AGRICULTURE FORM 1 – SOIL AND ITS AGRICULTURERAL UTILIZATION

SOIL AND ITS AGRICULTURAL UTILIZATION

Soil originates from Latin word “solum” meaning floor. Thus it’s a natural consolidated material that originates from weathered mineral rock and decomposition organic matter which supports plant and animal life.

Importance of soil to agricultural production

• It is a natural medium within which seeds germinate and roots grows

• It supplies plant with the mineral nutrients necessary for crop growth

• It provide anchorage for higher plants

• It provide water, air, and warmth for small small animals, microorganisms and plant roots to sustain life

• It shelters many small animals and microorganisms within the top soil

COMPOSITION OF SOIL

Soil is a complex body composed of five major components:

• Mineral matter obtained by the disintegration and decomposition of rocks;

• Organic matter, obtained by the decay of plant residues, animal remains and microbial tissues;

• Water, obtained from the atmosphere and the reactions in soil (chemical, physical and microbial);

• Air or gases, from atmosphere, reactions of roots, microbes and chemicals in the soil

• Organisms, both big (worms, insects) and small (microbes)

Mineral matter

According to its size, soil can be separated into various fractions. Two common systems of classification are given in Table I.

Table I.: Classification of soil particles according to two systems (U.S.D.A and International)

Formation of mineral matter

1. Primary minerals are formed at high temperature during cooling and crystallization of magma, they are inherited from rock materials and have not been altered chemically and they range above 2mm in diameter e.g. quartz.

2. Secondary minerals, formed at ordinary temperatures and chemically alteration of rock and mineral precipitation of the weathering e.g. clay.

Importance of mineral matters

Contain plant nutrients for growth i.e. Macro nutrient and Micro nutrient. Examples of Macro nutrients are Calcium (Ca), Magnesium (Mg), Potassium (K), Sulphur (S), Phosphorus (P) and Nitrogen (n). Examples of micro nutrient are Copper (Cu), Zinc, Molybdenum (M), Chlorine (Cl. ), Boron(b), Manganese(Mn), Iron(Fe) and Cobalt (Cb).

Organic matter 

The organic matter of the soil is an active state of decomposition caused by soil microorganisms. These are farmed from two components;

1. Plant residues; These include plant tops, plant roots, shrubs, grass, plant debris, crop harvests, green manure and compost manure.

2. Animal residues; these include worms, insects, bacteria, fungus, algae, animal manure. The organic matter of the soil is in the soil, the process of breaking down the remains is known as decomposition.

Importance of organic matter

• Act as mulch; before decomposition it can act as mulch, by covering the soil surface hence reduce the rate of evaporation, impact of rain drops and check in water runoff.

• Improve soil structure; through soil binding when it turn into humus, forming granules which facilitate air and water movement.

• Increases soil fertility; when organic matter decompose nitrogen, sulphur, phosphorus element because available to plant use.

• Improve water holding capacity; Soil water retaliation is influenced by good soil structure due to formation of humus.

• Regulate temperature; Due to its colour being brown-black, it can easily absorb heat of the sun.

• Supplies plant elements to the soil; Organic matter has high C.E.C(Cation Exchange Capacity) of which helps to withdraw such cation as K+, NH4, Mg++, Co++, for plant use Soil Air

In nutrient management, soil aeration influences the availability of many nutrients. Particularly, soil air is needed by many of the microorganisms that release plant nutrients to the soil. An appropriate balance between soil air and soil water must be maintained since soil air is displaced by soil water.

Air can fill soil pores as water drains or is removed from a soil pore by evaporation or root absorption. The network of pores within the soil aerates, or ventilates, the soil. This aeration network becomes blocked when water enters soil pores. Not only are both soil air and soil water very dynamic parts of soil, but both are often inversely related:

An increase in soil water content often causes a reduction in soil aeration. Likewise, reducing soil water content may mean an increase in soil aeration.

Since plant roots require water and oxygen (from the air in pore spaces), maintaining the balance

between root and aeration and soil water availability is a critical aspect of managing crop plants.

Soil water

Physical Classification Gravitational water — -1/3 bar Capillary water — -1/3 to -31 bars Hygroscopic water — -10,000 bars

Gravitational water: free water that moves through the soil due to the force of gravity. Gravitational water is found in the macro spores. It moves rapidly out of well drained soil and is not considered to be available to plants It can cause upland plants to wilt and die because gravitational water occupies air space, which is necessary to supply oxygen to the roots.

Drains out of the soil in 2-3 days

Capillary water: Water in the micro pores, the soil solution. Most, but not all, of this water is available for plant growth Capillary water is held in the soil. Against the pull of gravity

Forces Acting on Capillary Water

Micro spores exert more force on water than do macro pores

Capillary water is held by cohesion (attraction of water molecules to each other) and adhesion (attraction of water molecule to the soil particle). The amount of water held is a function of the pore size (cross-sectional diameter) and pore space

(total volume of all pores) this means that the tension (measured in bars) is increasing as the soil dries out.

Hygroscopic water: This water forms very thin films around soil particles and is not available to the plant. The water is held so tightly by the soil that it cannot be taken up by roots. not held in the pores, but on the particle surface. This means clay will contain much more of this type of water than sands because of surface area differences.

Hygroscopic water is held very tightly, by forces of adhesion. This water is not available to the plant.

Gravity is always acting to pull water down through the soil profile. However, the force of gravity is counteracted by forces of attraction between water molecules and soil particles and by the attraction of water molecules to each other.

• Soil Moisture Constants

These are the terms most commonly used when working with soil water. Terms us will use when making soil moisture calculations.

Saturation – all soil pores are filled with water. This condition occurs right after a rain. – This represents 0 bars.

Field capacity – moisture content of the soil after gravity has removed all the water it can. Usually occurs 1-3 days after a rain. – This would be -1/3 bar.

Wilting point – soil moisture percentage at which plants cannot obtain enough moisture to continue growing. – This is -15 bars.

Hygroscopic water – when the soil is about air dry – Water held at water potential less than than – 31 bars. This water is not available to plants.

Oven dry – soil that has been dried in a oven at 105 degrees C for 12 hours. All soil moisture has been removed. This point is not important for plant growth but is important for calculations since soil moisture percentage is always based on oven dry weight.

Plant available water is that held in the soil at a water potential between -1/3 and -15 bars.

Soil formation factors and processes

Weathering of parent material

All rocks, when exposed for sufficient length of time to the atmosphere, undergo decay from disintegration and decomposition, together referred to as weathering.

Disintegration is the break down into small particles by the action of mechanical agents of weathering such as rain, frost etc, and decomposition is the breakdown of mineral particles into new compounds by the action of chemical agents such as acid in air and in rain and river water.

Denudation is the general term used for the wearing down of land areas by the processes originating and acting at the earth’s surface. It includes both weathering and erosion. In addition to the atmospheric processes, agents of erosion (rivers, moving ice, water waves) contribute to the deduction of the land in their particular spheres of action, they also transport weathered and eroded material away from areas where it is derived, to from deposits of sediments elsewhere.

The weathering of parent material takes the form of physical weathering (disintegration), chemical weathering (decomposition) and chemical transformation. Generally, minerals that are formed under the high temperatures and pressures at great depths within the earth’s mantle are less resistant to weathering, while minerals formed at low temperature and pressure environment of the surface are more resistant to weathering. Weathering is usually confined to the top few meters of geologic material, because physical, chemical, and biological stresses generally decrease with depth. Physical disintegration begins as rocks that have solidified deep in the earth are exposed to lower pressure near the surface and swell and become unstable. Chemical decomposition is a function of mineral solubility, the rate of which doubles with each 10 °C rise in temperature, but is strongly dependent on water to effect chemical changes. Rocks that will decompose in a few years in tropical climates will remain unaltered for millennia in deserts. Structural changes are the result of hydration, oxidation, and reduction.

Physical disintegration is the first stage in the transformation of parent material into soil. Temperature fluctuations cause expansion and contraction of the rock, splitting it along lines of weakness. Water may then enter the cracks and freeze and cause the physical splitting of material along a path toward the center of the rock, while temperature gradients within the rock can cause exfoliation of “shells”. Cycles of wetting and drying cause soil particles to be abraded to a finer size, as does the physical rubbing of material as it is moved by wind, water, and gravity. Water can deposit within rocks minerals that expand upon drying, thereby stressing the rock. Finally, organisms reduce parent material in size through the action of plant roots or digging on the part of animals.

                

Chemical decomposition and structural changes result when minerals are made soluble by water or are changed in structure. The first three of the following list are solubility changes and the last three are structural changes.

• The solution of salts in water results from the action of bipolar water on ionic salt compounds producing a solution of ions and water.

• Hydrolysis is the transformation of minerals into polar molecules by the splitting of the intervening water. This results in soluble acid-base pairs. For example, the hydrolysis of orthoclase-feldspar transforms it to acid silicate clay and basic potassium hydroxide, both of which are more soluble.

• In carbonation, the reaction of carbon dioxide in solution with water forms carbonic acid.

Carbonic acid will transform calcite into more soluble calcium bicarbonate.

• Hydration is the inclusion of water in a mineral structure, causing it to swell and leaving it more stressed and easily decomposed.

• Oxidation of a mineral compound is the inclusion of oxygen in a mineral, causing it to increase its oxidation number and swell due to the relatively large size of oxygen, leaving it stressed and more easily attacked by water (hydrolysis) or carbonic acid (carbonation).

• Reduction the opposite of oxidation, means the removal of oxygen, hence oxidation number of some part of the mineral is reduced, which occurs when oxygen is scarce. The reduction of minerals leaves them electrically unstable, more soluble and internally stressed and easily decomposed.