Relevance of Biofertilizers to Agriculture - … · Relevance of Biofertilizers to Agriculture Tunde Ezekiel Lawal and Olubukola Oluranti Babalola * Department of Biological Sciences,

© Kamla-Raj 2014 J Hum Ecol, 47(1): 35-43 (2014)

Relevance of Biofertilizers to Agriculture

Tunde Ezekiel Lawal and Olubukola Oluranti Babalola*

Department of Biological Sciences, Faculty of Agriculture, Science and Technology,North-West University, Mahikeng Campus, Private Bag X2046, Mmabatho 2735, South Africa

Telephone: +27183892568, Fax: +27183892134,*E-mail: [email protected]

KEYWORDS Bacteria. Carriers. Inoculants. PGPR. Phytostimulant. Soil

ABSTRACT In the rhizosphere (the crop root or its close vicinity) microorganisms are abundantly present andthey are in millions. The rhizobacteria do not only gain from the nutrients produced from the crop root but alsopositively affect the crop and this result in stimulation of the crop’s growth. These bacteria are referred to as PlantGrowth Promoting Rhizobacters (PGPRs) and they have been grouped according to their activities. PGPRs havethe potential of acting as crop strengtheners, phytostimulators and plant health improvers. The objective of thiswrite up is to shed light on the possibility of using these for the improvement of agriculture. Undoubtedly, if the useof these organisms is appropriately managed by farmers, it will help in effecting better wellbeing of crops and it willthereby improve food safety.

INTRODUCTION

In the last century, farmers were happy aboutthe discovery and the use of chemical fertilizersdue to increase in crop yield and subsequentfinancial benefits that accrued to them. Over-time, demerits in the use of chemical fertilizerscame to surface and some of the disadvantagesinclude: leaching and pollution of water basins,destroying microorganisms and friendly insects,making crops more susceptible to attack fromdiseases, reducing the soil fertility and hence,farmers were being discouraged to use them (El-Lithy et al. 2014; Ofori-Boateng and Lee 2014).However, overtime, it has been realized that thealternative to chemical fertilizers may be biofer-tilizers which can help in enhancing the yieldand still avoid the harm inflicted by the use ofchemical fertilizers. Biofertilizers are live formu-lates which contain living microorganisms which,when applied to seed, plant surfaces, root orsoil, inhabit the rhizosphere and enhance thebioavailability of nutrients and increasing themicroflora through their biological activities andthereby promoting plant’s growth; they are prep-arations that readily improve the fertility of landusing biological agents (Babalola 2010; Schoe-bitz et al. 2014)

Biofertilizers are from biological wastes andthey are not hazardous to soil. They are veryuseful in that they help in enriching the soil withthe microorganisms and these microorganismsproduce organic nutrients for soil and help in

fighting pathogens. Besides accessing nutrients,for current intake as well as residual, differentbiofertilizers also provide growth-promoting fac-tors to plants and some have been successfullyfacilitating composting and effective recyclingof solid wastes. Biofertilizers, depending onavailable or present microorganisms have comeup as a replacement for chemical fertilizers toenhance soil fertility and crop yield in sustain-able agriculture. Symbiotic, free-living soil bac-teria are named as plant growth-promoting rhizo-bacteria (PGPR). They are involved in importantecosystem developments, and their activitiesinclude biological control of plant pathogens, Nfixation, mineralization of nutrients and phyto-hormones production because of these quali-ties, they occupy a unique place in the sustain-ability of agroecosystems. The main sources ofbiofertilizers are bacteria, fungi, and cyanobac-teria (blue-green algae). The relationship thatthese organisms have with plants is referred toas symbiosis. In this case, both partners derivebenefits from each other (Babalola and Glick2012a; Simmons et al. 2014).

Biofertilizers also stimulate plant growth,activate soil biologically, restore soil fertility andprovide protection against drought and somesoil borne diseases. Economically, it has beenobserved that biofertilizers are cost effective,eco-friendly, reduce the costs towards fertilizersuse, especially regarding nitrogen and phospho-rus. Rhizobacteria have exceptional ability toresist certain heavy metals and promote the

36 TUNDE EZEKIEL LAWAL AND OLUBUKOLA OLURANTI BABALOLA

growth of host plants by various mechanismssuch as nitrogen fixation, solubilisation of min-erals, production of phytohormones and sidero-phores and transformation of nutrient elements.They further stated that 1-aminocyclopropane-1-carboxylate (ACC) deaminase- producing bac-teria play an important role in the alleviation ofdifferent types of stresses in plants, includingthe effect of heavy metals. In this regards, dif-ferent heavy metal tolerance processes havebeen enumerated in different microorganisms.They include: exclusion, active removal, biosorp-tion, precipitation or bioaccumulation both inexternal and intracellular spaces. These listedmechanisms can enhance the solubility and themineralization of the metals and thereby makingnullification of the toxic metals possible. It hasbeen shown that the interaction between soiland its microorganisms positively influence thelevel of metal phytoextraction. For instance, thebacteria in the rhizoids facilitate the usage oftrace elements. It is also imperative to state thatthere are some instances when they have mini-mal or no effects. Table 1 shows the names, de-scription and possible locations of some mi-crobes that are useful as biofertilizers.

METHODS OF APPLICATION OFBIOFERTILIZER INOCULANTS FOR

AGRICULTURAL PURPOSE

It is important to be mindful of the method(s)to be used for the application of the preparedinoculants (bacteria-carrier mixture) to the crops.Factors to be considered include: the type ofplants or seeds to be biofertilized, availability ofthe biofertilizers carrier, season and age of thecrop. It must be ensured that appropriate andeffective strain(s) of organisms are used. Biofer-tilizers must not be used in company of strongdoses of plant protection chemicals and otherchemicals should not be mixed with the biofertil-izers (Balasubramanian et al. 2013; Son et al.2014). The environmental condition of agricul-tural soil must be taken into consideration: highsoil temperature or low soil moisture, acidity oralkalinity in soil, poor availability of phospho-rous and molybdenum, presence of high nativepopulation or presence of bacteriophages. Inorder to get good results from the use of biofer-tilizers, farmers must ensure that they use theright method of application and apply the fertil-izer at the appropriate time (See Table 2).

Characteristics of Suitable Carriers forAgricultural Biofertilizers

Development of suitable inoculants has todo with procurement or availability of a suitablecarrier substrate. This is important because it isthe carrier that will host and thereby determinethe growth of the organism and possibly main-tain the inoculants bacteria such that they cangrow and proliferate appropriately. A suitablecarrier must contain a considerable level of or-ganic matter and high nitrogen level and mustbe affordable and non-toxic. Deployment of in-oculants in a carrier grants long-term storageand makes the handling easier and effective(Schoebitz et al. 2014). The preparation of seedcarrier involves the milling of the material to pow-der to sizes which varies between 8-41 µm. Goodcarriers must: (1) maintain good moisture capac-ity (2) not be toxic to the inoculants bacterialstrain (3) be easy to process and devoid of lump-forming materials (4) be easy to sterilize by auto-claving and irradiation (5) be available in ade-quate quantity (6) be inexpensive (7) have goodadhesion to seeds (8) have good pH bufferingability and (9) not be toxic to plants. Apart fromthese mentioned criteria that determine the suit-ability of a material as a carrier, it must be stressedthat it is also important that the carrier must beable to support the survival of the bacteria evenbefore the seeds are sown or before the seed-lings are transplanted. This becomes importantbecause in the case of seed coating, the seedsare not always sown immediately. Besides, thesurvival of inoculant bacteria when the seedsare stored must be supported by the carrier ma-terials. Desirably, the carrier material must be ableto support the biofertilizers when in the soil. Thisis needed because the biofertilizers compete withthe native soil microorganisms for nutrient andthey will have to cope and survive in the soil inspite of the protozoa in the soil as well. There-fore, the carriers must offer a sufficient mi-croporous structures that will ensure the sur-vival of the inoculants bacteria (Singh et al.2013a).

Some of the known carriers now include:various clays, animal manure such as poultrymanure, composted plant materials or other com-plex organic matrices. Some users opined thatanimal manure possibly contains pathogens andantibiotic which can lead to serious soil degra-

RELEVANCE OF BIOFERTILIZERS TO AGRICULTURE 37

Table 1: The names, descriptions and habitats of some biofertilizers

Names Description Where it lives References

Rhizobium Genus of gram negative soil Root nodules and stem (Qin et al. 2014)bacteria. It fixes Nitrogen and nodulescolonizes plant cells within rootnodules.

Azotobacter Genus of motile; it can be oval Soil (Oldroyd and Dixon 2014)or spherical. They are aerobic,good in Nitrogen fixation. Usefulas food additives and biopolymers.Produces antibodies thatsuppresses root pathogens

Cyanobacteria Also called blue-green bacteria, Soil, ocean and fresh (Taylor et al. 2014)blue-green algae or cyanophyta. watersThey are photosyntheticnitrogen fixers.

Azospirillum Gram negative aerobic bacteria. Soil (Serelis et al. 2013)Beneficial at nitrogen fixationsand plant nutrition. Producesgrowth promoting substances.

Pantoea Gram negative bacteria formerly Plant surfaces and (Kouvoutsakis et al. 2014)agglomerans called Enterobacter agglomerans. animal faeces

It is a phosphate solubilizingbacteria.

Pseudomonas Gram-negative rod shaped Soil (Annesini et al. 2014) Putida saprophytic soil bacterium used

as soil inoculant to remedynaphthalene contaminated soils. Soil (Nawrocka and Ma³olepsza

Trichoderma A genus of fungi. They are oppor- 2013)tunistic avirulent plant symbionts.

Vermicompost It has N, P, K, S, hormones, Soil (Gutiérrez-Miceli et al. 2007)enzymes and antibiotics whichhelp to improve the quality andquantity of crop yield.

Bacillus A genus of gram positive rod- Non sterile soil (Kumari and Sarkar 2014)shaped bacteria and a memberof the phylum Firmicutes

Mycorrhiza They are a group of fungi that Soil (Vos et al. 2013).include a number of typesbased on different structuresformed inside or outside theroot. They are specific fungithat match with a number offavorable parameters of thehost plant on which it grows.

Burkholderia It is a genus of proteobacteria. In wet soil (Vandamme et al. 2002).Genus name refers to a groupof virtually ubiquitous gramnegative, motile, obligatelyaerobic rod-shaped bacteria.

Klebsiella Gram negative, non-motile, Soil (Ku et al. 2014)encapsulated, lactose fermen-ting, facultative anaerobic, rodshaped bacterium.

Enterobacter Gram negative, anaerobic, In water and soil (Jin et al. 2014)shaped bacterium. but are common

invaders of tissuesin contaminatedwounds of animals.

Herbaspirillum Gram negative soil and water- Soil and water (Lubambo et al. 2013)based bacteria that rarelycause human infections.

Gluconobacter The acetic acid bacteria are Air (Guo et al. 2013)usually airborne and areubiquitous in nature.


dation and phytotoxicity if uncomposted animalmanure is applied to soil (Babalola and Glick2012b; Yousefzadeh et al. 2013).

Sterilization of the Carrier Materials forBiofertilizers

It is important to sterilize carrier materials toensure that they are devoid of other microor-ganisms that may hinder the survival of the in-oculants and also to prolong the shelf life of theinoculants. Essentially, there are two major waysof sterilizing the carrier materials, gamma irradia-tion and autoclaving. But, the more adopted orrecommended is the gamma irradiation. This isbecause this method does not alter the chemicalor physical properties of the carrier materials(Babalola and Oladele 2011; Minaxi and Saxena2011). Most of the times, the irradiation is doneby packing the carrier material in a polyethylenebag and gamma irradiate at 50 k Gy (5 Mrads).Gamma rays effect the sterilization by collidingwith the atoms of nutrients such as protein,carbohydrates, lipids and nucleic acid. The rays

ionize the atoms and thereby damage them. Fromthe above listed materials, the most susceptibleto ionizing radiation from the gamma rays is thenucleic acid and it is only 1% of the total compo-sition of the cell. Furthermore, during the radia-tion treatment, the DNA strand breaks and baseis subsequently damaged. The break of the DNAstrands leads to break in the flow of genetic in-formation and this destroys replication processand subsequently, death of the cell results (Li etal. 2012). Moreover, the sterilization by autoclav-ing is carried out by putting the carriers in poly-ethylene bags and autoclaved at 121oC for 60minutes. However, it should be known that dur-ing autoclaving, some carriers undergo chang-es and their physicochemical properties maychange and produce toxic substances to somebacteria strains (Li and Yu 2011).

RHIZOSPHERE COMPETENCEOF BIOFERTILIZERS

Bacteria inoculation is often carried out whencoating the seeds or when placed very close to

Table 2: Methods of applying biofertilizers, descriptions and application of the methods to crops

Methods Description Application of the Referencesmethods to crops

Seed Treatment It is the most popular method. Pulses, oilseeds and fodder (Singh et al. 2013a)It is applied at the rate of crops100gm per 5kg of seeds.Slurry is prepared with waterand the biofertilizer is mixedwith water at ratio 1:2.

Seedling Treatment 1 part of biofertilizers in 10 Tomatoes, potato, onion (Andrade et al. 2013)parts of water is prepared. and paddy, marigold andThe roots of the seedlings jasmine.are dipped in the biofertilizersfor about 40 minutes.

Set Treatment The pieces of materials to be Sugarcane, banana and grapes (Shen et al. 2013)planted are immersed inbiofertilizers mixture for30 minutes. The sets aredried and planted in the field.

Soil Treatment The biofertilizers are applied Maize and Wheat (Schoebitz et al. 2014)to the soil before planting orsowing. The possible carriersare: compost, farmyardmanure, rice husks and ligniteat the rate of 1 kg per 25 kgof carrier. Irrigation followsimmediately after theapplication.

Spraying/Irrigation Biofertilizers is mixed with Recommended for standing (Singh et al. 2013b)water and other micronutrients citrus plants, vines, mango,in a tank. It reaches plants guava, custard apple andthrough irrigation or spraying. peach orchards.


the plant via a carrier. The inoculated bacteriashould have the ability to establish themselvesin the vicinity of the rhizoids at such a numberthat will be enough to have beneficial influenceon the plants. Expectedly, inoculants bacteriashould not only live in the vicinity of the rhiz-oids, but they should be able to maximally usenutrients produced from the root, multiply, andsubsequently colonize the entire root area (Sonet al. 2014). In summary, biofertilizers functionas soil microbes and they thereby convert ambi-ent nitrogen into forms that the plants can use(nitrate and ammonia), increases soil porosityby gluing soil particles together, defend plantsagainst pathogens by outcompeting pathogensfor food and worthy of note is the fact thatsaprophytic fungi in the soil break leaf litter downinto usable nutrients. It is important to note thatat present, biofertilization is responsible for ap-proximately 65% of the nitrogen supply to cropsall over the world. The biofertilizers bacteria forma host-specific relationship with legumes. Thisrelationship begins by initiation of root or stemnodules as a result of the presence of the bacte-rium. Lipooligosaccharide information activatesmolecules that are produced by the bacteriumand plays an important role in this process. Thebacteria percolate the cortex, stimulate forma-tion of root nodules, increase and eventuallybreak into bacteroids, which elicit the nitroge-nase enzyme production. In the root nodules,the plant provides a low oxygen concentration,which promotes bacterial nitrogenase to changenitrogen in the atmosphere into ammonia. As aresult of this, the plant supplies the bacteria withneeded carbon source for multiplication and ex-istence (Beneduzi et al. 2013).

Agricultural soil is said to be healthy if amongother things, it contains sufficient strains of mi-croorganisms that can terminate, prevent orhinder bacteria, fungi and species of nematodesthat can cause root rots. Moreover, organismslike mycorrhizal fungi produce compounds thatare antibiotic or bactericidal to many plant patho-gens (Babalola 2010). For the decomposition ofthe toxic materials, through the process of co-oxidation, bacteria and fungi need organic mate-rials to feed on with the toxic compounds (Asen-sio et al. 2013).

Use of Bacteria as Strengtheners ofAgricultural Plants

Plant strengtheners refer to “plant resistanceimprovers”. It is now commonly referred to as

agents and it maintains plant health or whichguide crop plants against non-parasitic adverseconditions. The association of Plant with micro-organisms explains important roles for plantwellbeing and health. Microbes are able to influ-ence plants’ health by improving nutrient up-take and hormonal stimulation. Different meth-ods or ways are involved in the minimization ofactivities of plant pathogens, and this will influ-ence and affect plant growth (Manivasagan etal. 2013). Although, bacterial genera Azospiril-lum and Rhizobium are known to be good forplant growth improvement, other microorgan-isms like: Bacillus, Pseudomonas, Serratia,Stenotrophomonas, Streptomyces, Ampelomy-ces and Coniothyrium are yet to be fully explored.Developments in recent agricultural practices aimat developing harvest yields and directed to-wards minimizing pre harvest and postharvestlosses occasioned by devastating abiotic andbiotic agents (Harish and Tharanathan 2007).These developments have potential of reducingthe effects of pests and diseases by about 20-40%. Recent pest management strategies in cropplants involve the use of classical and molecu-lar marker-based resistance breeding, geneticmanipulation of plant tolerance and the use ofchemicals as pesticides or strengtheners of plantwellbeing (Gomes and Silva 2007).

Use of Bacteria as Phytostimulators forAgricultural Plants

Phytostimulators promotes crops growth.Plant growth promoting rhizobacteria (PGPR)include Azospirillum, which is a popular genusamong the PGPR that exhibit positive effects onplants growth. A considerable volume of carbongets below ground through the activities ofplants’ roots. Invariably, plants release exudateswhich serve as nutrients (carbohydrates, pro-teins and other nutrients) for microbes aroundthe roots area (Ramos et al. 2011). By this, cropsget the right types of microbes around its roots.Eventually, these “well fed” microbes will pro-duce enzymes and growth hormones and pro-tect the plants against pathogens. It is estimat-ed that averagely, a gram of healthy soil shouldhave or contain 100 million organisms (Drogueet al. 2012). Meanwhile, in the proximity of cropsroots, there can be up to a trillion organisms pergram of soil and they live symbiotically. Manystrains of A. brasilense and A. lipoferum have


been explored in recent times as crop inoculantsto maximize yield. The results obtained whenAzospirillum was used as a phytostimulatoryPGPR has elicited comprehensive studies on thebiology of these bacteria. Among other benefitsfrom the use of Azospirillum are: nitrogen fixa-tion, deamination of the ethylene precursor1-aminocyclopropane-1-carboxylate (ACC) andproduction of nitric oxide properties and phyto-hormones, chiefly indole-3-acetic acid (IAA)(Aimey et al. 2013). Moreover, these benefitshave the ability to promote or enhance goodrooting system which enhances root hair densi-ty and invariably, this will lead to better watermineral uptake by crops.

Plant Growth Promoting Bacteria asCrop Health Improvers

For about sixty years, PGPR have been con-firmed to prompt development of various hostplants and they also benefit from the root exu-dates. The PGPR are classified into differentgroups according to their actions on crops. First-ly, the phytostimulating rhizobacteria that pro-motes crop growth directly by providing nutri-ents and phytohormones. Secondly, mycorrhizaand root nodule symbiosis which assist rhizo-bacteria and this positively affect functioningof plant and microorganisms in the symbioticrelationship; and thirdly, the biocontrol rhizo-bacteria that defend plants and crops from patho-gens via exudates from antimicrobial agents orby promoting plant resistance (Manivasagan etal. 2013). Due to their potential use as biofertiliz-ers and biopesticides, their modus operandi hasbeen largely studied in model bacteria such asAzospirillum spp. and Pseudomonas spp. Thegenotype of the host plant determines PGPRdensities both in terms of the number, size andcomposition. Furthermore, plant growth-promot-ing effects of these bacteria have been shownto rely both on host plant genes and bacterialstrain (Son et al. 2014).

Soil Food Web

The term soil food web means the interac-tions among the wide range of living organismsthat exist in the soil. It is made up of non-livingthings and the living things. The non-livingthings are the minerals while the living thingsare the creatures otherwise called soil biota

(Hansen et al. 2010). There are two types of soilbiota. The first ones are beneficial while the sec-ond ones are pathogenic ones which can causeroot rots and other diseases. However, both arevery important and needed in the cycle. Thebeneficial bacteria improve soil’s drainage, aera-tion, and texture and in general, they improvethe soil’s health by recycling organic matter (Bau-doin et al. 2010). In certain instances, they haveproducts that inhibit the growth of soil-bornepathogens to the advantage of the plant root.Beneficial bacteria are explored in the quest toimprove soil fertility and to reduce the negativeimpacts of chemical fertilizers on the environ-ment. One of the mechanisms by which bacteriaare absorbed into the soil particles is by simpleion exchange. In addition, a soil is accepted tobe fertile if its organisms release inorganic nutri-ents from the organic matter in order to effectand aid plant growth (Kalinina et al. 2013).

Destinies of Soil and Crops without Food Web

When chemical fertilizer is added to the soil,there is availability of much N. Plants will takethe needed quantity and use it for growth whilethe rest will be leached away. This will take placebecause there are no microorganisms that willretain and use the rest (Liu et al. 2013). Besides,other nutrients like phosphorus, copper, iron andzinc will be easily changed to unavailable or in-soluble forms. When and if there is no food web,the opportunistic organisms will increase andmultiply rapidly. The opportunity for the plantto have good water and nutrient holding capac-ity is abased. So, the plant may not be able tohave resistance during drought or adverse con-ditions (Sheelanere et al. 2013).

Levels in Agricultural Soil Food Web

There are levels in the soil food web. Thefirst level is the level of organisms that capturenutrients before they are lost. In this level, bac-teria and fungi play prominent role. They mopup the available nitrogen in the soil that theplants do not need at that time. They do this inorder to prevent them from being leached awayand besides, N in this form cannot be lost likegas (Creamer et al. 2014). They use the N for theformation of protein in their bodies. They con-vert nutrients like P, K and S to body biomassand their body wastes, like that of fungi are


summarily converted into humus soil. The soilfungi are able to biodegrade the cellulosic mate-rials. They degrade tougher biodegradable ma-terials like lignin and pectin. They actively dothis by the release of enzymes in course of feed-ing on these materials having formed rhizoidson the substrates (Bartley et al. 2014). By thisprocess or procedure, nitrogen and other nutri-ents are prevented from being lost in that theyare “kept” as part of biomass in bacteria andfungi, but plants do not also have access tothese nutrients. In order to make the nutrientsavailable, there must be predators that will feedon the bacteria and the fungi; hence we havethe second level in the soil food web. At thislevel, there are protozoa, nematodes and microar-thropods. These organisms feed on fungi andbacteria in the soil and the “kept” nutrients aremetabolized and gradually given back to the soilsuch that the soil will not lack the nutrients al-ways (Wang et al. 2014). Worthy of note is thefact that microarthropods, apart from feeding onbacteria and fungi in order to release nutrientsin them for plant use, also feed on some nema-todes that feed on plants’ roots and attack them.On top of the second level of soil food web arethe higher level predators that include millipedes,centipedes and earthworms. They feed on nem-atodes and protozoa and put them in check soas to prevent them from expanding in popula-tion and over feeding on bacteria and fungi. Otherhigher organisms like birds and rodents feed onearthworms and insects; then birds and rodentsmay be fed on by mammals (Arthur et al. 2013).On the whole, there are about six trophic levels.The above process results in continuous andsustained supply of nutrients to plants by pre-venting leaching and loss of nutrients in gas-eous form as bacteria and fungi would have usedthem and retained these nutrients in their bod-ies as part of the biomass (Senechkin et al. 2014).

CONCLUSION

Ultimately, the use of biofertilizers are attrac-tive because they act as plant strengtheners,phytostimulators, plant health improvers, andhave the potential to fix nitrogen, which plantscan use to improve its growth when there, isinsufficient quantity of nitrogen in the soil. Fur-thermore, because they originated from the soilthey seem to be rhizosphere competent.

RECOMMENDATIONS

Although farmers still prefer the use of chem-ical fertilizers, carrier material for biofertilizer andinoculum stability are the main obstacles for massproduction of biofertilizer. The research on thegenetically engineered microbes, will improvethe biofertilizers. Special attention needs to beaddressed towards N2O gas emissions which canbe minimized by developing sustainable manuremanagement strategy. Besides, it is important toembrace the use of biofertilizers because it hasthe potential of enriching farmers.

ACKNOWLEDGEMENTS

The first author thanks the North-West Uni-versity for the award of a post-doctoral fellow-ship. This work is based on the research sup-ported by the National Research Foundation,South Africa.

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Relevance of Biofertilizers to Agriculture - … · Relevance of Biofertilizers to Agriculture Tunde Ezekiel Lawal and Olubukola Oluranti Babalola * Department of Biological Sciences,