foundation of Agriculture of to - UC Agriculture & Natural ...cesantaclara.ucanr.edu/files/230251.pdf · Volcanic acrivicy has left nu-trient-rich lava deposits in many areas. Soils

INTRODUCTION

Earth Summit Alternative NongovernmentalSustainal¡le Agriculture Treat¡l

Sustainable agriculture is a model of social and economic organi-zation based on an equitable and participatoryvision of develop-ment that recognizes the environment and natural resources as

the foundation of economic activity. Agriculture is sustainedwhen it is ecologically sound, economically viable, socially just,culturally appropriate, and based on a holistic scientific approach. . . Sustainable agriculture respects the ecological principles ofdiversity and interdependence and uses the insights of modernscience to improve rather than displace the traditional wisdomaccumulated over centuries by innumerable farmers around theworld.

2

UxDE,RSTANDING THES orl E cosysrEM

The first step toward effective ecological soil management is an ap-preciation of the complex, living system known as soil. And to un-derstand soil is to be aware of how everything affects and is affectedby it. \We are all part of the soil ecosysrem.

Soil fertiliry can be described as its capaciry ro nunure healthyplants. Sustainable agriculture aims to prorec the soilt abiliry to re-generate nurrienrs lost when crops are harvested-without depen-dence on "ofÊfarni' fertilizers. This regenerative capaciry in turn,depends on the diversiry health, and vitaliry of the organisms thatlive, grow; reproduce, and die in the soil. Through the activities ofsoil microbes-which can number in the billions in every gram ofhealthy topsoil-the basic raw materials needed by plants are madeavailable at the right dme, and in the right form and amounr.

The basic aim of ecological soilmanagement is to provicle hospitable

conditions for life within the soil.

Your farm is both the product and producer of soil. Consideryour farm to be a living organism that achieves its greatest long-term

UNDERSTANDING THE SOIL ECOSYSTEM

productiviry when its natural rycles and processes are enhanced.Shortcutting these cycles for short-term control or economic gainwill eventually bear out the ecological maxim, "The creature thatwins against its environment destroys itself."

The place to start is where you are. Thousands of soil types havebeen named, classified, and described. Knowing their names can tellyou a lot about their general characteristics; but, like any living crea-ture, each individual is unique. Find out what soils live in your area,how they are classified and described by soil scientists, and how thatcompares with what you observe about them yourself.

Soil classification schemes organize soils according to their dif-ferent qualities, based on the kinds of minerals they contain, howthey were formed, and various physical characteristics. The individ-ual character of any soil arises from a combination of factors inher-ent to its particular geographic region (see table 1).

Tal¡le 1

ENVIRONMENTAL INFLUENCES ON SOIL

Climate. Temperature and precipitation affect the rate of organic matter ac-cumulation and the presence of soluble soil minerals. For example, moreorganic mâtter âccumulates where decomposition is slow due to coolert€mperatures, while high rainfall leaches mineral nutrients from topsoil.

Native vegetation. Grasslands, forests, and transition zones each affect soil de-velopment in a different way. Leaf litter from pine forests, fo¡ example, in-creases soil acidiry. Soil particles developed under grasslands are usuallybound into stable aggregates by the activiry of the plendful microorganismsand roots found there.

Parent material. Underlying rock rypes from which it was formed determinea soil's mineral content and basic textu¡al qualities. Limestone bedrock, forinstance, helps counteract soil acidiry. Red soils indicate that the parent ma-terial and derived soil is rich in iron. Volcanic ash eventually produces soilsheavy in amorphous clays.

Topography. Soil may be eroded from slopes and deposited in lowlands. TheIegendary fertiliry of river valleys such as the Nile resulted from deposits ofrich sediment carried from the highlands, while mountain farme¡s all overthe world have problems holding onto precious topsoil.

UNDERSTÂNDING THE SOIL ECOSYSTEM 9

Time. The availabiliry of minerals and the extent of humus development insoil is also influenced by how long the narive rock has been rubject torÀ/eathering. Young soils, such as rhose in Hawaii and other areas of volcanicactivicy, may be low in cla¡ which is produced by the chemical effects ofweâthefing on parent rocks.

Glaciation and geologic acriviry. In the north-temperare region, the advanceand retreat of glaciers, most recenrly a mere 12,000 years ago, has had a sig-niûcant effect on soil formarion and qualiry. Volcanic acrivicy has left nu-trient-rich lava deposits in many areas.

Soils worldwide have been classified into ren major orders (seetable 2). In humid remperare regions such as the northeasrernUnited States, where forests are the predominant natural yegetation,the soil order of spodosols is most common. These soils are generallyformed from coarse-rextured parenr marerial, and tend to be quiteacidic and low in mineral nutrients. Prairie soils, which have devel-oped under flat, grass-covered areâs with modest rainfall, are classi-fied as mollisols. They are âmong the most naturally productivesoils, with high native organic mamer and mineral conrenr. In tropi-cal regions with very high seasonal rainfalls, the heavily leached ulti-sol soils also tend to acidity. The Sahara, Gobi, and TirrkestanDeserts, as well as South and Central Australia and the AmericanSouthwest arelargely comprised of aridisols. If irrigated they can beproductive, but great care must be taken to prevent toxic accumula-tions of soluble salts.

Each order is further broken down into suborders, grear groups,and subgroups. Beyond this, soils are described in terms of families,associations, and series, which provide more information about theirplant growth characteristics, organic and mineral content, structure,drainage, and color. Series are often named after the places-rowns,rivers, or counties-where they are located.

Your local Extension or Soil Conservation Service office canprobably give you a soil map for your land. They can also show youyour countyt soil surve¡ which provides detailed information on lo-cal soils and their besr uses, as well as helpful climatological data.

Table 2THE TEN MAjORSOIL ORDERS

Entisols: Recently formed mineral soils with little evidence of horizon forma-tion. Found in a wide range of climate zones, including the Rocky Moun-tains, the Sahara Desert, Siberia, and ïbet. May be highly producive, butmost are relatively barren.

Vertisols: Mineral soils with a high content of swelling-rype clays, which indry seasons cause the soils to develop deep cracks. Found in some areas ofthe southern U.S., India, Sudan, and eastern Aust¡alia. Their physicalproperties make them difficult to rill and cuitivate.

Inceptisols: Young soils with limited horizon formation. May be very produc-tive, as those formed from volcanic ash. Found in the Pacific Northwest(U.S.), along the Amazon and Ganges Rivers, North Africa, and eâsrernChina.

Aridisols: Mineral soils found mostly in dry climates. Productive only if irri-gated, and may become saline. Found in the southwestern U.S., Africa,Australia, and the Middle East.

Mollisols: Characterized by a thick, dark surface horizon, they are âmong rheworld's most productive soils, with high natural ferriliry and tilth. Gener-ally found under prairie vegeration, such as the Great Plains (U.S.),Ukraine, parts of Mongolia, northern China, and sourhern Latin America.

Spodosols: Mine¡al soils characterizedby distinct horizons, including subsur-face organic matter, and aluminum and sometimes i¡on oxides. Coarse-textured, readily leached, and tending to be acid, they occur mosrly inhumid, cold temperate climates, generally under fo¡ests. Can be ve¡y pro-ductive if properly fertilized.

Alfisols: Moist mineral soils with high base status and presence of silicateclays. Found mostly in humid regions under deciduous forest or grassincluding parts of the U.S. Midwest, norrhern Europe, southern Africa,and Southeast Asia. Highly productive, good nutrient levels and rexrure.

Ultisols: Moist soils that develop under wa¡m ro tropical climates. Highlyweathered, acidic, with red or'yellow subsurface horizons. Found in the hu-mid southeastern U.S., Southeast Asia, and southern B¡azil. Can be highlyproductive, with good workabiliry.

Oxisols: The most highly weathered soils, with a deep subsurface horizon ofiron and aluminum oxides. High in cla¡ commonly deficient in phospho-


surfaces. Not well adapted ro mechanized farming, they have been poorly.researched.

Histosols: Organic soils thar have developed in a water-saturated environ-ment, with at least 20 percent organic content. Can be very productive ifdrained, especially for vegetable crops.

Source: adapted from Nyle Brady, The Nature and Properties of Soil:, 10th ed.

OncnNrc M¡rr¡n AND HuMUS

Soil health and humus are indivisible: health is the vitaliry of thesoil's living popularion, and humus is the manifesrarion of its activi-ties. As the cornerstone of the soil ecosystem, humus influences andis influenced by every other aspecr of the soil. Building soil humusimproves its physical and chemical properties as well as its biologicalhealth.

All humus is organic marrer, but not all organic matter is humus.Raw organic matter consists of the waste products or remains of or-ganisms that have not yet decomposed. Humus is one form of or-ganic matter that has undergone some degree of decomposition.There is no hard and fast dividing line, but a conrinuum, with fresh,undecomposed organic materials-manure, sawdust, corn stubble,kitchen wastes, or insecr bodies-at one end, and stable humus,which may resist decomposition for hundreds of years, at the other.Tâble 3 summarizes the attributes of different rypes of organic mat-ter and humus.

Humus is dark brown, porous, spongy and somewhar gummy,and has a pleasant earthy fragrance. Chemically, it is a mixture ofcomplex compounds, some ofwhich are plant residues that dont read-ily decompose, such as waxes and lignins. The rest are gums andstarches synthesized by soil organisms, primarily bacteria and fungi, asthey consume organic debris. Humus is highly variable in its compo-sition, depending on rhe narure of the original material and the con-ditions of its decomposition.

l1

I2 UNDERSTANDING THE SOIL ECOSYSTEM

"Humus" is actually more a generic term than a precise one. Itsqualities will reflect different origins and composition. Just as wine canvary widely in qualiry so can humus. And, just as different wines aresuitable for different culinary purposes, the varieties of humus servevarying soil functions.

Several classification schemes for humus have been suggested.Theories differ as to how it is formed, why it behaves as it does, andhow it should be measured. Humus that can still decompose readily is

known as effèctive or active humus. It consists of a high proportion ofsimple organic acids (fulvic acids), which will dissolve in either acids orbases. This type of humus is an excellent source of plant nutrients, re-leased as soil organisms break it down further, but of little conse-quence for soil structure and long-term tilth. This kind of humus ismainly derived from the sugar, starch, and protein fraction of organicmatter,

Humic acids, which dissolve in bases but not in acids, characterizemore stable or passive humus; humins, which are highly insoluble andmay be so tightly bound to clay parricles that microbes cant penerratethem, are the main constituents of the most stable humus. Becausestable humus resists decomposition it does little to add nutrients ro rhesoil system, but it is essential to improving the soil's physical qualities.Carbon-l4 dating has revealed that very stable humus complexes maysurvive unchanged for thousands of years. Stable humus originatesfrom woodier plant residues, which conrain lots of cellulose andlignin.

The status of soil organic matter and humus is a dynamic one,continually changing through the activities of all the crearures that livethere. Ideall¡ there should be a rough equilibrium among the differ-ent kinds of humus at any one time, with the more acrive fractionspredominant when plant nutrienr needs are highest, then giving wayto more stable forms after harvest or when plants are dormanr. Fungiand actinomycetes, which are more abundant than bacterial decom-posers under cool, damp conditions, are also more important in thecreation of stable humus.


The changes are fastest under optimum conditions for soil biolog-ical activiry, and fresh supplies of raw organic marrer must continuallybe added to keep the cycles moving. Anything that harms or disruptsone member of the soil community can lead to a form of "indigestion'in the soil. For example, if large amounts of nitrate fertilizer flood thesoil system, the bacteria responsible for converting protein fragmentsinto nitrates will be suppressed, in turn "backing up" the whole or-ganic decomposition process. Theywill recover after a while, but if thisprocess is repeated year after year, the capacity of that soil to digestfresh organic matter will be seriously damaged.

The process by which organic ma*er and humus breala down inthe soil is called mineralization. \Øhile humus is the producr of or-ganic matter mineralization, ir too can be mineralized under the rightconditions. organic mamer management, discussed in chapter 3, re-quires that you understand what conditions speed up or slow downmineralization.

Mineralization occurs quickly when conditions are perfect forbacteria to reproduce: high aerarion, adequate moisture, good pH,and balanced mineral nutrienrs. cultivation speeds it up by introduc-ing air; if soil is dry irrigation will also stimulate mineralization. In-creasing soil temperature with dark mulch or row covers, or actuallyheating the soil in a greenhouse bed, also encourages the faster releaseof nutrients to plants.

As is true with fertilizing, it's importanr to understand the conceprof "enough" when you choose to stimulate mineralization. Too quicka release of nutrients from organic matter can cause problems, whichparallel those of overfertilizing: excess plant nirrate uptake or possibleleaching of nutrients into groundwater. It's also important to avoid"burning up" viral, stable humus reserves by making sure to addenough organic mamer to replenish what is mineralized.

Humus tends to accumulate fastest under conditions unfavor-able to mineralization: cool remperarures, Iow pH, and poor aera-tion. \X/hile to some exrent this is desirable, rhe exrreme example ofgoing too far is the case of a peat bog, composed of almost pure hu-

13

I4 UNDERSTÁ.NDING THE SOIL ECOSYSTEM

mus. The key here is balance: an actiye, healthy biological popula-tion will continually be mineralizing humus at the same time that itis being formed. As you become attuned to the signs of biological ac-

tiviry and health in your soil, as well as the rhythms of growth andrest in your crops, you will develop a better sense of "enougli' whenit comes to humus formation and decay.

BeNr.nrs oF HuMus

. Humus can hold the equivalent of B0 to 90 percent ofits weight in water, so soil rich in humus is moredrought-resistan t.

. Humus is light and fluffy, allowing air to circulate easil¡and making soil easy to work.

. The sticþ gums secreted by microbes while forminghumus hold soil particles together in a desirable crumbstructure.

. Humus is extremely effective at holding mineral nutri-ents safe from being washed away in rain or irrigationwater, and in a form readily available to plants. Amplereserves of humus also provide additional plant nutri-ents in times of need.

. Humus is able, because of its biochemical structure, tomoderate excessive acid or alkaline conditions in thesoil-a quality known as buffering.

. Many toxic heavy metals can be immobilized by soil hu-mus, and prevented from becoming available to plantsor other soil organisms.

. Although the color of humus can yary, it is usually a

dark brown or black color, which helps warm up coldsoils quickly in the spring.

Table 3THE NATURE AND FUNCTION OF ORGANIC MATTERAND HUMUS

Raw organic matter Effectiue hurrtus Stable hamus

NATURE

Source \Vastes, residues, andremains of livingorganisms.

Composition Complexorganiccompounds, such asproteins, cellulose,lignins, fats, starchesand sugars.

Characteristics Heterogeneous, coâÍse,lumpy material.

Decomposed raworganic matter.

Characterized by highratio of fulvic acids(small, solublemolecules).

Decomposed raw organicmatter or effective humus.

Mostly long-chained humicacids, or humins bonded toclay particles.

A colloid, morehomogeneous intexture and color.

Homogeneous, resistant tochemical action.

FUNCTION

Physical

Chemical

Improves aeration,drainage, andmoisture retention.

"Tlash mulcli'protects soil fromweathering.

Iftoo coarse andabundant, mayhinder seed

Pfeparârion.Provides some soluble

nutrients, especiallyfrom manures.

Leaves a reserve supplyof nutrients in the soil.

Releases much carbondioxide as itdecomposes.

Provides food for mic-robial decomposers.However, if toocarbonaceous, canoverstimulâtemicrobes and lock upavailable nitrates.

Mobile in soil;readily releasesnutrients to plants.

Holds nutrient anionsin a form available toplants, but safe fromleaching.

Increases cation exchangecapacity.

Provides nut¡ients tomic¡obes as itdecomposes.

Releases vitamins,hormones, antibiotics,and other bioticsubstances,

Provides long-term nutrientstorage and maintains goodcation exchange capaciry.Toxic substances (as well asnutrients) can be chelatedand prevented from enter-ing the ecosystem.

Provides microbial habitatand evidence ofhealthy bi-ological âctiviry.

Creates'trumb srructure" Same as effective humus.

-spongy, porous, andsticþ-that makes anexcellent soilconditioner.

Dark brown colo¡improves heat retentionby soil.

Biological

t6 UNDERSTANDING THE SOIL ECOSYSTEM

PHvsrcRl FRcrons: Soll Srnucrunr RNo TIttn

Tilth is to soil what health is to people. A soil in good tilth is in goodphysical condition for supporting soil life. Good tilth also means soilis loose and easy to work, so tools as well as plant roots can readilydig in. Moisture and aeration are the key physical qualities of soil.The abiliry of soil to hold water without becoming soggy, and to al-low air to penetrate to plant roots and other soil organisms, is vitalto every aspect offertiliry.

The tilth of your soil is a composite of irs rexture, srrucrure, ag-gregation, densiry drainage, and water-holding capaciqr. No matterwhat kind of soil you starr out with, mosr of these qualities can beimproved by increasing its organic mater and humus conrent.

Soil Composition

About half the volume of a good, loamy soil is pore space-the areabetween particles where air and \Marer can penerrare. The pore spaceis generally an equal volume of air and warer, which clings to rhe sur-face of soil particles. Dont discount the importance of pore spaces.All the fertilizer in the world wont solve the problems created bydense, compacted soil that is deficient in pore space.

FIcun¡ I. Soil particlesand pore spøces, showing athin film of wøter coueringeacb pørticle.(Drawing by Tmothy Rice.)


Of the solid half of the soil, about 90 percent is composed ofsmall bits of the rocks and minerals from which the soil was formed,as well as clays created by the wearhering of the parenr rock. The re-maining 10 percent is the organic fraction. The influence of thissmall part of the soil on its abiliry to supporr plant growth is tremen-dous.

The sand and clay componenrs of a soil are largely unalterable-there's not much you can do to change them. But how you manageyour soil can have a profound influence on the amount and qualiryof organic matter it contains. The organic fraction of the soil is a dy-namic substance, constantly undergoing change. It consists of livingorganisms, including plant roots and bacteria, as well as dead plantresidues and other wastes. The total weight of the living organisms inthe top six inches of an acre of soil can range from 5,000 to as muchas 20,000 pounds.

Fundamental Qualities

Every soil has its own unique physical characreristics, which are de-termined by how it was formed. Some of these qualities can be im-proved with proper managemenr-or made worse by abuse-butothers must simply be considered the basic starting point you mustwork with. Theret not much you can feasibly do to change rhedepth of bedrock or warer table, or to eliminate a sreep slope. Youcan pick rocks out of a very stony soil, but in cold climates, frosr-heaving will only bring more rocks to the surface each spring.

Soil texture is one such inherent qualiry. Texture can range fromvery fine, mosdy clay particles, to coarse and gravelly ones. Any ex-treme is undesirable: the ideal loamy rexture is a balance of fine clayand silt, combined with coarse sand. The rexrure of your soil will in-fluence its nutrient srarus, workabiliry aeration, and drainage. Claysoils hold water and nutrients well, but can be poorly drained anddifficult to work. \Øhen they dry out, rhey form hard clumps, andcan take on the consistency of concrere. Sandy soils are generally

17

t8 UNDERSTANDiNG THE SOIL ECOSYSTEM

easy to work and well drained, but have poor nutrient- and water-holding ability. Their very high aeration means organic matterdecomposes too rapidl¡ and little stable humus is formed.(Refer tochapter 3 for instructions on evaluating your soilt texture and otherphysical qualities.)

Structure and Aggregation

Good tilth is less dependent on the composition of your soil than onhow it holds together. The abiliry of soil particles to form stable ag-gregates, giving it a crumbl¡ cake-like consistenc¡ determines itsstructural soundness. The ideal crumb structure, illustrated below isvery much a product of biological activiry. Humus plays a centralrole in forming soil aggregates, but many soil creatures-mosr no-tably earthworms-secrete the sticlcy gums that are crucial for hold-ing soil particles together. Structure and aggregation can bedramatically improved by increasing humus conrenr and stimulatingsoil biological activity.

'ø

FIcun¡ 2. Comparison of good crumb-liþe soil structure (lefi), with a poor,clod- liþe structure (rrghÐ.(Drawing by Stewart Hoyt.)

o


A good crumb srrucrure implies that soil is well aerared, sincethere will be plenty of pore spaces berween the granules. srructure isalso essential to the abiliry of soil to conducr soil moisture upwardtoward plant roots. This feature is referred to as capillary action, andit works in much the same way that oil is taken up into the wick ofa lamp. If soil structure is good, the surface may dry out, bur mois-ture will still reach the root zone from deeper soil levels. Adequatesoil moisture is crucial not only to replenish what is lost throughtranspiration from plant leaves, but also because plant roots take upmost of their nutrients when they are dissolved in the thin film ofwater that coats soil particles.

FtcuRe 3. Moisture rnouing upwørd in soil þt capillary action.(Drawing by Timothy Rice.)

19

Tal¡le 4PHYSICAL PROPERTIES OF SOIL

'roperty and def.nition Signifcance ín soíl Influence of organic matter

,ulk Density: The weight ofunit volume of dry soil, in-cluding pore spâces. Expressedas grâms"per cubic centimecer(gm/cm'¡.

ore Space: The portionoccupied by air and waterper unit voiume of soil.Expressed as a percentage ofvolume.lructure and Aggregation:Refers to the arrangementofsoil particles, their shape,size, and stabiliry.

txygen Diffirsion Rate:The rate at which oxygen cânbe replenished as it is used byrespiring organisms. Expressedas grams per cubic centimeterper min ute (gm/cm'/min).

ield Capacity: The amount ofwater held in pore spaces aftera fully saturated field has beenallowed to drain for 24 hou¡s.Expressed as a percentage ofvolume.

Indicates how dense the soilis and, therefore, how easilyair, water, and plant roots canpenetrate. (Optimum range:1.0-1.8 gm/cmJ for compactsubsoil).

Indicates specific âeration anddrainage qualities. (Optimumrange: 350/o-600/o for topsoil;25o/o-30o/o for compactsubsoil.)

The structure that encouragesthe most plant growth isgranular: rounded âggregatesthat stick together, but shakeapart easily. Especially porousgranules are called "c¡umbs."

Indicates the aeration statusofthe soil. In addition topore space for air to enter,there must also be continualdiffusion offresh air into thesoil to replace carbon dioxidewith oxygen. (Minimumlevel for root srowth is20 x 10-8 g-"/.-3l-i.,.)

Indicates the drainage qualities By improving the soil srrucrure,of the soil. A low fìeld organic matter modulates thecapacity means that water field capaciry of soils thatruns out too quickly; would otherwise be too wet orwith a high field capacity, too dry.water remains too longin pore spaces. ('\l'ellgranulated silt loam has afield capaciry of about 15%.)

Increased organic matter leadsto decreased bulk density,because organic matter is lessdense than soil minerals andgas is released duringdecomposition.

Increased organic matter leadsto increased pore space. Soilorganisms also increase porespace by burrowing and eating.

Biological activiry is virtually es-sential Êor proper granulation.Humus provides a perfectcrumb structure that resistscompaction.

Increased organic matter leadsto an increased oxygen diffu-sion rate. Decomposing or-ganic matter (especially plantroots) and mobile soil organ-isms c¡eate air passages in soil.


CHE¡rucnl FRcrons: Nurrur¡¡r Cycl¡s axo BelaNcrs

The conventional approach to soil management has been labeled"chemical," in contrast to the "organic" method, which rejects theuse of synthetic, petrochemical fertilizers and pesticides. The chem-ical approach holds that plant roors require cerrain chemical nutri-ents, but how these nutrients get to the roots and where they comefrom matters little. The nutrient elements must be presenr in a solu-ble, inorganic form in order for plants ro use them.

The ecological viewpoint holds that the effect of fertilizers onsoil organisms and the environment is of equal importance to theirvalue as plant food. "Feed the soil, not rhe planr," organic growersmaintain, and soil organisms will provide a balanced diet to crops.Highly soluble chemicals, though readily taken up by plants, can in-hibit or kill soil microbes, and can be washed away ro pollutegroundwater. Moreoveç plants are also able to absorb and benefitfrom complex biochemicals such as viramins and antibiotics, whichare not present in artificially synthesized fertilizers.

Public concern over groundwarer contamination by nitrate fertil-izers, as well as other agrichemicals, has stimulated greater interest insoil management practices that do nor rely on highly soluble materials.

Soil Chemistry Simplified

Regardless of which approach you adhere ¡e-çþç¡¡içal or or-ganic-the fundamentals of soil chemistry remain the same. Mostchemical interactions, in the soil or anywhere else, take place be-tween particles that carry either a positive or a negative charge whendissolved in water. Positively charged particles are called cations (pro-nounced "CAT-ions"), while anions ('AN-ions") carry a negativecharge. These particles may be a single elemenr, such as calcium(C^2*), or a compound, such as nitrate (NO3-). The behavior ofnutrients in the soil ecosystem is determined by whether they exist ascations or anions.

2l

t2 UNDERSTANDING THE SOIL ECOSYSTEM

RooÌ hair

Enters soil solution-suscept¡ble lo leaching

sites

-ll{ = Hydrogen ions (H+)

<C = Cation nutrients (Ca+1 Mg+1 K+, etc.)

FlcuRa 4. Cøtion Exchange Capaci4t (CEC). The higher the CEC, tlte nzorenatrients can be hept øuøiløble to plønts, yet søfe/ìom leaclting. Cations heldon the exchange sites øre søid to be absorbed.

The Basics of Cations

Cation nutrients tend to be metallic mineral elements, importantfor both plant and microbial nutrition as components of enzymes.They are generally quite water soluble, and enter the soil eitherthrough the recycling of organic matter or by addition of mineralnutrient sources such as limestone. Cations are called base ele-ments because they form bases in solution. In humid climates,where there is over thirty inches of precipitation a year, cationstend to become leached out of the topsoil-more slowly if they arestored by soil colloids such as clay and humus. Soils in arid cli-mates, conversel¡ are usually rich in minerals and so extremelyproductive when irrigated.

The major cation nutrients include calcium, magnesium, andpotassium; their functions are summarized in the chart that fol-lows. Other cation nutrients, needed in minute quantities, are de-scribed in the discussion of micronutrients (see page23).

(+t)

matter

Decomposingorganic

Limeor other

rock powder

Micelle:Colloidal clay orhumus particlewith negative

exchange sites

Table 5MAJORCATTON NUTRTENTS

NaturalNutríent soarces

Formsin soìl

Functìonin plants

Defciencyrymptoms

Calcium(c^2*)

Magnesium(Mg2+1

Dolomite,calcite, apatite,calcium feldspars,gypsum.

Mica, hornblende,dolomite,sefPentine,certain clays.

Most is Dresent^'¡i.as La"' lon oncation exchangesites, or in soilsolution.

At high pH,calcium formsinsolubleprecipitates withphosphorusand somemicronutrients.

Presenr as Mg2*ion on cationexchange sites,or in soilsolution.

Essential fornitrogen uptakeand proteinsynthesis.

Also has a rolein enzymeactivationand cellreproduction

Essential part ofchlorophyllmolecule.

Necessary forphosphorusmetabolism andenzyme activation.

Often concentratedin seeds.

Essential fo¡carbohydratemetabolism andcell division.

Regulatesabsorptionof calcium,sodium, andnitrogen.

Stunted roor growth,undeveloped terminalbuds, and leafcurl.

Pit rot in carrots,blossom'end rot intomatoes.

Yellowing of lowerleaves, with vena-tion in green; reducedyields.

'Weakened stems,scorched leafedges,necfotic sPots,stunted growth,susceptible todisease.

Potassium Feldspars,mica,lK*) granites,

certain clays.

Available asK+ on cationexchange sitesor in soilsolution. (Lessthan lo/o of totalsoil K+ is inavailable form.)

z4 UNDERSTANDING THE SOIL ECOSYSTEM

Cenoru ExcnRNcE Cnpacrv

Cation Exchange Capacity, or CEC, is an important measure-ment of the amount of cation nutrients a given soil is able tostore on its clay and humus colloid particles. Colloids, as de-scribed on page 25, have a large number of negatively chargedsites all over their surface. Positively charged cations are held onthese sites, largely protected from leachingaway in watet but stillavailable to plant roots. As plants give off hydrogen ions, a \¡/asteproduct that is also positively charged, it is exchanged for needednutrients like calcium, magnesium, and potassium. Nutrientsheld in colloidal exchange sites may not show up in soil tests be-cause they are not dissolved in water, but they are still available toplants through direct contact between roots and soil colloids.This process is referred to as adsorption.

Soils with a high clay and humus content will have the high-est CEC, which is measured by how many thousandths of a gramof hydrogen-called milliequivalents-can be held by 100grams of dry soil. Different kinds of clays have CECs rangingfrom 10 to as much as 100, while the CEC of pure humus canapproach 200.Yery sandy soils will have a CEC of 5 or less.

Think of your soil's CEC as a kind of nutrient savings ac-count. As nutrients are "withdrawn," whether by removing cropsor through the prolonged action of water, it is important to re-place them in order to maintain your reserves. These reseryesmust be well stocked before plants are able to draw on them, soa soil with a high CEC but depleted nutrients will require greaterapplications of mineral nutrients to restore its fertiliry than will asimilarly depleted but low CEC soil. A high CEC soil with anacid pH will require a larger amount of calcium, in the form oflimestone, to correct it than will a low CEC soil with the samepH. Knowing your soilb CEC will help you better understandand interpret your soil test recommendations, as discussed inchapter 3.


Colloros

One of the most important characteristics of humus is its col-loidal nature. Colloids are subsrances composed of many tiny par-ticles suspended in a gel-like mass, giving them a lot of surfacearea in proportion to their weight. Prorein, which makes up allliving cells, is a colloid. Other examples of colloids are milk,mayonnaise, rubber, and gelatin. Clay is also a colloid, and theclay component of the soil behaves similarly to humus. Physi-call¡ colloids tend to be sticþ and absorbent.

Colloids are important chemically because they are coveredwith negatively charged paticles. This makes them able to holdonto positively charged chemical particles, many of which areimportant soil nutrients. All soil chemical interactions are aÊfected by the soil's clay-humus colloidal content.

Cations, pH and FertilityrVhen cation or base nutrienrs are deficient in soil, it becomes acid.pH (which stands for "potential hydrogen") is a measure of the acid-ity or alkalinity of soil, determined by the concenrrarion of hydrogenions in a water or salt solution. Acidiry is indicated by a pH below7.0, which is neutral; pH values over 7 .0 indicate alkalinity.

As hydrogen ions replace the cation nurrients held in soil col-loidal reserves, soil pH decreases. The solubility, and thus availabilityto plants, of most nutrienrs is highest at a slightly acid pH-around6.3 to 6.8 is optimum. This is also rhe most favorable range for thefunctioning of most soil bacteria, though fungi can tolerate a widerpH range. At low pH-below 5.5-most major nurrienrs and somemicronutrients assume insoluble forms. Phosphorus becomes chem-ically immobilized at both low and high pH, requiring a range be-tr,veen 6.0 and7.0 for maximum availabiliry.

)5

26 UNDERSTANDING THE SOIL ECOSYSTEM

Many cation micronutrients, including iron, manganese, zinc,copper, and cobalt, become more soluble at low pH but are unavail-able under alkaline conditions. In some cases, acid conditions can

induce toxiciry of these elements. This is also ffue of certain heavymerals, most notably aluminum, which is naturally present in mostsoils, and lead, which can sometimes be a contaminant. Neutralizingthe pH also neutralizes the heary metal hazard.

-When correcting soil acidiry the object is not so much to neu-tralizepH as it is to replenish the appropriate cation ¡u¡¡iç¡¿5-usu-ally calcium and sometimes magnesium in the form of limestone.Applytrg other alkaline materials, such as sodium bicarbonate, mayneutralize the pH, but wont improve soil fertiliry.

'Acid soil syndrome" is a common problem in areas of high pre-cipitation, where soluble soil bases tend to leach out into the subsoil.Some of the problems associated with acid soil include:

. Interference with the availability of nutrients to plants.

. Increased solubiliry of iron, manganese, and especially alu-minum to undesirable levels.

. Reduced bacterial activiry especially of nitrogen-fixing rhizo-bia, and slower release of nutrients contained in organic matter.

. Lower total CEC, which further increases nutrients'leachabiliry.

Alkaline soils can be even more difficult to correct. The additionof acid-forming minerals like sulfur is more expensive and tempo-rary than the addition of limestone to acid soils. In many placeswhere soil is naturally alkaline, improper irrigation practices maycause salts to build up in the surface layer, a condition known as

salinization. This happens when nutrient-rich water rises to the sur-face through capillary action and then evaporates, leaving its miner-als behind. Some soils in arid areas âre naturally saline. It is difficultand costly to reverse these effects, and it is generally done by leach-ing the area with large amounts of fresh water-a resource usually inshort supply in affected regions.


Among the problems associated with alkaline soils are:

. Unavailability of many nurrienrs, especially mostmicronutrients.

' Saline seep, causing soil crusting.. Toxic levels of sodium, selenium, and other minerals.. Chemical destruction of organic matter.

Soil Anions and Their Cycles

Anion nutrients differ from carions in that they are not stored chem-ically by soil colloids, and form acids in solution. Reserves of anionnutrients are held in the organic portion of the soil, and are releasedto plants through the decay of organic marrer or through the air andwater. Depending on the status of soil organisms and the decay cycle,soil anions continually change in form and quantiry. As the majorbuilding blocks of proteins and carbohydrates, anions are required inlarger quantities than are carion nurrients. It is helpful to think of an-ions as large, soft, and changeable in form, while cations are small,hard, and durable.

tVhen added to the soil as soluble fertilizers, anion nurrientsmay be lost because they volatilize into the atmosphere, leach awa¡or revert to more stable, insoluble forms. These soluble fertilizersmay be acid-forming, or otherwise harmful to soil organisms. Vhensubstituted for nutrient sources rich in organic marrer, they can belikened to an addiction: Higher doses will be required to replace thenutrients that were previously supplied naturally by the soiltecosystem.

. Nitrogen tends naturally toward the gaseous srate as its moststable and plentiful form. The nitrare form, in which it ispresent in the soil solution, is exrremely transitory and willfluctuate significantly from day to day and even at differenttimes of day. Although plants cannor use atmospheric nitro-gen directl¡ certain soil microbes, most notably the rhizobia

27


F E RTI LIZE R.Nd RAIN

N.F IXATIONANIMALSáq

sor L oßGANtcMATT€N

GASEOUS LOSS^t

R ESI OUES,/---t MANURES,

WAST€S

\N¡ol{oNH¡

No¡ CLAYMINERALS

ll F IXATION

FtcuRn, 5. The Ninogen Cycle. Plants cannot utilize nitrogen in its gaseous

form. In order to pøss from atmosphere t0 plønt (and then to ønimals andpeoph), nitrogen must frst be fixed by soil microorganisms. To make syntheticfertilizers, atmoslteric ninogen is ørtificalþ fixed through use of huge quønti-ties of naturøl gøl (33,000 to 40,000 cubic feet of nøtural gøs is required toproduce one ton of ammonia.)Reprinted with permission of MacMillan Publishing Company from The Nature and Propertiesof Soils,9th Ed., by Nyle C. Brady. Copyright 19746yMacMiIlan Publishing Company.

bacteria that live on the roots of legumes, are able to caprure irfrom the air and transform it into a biologically useful form.Free-living soil bacteria such as azotobacter and closrridia areelren more important for nitrogen fixation. Still other bacteriatransform the nitrogen from ammonium to nitrite and then tonitrate form; each step in the natural nitrogen cycle is essendalfor plant nutrition.

. Carbon, the major constituent of plant (and animal) tissue, ismore truly the "food" consumed by plants than any mineral.Although abundant in the organic fraction of the soil, carbon

UNDERSTANDING THE SOIL ECOSYSTF,M

-,Animals

Green Manure

29

Crop

Farm

Soil

coä, Hcoä

& Residues

Microbial

reReactions I

CarbonDioxide

vrIDrainage LossesCOz+ çurbon",es & Bicarbonatesof Ca, Mg, K, etc.

FIcURB 6. The Carbon Cycle. Gøseous cørbon dioxide is trønsformed byplants into liuing tissue. Carbohydrates, proteins, andfats are decomposed bysoil orgønisms, thereby replenishing the suppþ of atmospheric cørbon dioxide.Reprinted with permission of MacMillan Publishing company from The Nature and propertiesof Soik, 8th Ed., by Nyle C. Brad¡ Copyright 1974by MacMillan Publishing Company.

is taken in by plants almost entirely from the armosphere, as

carbon dioxide. The carbon dioxide concentrarion close tothe ground can be substantially enriched by the presence ofacdvely decaying organic marre! which directly stimulatesplant growth. Lack of carbon dioxide can be a problem ingreenhouses, where air circulation musr be artificially main-tained. An indoor composr pile can add carbon dioxide-and6¡¡¡¿ þs¿¡-to the greenhouse environment.

. Phosphorus undergoes some of the most complex chemicalinteractions of all the elements of soil fertiliry. It is easily im-mobilized in the soil through its tendency to form insolublecompounds with calcium and other minerals. At a pH nearneutral, even highly soluble phosphorus fertilizers will soonrevert to forms indistinguishable from the "narural" rockpowders from which they were manufactured. Once added tothe soil, immobilized phosphorus srays rhere a long time;

Tal¡le 6MA,JOR ANION NUTRIENTS

NataralNutrient sot¿rces

Fotmsin soíl

Functionin plants

Defciencyslm?toms

Carbon(c)

Nitrogen(N)

Phosphorus(P)

Sulfur(s)

Organic mattetrespiration ofsoil organisms.

Organic matter,atmosphericnitrogenfixed by microbes,small amountsdissolved inrain water.

Organic matte¡mineral powders,some parentmaterials.

Organic mâtter,atmospheric sulfurfixed by microbes,pollutants inrain water.

Organiccompounds,carbon dioxidegas in airspaces, and weakcarbonic acids.

Organiccompounds,nitrites, nitrates,and ammonium(soluble forms).

Organiccompounds;solublephosphates;insolublecompounds ofiron, aluminum,manganese,magnesium,and calcium.

Organiccompounds;soluble sulfates,sulfites, andsulûdes.

Basic constituentof all livingcells.

Basic constituentofprotein andgenetic material.

Essential forgenetic material,membraneformation, andenergy transfer.

If atmosphericcarbon dioxideis limited,plant growthis slowed.

Thin stems;yellowing (chlorosis)ofleaves, beginningwith lower leaves;slowed growth.

Purpling ofleaves,beginning onundersides; stuntedroots; slowedgrowrh.

Importantconstituent ofproteins andcertain vitamins

Deficiency ishard to detect,but resemblesnitrogen deûcienc¡with yellowingof whole plant.

many soils, especially in the Midwest, have phosphorus re-serves created by government encouragement of excessive fer-tilizer applications.

Phosphorus is most readily available to planrs when re-leased gradually through the decomposition of organic mat-


ter. Its relative immobiliry means that distribution of phos-phorus throughout the soil is only accomplished through themovement of earthworms and other soil organisms. Other-wise, insoluble soil phosphorus reserves can become availableto plants through the activities of soil microbes.

. Sulfur, ân essential component of protein and fats, acts muchlike nitrogen in the soil ecosystem and is particularly impor-tant for nitrogen-fixing microorganisms. A special group ofmicrobes transforms organic sulfur into the sulfate form uti-lizedby plants. Sulfur deficienry is rarely a problem, especiallywhere adequate soil organic matrer levels are maintained. Airpollution also has had unintended beneficial effects on the sul-fur content of soils downwind. Deficiencies have risen withincreased use of highly concenrrared phosphate and nitratefertilizers, which lack the sulfur impurities found in the lowergrades. Sulfur is often needed as a nurrient and an acidifier foralkaline soils.

Micronutrients

Micronutrients are elements that are important in very smallamounts for the proper functioning of biological systems. Some-times called "trâce elements," over a dozen of them have been iden-tified as essendal in minute quantities for plant, animal, or microbialenzyme functions. Most of the important micronutrients, such as

iron, zinc, copper, and manganese, are cations; boron and molybde-num are the most important anion micronutrients.

Micronutrients occur, in cells as well as in soil, as parr of large,complex organic molecules in chelated form. The word chelate (pro-nounced "KEE-late") comes from the Greek word for "claw," whichindicates how a single nutrient ion is held in the center of the largermolecule. The finely balanced interactions between micronutrientsare complex and not fully understood. \Øe do know that balance iscrucial; any micronutrient, when present in excessive amounts, will

3t

32 UNDERSTA.NDING THE SOIL ECOSYSTEM

become a poison, and certain poisonous elements, such as chlorine,are also essential micronutrients.

For this reason natural, organic sources of micronutrients are thebest means of suppþing them to the soil: they are present in balancedquantities and not liable to be overapplied through error or igno-rance. \Vhen used in naturally chelated form, excess micronutrientswill be locked up and prevented from disrupting soil balances. Soilhumus reserves also serve to chelate excess metals-nutrients as wellas roxins. Unless a specific micronutrient deficiency has been diag-nosed by a soil test, the best way to provide adequate supplies is bybuilding organic matter and appþing balanced sources of mineralssuch as rock powders and seaweed. See chapter 4 for more informa-tion on micronutrient fertilizers.

-CHu -CH¡

_H

CH¡

H

H_

H

x+-H_H

H

H

X_H-C_H

IR

FIcuns 7. Chektion illasnated by the chlorophyll molecule.

NatrientDesired amountín soil (ppm)*

Table 7SOME IMPORTAM MICRONUTRIENTS

Functionín pknts

DeficienErymptoTfis

(F.)

(M")

Iron 25,000

Manganese 2,500

Zinc 100

Copper 50

Boron (B) 50

Moþdenum 2

Chlorophyll synthesis, oxidation,constituent of various enzymesand proteins.

Synthesis of chlorophyll andseveral vitamins, carbohydrateand nitrogen merabolism.

Formation of growth hormones,protein synthesis, seed and grainproduction and matu¡ation.

Catalyst for enzyme and chloro-phyll synthesis, respiration,carbohydrate and proteinmetabolism.

Protein synthesis, starch andsugaf transport, root develop-menr, f¡uit and seed Formation,water uptake and transport.

Essential for symbiotic nitrogenfixation and protein synthesis.

Chlo¡osis (yellowing), largerveins remain green; shortand slender stems.

Yellow mottling of leaves,pale overail coloring, poormaturation and keepingqualiry.

Late summer mottling ofleaves. Early defoliation offruit trees.

Yellowing and elongation ofleaves. Onions are soft,with pale yellow scales.

Growing tips die back.Heart rot of root crops,corþ core of apples, poorlegume nitrogen ûxation.

Pale, distorted, narrowleaves, leafroll, poor nitro-gen fixation.

(2")

(cu)

(Mo)

xApproximate values indicate relative proportions.

A Balancing Act

Balance is the crucial concept for understanding the relationship be-tvveen chemical nutrients and soil fertility. Fertility requires nor onlysufficient quantities of nutrients, but their presence in balancedform. In many cases, too much of one nutrient will lock up or inter-fere with the absorption of another. Phosphorus is rhe classic exam-ple; it will become immobilized at low pH by high concentrations ofzinc and iron, and at high pH by too much calcium. Potassium andmagnesium will each interfere with the availability of the other,when present in excess. In the case of carbon and niffogen, too much

Table 8NUTRIENT INTERACTIONS

NatrìentDefciency may be índacedfoi excæs of

Excess may inducedefciency of

CATIONS

Calcium Aluminum

Magnesium Calcium, potassium, ammonium

Magnesium, potassium, iron,manganese, zinc,phosphorus, boron

Potassium, zinc, boron,manganese

Magnesium, boronZinc, manganese

IronIron, copper, manganese

Iron, zinc, mânganese

PotassiumIron

ManganeseZinc

Copper

Magnesium, calcium, ammoniumPhosphorus (high pH),

mangânese (low pH),calcium, copper, aluminum, zinc

Iron, copper, zinc, calcium, magnesiumPhosphorus, nitrogen, magnesium,

iron, copper, calcium, aluminumPhosphorus, zinc, nitrogen

ANIONS

Carbon Sulfu¡ nitrogen, phosphorus

Carbon, phosphorusCalcium, nitrogen, iron, aluminum,

manBanese

Carbon, nitrogenCalcium, potassiumSulfur, copper

Sulfur, nitrogen,phosphorus

PhosphorusZinc, copper, nitrogen

Iron, copper

NitrogenPhosphorus

SulfurBoronMolybdenum


carbon will stimulate soil microbes to grow and take up all the avail-able soil nitrogen, resulting in a temporary deficiency until the mi-crobes die and release their nutrients to the soil system.

Cation balances have received a lot of scientific affention. Basesaturation refers to the percentage of a soil's CEC (see box, page24)occupied by bases-cations other than hydrogen or aluminum.

Some efforts have been made to find the "ideal" cation balanceor base saturation ratio. One very influential scientist was Dr.\Øilliam Albrecht, who conducred research at the Universiry ofMissouri in the 1940s. Albrecht's key contribution was to point outthe importance of calcium as a major ingredient of fertiliry con-tending that it was rhe calcium in limestone, nor its acid-neutralizing abiliry that made it an important fertilizer. He alsodeveloped a formula for an optimum base sarurarion ratio, empha-sizing calcium, which has been used by many soil labs to eyaluaremineral balances. \Mhile it is a useful guideline, Albrecht's ratio isnot universally accurate, and should not be relied on exclusively todetermine fertilizer needs.

The ideal proporrion of anion nutrienrs is the balance rhar isnormally found in humus: 100 parts carbon ro 10 parts nirrogen ro1 part phosphorus to 1 part sulfur. The importance of the carbon ronitrogen ratio was described earlier; however, the ratio of nitrogen tophosphorus is also important ro proper plant nutrition, since inade-quate nitrogen slows the growth of roors and therefore their abilityto reach phosphorus supplies.

Micronutrient problems are as ofren a result of imbalances as ofabsolute deficiencies. New information is continually being discov-ered about previously unknown interactions between major and mi-nor nutrients in the soil ecosystem. This is why rhe "cookbook"approach to soil chemistry can ger you into trouble; the best nutri-ent sources are those that are naturally balanced. The chart on page34 will give you some indication of the complexity of known nutri-ent interactions.

35

-36 UNDERSTANDING THE SOIL ECOSYSTEM

BrolocrcRl FRcroRs: The Living Soil Communit¡r

The cycles that permit nutrients to flow from soil to plant are allinterdependent, and proceed only with the help of the living or-ganisms that constitute the soil community. Soil microorganismsare the essential link between mineral reserves and plant growth.Animals and people are also part of this community. Unless theirwastes are returned to the soil, for the benefit of the organisms thatlive there, the whole life-supporting process will be undermined.

Soil organisms-from bacteria and fungi to protozoans andnematodes, on up to mites, springtails, and earthworms-performa vast array of fertility maintenance tasks. Ecological soil manage-ment aims at assisting these creatures in their work, rather thansubstituting a simplified chemical system to provide nutrienrs roplants. For, once disrupted by the excessive use of soluble fertiliz-ers, the restoration of a healthy soil ecosystem can be a long andcosdy process.

Soil Inhabitants

Microscopic plants and animals form the basis for the soil food web.Most contribute directly to humus formation and the release of nu-trients from organic matter. Stable humus, in fact, consists largely ofmicrobial remains. In cool, humid climates, fungi and molds aremore significant than bacteria for humus development. Beyond theirimportance for soil health, these microbial decomposers are essentialto all life on Earth, since they are responsible for virtually all organicwaste recycling.

Other creatures, both microscopic and visible, make importantcontributions to soil health, most notably the earthworm. Plantroots themselves are major participants in the soil ecosystem, andsignificantly affect the environment that sustains rhem. Once youbecome aware of the astonishing number and variery of life formsthat are constandy growing, reproducing, and dying in every crumb


of soil-billions in each gram of healthy topsoil-it is impossible topick up a handful of earth without feeling a sense of awe. Each or-ganism has a role to play in the soil ecosystem:

. Producers create carbohydrates and proteins from simplenutrient elemenrs, almost always by capturing energy fromsunlight through photosynthesis. Green planrs, including blue-green algae, are the producers ofthe soil. A few specialized bac-teria, known as autotrophs, are also able to synthesize their ownfood from carbon dioxide and mineral elemenrs in the soil.

. Consumers are just about everyone else, who all depend onthe food created by green plants for their nourishment. Pri-mary consumers eat plants directl¡ while secondary and ter-tiary consumers feed on other consumers. All animal life,from simple prorozoans to humans, as well as nonphotosyn-thesizing plants, such as yeasts and certain other fungi, fallinto this category.

. Decomposers perform the critical function of bringing rhebasic chemical nutrients full circle, from consumers back toproducers. They are all bacteria or fungi, and are found al-most exclusively in the soil; about 60 to B0 percenr of thetotal soil metabolism is accounred for by microbial decom-posers. \Øithout them, life would grind to a halt as we suffo-cated in our own wastes.

\W"hat They Need

If the surest route to improving soil fenility is to provide the mosthospitable conditions for soil life, understanding the basic needs ofsoil organisms is the first step. Successful composting requires thesame knowledge: Provide soil beasties with adequate food, air, andwater, and-depending on other enyironmental factors, such as

temperature-they will fl ourish.

37

Tal¡le 9SOIL ORGANISMS

OrganismApproximate SPecíalsoil popuhtion reqairements

Source ofnutrition

Role ìnecoslstem

MICROFLORA

Fungi: yeast,molds,mycorrhizae.

Actinomycetes.

BacteriaAutotrophs:Azotobacter,Rhizobia,Nitrobdcter.

Heterotrophs:decay organisms.

{Igae:green,blue-green

10t-106 Per gram 'üZill tole¡atewide PH andtemPeratufefanges.

107-108 per gram Need aeration,moisture, andpH 6.0_7.5.

10'-10' per gram Most need air(aerobes), andexchangeablecalcium.

Têmperature of70"-100'F,pH 6-8.

104-10t per gram

Humus formation,and aggregatestabilization. Createantibiotics, causeplant diseases,make phosphorusavailable.

Humus formation.

Organic matteror nutrientsfrom plantfoots.

Organic matter.

Autotrophs Autotrophs areconsumesimple nitrogen-fixers,nutrients from sulfur oxidizers,soil and air. nitrifiers. Some

Heterotrophs cause disease.break down Heterotrophs areorganicmatter. decomposers.

Photosynthesis. Add organicmatter to soil.Some fix nitrogen.

MICROFATTNA

\ematodes. 10-100 per grâm

?rotozoa, rotifers. 104-105 per gram

Organic matte¡other microbes,plant roots.

Secondaryconsumers,Some are pestsand parasites.

INSECTS E{ MOLLUSCS

It[ites, springtails, 103-10t per m,spiders, sowbugs,ants, beetles,centipedes,millipedes,slugs, snails,

Microflora,microfauna,other insects,plant roots andresidues,nematodes,molluscs,detritus (wastematter), organicmatte¡ weakplants.

Ae¡ate and mixsoil. Leaveremains fordecomposers.Cull weak ordiseased plants.

Tal¡le 9 (continuect)

SOIL ORGANISMS

OrganísmApproximatesoil popalatíon

Specialrequirements

Source ofnatr¡tìon

Role ìnecoslstem

EARTHTøORMS

Earthworms, 30-300 per m, Raw organicmatter,

Aerate andmix soil.

Leave nutrient-richcasts.

MAMMALS

Moles, mice,goundhogs.

Variable Earthworms, insects,molluscs.

Mix and pulverizesoil. Leave wastes.

PLANT ROOTS

Roots. 100-6,000 lbs.Per acre.

Photosynthesis;nutrientions andmolecule.s.

Remove water andnutrients; leaveresidues andexuclates.

Both soil and compost creatures need the same food: raw or-ganic mâtter with a balanced ratio of carbon ro nitrogen-approx-imately 25 or 30 pats ro 1. Carbon, in the form of carbohydrates,is really the main course for soil organisms: they will grow quicklygiven lots of it, and will scavenge every scrap of nirrogen from rhesoil system to go with it. That's why adding lots of high-carbonmateriâls to your soil can cause nirrogen deficiencies in plants. Inthe long term, though, carbon is the ultimate fuel for all soil bio-logical acdvity, and is therefore crucial ro humus formation andplant productivity. A balanced supply of mineral nurrienrs is alsoessential for soil beasties, and micronurrients are key ro the manybacterial enzymes involved in their biochemical transformations.Balanced nutrienrs also provide a favorable pH, though differentorganisms âre more sensitive to acid or alkaline conditions.

Air is also crucial for soil health, although certain bacteria canlive without it (see box, page 42).In fact, much effort in soil man-agement is directed toward improving soil aeration: no amount of

40 UNDF,R,STANDTNG THE SOiL ECOSYSTEM

fertilizing can compensate for lack of air, since plant roots cannottake full advantage of available nutrients if they are suffocating.

'W'ater is also strictly essenrial, but not to the extent that it dri-]¡es out air. The ideal biological environment consists of a thin filmof moisture clinging ro each soil particle, with lots of air circulat-ing berween them. Rain and irrigation, of course, play a centralrole in adding needed soil moisture, but good structure is also re-quired ro conduct moisture upward from reserves in lower soilstrata (see page 19).

Although temperature has critical effects on biological activiryevery specific soil communiry has evolved ro accommodate the nat-ural climate variations in its environment. Your only role in adjust-ing the temperature might be to moderate severe winter cold orsummer heat by mulching, or to heat up small areas with season ex-tension devices.

Soil Superstars

Despite the many volumes that have been written about soil biology,knowledge of the kinds of organisms that live in soil and how theyinteract is extremely limited. Although some scientists have tried towork out biological assays ro identify a soilt characteristics andneeds by examining its living population, such rests are still ex-tremely complex to carry out and difficult to interpret accurately.

The table on pages 3S-39 summarizes the major rypes of soil lifeforms. A few portraits of the more familiar and celebrated soil in-habitants, in order of size, follows:

Bacteria: Bacteria are the mosr numerous and varied of soil organ-isms, ranging from a few hundred million to three billion in everygram of soil. Under the right conditions they can double their pop-ulation every hour. The top six to eight inches of soil may containanywhere from a couple of hundred pounds ro rwo tons of live bac-teria per acre.


Bacteria vary in their requirement for air, but most beneficialones need it (see box, page 42). If enough moisture and food arepresenr, bacteria do best at remperarures of 70" to 100.F (21. to3B'C), and at a pH close to neurral. Adequate calcium is crucial, as isa balance of micronutrienrs, which are essential to the enzymes em-

4r

The three most imporrant plant microorganisms of the soil. Fungal mycelium(left), various rypes ofbacteria cells (center), and actinomycetes threads (right).The bacteria and antinomyceres are much mo¡e highly magnified than the fungus.

w a¡,.,¡¡ i

Wcommon rotifer

&

Parasitic nematodes (left), a ciliated prorozoan (center), and aGisht).

atr )7ù-r.

Jtr

Some soil organisms especially imporrant in the nitrogen cycle. Azotobacter (left),nitrate bacteria (center), and nodule organisms ofalfalfa (right).

FIcunr 8. Important Soil Orgønisms,(Drawing by Timothy Rice)


ployed by bacteria to perform their crucial biochemical tasks. Unfa-vorable conditions rarely kill bacteria off completely; they will eitherstop growing and form spores to wait for better times, or adapt to thechanged conditions as genetic mutations quickly spread to ne\M gen-erarions. This adaptabiliry can work against you when the organismin question causes a plant disease, though. If any soil nutrient is inlimited suppl¡ bacteria will be the first to consume it; plants thenmust wait to partake until the microbes die and decompose.

BR TETR AND BREATHING

The soil micro-universe is divided into two general types of bac-teria: those that need air and those that dont. The availabiliry ofair thus determines which kinds of bacteria will flourish, andhow vigorously they will grow.

. Aerobes require air in order to live. The bacteria thatmediate the soil nitrogen and sulfur cycles, as well as

many important decomposers, are aerobes. AII other soilorganisms are also aerobic, including plant roots. Somebacteria can survive in either aerobic or anaerobic con-ditions, but will only grow and thrive if they have air.

. Anaerobes can live happily without air, and in fact maybe killed if exposed to it. The bacteria responsible fordiseases like botulism and tetanus are famous examples.There are many anaerobic decomposers, which oftengenerate some foul-smelling waste products, such as hy-drogen sulfide, and the common term for the processof anaerobic decomposition is putrefaction. Anaerobicbacteria can also generate useful by-products such as

methane gas, which is sometimes used as an energysource.


Bacteria have a virtual monopoly on three basic soil processesthat are vital to higher plants: nitrification, sulfur oxidation, and ni-trogen fixation. Nitrifying bacteria transform nitrogen in the formof ammonium, a product of protein decomposition, inro nitrate, theform most available to plants. The sulfur oxidation process is analo-gous to nitrification. Nitrogen-fixing bacteria are able ro rransformelemental nitrogen from the atmosphere into protein, and evenru-ally make it available to other organisms-a process imitated by hu-mans at a high energy cost. They may live in symbiosis with plantroots, such as the members of the Rhizobium family, or rhey may befreeJiving soil dwellers, such as Azotobacter

Fungi: Yeasts, molds, and mushrooms are all fungi, and only yeastshave litde presence in the soil. Although they are plants, they do notcontain chlorophyll and so musr depend on other plants for theirnourishment. Molds may be as numerous as bacteria in soil, and willoutnumber them under conditions of poor aeration, low tempera-ture, and acidity, which they tolerate more easily. Although manyplant diseases are caused by soil-dwelling molds such as Fusariumand Aspergillu.r, those in the Penicillium family are well known as dis-ease fighters. Molds are especially important for humus formation,predominating in humus-rich, acid forest soils.

FIcun¡, 9. Myconhizae in-fecting a plant root and ex-trøcting nutrients jìom rockParticles.(Drawing by Timothy Rice.)

43

44 UNDERSTANDING THE .SOIL ECO.SYSTEM

One extremely significant group of fungi are called mycorrhizae,a term meaning "fungus root." These mushrooms enter into symbi-otic relationships with plant roots of many kinds, and are thought tobe essential for the health of trees such as pine and birch. The fungiare able to convert otherwise insoluble nutrients, most notably phos-phorus, into biological forms, and in turn receive carbohydratesfrom their host plants. Many crop plants are known to enter intomycorrhizal associations, but they are most significant for plantsgrowing on poor soils, where the fungal ability to extract nutrientsfrom rock particles is most critical to the host plant's nutrition.

Actinomycer¿s.'These microbes are like a cross between bacteriaand fungi, and are the most numerous soil organisms after bacte-ria. The characteristic aroma of freshly plowed earth is attributedto actinomycetes, which play a critical role in organic matter de-composition and humus formation. They need plenty of air and a

pH between 6.0 and 7.5, but are more tolerant than either bacte-ria or fungi of dry conditions. Their intolerance of low pH can beused to advantage in preventing potato scab, a disease caused byan actinomycete. Manure is especially rich in actinomycetes,which is why many people consider manure to be essential formaking high-quality compost.

Algae: A,lgae are single-celled plants, usually containing chloro-phyll, and are slightly less numerous in the soil than are fungi.Blue-green algae are common in many kinds of soils, but areparticularly important in paddy rice culture because of theirtolerance for high moisture levels and their ability to fix âtmos-pheric nitrogen. All algae growth is greatly stimulated by farmmanure.

Microfauna.: Microscopic soil animals including nematodes, proto-zoans, and rotifers. Nematodes, commonly called threadworms oreelworms, are extremely widespread and numerous in most soils.

UNDERSTANDING THE SOIL ECOSYSTEM 45

Pln¡¡r-Mtcnoee Sy ^srosrs

The mutually beneficial relationships berween planr roorsand soil microbes are complex and widely varied. Some of thesearrangements are well understood, but many remain mysterious.As we learn more about the workings of these microorganisms,they will undoubtedly be used more widely to improve cropproduction.

A few bacteria are able to convert nitrogen gas from the airinto a form usable by the roots of the plant with which they as-sociate. Rhizobium bacteria, for example, form visible nodules onthe roots of legumes, whose growth is greatly enhanced by the ni-trogen fixed there. 'ùØhen the legumes are rerurned to the soil, as

with green manure crops, the nitrogen fixed by the rhizobia be-comes available to the subsequent crop (see table 17). \Øhen a

legume is grown in association with another crop, for examplegrasses or grains, the nitrogen fixed by the rhizobia is available tothe associated crop while the two are growing togerher. Becausenot all soils contain these desirable nitrogen-fixing bacteria,when farmers plant legumes they often inoculate either their soilor seed with preparations containing rhizobia.

Other soil organisms, such as the actinomycere Franþia, thebacteria Azotobacter, and some blue-green algae, are likewise ca-pable of fixing nitrogen, with and without host species.

It has been estimated that more than B0 percenr of plantshave symbiotic associations with fungal mycorrhizae, whose per-vasive filaments extend the reach of plant roors in the soil, oftenimproving the roots' abiliry to absorb nutrienrs. Some of thesemycorrhizae are relatively nonspecific, able to penerrate the rootsof many different species of plants in various ecosysrems. Othersare quite specific, surrounding the roots of only one plant speciesin one type of soil.


Although they are often thought of as troublesome plant pests, themost common kinds help break down organic matter or prey onbacteria, algae, or other soil animals. Some parasitic nematodesare used as biological control agents for soil-dwelling pests such as

cabbage root maggots. Protozoans are one-celled animals that arelarger than bacteria, often using them as a food supply. Rotifersare common in wet soils, feeding by spinning around and sweep-ing food particles into their "mouths."

Earthuorrns and Other Macroføuna: Of the numerous animalswho make their homes in the soil, from miniscule spiders toprairie dogs, the earthworm is the most closely identified with soilhealth (see box, "The Noble -Vorrri'). Other small soil animalsinclude mites and spiders, beetles, springtails, flies, termites,ants, centipedes, and slugs, as well as the larval forms of manybutterflies and moths. Many of these creatures play an import-ant role in breaking down organic materials into smaller piecesand simpler compounds; some are significant as plant pests.Mammals such as moles, voles, gophers, and other burrowers cansometimes be a nuisance when they decide your broccoli looksgood to them; however, they too contribute to the soil ecosystemby keeping pest populations in check, mixing soil, and depositingtheir droppings.

Farrn Anirnals: The importance of farm animals to soil fertilitysometimes gets overlooked. Although modern "factory farming"concentrates too many animals on small areas, the ideal farmingsystem should include some animals. Besides contributing valu-able manure, farm animals are important for a number of ecolog-ical soil-building practices:

. Soil-building rotations require sod crops and legumes, whichare often only economically feasible when used as animal feed.

UNDERST¡.NDING THE SOIL ECOSYSTEM 4/

. Animals are often the only means to harvest a crop (besidestrees) on soils that are too wet, steeply sloping, or srony rocultivate.

. Pigs and chickens can improve the soil by acting as living ro-totillers, scratching and aerating a patch before crops areplanted. They also eat weeds and ground-dwelling pests.

THe Noslr Won¡n

A good earthworm population is universally considered a signof healthy soil. Unparalleled as soil excavators, earthwormsspend their lives ingesting, grinding, digesting, and excretingsoil: as much as fifteen tons of soil per acre goes throughearthworm bodies in a year. Earthworm castings are richer innutrients and bacteria than the surrounding soil, and may addup to as much as eight tons of nutrients per acre in cultivatedfields. Their contribution to drainage and aeration, soil aggre-gation, and transport of nutrients from the subsoil is signifi-cant as well. It is for good reason that Charles Darwin extolledearth\Morms as the "intestines of the soil."

Of about two hundred known earthworm species, Lum-bricus terrestris is the most common: interestingly enough, it isnot nâtive to North America, but came with the Europeansand turned out to be better adapted to cultivated conditionsthan its native predecessor. Earthworms, unlike the types ofworms used for composting, prefer cool temper¿¡Lr¡ç5-¿þsu¡50"F (10'C) is optimum. They need good aeration andenough but not too much moisture. Although some speciescan tolerate fairly acid soils, most require adequate calciumsupplies and thus more neutral pH. They are also sensitive tomany toxic pest- and weed-control chemicals, as well as fertil-izers with a high salt index.


Life in the Root Zone

plant roots themselves play an important role in soil ecology. Thelargest numbers and kinds of organisms are found in the upPer-mosr layers of the soil, closer to fresh sources of air, water, andfood. Tlue, some biological activity happens even at fairly deeplevels, especially where earthworms and other animals burrow, andwhere deep-rooted plants grow. However, in the area immediatelysurrounding plant roots, known âs the rhizosphere, there are con-centrations of ten to as many as one hundred times more organ-isms than can be found elsewhere in the soil. A soil such as rharfound under permanent grass sod, totally permeated by fibrousmasses of roots, will inevitably have a healthier, more robust mi-crobial population than one with cleanly cultivated row crops.

Most of the important soil biological rransformarions takeplace in the rhizosphere, especially nitrogen fixation and mycor-rhizal associations. The outer coating of the growing root rip,called the mucigel, is a fascinating subsrance, a product of boththe root and the microcommunity around it. A gelatinous sub-stance secreted by the roor, rhe mucigel is a rich mass of microbesand chemical nutrients rhat connects the plant directly to the lifeof the soil.

Some ways in which plant roots inreract with the rest of thesoil community include:

. Roots can take up cation nutrients directly from exchangesites on soil particles in exchange for hydrogen, through aprocess known as adsorption (see page 24).

. Plant roots giye off carbon dioxide in the process of respira-tion, just as animals do. Together with the release of hydrogendescribed above, this creates slightly more acid conditions inthe rhizosphere, since carbon dioxide forms a weak acid whendissolved in water. This slight acidiry helps to make otherwiseinsoluble phosphorus and micronutrienrs more available.

Root Hairs

Nematode ,Myco

t' Mucioel

:.'..,.hoot-fþ .,

& Actinomycetes' Worm Tunnel & Casti ngs

Ftcunr 10. The rhizosphere, a region approximateþ 3mm uide, and thezone of highest biologicøl ønd chemical øctiuity where soil, root, and mirobesinteract. Mucigel is a gelatinous substance surrounding the root thøt is boththe mediøtor and the product of the root-soil-microbe community.(Drawing by ïmothy Rice.)

50 I]NDERSTANDING THE SOIL ECOSYSTEM

. Roots give off certain biochemical compounds called exudates,which sometimes act as phytotoxins, chemical inhibitors ofcompeting plant species-a process called allelopathy. lVinterrye, for example, gives off exudates that suppress couchgrassgrowth.

. The dead tissue continually sloughed off by growing roots isexcellent food for microorganisms. The organic contributionof the root portion of a green manure crop often is more sub-stantial than the part you see above the surface.

. Plant roots are able to take up many complex organic com-pounds such as hormones, vitamins, antibiotics, and humusfractions, as well as toxic substances like pesticides and herbi-cides. This is an important counter to the argumenr that thesource of plant nutrients-whether chemical fertilizers orcompost-is irrelevant to plant health.

Soil and Civilization

All land-dwelling animals, including humans, are members of thesoil community. Human societies disregard this fact at their ownperil: soil fertility has historically been squandered for the imme-diate enrichment of a few at the expense of future generations.Cultural values-ethics, aesthetics, and spiritual beließ-have aprofound influence on how soil is treated.

Not only the farm itself, but also the society of which it is apart must be viewed as components of the soil ecosysrem, for allsupport and maintain one another and none can exisr indepen-dently. \Without a good-sized nonfarm communiry nearby, for ex-ample, marketing becomes a problem for the farmer; yet too largea nonfarm community exerts pressure to convert productive farm-land to other uses. A whole book could be written about the effectsof political and economic pressures on soil fertiliry-especially inthe "Third 'Sü'orld," where peasanrs are forced to produce exportcrops for foreign exchange instead of food for their families.

UNDERSTANDING THE SOIL ECOSYSTEM 5t

The 1992 United Nations Earth Summit acknowledged theimportance of sustainable agriculture âs a means of reversingworldwide environmental degradation. Implemenring its recom-mendations will require widespread public consciousness-raising.Political and social acdvism are, more clearly than ever, essentiâlcomponents of soil stewardship.

Documents

foundation of Agriculture of to - UC Agriculture & Natural ...cesantaclara.ucanr.edu/files/230251.pdf · Volcanic acrivicy has left nu-trient-rich lava deposits in many areas. Soils