CHEMISTRY FORM FOUR TOPIC 3: SOIL CHEMISTRY

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SOIL CHEMISTRY

Soil Formation
Describe soil formation
Soil is formed by the process of weathering. All types of weathering (physical, chemical or biological) result to disintegration of rocks into smaller particles. Air and water enter the space between these particles and chemical changes take place, which lead to the production of chemical substances. Bacteria and plant life soon appear. When plants and animals die, they decay and produce humus. Bacteria and other decomposers play a vital role in the decomposition of plant and animal substrata. The end product of these mechanical, chemical and biological processes is soil.
Therefore, soil can be defined as unconsolidated mineral (inorganic) and organic material on the immediate surface of the earth‟s crust that serves as the medium for plant growth.
All soils contain mineral matter, organic matter, water, air and living organisms, especially bacteria. If any one of these is substantially reduced in amount or is removed from the soil, then the soil deteriorates. There are many types of soil and each has specific characteristics related to the climate, the vegetation and the rock of the region in which it forms. The weathering processes of a region also play an important part in determining soil characteristics. The relationship of these factors is as shown in figure 3.1.

The Factors Influencing Soil Formation

Describe the factors influencing soil formation
Information about soil formation can lead to better soilclassification and more accurate interpretation of soil properties.There are several factors responsible for soil formation. Thefactors include climate, living organisms, relief (topography),parent material and temperature. All the factors, except time,depend to a greater or lesser extent upon each other, upon the soilitself or upon some other factor. None of the factors can beconsidered more important than any other, but locally one factormay exert a particular strong influence. These factors areexplained in details below.
Parent material
Parent materials are made up of mineral material or organic matter or a mixture of both. The organic matter is usually composed predominantly of unconsolidated, dead and decaying plant remains. The mineral material, which is the most widespread type of parent material, contains a large number of different rock– to form Climate which decays to form results in weathering of influences the type of climate rocks vegetation humus Climate Soil mineral soil Climate forming minerals and can be in either consolidated or unconsolidated state.
Some rocks are more easily weathered than others. Acidic rocks are more resistant to weathering than basic rocks. The parent rock affects soil texture and water permeability.
Parent rock with fine particles is more resistant to chemical weathering than mechanical weathering. Very compact parent rocks like sandstone are very much resistant to weathering. Porous rocks weather easily by chemical processes. This is because they have large surface areas for weathering agents to act upon.

Climate

Climate is the principal factor governing the rate and type of soil formation as well as being the main agent determining the distribution of vegetation. The dead vegetations decay to form humus as one of the components of the soil.
To understand well the influence of climate on soil formation let us have a look at its components and how each of these components affects soil formation.
Temperature
The main effect of temperature on soil is to influence the rate of reactions; for every 10°C rise in temperature, the speed of a chemical reaction increases by a factor of 2 or 3 (twice or thrice). Temperature, therefore, influences the speed of disintegration and decomposition of the parent materials and its consolidation to form the soil.

Rainfall (water)

The water in soils includes all forms of water that enter the soil system and is derived mainly from precipitation as rain. The water entering soils contains appreciable amounts of dissolved carbodioxide, forming a weak carbonic acid. This dilute, weak acid solution is more reactive than pure water. It thus reacts with unconsolidated minerals and organic matter, breaking them down into mineral (clay, sand) and organic debris (humus) respectively.

Organisms

The organisms influencing the development of soils range from microscopic bacteria to large mammals including man. In fact, nearly every organism which lives on the surface of the earth or in the soil affects the development of soils in one way or another. More important soil organisms of interest to soil formation are as follows:

Higher plants.

Higher plants (particularly grasses) extend their roots into the soil and act as binders. So they prevent soil erosion. The roots also assist in binding together small groups of particles hence developing a crumby or granular structure. Large roots are agents of physical weathering as they open and widen cracks in rocks and stones. When plants die they contribute organic matter to the soil, which acts as a binder of the soil particles. Higher plants intercept rain and they shelter the soil from the impact of raindrops. They also shade the soil and hence reduce evaporation.

Vertebrates

Mammals such as moles, ground squirrels and mice burrow deeply into the soil and cause considerable mixing up of the soil, often by bringing up subsoil to the surface, and creating burrows through which the top soil can fall and accumulate within the subsoil.

Microogarnisms

These include bacteria, fungi, actinomycetes, algae and protozoa. These organisms act as decomposers of organic and even mineral matter.

Mesofauna

These include earthworms, nematodes, millipedes, centipedes and many insects, particularly termites and ants. Activities of mesofauna include:
  • ingesting organic mineral materials e.g. earthworms and millipedes;
  • transportation of materials e.g. earthworms, millipedes, termites, beetles, etc; and
  • improvement of soil structure and aeration.
Man
Activities of man are too many and too diverse. Man‟s roles include:
  • Cultivation of soils for production of food and tree crops, which in many cases has negative effects causing impoverishment of the soil and erosion.
  • Indiscrimate grazing, casual burning, cutting of trees, manure and fertilizer use, all of which alter the soil characteristics.

Relief (Topography)

This refers to the outline of the earth‟s surface. All land surfaces are constantly changing through weathering and erosion. It may take millions of years, in the case of Himalayas and the Andes, to be worn down to flat undulating surfaces. The soils on steep mountain slopes are shallow and often stony and contain many primary minerals. In areas where the difference in elevation between the highest and the lowest point is great, then climatic changes are introduced. These differences in elevation, slope, slope direction, moisture and soil characteristics lead to the formation of a number of interesting soil sequences.

Time

Soil formation is a very slow process requiring thousands and even millions of years. Hence, it is impossible to make definite statements about the various stages in the development of soils.This is because it takes a considerable period of time for a particular soil type to be formed and categorized.

 

Soil Fertility and Productivity
The Concept of Soil Fertility and Soil Productivity
Explain the concept of soil fertility and soil productivity
Soil fertility
Soil fertility is the ability of the soil to supply the essential nutrient elements in adequate amounts, forms, and proportions for maximum plant growth.
There are three types of soil fertility. These are:
  • Chemical soil fertility: this is the fertility due to chemical processes that contribute to soil fertility. The chemical soil fertility falls under two categories namely,(i) potential chemical fertility, due to cations in soil solution; and(ii) active chemical fertility, which is due to exchangeable cations adsorbed to the soil colloidal surface or negatively charged plant roots.
  • Physical soil fertility: the fertility contributed by soil moisture, texture, structure, temperature, etc.
  • Biological soil fertility: this is due to organic matter content, and soil microorganisms.
Soil productivity
Soil productivity is the capacity or ability of a particular soil to sustain plant growth and development. It is measured in terms of yield of a particular crop which is a reflection or consequence of nutrients taken up by plants from the soil. Soil productivity is an interaction of three main factors.
  1. Soil fertility. This refers to the ability of the soil to supply the essential plant nutrients required for maximum plant growth.
  2. Plant factors. These includes yield potential, root growth characteristic and genetic make up of a particular crop plant. This means that some crop plants are high-yielding than other plants of the same species and are thus likely to give more crop yields. Also plants with good root development are likely to absorb more nutrients from the soil, grow better and give good yield as compared to plants with poor root development. Genetic make up of a plant also plays a vital role in this respect. For example hybrid maize will always survive harsh146soil and environment conditions than local varieties of maize and, therefore, will give high yields.
  3. Environment factors. These factors include climatic factors and agronomic practices.Climatic factors – These are factors such as temperature, precipitation (rainfall), radiation, humidity, altitude, etc.Agronomic practices include weed control, pest and disease control, good soil preparation, plant population, etc. Yield can be measured in terms of grain yield, tubers yield, dry matter, height of plants, number of leaves, number and size of fruits, berries, etc.
A soil is considered to productive if:
  • It has adequate water retention capacity;
  • It is well aerated; and
  • It is able to supply adequate amounts of the nutrients to plant.
Water retention capacity is influenced by soil organic matter, soil texture (loam, clay, sand, etc), soil structure, and proportions of macro and micro pores in the soil. Aeration is influenced by soil structure and texture.

Difference between Soil Fertility and Soil Productivity

Differentiate soil fertility form soil productivity
We have learned that soil productivity depends on soil fertility in one way or another. However, plant and climatic factors have their roles play too.
From this point of view, therefore, it is correct to assert that a fertile soil is not necessarily productive simply because soil productivity does not rely singly on the fertility of the soil. It depends on several other factors such as soil moisture, which is determined by climate and even altitude, which influences plant development a great deal. In concise, it should be understood that there are several other factors apart from soil fertility which affects the productivity of the soil. Even soil management practices can affect soil productivity to a large extent. Soil fertility is accounted for by the type and quantity of the nutrient elements present in a particular soil which are available to crop plants.
Soil productivity is a measure of the amount of harvest or yield that can be obtained from a given piece of land under certain agronomic conditions and practices. For example, suppose a farmer grows maize on one acre of plot A and manages to harvest 20 bags of maize. On another one-acre plot, plot B, he harvests only 10 bags of maize. Of the two plots, A is said to be more productive than B. This is one among many means for determining soil productivity.

The Factors which Determine Fertility and Productivity of the Soil

Explain the factors which determine fertility and productivity of the soil
A fertile soil provides all essential plant nutrients in amounts and proportions which are suitable for growth of most plants. Soil fertility depends on a number of factors, namely:
The texture and structure of the soil
This affects water and nutrient storage, and aeration. The soil with a fine texture such as clay contains small airspaces. The movement of air in and out of such a soil is thus minimal. However, these soils have a great capability of holding water and nutrients. Its structure can be corrected by addition of organic matter, such as farm yard manure and compost, and heap manure.
On the other hand, soils having a coarse texture such as sand are quite porous. Sand allows water to pass through it very quickly. It is poor at water retention and nutrient storage. It has wide air spaces and thus well aerated. It can also be improved by adding organic manures.
The depth of the soil profile
The deeper the soil the better the plant root development and the greater the water and nutrient supply potential it has. Shallow soils do not normally allow roots to penetrate deep through the soil. This leads to poor root development and poor plant growth.
The mineral and organic matter content
The chemical composition of the parent material of the soil provides the natural inorganic nutrient supply due to minerals present. A soil formed from the decomposition of limestone will probably contain reasonably high concentrations of Ca2+ ions in their exchangeable sites due to inherent Ca2+ ions derived from limestone. The same case can apply to high contents of nitrogen and phosphorus in humid soils due to high decomposition of organic matter. The soil organic matter helps to cement the soil particles therefore aiding to create a crumbly structure which is ideal for most agronomic practices.
Cation Exchange Capacity (CEC)
This depends very much on the content of the soil and soil pH. A soil well supplied with organic matter has an optimum cation exchange capacity. The cations adsorbed to the soil colloids are easily exchanged with those in the soil solution. Also humus contains humic (organic) acids which can donate protons if the pH is low.
Soil pH
This affects nutrient storage and availability. The availability of N, P, K, S, Ca, Mg, and Mo decreases with increase in soil acidity. Below pH 5 and above pH 7, Al3+ and Fe3+ ions form complexes with soluble phosphates so that the phosphates are no longer available to plants. This is called phosphorus fixation. Below pH 4.8, Al3+ becomes so soluble that it appears in high concentrations in the soil solution which are detrimental to most plants. This aluminium toxicity is a problem in some tropical soils.
Climate
The climate affects water availability, temperature, weathering as well as the physical and chemical properties of the soil. In humid tropical climates the rate of weathering and organic matter decomposition is very high. Soils in wet tropical and equatorial regions are well supplied with water because these regions normally receive sufficient rainfall.
The position of the ground water table
Water table position affects drainage. Normally, the lower the water table the wet is the soil. Such soils have a good content of moisture and soil microorganisms. Usually water rises from the bottom to the upper parts of the soil profile by capillarity action. The presence of a water table can be detected by vegetations growing directly above it, which always remain greenish even during the dry spell.

The Causes of Loss in Soil Fertility

Explain the causes of loss in soil fertility
All factors that contribute to loss of nutrients from the soil cause loss in soil fertility. These factors include:
  1. Soil erosion: Most plant nutrients are contained in top and sub soil. It is these nutrients that are available to plants for growth and development. When the top player of the soil is removed by erosion, the nutrients in it are lost too. Erosion agents tend to clear and transport the soil from its original site to another site far away. By so doing, the nutrient elements that are contained in this soil are also carried together with the soil. This process then leads to loss of nutrients from the soil and hence loss in soil fertility.
  2. Leaching: This refers to the flushing of plant nutrients from the top to the lower layers of the soil and beyond the reach of plant150roots. This is caused by heavy rainfall or flood irrigation. The process makes the nutrients unavailable to plants as it washes them for beyond the root zone.
  3. Monoculture: The cultivation of the same type of crop on a piece of land year after year, leads to soil depletion if manures or fertilizers are not added. Different plants have specific needs for particular mineral compounds. If the same type of crop is grown on as similar field continuously over a number of years, then the soil will become deficient of the minerals taken up by that crop.
  4. Denitrification: Denitrifying bacteria convert nitrates of the soil to gaseous nitrogen which escapes to the atmosphere, thus depriving the soil of nitrogen. However, this process is counteracted by another type of beneficial bacteria called nitrifying bacteria, which again convert nitrogen of the air back to soil nitrates by a process called nitrification.
  5. Nutrient uptake by plants: Growing plants absorb nutrients from the soil for growth and development. If the nutrients taken up by plants are not replaced through adding manure or inorganic fertilizers, the soil becomes deficient of these minerals.
  6. Volatilization: This refers to the conversion of ammonium compounds into ammonia gas. Nitrogenous compounds in the soil are decomposed by heat into ammonia gas. Also when nitrogenous fertilizers are added to highly basic soils, they react with sodium hydroxide in the soil to release ammonia gas which simply escapes to the atmosphere.NH4+(aq) + OH–(aq) → NH3(g) + H2O(l)
  7. Accumulation of salts: Under normal conditions, rain water washes the mineral salts away, thereby keeping their concentrations in the soil low. However in arid and semi arid151regions the salts accumulate in the soil as the rain falling there is irregular and is insufficient to wash away the salts. This, together with the high evaporation rate and poor drainage, leads to excessive accumulation of salts on or below the soil surface.
  8. Change in soil pH: Use of acidic fertilizers over a long period of time can make the soil acidic. Change in soil pH affects the activity of soil microorganisms and availability of some plant nutrients. This, in turn, affects the fertility of the soil.
  9. Burning of vegetation: Burning crop residues and vegetation deprives the soil microorganisms of the organic matter they require for survival and proliferation. This affects microbial activities such as nitrogen fixation and decomposition of organic matter

The act of burning vegetation also exposes the soil to erosion agents such water and wind. The resulting ash also may cause imbalance of nutrients in the soil or make the soil alkaline.



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