Soil consists of particles of different sizes that are a result of weathering of rocks. Rocks break down into small particles by the action of temperature changes, frost, water, ice and wind. In cold climates, water flowing through rock crevices or cracks freezes during the cold months of winter and thaws when the temperatures become warm. The stress exerted by continual freezing of the water and the thawing of the ice over many years will eventually break the rocks.
In the deserts, the action of winds and hot temperatures during the day and near freezing temperatures at night, result in the formation of sand particles.
In addition, plant roots break up large soil particles into small particles. Carbon dioxide and nitrogen can dissolve in water into acids and oxides respectively. These break up rock and soil particles into smaller particles.
Soil Composition
1. Inorganic particles formed from weathering
2. Humus which are decayed and partly decayed remains of plants and animals
3. Water plays part in weathering and is needed by plants and microorganism
4. Air spaces contain oxygen needed for the respiration of plants and microorganisms present in the soil.
5.Dissolved inorganic mineral salts formed from rock weathering and humus by the action of rainwater or microorganisms.
6., Living organisms that are the major decomposers. These are bacteria, fungi, and earthworms.
7.Nitrates from released by decomposing humus and by nitrogen fixing bacteria necessary for the fertility of the soil.
Soil particles
A rough idea of the composition of soil particles can be obtained by scooping soil from the ground and putting it in a beaker and filling the beaker with water and allowing the beaker to remain undisturbed for some time. The various particles of the soil will settle at different rates according their weights and sizes.
Insert diagram
The largest particles settle to the bottom quickly while the smallest particles remain suspended in water for a long time. The largest soil particles consist of grave, coarse sand. fine sand, ,silt and clay. At the topmost layer are floating particles of plant and animal remains.
Properties of Soils
Top soil is usually dark in colour because it has high proportion of organic matter .It is made up of rock particles, mineral ions, living organisms, humus, water and air.
Humus is undecomposed or partly decomposed organic material by the activity of decomposers in the soil. Among the major decomposers are various micro-organisms including bacteria, fungi, nematodes and earthworms. Ideal conditions for decomposition are high temperatures and high humidity. These conditions prevail in the tropics particularly in tropical forests.
Decomposition releases important inorganic salts, water and CO2 .The importance of humus is to improve soil fertility and water retention. Secondly, humus binds soil particles together into crumbs and thus creating spaces between soil particles. This improves the soil drainage and promotes bacteria growth. Humus prevents waterlogging and acid formation.
Clay soil has small, packed together particles, which are able to retain water through capillary attraction. Because sand particles are negatively charged they attract positively charged ions such as K+ and Ca++ and thus prevent the leaching out of such ions from the soil.
Unfortunately, because of the small spaces between the particles clay soils have poor aeration and become easily waterlogged under wet conditions. They are also hard to dig when they are dry.
However, clay soil can be improved by the addition of lime (calcium hydroxide). This makes the particles form clumps, which increases the spaces between them. This improves the soil drainage and air content. This condition is known as flocculation.
Addition of manure not only improves aeration and drainage but also the mineral content, as well as the population of living organisms the population of living organisms.
Loamy soils contain a good balance of sand and clay, including humus. In addition, they have sufficient air, mineral content and good drainage. Humus is derived from the remains of dead plants, animals and other organic refuse that have been subjected to decomposition by fungi and bacteria.
Humus, also known as compost, is rich in mineral salts and is a major source of food for millions of soil organisms, including earthworms.
Acid soils are characteristic of areas that receive plenty of rainfall, rendering the soil waterlogged and poor in bacterial and fungal organisms. This results in partially decayed acidic humus known as peat. Soil acidity also influences solubility of certain minerals like iron, manganese, and phosphate causing them to leach. Acidity inhibits activity of microorganisms that play an important part in improving the texture of the soil by mixing and breaking down organic matter.
Soil Erosion and its Causes
Soil erosion is due to both man made and natural factors. The made factors include burning, overgrazing and poor farming methods. Soil erosion removes minerals from the soil, thus depriving plants of nutrients.
In overgrazing the animals not only eat up all the vegetation but they also trample on the surface of the soil, exposing it to the action of winds and water.
Burning removes the soil cover and thus makes the soil easily blown away by winds and rainstorms.
Poor farming methods affect especially the high ground such as hilltops. Ploughing down the slope promotes erosion so that the soil can easily be washed away by water or blown by wind.
Heavy rainstorms are usually very destructive destroying crops and washing away large quantities of the soil surface and minerals. In addition, rainstorms create large gullies in the ground with much of the soil and minerals being lost into the rivers, and streams.
Prevention of Erosion
Prevent overgrazing where this is possible.
Fertilisers
Organic fertilisers include manure and fishmeal among others. They are added to the soil in order to improve its mineral content as well as its texture. Organic fertilisers, also known as compost, must be broken down by decomposition in order to release the mineral salts.
Commercial fertilisers that are widely used in agriculture consist mainly of nitrates, phosphates and potassium. The major drawback in using commercial fertilisers is that they tend to leach rapidly from the soil by rainwater and irrigation.
They can also alter the pH of the soil and increase the loss of some soil minerals. For example, at a pH of 8 iron is virtually absent for plants due to leaching although other elements such as calcium are readily available. Leaching fertilisers can drain into rivers and lakes and pollute them. In addition to polluting the lakes, they can cause their autotrophication or massive population explosion of plant and animal organisms.
Nitrogen Fixation
Plants need nitrogen for the synthesis of protein and nucleic acids. Before atmospheric nitrogen can be of any use to the plant, it is first converted to ammonium compound such as ammonium nitrate by the chain of bacteria living in the soil. This process is called nitrogen fixation. An enzyme present in bacteria called nitrogenase catalyses the conversion of atmospheric nitrogen to an ammonium compound.
N2 + 6H → 2NH3.
Nitrogen fixation starts with the bacterial conversion of nitrogen to ammonia. Next ammonia is converted to an ammonium ion when it picks up a hydrogen ion from the soil (NH4+). The ammonium ion is oxidised to a nitrate ion, (NO3)-by bacteria. The plants can now absorb the nitrate.
Insert diagram from the draft and explanations
Symbiotic N – fixation
Legumes (peas, beans, Soya beans, etc.) have swelling called nodes on their roots that contain nitrifying bacteria. These bacteria belong to genus Rhizobium. Bacteria supply the plant with fixed nitrogen in form of nitrates as we have already seen. In return, the plants provide the bacteria with manufactured foods.
Improving the protein yield of crops.
People depend on plant material for food but plants are poor in protein. In addition, plant protein may be deficient in one or more of the essential amino acids.
Plant breading has resulted in enriched protein yielding cereals. The problem is that these plants need a lot of nitrates that can only come from fertilisers, which are expensive to make. Unfortunately, the poor people in the developing countries need these fertilisers most. Taking advantage of the symbiotic nitrogen fixation by the Rhizobium may increase protein-yielding crops.
Genetic engineering has a great potential to increase the world’s food production, including food proteins. Without decomposers, leaves, logs, faeces and dead animals would accumulate and pile up into mountains of organic waste. Such a situation would deprive plants of nutrients needed for photosynthesis. With plants starved of minerals and no photosynthesis, there would be no food for herbivores and carnivores.
Study Questions
1. Describe the composition of the soil. What processes account for the formation of the soil?
2. State the characteristic of each of these soils types: topsoil, loam soil, clay soil and sandy soil.
3. In which way is the soil affected by the presence of bacteria, fungi and invertebrates?
4. Explain the role of symbiotic bacteria in soil fertility.
What human activities contribute to soil erosion?
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