A review

By Dr. S.M. ALAM, NIA, Tando Jam.
Nov 11 - 17, 2002

Soil structure is concerned with the living space in soil; the size, shape and presence of the pore space. Similarly, soil texture is concerned with the nature and working properties of the soil particles, which by the manner of their arrangement enclose the living space. The term "soil structure" and "soil texture" are frequently confused. When assessing the quality of cloth by its feel, we should try to distinguish between characteristics determined by the nature and thickness of the thread and characteristics determined by the nature and closeness of the weave. Similarly, when feeling soil, we should try to differentiate between characteristics determined by the nature and size of the fundamental particles by the manner of their arrangement. In soil, texture is primarily a matter of particles; structure and their arrangement. Texture is a fundamental, stable characteristic of a soil that the farmer has to accept; structure is temporary and can be manipulated. In a fertile soil the living space consists of an abundance of small pores retentive of moisture and an interconnected system of large pores that drain to admit air. The ideal soil would have about 30% of its pore space unblocked by water and well aerated. The rest of the pore space (70%) would be full of water available to plant roots. The problem is to achieve the ideal mozaic of air-filled and water-filled pore space in a medium consisting of particles all roughly spherical, blocky or platy in form. When soil is compact, particle size controls pore size, and hence, the rate of water flow and the tenacity with which water will be held.

In soil structure formation, large particles generally provide large pores in the soil system and thus permitting a rapid flow of excessive moisture through the soil, i.e. rapid drainage, but this advantage in respect of drainage is cancelled by the inability of large pores to retain moisture against the pull of gravity once the excess moisture has been cleared off the surface. In a large particle system, therefore, there is a risk of an inadequate moisture reserve to sustain plant life over periods of drought. Small particles provide small pores resulting in much slower rates of water movement and a much greater tendency for the pores to remain water-filled, once excess moisture has been cleared through them. Therefore, a fine particle soil will drain slowly and the pore system may well remain completely waterlogged after drainage has ceased unless, by some means, large diameter, free-drainage channels can be established through it. Organic materials increases the total porosity with a decrease in bulk density increases cation exchange capacity (CEC).

Decomposition of different organic amendments may be affected by organic matter characteristics in terms of soil properties such as soil texture, structure and pH. Soil structure influences soil moisture, air-water ratio, capacity to infiltrate, conduct store and drain water in addition to plant root growth development and penetrations. Soil is formed from the weathered products of rocks resorted and redistributed by the forces of gravity, wind, water and ice. The end product is a surface mantle of loose mineral debris consisting of particles far from uniform in shape and size. Neglecting the very large particles that we might all agree to call either boulders, stones or gravel there is, within what soil scientists define as the "fine earth", a thousand-fold range in particle size. The largest particles included within the sand category are just under 2 mm. in diameter (2000 micron). The smallest particles within the range of particle size, which includes the clay minerals, are 0.002 mm (2 micron) in diameter. A useful and generally accepted system of defining soil particle size categories is given below: - 2000-1000 mm, coarse sand; 1000-500 mm, coarse sand; 500-250 mm, medium sand; 250-125 mm, very fine sand; 125-50 mm, very fine sand; 50-20 mm, coarse silt; 20-2 mm, fine silt; less than 2 clay.

All the sands feel gritty and make a rasping sound when rubbed between the fingers. The sharp, searing sound of the word "sand" reflects this property. The silts, by contrast, feel smooth and powdery when dry, oily smooth and slimy when wet. Again the material being described seems to have the characteristics implied by the sound of the word evolved over time to identify it. The sands and the silts are essentially the product of the physical commination of rocks. They are particles all roughly spherical or cubical in shape hence, the finest of them, the very fine sands and the silts, readily flow when saturated, clogging pores between larger particles, in-filling drains and lubricating the erosion of soil down-slope. The clay minerals are the particles typical of the finest particle size category recognized in a textural description of soil, but clay particles are not just very small particles less than 2 microns in diameter, and they are not simply very fine particles, the ultimate product of physical commination. Clays are formed directly from rock minerals by chemical transformation, and considerably in form, chemical composition and physical properties according to their mineral origin and the conditions under which they were formed. Like the rust on iron they are not just small particles of iron but are new materials formed by chemical changes involving the regrouping of atoms.

However, the change from rock mineral to clay mineral is not simply a change in particle size. This change also involves a change in particle shape and in some important physical properties of considerable significance for the development of structure. Clay minerals generally are platy in form and like sheets of glass, cohere when stacked moist whereas silt particles, which are more isodiametric in form, would slump like a pile of minute billiard balls. But clay particles not only stick to each other; they can serve as a binding agent between other particles. Thus the clay fraction, if present as at least ten percent of the whole, can confer to the soil, when moist, qualities of stickiness and stiffness, and a degree of rigid cemented when dry.

But a stack of platy clay particles has other properties than cohesiveness: it will swell when able to absorb water freely and shrink again as it dries out. On drying, clays will tend to cause shrinkage to the point of cracking and the creation of large compact clods but on re-wetting, moisture will cause swelling and clods will tend to disintegrate into fragments producing a tilth similar to that achieved by frost. With clay contents of thirty percent and more, therefore, soils will, to some extent be self- mulching. Ultimately, of course, excess moisture can cause such a tilth to collapse completely but providing drainage is adequate to allow rapid clearance of excess moisture from the large cracks that separate the clods, this is unlikely to occur. Finally, clay particles, unlike sand and silt particles, are charged. Like little magnets they have another mechanism by which they can form links with other particles, and this mechanism, though not rigid, can still function through water. But the charge is negative and since like charges repel, this would seem to favour the dispersion of clay particles rather than their clumping. However, there are many positively charged particles in soil, in particular, calcium ions that dissociate from the calcium carbonate in lime, and these by being attracted to the negatively charged surfaces of clay particles, can serve as a bridge between them, causing them to clump into loose aggregates large enough to settle out of fluid suspension, like curds in milk. Thus, though clay particles, were they to function as individual units, would be small enough clay particles, were they to function as individual, would be small enough to be washed out of the surface of the soil and form drainage pans in the subsoil, this is unlikely to happen, unless the lime status of the soil is allowed to fall.

The clay fraction, therefore, is a very important fraction; not indispensable, as we shall see later, but very important where it is present in sufficient quantity to influence working properties. Soils with over forty percent clay are likely to be sticky when wet and hard when dry, but also, they may be self mulching and easily cultivated to a satisfactory, cracking tilth if worked at the right moisture state. Given good subsoil drainage plant roots can penetrate the large, well-aerated cracks, keeping them open and deepening the zone of biological exploitation. Clay soils are naturally retentive of moisture and of nutrients, and both can be exploited for crop production provided weather conditions and cultivation techniques preserve and encourage the cracking structure, required for drainage, aeration and deep root penetration.

The sands and the silts provide only a mechanical framework which, were it not for the cohesive properties of clay, would be liable to slump into a system of particles individually packed to the densest possible arrangement. If the predominant particles are of sand size the soils, though quick draining and loose enough to be easily worked will be thirsty and hungry. With insuff'cient small pores they will be unable to hold an adequate reserve of water and, without negatively charged surfaces on which to hold positively charged, basic, nutrient ions such as calcium, potassium and ammonia, will rapidly frequent coddling with carefully metered doses of water and nutrients. Alternatively, they need reinforcing with organic matter.

Soils strongly biased in texture towards sand, silt or clay are not typical of our soil. There are many soils with less than seventy per cent sand and less than forty per cent clay. These, as shown on the triangle of texture are the loam. Typically they contain twenty to sixty percent silt. For example, the loam, clay loam and silty clay loam of central Wales contain up-or-down of 35 %, 25 % clay and 40 % silt. In the loam, the nature of the living space is greatly affected by the possibilities that exist for inter-particle packing and the uniformity with which this is achieved. The clay content is insufficient to confer much stability to any favourable clustering of particles that could be expected to occur in a heterogeneous mix so, unless periodically disturbed by cultivation, they are liable to pack into a close packing arrangement. In this state they will have fine silt particles blocking any large pores created by the close packing of the sand and will behave, in their pore characteristics, as if they were dominantly silts. In fact, under these conditions, the sand particles present would appear to be the equivalent of inert filler, occupying space but not contributing to drainage or water retention. This no doubt accounts for the fact that some of our soils that are almost entirely silt are not as sow to drain as might be expected. It may also explain why some of our problem sports filed soils are structurally degraded, close packing, sandy loam.

The loams are like the sands in their readiness to slump into close packing arrangements but, unlike the sands, and like the clays, when close packed, they are dominated in their moisture characteristics by particles so fine that drainage cannot be other than slow and water is retained, after drainage has ceased, to an extent detrimental to aeration. These, then, are the soils that would appear to be most at risk to the hazards of structural collapse. These are the soils to which British agriculture as a whole has had to become adjusted and it would be surprising, therefore, if our traditional methods of farming these soils not cherished accordingly. So far I have only considered the conclusions that seem to arise from what we know of the properties of the mineral skeleton, but most soils consist of more than mineral particles. A pile of mineral particles is not a soil until it has acquired a biological component of living and decaying organic matter. Nitrogen is not a component of the normal range of rock minerals and is built into the soil largely as a result of the activities of soil organisms. For this important element at least, therefore, natural soils are dependent on the presence of the organic fraction. However, it may be we need not do things Nature's way and, with the advent of chemical fertilizers, it could be argued that the organic fraction is no longer essential for the supply of nutrients. Likewise, though organic matter would seem to be of benefit to sands for water and nutrient retention it could be argued that well-managed, clay soils, such as those at Rothamsted, for example, are not dependent on organic matter either for nutrients or water. However, for the loam, though organic matter is not essential for water or nutrient retention, it is absolutely essential for structure.

The unstable but open structure so essential for the aeration of loamy soils is a consequence of particle aggregation granulation, if you like, rather than disintegration. Unlike the purely physical process of clod disintegration by cracking, granulation is a biological process involving organisms and ephemeral organic agents, which arise as, intermediate products in the process of organic decay. The effect is summarized diagrammatically. Thus a small particle system that would form a uniform network of small, water-holding pores in close packing is converted into aggregates of sand or gravel size with small pores retentive of moisture within the aggregates and large pores for drainage, aeration and root penetration between. The organic bonding agent involved in this type of aggregation is not very strong but, if the aggregate is not physically sheared, it remains effective in water. The bonding is electrostatic involving negatively charged sites on long chain organic molecules capable of threading together clay particles through the intervention of positively charged bridging particles such as calcium ions. However, the effect of the organic bonding agents is only temporary, as they are themselves subject to microbial attack. They need to be periodically replaced and for this, the soil needs to be continuously replenished with fresh organic residues. These residues must then be decomposed but not too rapidly or, as in the Tropics, their effective life will be too short to be of any consequence for structure. In addition, fresh organic residues must not only be decomposed to provide bonding materials, to be effective, these materials must be mixed into the soil by some churning agent. Being gums, they are unlikely to be able to migrate into position of their own accord.

The most effective natural, churning agent for soil is the burrowing earthworm. These organisms are present to the extent of some fifty to a hundred per square yard in a fertile soil and may process each year through their gut some twenty tons per acre of soil. The end product of their activities is not only their numerous deep burrows but soil, plus calcium from a special lime gland in their gut, intimately blended with fresh organic residues and moulded into a continuous cylindrical ribbon that coils into a water-stable, open cast easily permeated by roots. These casts, though obvious when they appear above ground, ate also found below ground and, when reinforced and maintained by root binding, are responsible for the crumb-sized, aggregate tilth that is a feature of well-managed, permanent pasture. Cracking, root penetration and careful cultivation no doubt can all play their part in preparing the way for earthworm activity and reinforcing its effects but, for the job of positively creating the water-stable aggregation so vital for loams, the earthworm is essential.

Worms are very sensitive to acidity and are absent from soils with a pH of less than five. They are also unable to cope with long immersion in water. As food they seem to prefer the fresh, soft tissues of leaves. If they are to be encouraged, therefore the soil needs first to be drained to avoid persistent water logging then brought to an adequate lime status and cropped in a manner ensuring an adequate return to the soil of fresh, or only partially decayed, organic residues. To summarize: for healthy, vigorous plant growth we should aim to create soil conditions favourable to the thorough exploitation of the soil in depth, both for its moisture and its nutrient reserves. This seems to require an actively growing root system that can forage for itself. To achieve the desired root growth the pore space system must be such as to allow for easy root penetration through a network of relatively large, well-aerated pores within a structure of fine particles that are water-retentive. A compact, sandy soil might approximate to this when heterogeneously packed, and so might a strong clay when split open by intensive cracking, but the loams, unless aggregated, have too little sand to avoid being completely blocked by the fine material present and too little clay to form an adequately fine cracking structure. For the loams, therefore, Nature has had to evolve a system of fine particle clustering and it is this that is achieved by organic bonding agents organized through the casting activities of earthworms. For the farmer this means he must try to maintain conditions favourable to earthworms and then plan his management so as to preserve their handwork, avoiding excessive foot vehicle traffic over the surface when the soil is wet, and using skill in the timing of his cultivations. If a farmer can maintain his soil both adequately aerated and adequately watered in depth, he can then enjoy the consequences of deep, exploitive rooting in the robust growth of healthy plants.


Stable aggregation requires that individual soil particles do not disperse in water, but aggregation is a step beyond this to the creation of peds, which exist as units within the soil mass, i.e. the creation of aggregates out of flocculated particles.


The Influence of Lime: Lime, organic matter an adequate supply of fine textured particles are generally considered to be the essentials for aggregate formation: i) Ca++ will ensure clay flocculation but it is not essential for this purpose as H+ is as effective and even Na+, in the presence of an excess of NaC1 in solution, will not achieve dispersion; ii) Calcium humates, though stable, are no more stable than H- humates; iii) Calcium seems to assist in stable aggregation by acting as a cation bridge between negatively charged sites on clay particles and similarly charged sites on organic matter. However, it has been reported that hydrogen saturated humus forms more stable aggregates with clay than calcium saturated humus; iv) Little mention is ever made of the possible role of earthworms in aggregated formation but if a mixing agent is essential then it is not without significance, that an adequate level of calcium is required for the presence of burrowing earthworms; v) Calcium, through its influence on pH and the churning activity of earthworms can have an influence on the decomposition of organic matter and therefore, perhaps, the production of organic compounds with an influence on aggregation.

The Influence of Clay: Clay particles, being plate-like, will become oriented parallel to each other and to other flat surfaces on drying. When dry it forms a very effective cement. Rehydration requires considerable agitation. A degree of cementation is apparent long before the air dry state has been reached. Oriented water molecules may be involved in linking clay particles through bridging cations. The tenacity of such links will increase with dehydration and finally direct links may develop between adjacent clay surface through peripheral hydroxyls or bridging cations. The smaller the clay particles and the smaller the adsorbed cations the harder the aggregates.


i) From a detailed study of the clay and organic matter contents of water-stable aggregates it has been reported that organic colloids not only cause a high degree of aggregation of clay particles but also produce large aggregates; ii) Only in the wet tropics does organic matter seem to be an insignificant factor in stable aggregate formation. There dehydrated oxides of iron and aluminium act as the principle cements; iii) Hydrogen saturated colloidal organic matter will form effective stable aggregates with clay, fine quartz or orthoclase, the link being a consequence of electro-station forces, stabilized by dehydration. The aggregation of calcium saturated organic matter is slightly less stable; iv) Direct link may be possible between negatively charged organic molecules and positively spots along the broken edges of clay particles; v) It has been suggested that the final stages of dehydration in base-rich soils involving the migration of alkali soluble organic colloids to the drying surface of the aggregate which on dehydration is to confer a measure of stability. Under acid conditions the end-product of a drying phase might be a surface coating of iron and aluminium oxides; vi) Fresh or only partly decayed organic matter is much more effective in promoting stable aggregation than stable soil humus. Thus, if granulating substances are involved they would appear to be ephemeral products of decay.


i) Soil bacteria will remain attached to soil particles or dispersed in the soil solution according to the pH of the medium. Under acid conditions they may be leached from the soil. Thus, the proteinaceous bodies of soil bacteria may themselves act as a cementing agent when the medium is not acid; ii) Fungal hyphae, like fine roots, may contribute an element of mechanical binding particularly effective in the stabilization of large aggregates; iii) As already discussed, with reference to the influence of calcium, many products of organic decay may contribute to aggregation through ionic linkages to charged mineral particles, possibly involving bridging cations of oriented water molecules; iv) Polysaccharide gums, apparently synthesized by micro-organisms when stimulated by the addition of sucrose, have been shown experimentally to have a marked effect on the stability of aggregates (i.e. when mixed with soil and moulded into aggregates). Such organic compounds would be readily attacked by other soil organisms and the duration of their effect would depend, therefore, on the general level of microbial activity. Where this is high throughout the year, as in the moist tropics, polysaccharide gums are likely to survive long enough to have any great influence on aggregate stability. Under the extreme continental conditions of Steppe and Prairie countries there may well be two periods of reduce microbial activity one due to summer drought and another due to winter cold. Under these conditions an adequate reserve of stabilizing polysaccharide gums may be preserved and may account for the very stable structure of natural, organic rich, chernozem soils.

The Influence of Iron and Aluminium Colloida: Ferric hydroxide precipitated as an hydrated gel at pH's around neutral will become an effective, stable cement by dehydration. As dehydration will take place at the periphery of fine textured structural units this process will tend to create a stable skin around such aggregates but in coarse textured materials, the iron will be concentrated in residual moisture films around particles and pore outlets. These contrasting end-products are to be seen in laterites and the B horizons of podzols. Colloidal alumina probably behave in a similar manner.

The Influence of the Type of Clay Present: Kaolinite, in the absence of significant amounts of montmorillonite, hUmus, calcium or iron oxides will, with alternate wetting and drying, tend to produce a platy structure. This no doubt accounts for the platiness of the A2 horizon in podzols. All the above processes have really been concerned with cementation after aggregate formation has taken place. The processes now to be considered are those likely to contribute to fragmentation or granulation.


The Influence of Alternate Wetting and Drying

i) Drying of a moist soil mass will cause shrinkage, which if it is uneven, will cause cracking. At a later stage dehydration of soil colloids will result in cementation within the separated fragments; ii) Re- wetting involves the dragging of water through fine pores and between platy sheets by capillary forces with consequent swelling and compression of occluded air; iii) The bouyancy of occluded air will contribute to aggregate disintegration should the method of re-wetting be rapid immersion in water. Wetting by capillarity from below may well allow time for occluded air to be released through the existing system of large pore spaces; iv) Infiltrating with a non-polar liquid such as carbon tetrachloride will reduce the disruptive effect of wetting by reducing the affinity of the infiltrating liquid for charged soil particles; v) Repeated wetting and drying will cause the disintegration of soil clods through unequal stains and stresses brought about by swelling and shrinkage and the disruptive effect of escaping air bubbles, however, unless there is periodically a phase of excessive desiccation leading to cementation through the dehydration of soil colloids, the fragments released may not be sufficiently water-stable to result in any long-term structural benefit; vi) Prolonged slow desiccation in fine-textured soils will result in lateral shrinkage and the production of prismatic units. This stage presumably precedes the development of fragmental cracking within the large prismatic units; vii) The prismatic units produced by the progressive drying out of soils in depth with the production of lateral as well as vertical shrinkage may create a permanent system of cracks. Dehydration effects localized to crack faces may stabilize these surfaces. They may be in-filled from the surface by fine material carried down by gravity or percolating water. Eventually such cracks provide an effective net-work of drainage channels but, though plant roots and earthworms may be found within them, they seem unable to penetrate the surrounding compact, prismatic, blocks. Thus, prismatic cracking may be said to provide only very limited access to the subsoil environment.

The Influence of Roots: Good granulation is often to be associated with a good grass sword but how far grass roots actually cause fragmentation and how far they merely exploit its presence is not clear. Clearly grass roots, by incorporating an abundant supply of fresh organic matter within the soil, could help to maintain good structure but, only by physically moulding the soil could they be said to be actually responsible for the creation of the porous, granular, aggregates with which they are commonly associated. It seems most probable that roots penetrate soil masses along pre-existing cracks, which are large enough to be aerobic and thereafter, their presence ensures a continuous supply of fresh organic residues, which may contribute to the continued stabilization of exposed structure faces.

Work with natural and synthetic compounds has shown the wide range of substances, which can act as particle cements and thereby might have an influence on aggregate stability. This type of approach has also produced evidence of the importance of resistance to decay in the aggregating compound itself. In this many synthetic polyelectrolytes, such as Krilium have a very real advantage. However, since the very property which renders them useful their adhesiveness is likely to prevent their penetrating existing structures or mixing freely with the soil it is clear that unless they are mixed into aggregates at the time of formation they will be effective only in stabilizing the surface of existing structures.


Any process leading to localized pockets of cementation, when combined with a process involving differential swelling and shrinkage and the creation of cracks, will undoubtedly result in fragmentation. This then could be stabilized by the invasion of plant roots and the further stabilizing effects of products of organic decay. However, local pockets of cementation are unlikely to occur in the uniformly moist conditions of fine-textured possibly around a nucleous of organic matter. What is required is a granulating mechanism capable of moulding fine particles into immediately stable aggregates, 2-3 mm. in diameter.

The Influence of the Earthworm: Though ignored as an agent which might possibly be involved in the creation of stable soil aggregates the burrowing earthworm would appear to be the ideal agent for soil granulation, viz: - i) Burrowing earthworms can invade moist, structureless, fine-textured soil by ingesting mineral particles; ii) Ingested mineral matter is cast within the soil to form a coherant nutty type of aggregate similar to a shellod walnut in size and form. Such units can be found with the tip of the earthworms tail imprinted as a perfect countersunk hollow in the surface. Surface casts will rapidly fragment into cylindrical units 2-3 mm. in diameter; iii) Earthworms feed on fresh organic residues, which are intimately mixed with ingested mineral matter and lime secreted from a special gland within the gut. The normal cast of the feeding earthworm is, therefore, an ideal size and composition for stable granulation; vi) Under humid tropical conditions a high level of microbial activity would lead to a rapid loss of any organic component but solubilized iron and aluminium would migrate to the surface of aggregates during periods of drought and there, through dehydration, form a stable coating; v) Studies of the extent of earthworm casting in British soils suggest that the amount of soil consumed by the earthworm population under established grass may generally be of the order of 20 tons/acre and may rise to 30. This means that, if most of the soil consumed comes from the top 6 inches, something like 5 % of the topsoil may pass through the gut of an earthworm each year under established grass. The equivalent figure for arable would be much less, approx. 1 %. The question is, is this enough to establish a structural pattern, which can be maintained by grass roots.


With drying and wetting, freezing and thawing, dehydration, clay and organic matter content, soluble iron and aluminium, level of microbial activity, earthworm activity and surface vegetation all cited as possible influences on the development of soil structure it may well be that climate, which has an influence on all these things, may be correlated with particular manifestations of structural development; and so it would appear: i) Cold winters with cracking promoted by freezing; ii) Cold winters ensuring arrested decay of fresh organic residues; iii) Short period of intense earthworm activity and microbial decay during a short warm, moist spring; vi) Desiccation and arrested decay during the hot, dry and summer; v) High base status, high organic matter content and adequate clay.

Conditions necessary for the development of wheat is called"good crumb structure"

i) Aluminium content of flocculated clay - 10 per cent; ii) An adequate and fairly continuous supply of fresh organic residues; iii) A base-status, soil temperature and moisture state conducive to the presence of burrowing earthworms and a moderate rate of organic decay; vi) Ideally, deciduous forest or permanent pasture, where cultivations do not damage what would appear to be a more delicate structure than that developed in chernozems, and where the surface is continually protected and there is time for the development of extensive root system.