Topic 5: Introduction to Soil Science | Agriculture Form One

Topic 5: Introduction to Soil Science | Agriculture Form One

Introduction to soil science | Topic 5 | Notes za Agriculture Form One | Topic ya Tano | Free Notes | 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 | Agriculture Form One:

Importance of soil to agricultural production

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

2. It supplies plant with the mineral nutrients necessary for crop growth

3. It provide anchorage for higher plants

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

5. 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)
Soil separates
U.S.Dept.of Agric. System
Diameter(mm)
International system diameter (mm)
Number of particles per g
Very coarse

Sand

2.00-1.00 90
Coarse sand 1.00-0.50 )

)2.00-0.20

720
)
Medium sand 0.50-0.25 5,700
Fine sand 0.25-0.10 ) 46,000
)0.20-0.02
)
Veryfinesand 0.10-0.05 722,000
Silt 0.05-0.002 0.02-0.002 5,776,000
Clay below 0.002 below 0.002 90,260,853,000

Formation of mineral matter

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.

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;

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

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

1. 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.

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

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

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

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

6. 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.

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

2. 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.

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

4. 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.

5. 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.

6. 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.

Agriculture Form One: Introduction to Soil Science | Topic 5

 

 

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.

AGENTS OF WEATHERING

1. Rain

The mechanical action of rain consists mainly in the washing of loose particles of soil and rock to lower levels. This phenomenon is known as rain-wash.

It is the means by which rivers receive much of the sediments they carry in suspension. The chemical weathering effects of the rain are seen its solvent action on some rocks notably limestone.

The process depends on the pressure of feeble acids, derived from gases such CO2 and SO2 which are present in air in small quantities and which enter into solution in rain water.

The denuding effects of heavy showers and rain-storms may be very severing, especially in regions where a covering of vegetation is lacking.

It cuts gullies in the surface of the ground, some of considerable size and may cause great damage by the destruction of roads and livestock.

Heavy rains also promote landslides. Vegetation protects the ground from the immediate disintegrating effects of rainfall.

2. Frost

In cold climates the action of the frost is to break off angular fragment from exposed rock surface, a process sometimes referred to as ice-wedge.

Water enters rock along pores, cracks and fissures. On freezing it expands and occupies about 10% greater volume exerting a pressure of about 2000 lbs per square inch. This is therefore like a miniature blasting and brings about the disintegration of the rock.

The loosened particles fall from the mass and accumulate as heaps of talus at lower levels and this material may later be consolidate into a deposit known as breccia.

3. Wind

It is one of the two natural agents which transport rock material against gravity. Its effect is three-fold.

First it removes loose particles of rock decay as it blows over a surface, then charged with these grains the wind act as an abrading sand-blast driving the grains against rock surfaces which becomes worn and polished in course of time. Thirdly the blown grains are accumulated to from sand-dunes.

Lines of communication may be seriously affected by wind-blown sand in arid countries. It is on record that the telegraphic wire on the trans-Caspian railway was worm down to half of its diameter in eleven years, and renewal was then made. To avoid accumulation of sand alongside railway embankments in Sudan, culverts have been made to allow for easy passage of the wind and its load sediments.

4. Insulation

When a rock surface is exposed to a considerable daily range of temperature, as in arid and semi- arid regions, the expansion which occurs during the day and contraction at night, constantly repeated have a weakening effect on the texture of the rock over a period of time . The outer heated layers tend to pull away from the cooler rock underneath a process known as exfoliation.

By the unequal expansion and contraction of its mineral constituents the strain is set up in a rock and its texture is loosened. This kind of weathering is prevalent in climates where high day and low night temperatures are prevalent.

Weathering by organic materials

Plants retain moisture and any rock surface on which they grow is kept damp, thus aiding the solvent action of the water. The chemical decay of the rock is also promoted by the formation of vegetable humus organic product of the decay of plants.

The mechanical break up of rocks is helped by the roots of plants which penetrates into cracks and crevices and tend to wedge apart the rock.

FACTORS AFFECTING SOIL FORMATION PROCESSES

1. Climate

The principal climatic variables influencing soil formation are effective precipitation (i.e., precipitation minus evapo-transpiration) and temperature, both of which affect the rates of chemical, physical, and biological processes.

The temperature and moisture both influence the organic matter content of soil through their effects on the balance between plant growth and microbial decomposition. Climate is the dominant factor in soil formation, and soils show the distinctive characteristics of the climate zones in which they form.

For every 10 °C rise in temperature, the rates of biochemical reactions more than double. Mineral precipitation and temperature are the primary climatic influences on soil formation.

If warm temperatures and abundant water are present in the profile at the same time, the processes of weathering, leaching, and plant growth will be maximized. Humid climates favor the growth of trees.

In contrast, grasses are the dominant native vegetation in sub humid and semiarid regions, while shrubs and brush of various kinds dominate in arid areas.

2. Water

Water is essential for all the major chemical weathering reactions. To be effective in soil formation, water must penetrate the regolith.

The seasonal rainfall distribution, evaporation losses, site topography, and soil permeability interact to determine how effectively precipitation can influence soil formation.

The greater the depth of water penetration, the greater the depth of weathering of the soil and its development. Surplus water percolating through the soil profile transports soluble and suspended materials from the upper to the lower layers.

It may also carry away soluble materials in the surface drainage waters. Thus, percolating water stimulates weathering reactions and helps differentiate soil horizons. Likewise, a deficiency of water is a major factor in determining the characteristics of soils of dry regions.

Soluble salts are not leached from these soils, and in some cases they build up to levels that curtail plant growth. Soil profiles in arid and semi-arid regions are also apt to accumulate carbonates and certain types of expansive clays.

The direct influences of climate include:
  • A shallow accumulation of lime in low rainfall areas as caliche
  • Formation of acid soils in humid areas
  • Erosion of soils on steep hillsides
  • Deposition of eroded materials downstream
  • Very intense chemical weathering, leaching, and erosion in warm and humid regions where soil does not freeze

Climate directly affects the rate of weathering and leaching. Wind moves sand and smaller particles, especially in arid regions where there is little plant cover.

The type and amount of precipitation influence soil formation by affecting the movement of ions and particles through the soil, and aid in the development of different soil profiles.

Soil profiles are more distinct in wet and cool climates, where organic materials may accumulate, than in wet and warm climates, where organic materials are rapidly consumed.

The effectiveness of water in weathering parent rock material depends on seasonal and daily temperature fluctuations. Cycles of freezing and

thawing constitute an effective mechanism which breaks up rocks and other consolidated materials.

Climate also indirectly influences soil formation through the effects of vegetation cover and biological activity, which modify the rates of chemical reactions in the soil.

3. Topography

The topography, or relief, is characterized by the inclination (slope), elevation, and orientation of the terrain. Topography determines the rate of precipitation or runoff and rate of formation or erosion of the surface soil profile. The topographical setting may either hasten or retard the work of climatic forces.

Steep slopes encourage rapid soil loss by erosion and allow less rainfall to enter the soil before running off and hence, little mineral deposition in lower profiles. In semiarid regions, the lower effective rainfall on steeper slopes also results in less complete vegetative cover, so there is less plant contribution to soil formation.

For all of these reasons, steep slopes prevent the formation of soil from getting very far ahead of soil destruction. Therefore, soils on steep terrain tend to have rather shallow, poorly developed profiles in comparison to soils on nearby, more level sites.

In swales and depressions where runoff water tends to concentrate, the regolith is usually more deeply weathered and soil profile development is more advanced. However, in the lowest landscape positions, water may saturate the regolith to such a degree that drainage and aeration are restricted. Here, the weathering of some minerals and the decomposition of organic matter are retarded, while the loss of iron and manganese is accelerated.

In such low-lying topography, special profile features characteristic of wetland soils may develop. Depressions allow the accumulation of water, minerals and organic matter and in the extreme; the resulting soils will be saline marshes or peat bogs. Intermediate topography affords the best conditions for the formation of an agriculturally productive soil.

4. Organisms

Soil is the most abundant ecosystem on Earth, but the vast majority of organisms in soil are microbes, a great many of which have not been described. There may be a population limit of around one billion cells per gram of soil, but estimates of the number of species vary widely.

Estimates range from 50,000 per gram to over a million species per gram of soil. The total number of organisms and species can vary widely according to soil type, location, and depth.

Plants, animals, fungi, bacteria and humans affect soil formation. Animals, soil mesofauna and micro-organisms mix soils as they form burrows and pores, allowing moisture and gases to move about.

In the same way, plant roots open channels in soils. Plants with deep taproots can penetrate many meters through the different soil layers to bring up nutrients from deeper in the profile.

Plants with fibrous roots that spread out near the soil surface have roots that are easily decomposed, adding organic matter. Micro-organisms, including fungi and bacteria, affect chemical exchanges between roots and soil and act as a reserve of nutrients.

Humans impact soil formation by removing vegetation cover with erosion as the result. Their tillage also mixes the different soil layers, restarting the soil formation process as less weathered material is mixed with the more developed upper layers.

Earthworms, ants and termites mix the soil as they burrow, significantly affecting soil formation. Earthworms ingest soil particles and organic residues, enhancing the availability of plant nutrients in the material that passes through their bodies.

They aerate and stir the soil and increase the stability of soil aggregates, thereby assuring ready infiltration of water. As they build mounds, some organisms might transport soil materials from one horizon to another.

In general, the mixing activities of animals, sometimes called perturbation, tend to undo or counteract the tendency of other soil-forming processes that create distinct horizons.

Termites and ants may also retard soil profile development by denuding large areas of soil around their nests, leading to increased loss of soil by erosion. Large animals such as gophers, moles, and prairie dogs bore into the lower soil horizons, bringing materials to the surface.

Their tunnels are often open to the surface, encouraging the movement of water and air into the subsurface layers. In localized areas, they enhance mixing of the lower and upper horizons by creating, and later refilling, underground tunnels.

Old animal burrows in the lower horizons often become filled with soil material from the overlying A horizon, creating profile features known as crotovinas.

Vegetation impacts soils in numerous ways. It can prevent erosion caused by excessive rain that might result from surface runoff. Plants shade soils, keeping them cooler and slow evaporation of soil moisture, or conversely, by way of transpiration, plants can cause soils to lose moisture.

Plants can form new chemicals that can break down minerals and improve the soil structure. The type and amount of vegetation depends on climate, topography, soil characteristics, and biological factors. Soil factors such as density, depth, chemistry, pH, temperature and moisture greatly affect the type of plants that can grow in a given location.

Dead plants and fallen leaves and stems begin their decomposition on the surface. There, organisms feed on them and mix the organic material with the upper soil layers; these added organic compounds become part of the soil formation process.

Human activities widely influence soil formation. For example, it is believed that Native Americans regularly set fires to maintain several large areas of prairie grasslands in Indiana and Michigan. In more recent times, human destruction of natural vegetation and subsequent tillage of the soil for crop production has abruptly modified soil formation.

Likewise, irrigating an arid region of soil drastically influences the soil-forming factors, as does adding fertilizer and lime to soils of low fertility.

5. Soil water

Further information: Water content and Water potential

Water affects soil formation, structure, stability and erosion but is of primary concern with respect to plant growth. Water is essential to plants for four reasons:

It constitutes 80%-95% of the plant’s protoplasm. It is essential for photosynthesis.

It is the solvent in which nutrients are carried to, into and throughout the plant. It provides the turgidity by which the plant keeps itself in proper position.

In addition, water alters the soil profile by dissolving and re-depositing minerals, often at lower levels, and possibly leaving the soil sterile in the case of extreme rainfall and drainage. In a loam soil, solids constitute half the volume, gas one-quarter of the volume, and water one-quarter of the volume of which only half will be available to most plants.

A flooded field will drain the gravitational water under the influence of gravity until water’s adhesive and cohesive forces resist further drainage at which point it is said to have reached field capacity.

At that point, plants must apply suction to draw water from a soil. When soil becomes too dry, the available water is used up and the remaining moisture is unavailable water as the plant cannot produce sufficient suction to draw in the water.

A plant must produce suction that increases from zero for a flooded field to 1/3 bar at field dry condition (one bar is a little less than one atmosphere pressure). At 15 bar suction, wilting percent, seeds will not germinate, plants begin to wilt and then die.

Water moves in soil under the influence of gravity, osmosis and capillarity. When water enters the soil, it displaces air from some of the pores, since air content of a soil is inversely related to its water content.

The rate at which a soil can absorb water depends on the soil and its other conditions. As a plant grows, its roots remove water from the largest pores first. Soon the larger pores hold only air, and the remaining water is found only in the intermediate- and smallest-sized pores. The water in the smallest pores is so strongly held to particle surfaces that plant roots cannot pull it away.

Topic 5: Introduction to Soil Science | Agriculture Form One

Consequently, not all soil water is available to plants. When saturated, the soil may lose nutrients as the water drains. Water moves in a drained field under the influence of pressure where the soil is locally saturated and by capillarity pull to dryer parts of the soil.

Most plant water needs are supplied from the suction caused by evaporation from plant leaves and 10% is supplied by “suction” created by osmotic pressure differences between the plant interior and the soil water.

Plant roots must seek out water. Insufficient water will damage the yield of a crop. Most of the available water is used in transpiration to pull nutrients into the plant.

Water retention forces; Water is retained in a soil when the adhesive force of attraction that water’s hydrogen atoms have for the oxygen of soil particles is stronger than the cohesive forces that water’s hydrogen feels for other water oxygen atoms. When a field is flooded, the soil pore space is completely filled by water.

The field will drain under the force of gravity until it reaches what is called field capacity, at which point the smallest pores are filled with water and the largest with water and gases.

The total amount of water held when field capacity is reached is a function of the specific surface area of the soil particles. As a result, high clay and high organic soils have higher field capacities.

The total force required to pull or push water out of soil is termed suction and usually expressed in units of bars (105 Pascal, about one atmosphere) which is just a little less than one-atmosphere pressure. Alternatively, the terms “tension” or “moisture potential” may be used.

The Writer