)

170 Plant Soil (2010) 335:155–180

root growth. This need for K is evident from thefindings of Egilla et al. (2001) in experiments withChinese Hibiscus, (Hibiscus rosa-sinensis cv Lepre-chaun), growing under various K regimes. Rootsurvivial was markedly reduced when water supplywas limited and K supply low, an adequate K supplybeing essential to enhance drought resistance andincrease root longevity. The benefits of deep Kfertilizer placement have already been demonstratedin some field experiments in which K fertilizerplacement was achieved at a depth between 25 and45 cm. The technique and the economical evaluationof deep rooting still need further investigation undervaried conditions including high and low inputagricultural systems.

Improved rainwater capture To increase plant Kacquisition particularly from depth from the soilprofile requires high rainfall infiltration as well ashigh water storage capacity within the profile, thelatter being dependent on soil texture and structureand to some extent also on soil organic matter(Hermann et al. 1994). In this respect the recyclingof K-rich crop residues serves a double function insupplying K and supporting the organic matter statusof the soil.

Protection against tissue dehydration It is welldocumented that under low K nutritional status,particularly during the midday period, leaf damagecan take place due to wilting with subsequent tissuedehydration and necrosis. The general physiologicalfunction of K in plants in maintaining water relations(osmotic regulation) has been discussed in “Potassiumin plants: present knowledge” and is particularlyimportant for optimal photosynthetic activity asshown by the findings of Sen Gupta et al. (1989)(Table 6). Under drought stress events it is essentialthat leaf K status is adequate to counteract the“vicious cycle” mentioned above.

Regulation of stomatal opening and closing AdequateK nutritional status of crop plants is closely associatedwith plant water use efficiency as already discussed in“Potassium in plants: present knowledge” (Thiel andWolf 1997). Much work has been carried out on thephysiology of K in relation to stomatal movement.The transport of water and potassium from roots toshoots mediates in CO2-water exchange governed by

transpiration through the stomatal pores. In natro-philic crop plants like sugar beet, Na as well as Khas to be considered in adaptation of stomatalclosure and opening under drought (Hampe andMarschner 1982). It seems to the authors that nofurther basic research is needed in this area.

Detoxification of oxygen radicals Under high lightintensity, increased formation of toxic oxygen radicalscan be bring about damage to leaves as chlorosisparticularly if photosynthate transport is limited as aconsequence of K, Mg or Zn deficiency (Cakmak2005). Such damage by high sunlight has beenreported as sunscald in fig fruits of low K status inTurkey by Irget et al. (2008) clearly showing theneed for adequate K nutrition under these conditions.In addition to K, however, various micronutrientsincluding Zn, B, Cu and Mn are also of vitalimportance in the detoxification of oxygen radicals(Marschner and Cakmak 1989; Cakmak 2005). Thisis another example showing the need to consider Knot in isolation but together with other mineralnutrients in mitigating heat/light stress as well asdrought stress in future applied research.

Enhanced translocation of photoassimilates Duringthe reproductive stage of crop growth the highdemand for photoassimilates by developing seedsand fruits is often accompanied by severe chlorosis inthe leaves (Table 1). These chlorotic symptoms arethe consequence of inhibited translocation of photo-assimilates from leaves via the phloem to the seeds orfruits and are observed particularly at low nutritionalstatus in K, Mg or Zn (Marschner and Cakmak 1989,see also above “Soil aspects”). As proposed byCakmak (2005) farmers should ensure that leafconcentrations of both K and Mg are adequate andif necessary make foliar applications of K and Mgseparately to mitigate against such chlorotic symp-toms during reproductive growth. In wheat such latefoliar application of Mg has been shown to preventMg chlorosis under drought events (Römheld andKirkby 2007). As discussed during the IPI Intern.Symposium (2006) a late K foliar application inbanana and sugar cane increased the yield or at leastthe sugar content in harvested products (Yadov 2006;Kumar et al. 2006). Without doubt further fieldstudies with different crops are needed in this areaof applied research.

Plant Soil (2010) 335:155–180 171

Role of potassium in salt stress resistance

Detrimental effects of salt stress on growth of cropplants are an increasing problem for agriculture,particularly on irrigated land (Kant and Kafkafi2002, Shabala and Cuin 2008). There are twocomponents to this detrimental effect, a short termosmotic effect with consequence of decreasing wateravailability to plants and a long term ionic effect,which results in salt toxicity (mainly Na and Cl) anddeficiencies of other mineral nutrients particularly Kand Ca (Kafkafi and Bernstein 1996). Roots, directlyexposed to a saline environment, react by restrictinggrowth as a consequence of a water deficit i.e. lowerwater availability caused by the more negative waterpotential in the rooting medium. This in turn results inlower nutrient uptake and inhibited translocation ofmineral nutrients to the shoot in general and of K inparticular. A lowering of photosynthetic activity is aconsequence. Thus under salinity, closure of stomataand inhibited photosynthetic activity due to lower Knutritional status induces the formation of toxicoxygen radicals (Cakmak 2005). A higher K supplyis thus needed to counteract this effect under salineconditions (Abogadallah et al. 2010). As pointed outby Shabala and Cuin (2008) measures for mitigationof salinity should not focus only on lowering Naaccumulation in photosynthetic active shoot tissue butrather on K homeostasis maintaining a high K/Naratio (Rubio et al. 2010) by preventing K losses by Naand/or Na-induced Ca deficiency.

Programmed cell death (PCD) has been proved tooccur in response to biotic and particularly to variousabiotic stresses such as salinity (Shabala 2009). Thisresponse seems to be ion-specific (induced by Na+)and not due to the osmotic component of elevated saltconcentration (Huh et al. 2002). As a consequence ofmembrane depolarization, massive K efflux by theoutward-rectifying K+ channels, (KORCs) can beobserved. In this PCD induced by salinity (NaCl),Zn can play an additional role by increasing thecytosolic K+/Na+ ratio (Shabala 2009). It is alsosuggested that ROS and some plant hormones (e.g.ethylene, jasmonic acid) are involved in regulatingsalt-induced PCD. However, further direct experi-ments particularly with crop plants rather thanArabidopsis are needed to reveal the full complexityand cross-talks between multiple pathways control-ling salt-induced PCD in plant cells (Shabala 2009).

Of special interest is that as a consequence of salt-induced PCD, primary roots might be eliminatedand new better salt-adapted secondary roots formedfor an adequate nutrient and water acquisition (Huhet al. 2002).

Common measures in practical agriculture toreduce salinity problems for crops include Casupplementation as gypsum and supplying adequaterates of K fertilizer application rate, both of whichact to reduce salinity problems by maintaining Khomeostasis. The ameliorating effect of Ca isdependent on its role in improving soil structure viaclay flocculation as well as in preventing NaClinduced loss of K from plant roots as K+ efflux viaKORCs (Shabala et al. 2006). Kaya et al. (2001)have shown in tomato that foliar application of Ksalts is also able to counteract salinity-induceddetrimental effects on plant growth, water use andmembrane permeability. Besides raising K and Casupply there are also reports of positive effects ofboron and specific biofertilizers (Nabti et al. 2007)on mitigation of salt problems, particularly via seedpriming. The positive effect of Si supplementationto barley plants grown in nutrient solution under saltstress has been shown to result from increasing plantK nutritional status and antioxidant enzyme activi-ties (Liang 1999; Liang et al. 2003). In view of thefunction of various mineral nutrients in relationto salt tolerance by stabilizing plant membranes(Ca, B) and by depressing the formation of stressinduced oxygen radicals or their detoxification (K,Zn, Cu, Mn, P), an integrated approach is needed toconsider all these nutrients both in basic andapplied research.

Role of potassium in crop quality

The physiological basis for the need of adequate Kstatus of plants in quality development of crops iswell recognised. To a large extent it relates to thespecific effects of K which include: increasingphotosynthesis as consequence of a more efficientphotosynthetic activity, increasing leaf size andnumber and more effective translocation of photo-assimilates and amino N compounds into reproduc-tive organs via the phloem (Cakmak 2005; Pettigrew2008). An immense number of publications reportthis positive role of adequate K supply in raising thequality of various crop plants (e.g. Kumar et al. 2006;

172 Plant Soil (2010) 335:155–180

Pettigrew 2008). Yadov (2006) very appropriately hasdescribed K as the “quality element”. From thesenumerous reports on the role of K on crop quality itseems reasonable to conclude that, rather thancarrying out more applied research in this area, thereis a much greater need to make the farmingcommunity more aware of the importance of thebenefits of maintaining an adequate K status in cropplants. The widespread lack of informed recommen-dation to farmers regarding K fertilizer use as referredto in the examples of kiwi orchards in Italy andtomato production in Chinese greenhouses (Table 1,chapter 2) underlines this conclusion.

In order to enhance crop quality, there is a need forboth a greater as well as a more efficient use of K(Pettigrew 2008). Increasing uptake efficiency canraise both yields and quality, particularly underdrought. However, in using K more efficiently thereis a need to avoid long-term K mining by balancing Kremoval from the soil by appropriate K replacementas already considered in “Potassium in soils: presentknowledge”. As discussed by Cassman (1998), cropgenotypes with longer or improved root systemscould be used to achieve more efficient removal ofnative soil K or applied K fertilizer. Raising efficiencyof K utilization by developing specific crop genotypeswith lower K demands, has the possible consequenceof an undesired decline of the K/Na ratio in crops andhence also in the human diet. The impact of thisdecline on human health is discussed below in“Human and animal nutritional aspects”.

Human and animal nutritional aspects

As well as being of fundamental importance for plantsand crops, K is an essential element for animals andhuman beings, responsible among other things for anadequate electrolytic and energy status of cells and inparticular of muscle cells. The K status of the animalbody is well regulated via intake or resorption fromthe gastrointestinal tract and excretion (Serfass andManatt 1985; Preston and Linsner 1985). Thisoptimal regulation means that there are no majorproblems associated with the K status of animals andhuman beings as long as the K intake from the diet isguaranteed by an adequate supply of fodder or in thecase of humans, by fruit and vegetables.

The human dietary intake of K, however, is oftentoo low at about one third of evolutionary intake (He

and MacGregor 2008). It is very low for example inrural populations in developing countries in whichthe staple diet is dominated by low K cerealproducts. In the more affluent modern societies thestrong decline relates to great increase in consump-tion of processed food and decrease in fruit andvegetables in the daily diet, a change also linked toan increase in prevalence of health problems. Theseproblems relate not directly to the lower K intake perse but are rather associated with the dramaticallyincreased intake of sodium (Na) mainly as NaClcausing elevated blood pressure, cardiovascular andkidney diseases, hypercalciuria and osteoporosis (Heand MacGregor 2008).

The benefits of increasing intake of K in the humandiet (Demingé et al. 2004; He and MacGregor 2008)may be achieved by raising K concentration of foodcrops and/or K salt additions to processed foods. Thismay be regarded as an important challenge for thefood industry, however, account must also be taken ofK-induced Mg deficiency effects in animals andhuman beings. Raising dietary K status restricts Mgre-sorption from the gastrointestinal tract and thusalso the metabolic function of Mg in cells. The realchallenge for the food industry and for plant researchis to reduce Na intake and at the same time to increasedietary intake of a well balanced supply of Mg and K.Food supplements on the market focused on humanhealth take into account this aspect of balanced foodfortification by supplying Mg and K together with Zn.For future research the following areas need to beconsidered:

Potassium/magnesium ratio in food and fodder

As with the lack of understanding of the importanceof K/Mg ratio in plant production (see example kiwiorchards, Table 1), there is similarly an undervalua-tion of Mg in the human diet in relation to K,although the significance of this interaction has beenknown for many years in animal nutrition as inrelation grass tetany in lactating grazing cattle (seeGunes and Welch 1989). It is of high interest that overthe years from 1880 until 1960 the Mg content insome home produced feeding stuffs of farms inSouth-West Germany declined by up to 40% whereasthe K content increased by up to 80% leading to achange of the K/Mg ratio from 1 to about 2.4 (Fig. 4,Arzet 1972).

Plant Soil (2010) 335:155–180 173

A comparable change in K and Mg content inleafy vegetables may also be assumed to have takenplace as a consequence of a one-sided elevatedapplication of K fertilizers which occurred in thepast. Documenting changes in concentration of K andMg, as well as Na and Ca in the main leafy vegetablessuch as lettuce would be of value particularly toinclude periods in which K fertilizers supplementedbyMg have been applied as has occurred over the pasttwo decades. Here field research is urgently needed.

Potassium/sodium ratio in processed foods

As discussed above there is an increasing healthburden for human beings as a consequence of thedramatic increase in Na intake resulting mainly fromprocessed food products. The first essential require-ment for the food processing industry is therefore tolower Na supplementation and partly replace it by amixture of K and Mg. Current labelling of foodproducts giving quasi “nutrition facts” is of littlevalue concerning mineral nutrients, for among thecationic nutrients only the Na or NaCl composition isgiven. In view of the major importance of K to human

health, the Na/K ratio should be clearly visible on theproduct label. In summary there is scarcely any needfor future research in crop production, the onus ofsupplying healthy food in this respect lies with thefood processing industry!

Effect of potassium chloride on cadmium uptakeby crop plants

Cadmium (Cd) is an undesirable heavy metal in foodproducts because of its high toxicity. Contradictoryreports appear in the literature on the effect of Kfertilizers on Cd availability in soils particularlywhen applied in the chloride form (Grant et al.1996; Umar et al. 2008; Blank 2009). Chloride canform easily soluble chloro-Cd complexes so that plantuptake of Cd can be enhanced (Smolders andMcLaughlin 1996). Chloride is also effective inincreasing Cd transport within plants (Ozkutlu et al.2007). Increased Cd uptake by application of KCl wasfound in barley (Grant et al. 1996) and under salinityin wheat (Norvell et al. 2000). At conventional ratesof application (100–200 kg chloride ha-1), Blank(2009) was unable to find any difference in effect ofchloride as compared with sulphate on Cd extract-ability from soil. The influence of the K appears to bemore important by its effect in desorption of Cd fromthe soil. From these findings it can be concluded thatthe chloride effect occurs particularly at very highapplication rates. At normal application rates, theeffect of cations (K+ or Na+) are of higher relevancethan the mobilization of Cd by formation of Cd-chloro-complexes. The findings of Zhao et al. (2003)showing no differences in Cd uptake by spring wheatbetween K salts in chloride or sulphate form supportthis conclusion. Thus in summary, in order to givesound recommendation in agricultural practice agreater understanding is needed of the mechanismsinvolved in Cd / K and Na / Cl interactions.

Summary and prospects

The various aspects of plant K discussed above suchas general stress signalling, enhanced disease resis-tance and adaptation to drought stress, clearly indicatethat progress in our understanding of physiologicalaspects of K acquisition and its utilization, as well asthe adoption of K fertilization strategies in farming

100

120

80

%

100

140

180

%

Mg

K/Mg

K

meadow hay

oat straw

fodder potatoes

0

0,5

1

1,5

2

2,5

3

1860 1880 1900 1920 1940 1960 1970

Fig. 4 Changes in the K and Mg concentrations and K/Mgratio in fodder of farms in the Southwest of Germany from1880 to 1970 (according to Arzet 1972)

174 Plant Soil (2010) 335:155–180

practice, are closely linked to current researchfindings in molecular biology. Plant nutritionists,extension service scientists and progressive farmersneed to be more aware of the continuous achieve-ments being made in the understanding of basicplant physiology and its signalling network asdeduced from results from this molecular approach(Fig. 2). On the other hand it is also becomingobvious that molecular biologists themselves needto have a basic understanding of the farmers’problems on a global scale so that they may becomemore proactive in addressing these problems andin considering the practical relevance and applica-tion of their research findings to the real world ofagriculture. We are convinced that a closerinteraction between those working in molecularbiology and those in farming practice (Fig. 5) willhelp to improve the urgently needed managementstrategies for stress mitigation. In general, through-out agriculture there is an urgent need for laboratoryspecialists to have a greater appreciation of allaspects of practical crop production. As a part ofthis integration, a great challenge exists to be betterable to deal with stress events involving K infarmers’ fields.

In the use of K in crop production, there is a need toensure balanced fertilization and efficient usage of Kin relation to the supply of other nutrients especiallyN and Mg. The reported mitigating effects ofparticular micronutrients acting in conjunction withK on various stresses is of immediate importance tocrop production and requires investigation. Thevarious proposed basic and applied research aspectsrelating to adequate frost resistance are goodexamples of these needs. From the plant viewpoint,more basic research is required on the drought-heat

syndrome to understand the separate influences ofthese two often combined stress factors.

In human and animal nutrition research, all studiesproviding more information on changes in the mineralcomposition of food (vegetables, fruits) and fodderover the last couple decades are of value in the aim ofimproving health.

From the soil viewpoint, determination of ex-changeable K by soil extractants, as a measurementof K availability in predicting K fertilizer response tocrops has been used extensively and successfully onmany soils. On soils containing 2:1 clay mineralswhich can both release and fix K at interlayer sites,however, the exchangeable K extraction method hasproved unsatisfactory as a guide to K fertilizerrecommendations. Prediction of crop requirementsby chemical K extraction from soils also takes noaccount of the limitation of spatial availability of K toliving roots which may be affected by physical andchemical soil factors such as low pH, drought,compaction or salinity as well as by plant factorsincluding crop species or genotype. From this

Extension service

Science

Farmers

Agrochemicalindustry

Fig. 6 Required interactions between scientists, agrochemicalindustry and farmers including the extension service to improvefarming practice

MolecularPlant Nutrition

(Plant) Root Physiology

Better Understanding of General Fertilization Techniques

Implementationfor Practice

Application and Demonstrationby Progressive Farmers

Use by Conservative,Ordinary Farmers

Deduction/Development of New/Innovative Fertilization Techniques

Fig. 5 Various research areas and interactions needed to improve farmers’ practice

Plant Soil (2010) 335:155–180 175

complexity of factors relating to potassium availabilityincluding soil extraction, clay mineralogy, weather,crop species and genotype, root distribution within thesoil profile etc, we suggest that there is a need for areappraisal for the estimation of K availability.

Much is already known about the behaviour of Kin soils and plants (see “Potassium in soils: presentknowledge” and “Potassium in plants: presentknowledge”), but in general, on a global scale thisinformation is not well passed on to or applied by thefarmer. This big gap between scientific knowledgeand its lack of use by farmers has to be bridged bybetter and more intensive knowledge disseminationas appreciated by Krauss (2003b) (see also Gill andGill 2006). In order to achieve this aim there is anurgent need for a more responsible co-operationbetween scientists, the agricultural chemical industryand farmers together with involvement of an extensionor advisory service (Fig. 6). Improving the interactionbetween these various bodies is particularly neededin a global world in which enormous progress inbeing made in basic sciences coupled with everincreasing demands on the farming industry to feedthe rapidly and hugely expanding world population.

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