Soil physics is a branch of soil science that deals with the study of soil physical conditions. It is necessary that persons dealing with soil in any way should be acquainted with the physical nature of different soils-such as moisture retention capacity, plasticity, and ease of penetration by roots or compaction.
The knowledge of the physical concepts will enable the user of soil to know how to alter these properties to an advantage. That is, knowing about the physical properties of the soil will form a basis for much of what you learn about and do with soils.
Soil Physical Properties
Soil Texture and Textural Composition
Soil texture could be defined as the relative proportion of particle size groupings in the soil on a percentage basis. It describes the sand, silt, and clay composition of the soil.
The smaller the particles in the soil, the larger the internal surface area. Similarly, the smaller the particles in the soil the more water and nutrients the soil can retain.
In the field, a soil is composed of a mixture of separates which are groups of soil particles of a given size range (i.e. different size particles which together make up a given soil).
Physical test for these three groups reveals that sandy soil is generally coarse, gritty, and non-sticky with low cohesion; silt is smooth like flour while clay is sticky and plastic when wet but very hard when dry. A loam texture soil usually exhibits a combination of the three properties.
A soil sample usually contains a combination of at least two separates thus there are classes of soil texture. These combinations are therefore classified into twelve different combinations called soil textural classes. Table 1 shows two schemes of classification used for defining various separates in soils.
Definition of Soil Texture Classes
Apart from the modification of sandy textures with terms such as gravelly, coarse, very fine, fine, etc., there are twelve basic soil textural classes recognized. In order of increasing proportions of the fine separates, the classes include sand, loamy sand, loam, loam, silt, loam, sandy clay loam, clay loam, silty clay loam, sandy clay, silty clay, and clay.
The basic soil textural class names, in use presently, are defined in terms of particle-size distribution as determined in the laboratory by a procedure termed Particle Size Distribution Analysis or Mechanical Analysis, or Gravimetric Analysis.
In general, the twelve textural classes may be defined as follows:
Sand – Soil material that contains 85% or more sand and a percentage of silt plus 1 ½ time the percentage of clay not exceeding 15.
Loamy Sand – Soil material that contains at the upper limit 85 to 90% sand, and the percentage of silt plus 1 ½ time the percentage of clay is not less than 15; at the lower limit it contains not less than 70 to 85% sand, and the percentage of silt plus twice the percentage of clay does not exceed 30.
Sandy Loam – Soil material that contains either 20% clay or less and the percentage of silt plus twice the percentage of clay exceeds 30, and 52% or more sand; or less than 7% clay, less than 50% silt, and between 43% and 52% sand.
Loam- Soil material that contains 7 to 27% clay, 28 to 50% silt, and less than 52% sand.
Silt Loam- Soil material that contains 50% or more silt and 12 to 27% clay (or) 50 to 80 percent silt and less than 12% clay.
Silt – Soil material that contains 80% or more silt and less than 12% clay.
Sand Clay Loam – Soil material that contains 20 to 35% clay, less than 28% silt, and 45% or more sand.
Clay Loam- Soil material that contains 27 to 40% clay and 20 to 45% sand.
Silty Clay Loam – Soil material that contains 27 to 40% clay and less than 20% sand.
Sandy Clay- Soil material that contains 35% or more clay and 45% or more sand.
Silty Clay- Soil material that contains 40% or more clay and 40% or more silt.
Clay- Soil material that contains 40% or more clay, less than 45% sand, and less than 40% silt.
In the field, however, a method known as the “Feel Method” is used in assessing soil texture. In this method, a sample of the soil usually moist or wet is rubbed between the fingers, and the texture is assessed by the behavior of the soil particles using knowledge of the behavior of the various quantities of the separates present in the soil sample.
In other words, the differing sizes of the constituent particles give each soil a characteristic feel. Thus, soil composed mainly of coarse sand particles feels light and gritty; one composed mainly of clay feels heavy and sticky.
In general, the twelve textural class names already established, form a more or less graduated sequence from soils that are coarse in texture and easy to handle to the clays that are very fine and difficult to manage, while the loams are in between the two extremes.
Read Also: Movement of Nutrients in Soil Moisture and Measuring Soil Water
Why Study Soil Texture
Soil texture is studied because:
- The rate at which water enters the soil (infiltration) and drains through (percolation) depends on whether it is sandy, silt, or clay soil.
- The rate of nutrient leaching also depends on the rate of water infiltration e.g. clay soils have the best holding ability for water and chemical nutrients.
- Soil texture influences the ease at which soil can be worked; clay soils are more difficult than sandy soils.
- The knowledge of soil texture and crop requirements of soil enables the grower to select suitable soils/land for his crop.
- Growers would be able to know management practices suitable for the soil types, especially in terms of fertilization, irrigation, and organic materials incorporation.
Soil Structure and Aggregates
Soil structure may be defined as the organization of sand, silt, clay, and humus particles into somewhat stable groupings (peds). It can also be defined as the aggregation of primary soil particles (sand, silt, and clay) into compound particles termed peds or aggregates, which are separated by adjoining peds by lines of weakness.
Three groups of characteristics are used to classify different kinds of structures:
Type refers to the shape of the soil aggregate e.g. granular, platy, crumb, etc.
Class refers to the size of the peds e.g. fine, medium, coarse, etc.
Grade describes how distinct and strong the peds are. It expresses the differential between cohesion within aggregates and adhesion between aggregates e.g. weak, moderate, and strong or structure-less terms are used for a grade. A structureless condition exists when there is no observable degree of aggregation.
Thus, the full description of the structure of a given ped could be strong, coarse, prismatic structure; moderate fine granular structure; weak fine crumb structure or structure less (massive) or structure less (single-grained).
Soil structure is important in agriculture from the point of view that well-aggregated soil is often well drained, and has good permeability to water, air, and roots. Such soil is also easily worked or tilled and thus serves to control erosion.
All these are made possible because of the numerous macrospore spaces created by the existence of numerous lines of weakness between aggregates or peds.
Common agents of aggregation which are responsible for binding primary soil particles into peds include the following:
1. Colloidal clay minerals consist of the finer, more reactive part of clay in soils.
2. Colloidal oxides of iron, aluminum, and manganese, which are collectively termed sesquioxides. These are especially typical of tropical soils.
3. Microbial gums; which are gums secreted by micro-organisms in soils.
4. Organic compounds, especially humus which is also colloidal in nature.
5. Carbonates.
Soil Consistence
Soil consistence refers to the behavior of soil when pressure is applied, especially at various moisture contents, usually when the soil is wet, moist, or dry.
The terms used to describe consistency include:
Wet Soil: Often described in terms of stickiness, as non-sticky, slightly sticky, sticky, very sticky; and in terms of plasticity as non-plastic, slightly plastic, plastic, and very plastic.
Moist Soil: This is very important because it best describes the condition of soils when they are tilled in the field.
Consistency of moist soil is described in the following terms; going from the material with least coherence to that which adheres so strongly as to resist crushing between the thumb and forefinger: loose, very friable, friable, firm, very firm, and extremely firm.
Dry soil: Terms used to describe the degree of rigidity or brittleness to crushing or manipulations include the following: loose, soft, slightly hard, hard, very hard, and extremely hard.
Cementation is also a type of consistency and is caused by cementing agents such as calcium carbonate, silica, or oxides of iron and aluminum.
Cementation is little affected by moisture content. The terms used to describe cementation include: weakly cemented i.e. cemented units can be broken in the hand. Strongly cemented units cannot be broken in the hand but can be broken easily with a hammer. Indurated units are breakable only with sharp blows of a hammer.
Cutans
Cutans may be defined as modifications of the fabric of natural surfaces in soil materials due to the concentration of particular solid constituents such as clay, sesquioxides, and organic materials notably hummus.
The presence of cutans in subsoil horizons (B or C) of a soil profile is of specific pedological significance. For instance, the presence of recognizable amounts of clay cutans or clay skins on ped faces or in pores is a direct indication that an argillic horizon (Bt) is present and that such soil is mature.
The presence of both Fe-oxyhydroxide (sesquioxides) and humus (organic) in the B-horizon may indicate the occurrence of a spodic horizon and the onset of the process of podzolization in soil.
Soil Compaction
This is the natural packing of soil particles by natural force into a denser or closer pack. The forces acting to compact soil are:
– Overburden the weight of material above the soil.
– Implement traffic from mechanized agriculture
– Foot traffic
– Tillage pressure due to implement moved through the soil
– Micro-organism as in the ant-hills.
Over the years, the implements cause pressure on the lower layer of tillage soil. There is thus a low infiltration rate on this layer of soil. That is, the porosity becomes low, and bulk density increases, other effects of compaction are:
– Reduced permeability, aeration, and water infiltration
– Difficulty in root penetration
– Severe compaction inhibits the production
Soil compaction is measured by the bulk density and the use of a cone penetrometer.
Porosity and Pore Spaces
Pores are spaces or voids between solid soil materials. The occurrence or abundance of pores in soils is of pedological significance because soil with many coarse pores will be much more aerated and better drained than one with few very fine pores.
When describing pores in detail, a definite sequence of terms should be consistently followed. A usual sequence is a number (Few, common, many), size (very fine, fine, medium, coarse), continuity (discontinuous, constricted, continuous), orientation (vertical, horizontal, random, oblique), shape (vesicular, irregular, tabular) and location (impede, exped).
The pores allow the soil to act as a medium for air and water transport and it is within the pores that physical, chemical, and biological processes occur in the soil.
Pore Space could also be defined as the portion of a given volume of soil that is not filled with solid matter.
Porosity (pore space) refers to total pore space per volume of soil.
Macropores are the big pores that are mainly meant for aeration while micropores (small pores) are meant to transmit water after wetting. Pores are connected with one another in the soil and are usually described by their retention of water and air.
An ideal soil for agricultural purposes has a fairly equal proportion of macropores and micropores. This is usually put at 25% for each of the two-pore sizes.
Porosity also depends on both the texture and structure of the soil and on the shape of the particles.
Soil Density and Permeability
The density of a soil is its weight per natural volume or bulk volume and it is related to the amount of empty space in the soil. The soil density is expressed in two ways; namely, particle and bulk density.
Particle Density
Is the mass per unit volume of soil solids. For example, one cubic centimeter of soil solids weighing 2.0g, has a particle density of 2.0g cm-3.
Mineral soils have a particle density range of 2.60-2.75g cm-3 with an average value of 2.65gcm-3. Organic matter tends to lower particle density.
Bulk Density
Is the mass of soil per unit bulk volume of dry soil:
Bulk density = Ms or weight of dry soil = g
Vb volume of dry soil cm3
Where Ms is the mass of soil, Vb is the natural volume or bulk volume.
Particle density = Ms
Vs
Where Vs indicates the volume of solids (Solid space) and solid space is bulk volume – air space. Organic matter which promotes soil aggregation tends to lower the bulk density.
Permeability
Is the ease with which air water and roots move through the soil. The number, size, and continuity of soil pores determine the permeability of the soil. Since pore space depends on texture and structure permeability also depends on soil texture and structure.
The permeability of a soil is measured by measuring the movement of water through the soil; this is known as Hydraulic conductivity.
Soil Tilth
Tilth is the physical condition of the soil in relation to ease of tillage and permeability. It is a function of the texture and structure of the soil. Soil with good tilth makes room for rapid root growth and ease of seedling emergence.
Tilth can be improved by improving soil structure and avoiding compaction by adopting the following practices:
– Avoid working too wet or very dry soils.
– Reduce traffic or overburdened weight on the soil.
– Number of tillage operations could be reduced to minimum tillage.
– Subsoil or deep ploughing is good to break up hardpans
– Mulching or cover-cropping protect the soil from raindrop impact.
– Incorporation of organic matter into the soil such as compost, FYM, or green manures.
– Acid soils should be limed to enhance rapid organic matter decomposition.
Soil Colour
Soil color is an important indicator of soil conditions. It reveals considerable information about any given soil. The soil can take several shades of colors such as:
Brown-Black where dark soils result from the level of organic matter contents which is usually high in waterlogged soils.
White-Light grey indicates the leaching of coloring materials such as organic matter, or due to the accumulation of lime or salts.
Yellow-Red soils contain iron oxides in well-drained soils
Mottling Color develops when the soil is waterlogged for part of the year. Patches of different colors are shown. Soil color is described by using the Munsell system in a soil color chart.
In conclusion, our understanding of soil’s physical properties is imperative to the proper management of soils both for present use and future generations.
The physical properties of soils could adequately be harnessed for sustainable agriculture through optimum utilization of the soil physical processes and water resources.
Read Also: General Importance of Soil Organic Matter
