Nutrients are elements that are required by crops in other to grow and complete their life cycle. When they are not available in the soil in adequate amounts, they can be added to the soil in the form of fertilizers. The soil nutrients required by crops are nitrogen, phosphorus, potassium, calcium, magnesium, sulphur, iron, manganese, boron, molybdenum, copper, zinc and chlorine.
Losses of water from soil
There are two forms in which water can be lost from the soil:
1. Losses in vapour form
2. Losses on liquid form
Losses in vapour form
Losses of water in the form of vapour occur when the water evaporates from the soil surface. The factors that affect evaporation from the soil surface are :
(I) Sunshine - more water is evaporated from the soil and surface on sunny days because of the heating effect of the sin.
(ii) Temperature - the higher the temperature of the atmosphere the greater the evaporation.
(iii) Humidity - less water is evaporated when the atmospheric humidity is high, and more water is evaporated when the atmospheric humidity is low.
(Iv) Wind - increases the amount of evaporation from the soil and from plant surfaces.
(v) Crop cover - a dense crop cover reduces the amount of direct evaporation from the soil but the more dense the cover the more water is used by the crop through transpiration ( a process by which water vapour passes to the atmosphere from plant cells). Water that is used by weeds is lost through transpiration and can no longer be used by the crop.
(vi) Water Supply - more water is evaporated from the soul when it is very wet than when it is just moist.
Losses in liquid form
There are two ways by which water in liquid form is lost from the soil.
(a) Deep percolation - percolation is the vertical movement of water through the soil. Deep percolation occurs when water moves down the soil and out of the crop root zone.
(b) Runoff - Water moving on the surface and out of the area on which it fell as precipitation or was applied as irrigation.
Several factors govern losses of liquid water from the soil:
(I) Soil Texture - in a sandy soil, water moves down the profile very fast because of the large air spaces. In a clayey soil water does not percolate fast enough and may be lost as runoff.
(ii) Soil structure - a soil that has a well granulated structure takes in more water than a puddled soil. A puddled soil will have more runoff.
(iii) Organic matter - A soil that has a high organic matter content has a stable structure. The granules will not break up and close the air spaces when the soil is wetted. Water loss through runoff will therefore be less.
(iv) Slope - Slope is land that lies in a slanting position. Both the length and degree of slope affects runoff.
(v) Crop cover - Less water will runoff from a bare soil than when there is a crop. The crop roots tend to prevent percolation.
(vi) Water supply - When rain falls at a faster rate than the soil can take in, the water collects and begins to run off.
Nutrient Losses
The thirteen nutrient elements listed at my previous write up are found in the soil in various forms. Nutrients are either absorbed I.e attached to the surfaces of the soil solids or they occur in solution in the soil water. There are four basic ways in which nutrients are lost from the soul. There are Leaching, Crop removal, Burning and Erosion.
1. Leaching - All conditions that enhance deep percolation also lead to loss of these nutrients that are present in the soil water. Such removal of materials in solution from the soil is called leaching.
2. Crop removal - Any growing plant take up nutrients from the soil. When such crops are harvested and taken away the nutrients are lost.
3. Burning - When crop residues are burnt, most of the nutrients remain on the soil surface in the form of ash. Only sulphur and nitrogen are oxidized to the gaseous form and lost directly from the soil. The ash may however be lost before a crop can use it. It maybe washed away by run off water or leached through deep percolation.
4. Erosion - This is the removal of soil by water or wind. Thus it is the lost of the soil itself with any nutrients contained in it.
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