There are four main components of soil, which are: those resulting from the breakdown of rocks, organic matter, water and air. In addition, soil contains microbes or micro-organisms.
Soil is made up of inorganic particles of different sizes such as stones, sand, silt and clay. Each of these is surrounded by microscopic film of moisture in which nutrients are dissolved. These particles aggregate together to form larger lumps of soil crumbs about l-5cm diameter. The presence of organic matter facilitates the formation of soil crumbs. The presence of these crumb structures indicates the ideal soil structure for cultivation and planting. If thesoil textureis too friable, then the roots will not be held firmly and crops will fall over, be blown or knocked out of the soil.
Water and Air
Water comes down from the soil surface trickling between the soil particles and the soil crumbs. Some water will be held but there are some spaces filled with air (soil contains water and air). Roots of crops penetrate the soil always by growing downwards (the direction of gravity). The roots touch the nutrient solution; roots take in water and they take the nutrient solution as food. Scattered about the soil are the soil organisms and dead plant and animal remains.
Air is not solid or liquid, but a combination of gaseous elements that are found naturally in Earth’s atmosphere. In soil, air pockets allow water to pass through the soil and into the plants growing above and below the soil line. Water in soil usually contains dissolved salts and other chemicals. Water is an essential part of components of soil; plants cannot survive without it. Some soils, like clay, retain water much better than others. When water lingers in soil instead of passing through it easily, the soil becomes more densely packed. Some plants cannot grow in heavy, moisture-rich clay soils.
Minerals
All soil is composed of sand, silt and clay, though some types of soil have heavier concentrations of these minerals than others. Rocks and minerals make up the largest chunk of soil’s composition. The rocks and minerals found in soil come from non-living, inorganic materials. Sand comes from small fragments of quartz and other minerals, and by itself is not rich in the nutrients that plants need.
Sand is the largest and coarsest of soil particles; water passes through sand more easily than other soil types. Silt is a combination of quartz and other rocks. Silt particles are smaller than sand but larger than clay. Clay is the richest of soil minerals, containing nutrients like iron, potassium and calcium. The smallest soil particles come from clay, which can become very dense and hard to work.
Organic
Decayed plants and animals provide the organic materials found in soil. Through decomposition, organic materials are broken down and turned into nutrients that plants can use. Mineralisation also occurs through decomposition, and through this process organic materials become inorganic.
Biological Materials
The fifth of components of soil, which is not always recognised, is the living world that exists under the ground i.e., the biological component. Bacteria and fungi are microscopic organisms; those living in the soil decompose and are beneficial (but there are many others which cause diseases).
The biological world, provides these important organic elements that are so essential. Plants and animals, when they die, become part of the ground once more, and so the cycle continues. Soil gives life; life gives itself back to the soil.
All parts of soil are essential to plant development, and each is necessary for plants to survive. The composition of the components of soil varies to create different soil types. Soil that is classified as clay, for example, contains more water than light, sandy soils.
Experiment:
Carry out experiments to show that the components of soil include organic matter, air, water, and rock particles.
You may find the following materials useful in your experiment. These are:
Samples of garden soil, scale, water, clear plastic or glass bottle, cans, Bunsen burner/charcoal brazier, cotton wool, funnels, stirring rod, etc.
After carrying out theexperiment, now answer the question below:
Describe the importance of the following components of soil to a growing plant: –
(i) Rock particles
(ii) Soil water
(iii) Soil air
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