Soils are derived from bedrock. The bedrock may be that which is directly underlying the soil, or it may be some distance away and the soil from which it was derived been moved by glaciation, volcanic action, wind or sedimentation from floodwater. The chemical composition and the arrangement of molecules of the original bedrock determines how hard or soft it is, and how easily it is broken down into small particles which go to make up the soil. Soils are constituted of a mixture of different sized rock particles. The ‘texture’ of a soil is determined by the size of the mineral particles that predominates within the soil and it is this which influences its physical characteristics, its drainage, ‘workability’ and its suitability for plants and their growth.
Mankind has created its own classification for soils in order to study them or modify them. Plants and other organisms that live in the soil just had to adapt to whatever happened to be available. However, anything that man is able to do to improve the properties of soils enables plants to thrive and flourish. Broadly, soil is classified as having three main textures, clay, silt and sand. Clay has a particle size of 0.002mm and below, silt a particle size of greater than 0.002mm but less than 0.2mm, and sand has a particle size of between 0.2mm upto 2.0mm. There are subdivisions within these bands, for example fine, medium and coarse sands, but these are usually only applied for more detailed scientific study and for most horticultural purposes the three broad classifications suffice. Above 2.0mm are the grits and gravels and so on.
There is also an organic constituent present in most soils, made up of living and dead vegetable and animal matter. A soil composed of a high proportion of dead and decaying vegetable matter is usually called a ‘peat’ or ‘fen’ soil. Most soils only contain around no more than 5% (often less) organic matter, but even this low content is critical in determining its ‘structure’ and how fertile the soil is.
The ‘structure’ of a soil can be roughly defined as how well it forms crumbs of the agglomerated particles. None of the particle ranges on their own provides a good structure. Clay forms clods of a blocky and intractable nature, silt slumps and collapses with a capping layer of a crust, while sand just won’t form crumbs at all. It is only when there is a mixture of the various particle sizes together that the soil will agglomerate into crumbs to give it structure. A mixture of roughly equal parts of the three particle ranges gives a soil with an optimum structure. This is called a ‘loam’. If one particle range in a loam predominates the loam may be subdivided into, for example, a sandy loam or even a loamy sand. Organic matter within the soil forms humus which has ‘sticky gel-like’ properties which helps the particles to bind together to form crumbs. That is why humus helps form soils having a good structure.
Soil particles do not mix or pack together to form one continuous mineral block, they have spaces or voids between each other called pores. The finer the particles, the tighter they pack together, the smaller the size of the pores, and similarly the larger the particles the less easily they pack tightly together, resulting in larger size pores. Like soil particles the size of these pores is divided into three ranges, the smallest being ‘micropores’, the largest ‘macropores’ and those in between ‘mesopores’.
These pores are not empty spaces of ‘vacuum’, they contain a mixture of gases and or water. The mixture of gases can be air, or other gases may be present in greater or lower proportions. The water can be present as water vapour or liquid water.
It is the quantity and size distribution of the pores within the soil that determines its water drainage, water holding and aeration properties. Not all pores are isolated from each other, there will be many interconnections between them, and these form conducting pathways that allow water to drain through them. The larger the particles, the larger the pores, and the larger the conducting pathways the greater the ease with which water can pass through them, and the greater and faster the drainage. The speed and amount of drainage of water through a soil is uneven and soils retain a certain amount of water. As the water drains downwards it draws air from the atmosphere on the surface down into the soil behind it, but some water ‘sticks’ to the soil particles and the soil retains a quantity of water in the following manner. Each soil particle attracts a thin layer of water to its surface by polar attraction. This is called adhesive water and it is bound quite strongly to the particles by the polar charges on the surface of the particles and the water. Outside this thin layer is a further layer of water which is bound to itself by the cohesive forces that give the water surface tension. This is cohesive water. As the volume of cohesive water increases the attractive forces between the water molecules are no longer able to counteract the force of gravity and consequently are no longer held by the soil but drain away under the influence of gravity. This is gravitational water.
Assuming a soil composed of 50% by volume of mineral and organic matter and 50% of pore spaces. If all the pore spaces are filled with water the soil is saturated (i.e. waterlogged). If all the free-draining water (gravitational water) is allowed to drain away under the influence of gravity, then the water that is left is the adhesive and cohesive water. This represents the ‘water-holding capacity’ of the soil and the soil is said to be at ‘field capacity’ This water might represent 30% of the total volume of the soil, depending upon its texture and its structure, but these percentages will vary from soil to soil; a sandy loam will retain a lower percentage than a clay loam. If a soil contains less water than its field capacity it is said to be in ‘deficit’ and the amount of water required to bring it back to field capacity is the ‘soil water deficit’. This is expressed as the number of inches (millimetres) of rainfall, or irrigation, required to restore the soil to its field capacity. Not all of the water held by the soil can be used by plants; a small proportion of it, that held by adhesive forces to the soil particles, is held so tightly that not even the absorbing forces exerted by the plant roots can ‘prise’ it from the soil. This is ‘unavailable’ water, and that which it is possible for absortion by the plant roots is termed ‘available’ water. It used to be measured in ‘inches’, but nowadays is measured in millimetres, like rainfall. The difference in the quantity of maximum available water and the actual available water is termed the ‘available water deficit’ (AWD). As the plant absorbs the available water, the remainder becomes increasingly more difficult to absorb and the plant has to work increasingly harder to counteract the attractive forces between water and soil particles, and water and water. Eventually it reaches a point when the plant is unable to absorb further water from the soil, even though it is present. This point is termed the ‘wilt point’. To make it easier for the plant to absorb water it is more beneficial to keep the ‘available water’ topped-up to near its maximum ‘available water capacity’ rather than undergo a cycle of water shortages. Large commercial growers monitor the water content of their soil with water ‘tensiometers’ to determine irrigation needs.
The remaining 20% of the soil will be filled with air and other gases.The volume of soil pores occupied by air or other gases when the soil is at ‘field capacity is called the ‘air-filled porosity’ (AFP). The air-filled porosity of a soil is very important as it allows oxygen to reach the root cells and allows micro-organisms to thrive. It also allows for the carbon dioxide and other gases produced by respiration, metabolism and decomposition to diffuse away to the surface of the soil. Plant roots are generally not completed immersed in soil water, but a large proportion of them are surrounded by moist soil air. This allows oxygen and carbon dioxide to diffuse into and out of the root cells more rapidly as these gases can move through air more rapidly than they can through water. When a soil is waterlogged the roots cannot exchange the gases as readily and thus cannot ‘breathe’ properly. Continuous waterlogging for any length of time will lead to the death of the plant through ‘drowning’, so ‘over-watering’ can be just as bad for the plant as insufficient water.
A clay soil, because of the small particle size, has a predominance of smaller pores and a greater proportional surface area of the particle to it volume. Thus the water that is there in the soil at field capacity has a high proportion of adhesive water in relation to cohesive water, and thus a lower available water content. Only a very small proportion will be gravitational water and free-draining. Because it isn’t free-draining soil, air from the atmosphere, is not drawn into the soil as readily, and the air-filled porosity is low. As most living matter requires oxygen for respiration, the plant roots cannot get as much as they need to sustain adequate growth, nor can the micro-organisms gain oxygen for their growth. Thus clay soil will be low in organic matter. Because water takes more heat to warm up than air, a clay soil, having a low AFP will take longer to warm up in the spring and be a ‘cold’ soil. Because the clay particles are able to pack more densely together the mineral proportion relative to the pore proportion will be greater, and those pores will largely contain water rather than air. As a consequence the bulk density of the soil is higher, and termed a ‘heavy’ soil. But as the water that is present is held more tightly, the mineral nutrients dissolved in it are not leached away through drainage as quickly, so a clay soil is potentially a nutrient rich soil.
On the other hand, a sandy soil, having a large particle size which cannot pack so closely together will have correspondingly larger pores and the particles will have correspondingly lower surface area relative to its volume. Therefore the water present will drain more easily, and because of the lower adhesive water proportion, will drain more thoroughly. This will give a high AFP but a low available water content, so it will dry out and reach its ‘wilt point’ more speedily. As there is a high air-filled porosity the bulk density is lower and the soil is ‘light’ and quicker to warm up in the spring. But as any water from rainfall or irrigation passes through the soil quite quickly its water holding capacity (field capacity) is low and dissolved nutrients will quickly be leached out of it. This leads to sandy soils being termed ‘hungry’ soils as they will require additional feeding to replenish those nutrients lost through drainage.
Organic matter is often fibrous in nature and has a strong affinity to water as well as a large surface area relative to its volume. So the addition of organic matter to sandy soil increase its water holding capacity. By adding the organic matter to clay soil it agglomerates the clay particles, so that they behave as though they are larger sized particles, thus increasing the air-filled porosity and drainage. The organic matter does not hold onto water as strongly as clay particles so it doesn’t become as tightly ‘locked up’ and thus more ‘available water’ is present for assimilation by the plants. Both clay and sandy soils are usually very acid, which tends to lower the solubility of nutrient minerals. Adding lime to both clay and sandy soils lowers the acidity and increases the solubility of mineral nutrients, thus increasing their availability to the plants. However, just adding lime to a sandy soil without a corresponding addition of organic matter would merely increase the rate at which minerals were leached and depleted. Adding lime to a clay soil also helps the clay particles to agglomerate (flocculate) and increase the solubility of mineral nutrients. However, just adding lime, without a corresponding addition of organic matter, will improve drainage and availability of nutrients but it will also increase leaching of both the nutrients including the lime. Once the lime has been leached or neutralised, the soil will revert to behaving like clay. The organic matter holds onto the water and slows down leaching of the lime so maximising its flocculating action. The raising of the pH (reducing the acidity) of the soil also favours increased biological activity in the soil, leading to increased production of humus.
As water drains down through the soil under the influence of gravity it reaches bedrock which is relatively impermeable. Thus water collects above it, gradually filling all the pore spaces. The point where all the pore spaces become filled with drained water is the ‘water table’ and this will move up or down according to the supply of water from above and any drainage downwards or sideways. Sometimes a ‘false’ water table may collect above a layer of slower draining soil, such as a compacted layer, or a seam of clay. This will create what is called a ‘hanging water table’, for if the layer is perforated by deep digging, the hanging water table will drain downwards to the true water table below. Poor cultivation by continually ploughing or rotavating at the same depth can give a ‘smear’ layer of compacted ground which then could lead to the formation of a ‘plough pan’ and a hanging water table. A similar occurrence can happen by dissolved iron salts being precipitated out in an insoluble form by acid conditions in the absence of air. This forms an ‘iron pan’.
When soil is wet its structural strength is at its weakest. Using equipment, or even walking, on soil can lead to it becoming compressed (compacted). This reduces the number of pores within the soil causing reduced drainage and reduced aeration. Both these are detrimental to optimum plant growth. Thus it is better, when soil is very wet, to leave it alone to drain naturally before venturing upon it. Trying to cultivate soil that is too wet will damage its structure and actually do it more harm than good.