Essential elements for plant growth:
H (95% of plant matter)
Ca, S, Fe, Mn, Zn, Cu, B, Mo, Cl (5%)
Sources of essential elements:
C from atmospheric CO2
CO2 from atmospheric air is taken in by plant leaves and with the use of
chemical energy (from light energy) produces sugars and carbohydrates
O and H from water
Water is absorbed by plant roots from the soil
P, K, Ca, Mg, S, Fe, Mn, Zn, Cu, B, Mo, Cl
Mineral nutrients are absorbed by plant roots from the soil
Biophysical factors
Radiant energy (sunlight)
Weather (rainfall, temperature, wind, velocity, humidity)
Soil (supply of nutrients, water, O2, absence of toxic substances)
Definition of soil
Soil is a natural body on the landscape which has been formed from the weathering of rocks and minerals and whose composition and properties influence the growth of plants.
Fertile soil provides available forms of mineral nutrients in adequate quantities and proper proportions. It is free from toxic substances.
Productive soil is a fertile soil and provides adequate moisture and aeration.
Components of the soil
Soil solids:
Organic matter
Soil water: solution of salts and gases
Soil air: 20% O2, 78.6% N2, 0.9% Ar, 0.5% CO2
Soil organisms: macro and micro
Increasing organic matter in soil
In general organic matter and soil fertility are linked. If soil has a large amount of organic matter, it is also high in fertility. Increasing the supply of one will ultimately increase the amount of the other.
1. Increasing biomass production
Increasing biomass production is the first step to attain higher organic matter content in soil.
The amount of biomass produced is in turn increased by increasing the soil fertility and by
improving the water use efficiency of soil.
a) Increasing soil fertility
One of the fastest, though most expensive, ways to increase soil fertility is by using chemical fertilisers. However the value of the chemicals lies in their use as catalysts when they are used in very small amounts to supply any nutrient which is lacking in the soil (and which is very difficult to make available in any other form).
Biofertilisers such as green manure, azolla and micro-organisms which can ‘fix’ nutrients in forms that can be used by plants are more practical for small farmers with little capital.
Crops or trees which produce food for human, feed for livestock, or other useful items and at the same time increase soil fertility can also be grown. These are usually legumes.
The application of animal manure and other farm by-products which would otherwise be thrown away can also significantly increase soil fertility.
b) Improve water use efficiency
Water is essential for plant growth. Making the best use of water will increase total biomass production.
When water is in excess (during the rainy season) plants that are able to use a lot of water and which grow very fast should be cultivated.
Excess water can also be stored on the surface in ponds, underground reservoirs or on the soil's surface layers for use by crops.
Preventing stored water from evaporating is important. If possible, water stored in ponds should be shaded and windbreaks planted around it. The ground should be mulched if the water is stored in the ground for the use of crops.
When water is scarce, plant crops which need very little water for good growth and production (e.g. sorghum and forage legumes) or deep rooted plants which can tap soil reserves (trees) should be cultivated.
2. Return biomass to the soil
The biomass produced must be returned to the soil in some form, either through direct incorporation, using mulch, passing through livestock as feed or by composting. Although the choice of method used will be determined by any number of factors, one of the most important factors to consider is how the biomass can be used to meet as many needs as possible in the most efficient way before ultimately serving as organic matter in the soil.
G. Effects of organic matter on soil and crops
Conserve water
Organic matter increases the size of macropores in the soil. This allows for more water absorption and retention. Consequently, the greater rate of absorption reduces runoff during heavy rains, decreasing erosion.
