General review on concepts, components and textural classes of soils

Soil as the “biological engine of the Earth” is one of the most diverse habitats and contains the most diverse collections of living organisms. Scientists have defined soil in different ways because they think of soils in different ways, or they use soils for different purposes.

31 Soil is the outermost solid layer of the Earth which serves terroir of plants. Basic property of the soil is its fertility i.e. soil is able to supply its own living vegetation with water and nutrients at appropriate time and in appropriate amount allowing thus primary biomass production (Stefanovits et al., 1999).

According to USDA, NRCS (2003) soil is a natural body comprised of solids (minerals and organic matter), liquid, and gases that occurs on the land surface, occupies space, and is characterized by one or both of the following: horizons or layers, that are distinguishable from the initial material as a result of additions, losses, transfers, and transformations of energy and matter or the ability to support rooted plants in a natural environment.

Often land is used synonymously with soil, but the two are not the same. Land is the non-water part of the earth’s surface, while soil occupies only a thin upper part of some land (Osman, 2013).

Land refers not just to soil, but to the combined resources of soil, water, vegetation and terrain that provide the basis for land use (Arshad and Martin, 2002).

The soil is a component of all terrestrial ecosystems. However, the soil is itself an ecosystem in that it harbours a large number of organisms which interact among them and with the physical and chemical soil environment (Osman, 2013). The materials from which the soils are formed are called parent materials. Parent materials may be organic and inorganic, although most soils (more than 99% of world soils) develop from inorganic or mineral parent materials (Osman, 2013). Soil is a great recycler of materials and also has a huge storage of organic matter. Therefore, soil acts as an environmental buffer. Inorganic substances are also transformed by chemical or biological processes and are rendered soluble/insoluble, mobile/immobile, and active/inactive (Osman, 2013). Soil is made of minerals, water, organic matter, sol biota and air; however, the volume composition of these basic components highly varies with soil types (DeGomez et al., 2015;

Várallyay, 2013).

1. Minerals

The largest component of soil is the mineral portion, which makes up approximately 45%

to 49% of the volume. Soil minerals are derived from two principal mineral types. Primary minerals are those soil materials that are similar to the parent material from which they formed. Secondary minerals result from the weathering of the primary minerals. The mineral components of soil are chlorides, sulfides, sulfates, nitrates, phosphates, borates, carbonates, oxides, hydroxides and silicates. Most soil minerals contain silica as their major structural constituent (silicates), while non-silicates are composed mainly of oxides, carbonates and sulphates (Stefanovits et al., 1999).

32 2. Water

Water can make up approximately 2% to 50% of the soil volume. Soil water availability is the capacity of a particular soil to hold water that is available for plant use. Water holding capacity (WHC) is largely dependent on soil texture.

Soils with smaller particles (silt and clay) have a larger surface area than those with larger sand particles, and a large surface area allows a soil to hold more water.Additionally, organic matter also influences the WHC of soils because of the high affinity of organic matter to water. The addition of organic matter to the soil usually increases the water holding capacity of the soil. This is because the addition of organic matter increases the number of micropores and macropores in the soil either by “gluing” soil particles together or by creating favourable living conditions for soil organisms (Bot and Benites, 2005).

3. Organic matter

Organic matter is a keystone component of soil, which has many attributes that influence important soil characteristics. Most mineral soils contain <5% by weight organic matter, but some soils (for example Histosols) contain high organic matter, even more than 80%

by weight. Carbon (50–58%) is the most abundant constituent of soil organic matter, therefore it provides the congruence between soil organic carbon and soil organic matter.

Organic matter is derived from dead plants and animals and as such has a high capacity to hold onto and/or provide the essential elements and water for plant growth. These residues may be at various stages of decomposition, ranging from fresh undecomposed materials through partially decomposed and short-lived products of decomposition to well-decomposed humus (Osman, 2013).

4. Soil biota

The soil biota consists of the microorganisms, soil animals (protozoa, nematodes, mites, springtails, spiders, insects, and earthworms) and plants (plant roots) living all or part of their lives in or on the soil or pedosphere, and performing a variety of functions for their growth and reproduction. Microorganisms are found in the soil in very high numbers but make up much less than 1% of the soil volume. A common estimate is that one thimble full of topsoil may hold more than 20,000 organisms. The largest of these organisms are earthworms and nematodes and the smallest are bacteria, actinomycetes, algae, and fungi.

Microorganisms are the primary decomposers of raw organic matter. Decomposers consume organic matter, water, and air to recycle raw organic matter into humus, which is rich in readily available plant nutrients.

33 5. Air/Gases

Gases or air can make up approximately 2% to 50% of the soil volume. Oxygen is essential for root and microbe respiration, which helps support plant growth. Carbon dioxide and nitrogen also are important for belowground plant functions such as for nitrogen-fixing bacteria.

The ideal loam-textured surface mineral soil contains 45% mineral matter, 5% organic matter, 25% water, and 25% air by volume (Osman, 2013). Air and water contents in soils are more variable. The porous component divided between water and air will vary with the moisture conditions of the soil (Murphy, 2014).

Soil texture and structure are also very important features, as determine the pore-size distribution, soil water holding capacity and the amount of water to air-filled pore space in soil aggregates that provide habitat for soil organisms. Soil textural classes can be determined on particle size distribution data, using the triangular diagram, shown in Figure 2.10. On the basis of the directly measured percent of sand (the sum of all single grain fractions with the size of 0.05–2.0 mm), silt (0.002–0.05 mm) and clay (< 0.002 mm) the textural class can be read from the triangle

(Várallyay, 2013). Figure 2.10 Soil texture triangle

(nrcs.usda.gov)

Some simple methods and indices can be also used to determine textural class instead of the particle size distribution analysis. These are as follows: finger test, upper limit of plasticity according to Arany (K(A)), hygroscopic moisture content (hy1) and simplified mechanical analysis (L). Limit values of these simple tests are summarized in Table 2.3 (Várallyay, 2013).

34 Table 2.3 Limit values for various soil textural classes (Várallyay, 2013)

Textural class L % K(A) hy1

Coarse sand <10 <25 <0.5

Sand 10–20 25–30 0.5–1.0

Sandy loam 25–30 30–37 1.0–2.0

Loam 30–60 37–42 2.0–3.5

Clay loam 60–70 42–50 3.5–5.0

Clay 70–80 50–60 5.0–6.0

Heavy clay >80 >60 >6.0

Simplified mechanical analysis (L): determination of only the sum of soil fine fraction (<0.02 mm) after a distilled water disintegration. Upper limit of plasticity according to Arany (K(A)): the quantity of water (cm³) that is necessary for preparing a fully water saturated “aggregate free” soil paste from 100 g oven-dry soil sample to reach the “endpoint indicator”, which is the “sticky point”. Hygroscopic moisture content (hy1): The weight-percentage moisture content of soil, equilibauxite residueated with the air of certain relative humidity: “air-dry”.