soil science

WHAT IS SOIL

Soil is a natural material that covers the earth’s surface. It is made up of different sizes of weathered parent rock materials and decomposing organic matter. These two components form the solid part of soil. Spaces between the solid particles are filled with water and/or gases. Also existing in the soil are numerous species of living organisms, some of which are microscopic (such as protozoa, bacteria and fungi), and others are larger organisms (such as earthworms, termites, rodents and nematodes). 

SOIL COMPOSITION

Soil is made up of five major components. These are water, organic matter and humus, air, living organisms and inorganic or mineral matter. 

Soil water 

This is found in the spaces between the soil particles that are not filled with air. It constitutes 25% of the soil by volume. Sources of soil water are precipitation and irrigation. 

Importance of water in the soil 

•It dissolves mineral nutrients so that the roots of plants can take them in. 

• It keeps soil temperatures cool, thus enabling living organisms to survive in the soil. 

•It helps in germination of seeds. Water is one of the requirements for seed germination. 

• It softens the soil particles, thus enabling plant roots to grow between the soil particles. 

•It provides hydrogen which is useful in photosynthesis, process used by green plants to make their food. 

•It cools plants through transpiration. This is a process whereby excess water and heat are lost from the plant through the leaves. 

• It maintains the turgidity of leaves and young stems of plants. 

• It helps to carry away excess salts and chemicals from the soil. 

Types of water in soil 

•Hygroscopic water; this is the water that is strongly held by soil particles on their surface by hygroscopic forces. This water is not available to plants. 

•Gravitational water; It is the one that moves down the soil under the influence of gravity. It is available to plants when it is still in the root zone. 

•Capillary water; It is the water that raises up the water table and it is held in the fine and medium pores. This water dissolves mineral nutrients in the soil particles. It is available to the plants. 

•Ground water; This water is permanently found in the saturated zones below the water table. It is not readily available to the crops.

•Water vapour; It is the water that exists in gaseous form. It is available to crops only when it is condensed into liquid. 

Ways through which water is lost from the soil 

Transpiration, This occurs mainly through the stomata found on the leaves of plants. Transpiration is high when the weather is hot and windy. 

Evaporation, this occurs when the soil temperature is high and the soil surface is bare. 

Runoff. This is part of rainwater that runs on the ground surface instead of sinking in the soil. It occurs when rainfall intensity is high, and also in places where soil 

capping has occurred. 

Ways of preventing water loss from soil 

Mulching. Mulches cause water to sink into the soil instead of being lost as runoff. They also control evaporation of water from the soil by reducing the sun’s heat reaching the soil surface. 

Cover cropping. Cover crops such as beans also play the same roles as mulches. 

Manuring, Manure increases the water holding capacity of soil.

Soil structure, soil with good particle arrangement holds and retains more water.

Type of soil, clay soils hold more water than the sand soil because of the small sizes of the soil particles.

Amount of organic matter; high levels of organic matter improves on the rate of water retention.

Soil depth, Drainage and irrigation

Farming practices like mulching, minimum tillage helps to improve the water retention.

Amount of rainfall received.

Experiment to find out the amount of water in the soil 

Apparatus needed 

a sample of fresh soil weighing scale 

* evaporating dish 

• source of heat (e.g. bunsen burner or oven) 

thermometer stirring rod 

• desiccator 

Procedure 

• Weigh sample of fresh soil from the garden. Also weigh an empty evaporating dish. 

• Put the weighed soil sample onto the dish and weigh again. 

• Heat the dish and soil at about 100-105°C for one hour. 

• Keep stirring the soil to allow the moisture to evaporate. Then cool the dish with the soil in a desiccator. 

•After cooling, weigh the dish and its contents. This should be repeated two or three times until a constant 

weight is obtained. 

Observation: 

The final weight of dish and soil will be less than the original weight recorded before heating. 

Explanation 

The reduction in weight is due to loss of moisture from the soil. The difference of the weight before and after heating is the weight of water lost during heating. 

Organic matter or humus

Soil organic matter is that fraction of the soil composed of anything that once lived. It includes plant and animal remains in various stages of decomposition, cells and tissues of soil organisms. It constitutes about 5% of the soil by volume. Organic matter in the soil is lost mostly by microbial decomposition and sometimes by erosion. 

Importance of organic matter or humus 

• Increases the water holding capacity of soil. 

• Binds the soil particles together, thereby improving soil structure. 

• Because of its black colour, it absorbs sun’s heat, making the soil warm 

• Has a buffering characteristic on soil, which prevents rapid changes in the pH of soil. 

• Provides food and shelter to the living organisms found in the soil. 

• Releases nutrients such as nitrogen, carbon, potassium, etc. into the soil, which are essential for plant growth. 

• Holds and retains nutrients that would otherwise be easily lost from the soil by leaching. 

• Improves water infiltration by reducing soil surface crusting. 

• When not fully decomposed, acts as mulch that prevents weed growth and soil erosion. 

• Improves soil aeration. 

How to increase organic matter content of soil 

1. Increasing the production of plant materials, for example by using irrigation in dry areas and growing cover crops. 

2. Decreasing the decomposition of organic matter by practicing minimum tillage. Cultivation increases aeration and soil temperature, which in turn increases the rate at which organic matter is oxidized. 

3. Increasing the supply of organic materials in the soil by applying compost, green manure and farmyard manure. 

Soil air 

This component constitutes 25% of the soil by volume. It occupies the pores between soil particles that are not filled with water. The more water there is in soil, the less the air content, and vice versa. The most important component of air is oxygen which is used for respiration, followed by nitrogen that is converted to nitrates by organisms living in the soil. 

Importance of air in the soil 

•It is needed by soil living organisms and plant roots for respiration. 

•It oxidises organic matter forming nitrates which are taken in by plants 

•It is needed for seed germination. 

•It is used by plants to carry out photosynthesis. 

•It contains nitrogen that can be converted into nitrates for use by plants. 

•It expels excess carbon dioxide from the soil. 

Living organisms

These include worms, insects such as ants and termites, bacteria, fungi and nematodes. 

They are divided into two categories, namely which belong to the animal kingdom, and which belong to the plant kingdom. They can also be classified as microorganisms (those which cannot be seen by our naked eyes but with a microscope), or macro organisms (those which can be seen by our naked eyes). 

Importance of living organisms 

•They decompose organic materials into organic matter (humus). 

•They aerate the soil by making pores in the soil which facilitates the circulation of air within the soil. 

•Living organisms like Rhizobia bacteria convert nitrogen gas in the soil into nitrates. 

•They transform organic compounds which cannot be used by plants into usable forms e.g. insecticides, herbicides and detergents. 

•Some of them produce antibiotics that kill pathogens, such as harmful bacteria. 

•They may act as pests to crops and animals. For example, nematodes destroy plant roots. 

•They cause diseases to plants. 

•They compete with crops for nitrogen and other nutrients. 

•They transmit diseases from one area to another and from crop to crop. 

Factors influencing the abundance of soil living organisms 

Tillage practices: Tillage of the soil directly kills the soil living organisms. it also improves on the drainage and aeration of the soil, which promotes rapid break down of organic matter by aerobic bacteria, eventually causing a fall in the population of living organisms due to shortage of food, 

Soil depth: Deep soils favor abundance of living organisms because they afford the living organisms space for escape from extreme environmental conditions which cause fluctuations of populations. However, the deeper you go into the soil the fewer the living organisms present due to shortage of food, air etc. 

Types of crops grown: Some organisms are abundant when certain crops are grown Blue- green alga is common in rice fields while mycorrhiza is common in tree crops such as pine and oak. 

Amount of water in the soil: Most soil organisms require moist conditions to survive. However, increasing moisture content favours continuously fewer organisms e.g. very high moisture content a ray favours alga but discourage moulds. 

Soil aeration: Most organisms require oxygen for survival. Absence of oxygen then reduces the population of aerobic organisms but increases that of anaerobic and facultative organisms. 

Soil temperature: Most organisms live only within a given temperature range. 

Soil pH: Most bacteria and earthworms do well in slightly alkaline soil while the fungi will be favoured by acidic soils.  

Mineral nutrients in the soil: Especially Ca and N most if not all organisms require these nutrients for their biochemical activities.

Competition between organisms: Some organisms produce substances that kill others e.g. some fungi produce antibiotics that kill bacteria. 

Organic matter content: Many soil organisms use organic matter for food. The higher the amount of organic matter present therefore, the higher the population of soil living organisms present

Pollution: Use of pesticides and other agricultural chemicals and the dumping of industrial and other wastes in the soil reduces the population of living organisms. -  

Introduction by man: Addition of manures introduces more living organisms into the soil. 

Inorganic or mineral matter

This includes the mineral particles which vary in size from 

large rock particles to fine particles like clay. It constitutes 45% of soil by volume. 

Importance of inorganic matter 

Mineral matter: 

•Supplies nutrients to plants. 

•Offers anchorage to plants. 

•Makes the framework of soil. 

•Offers a surface onto which water is held.