General Concept of Soil Fertility
The plant depends on soil not only for anchorage but required elements for its structural build-up and physiological processes. All the elements that have been identified as essential for crop-plant growth and yield are obtained from the soil with the exception of C which is obtained from the air through the stomata. Hydrogen and oxygen are obtained from water absorbed through plant roots. The remaining elements – N, P, K, Ca, Mg, S, and the micronutrients, which are referred to as plant nutrients, are obtained directly from the soil. Thus the plant depends on the soil for its nutrition.
All the plant nutrients are present in the soil. However, the mere presence of plant nutrients in the soil does not mean that such a soil is fertile. Plants absorb nutrients in ionic forms dissolved in soil solution. Also, for optimum performance of crop, there must be sufficient amount of all the nutrients enough to meet the minimum requirement of the crop. Thus the soil must be able to supply adequate amount of plant nutrients, in forms which can be absorbed by the crop, within its lifespan. Thus soil fertility is defined as the ability of the soil to supply plant nutrients in adequate quantity and in available forms
Available forms of plant nutrients are the forms (ionic) which can be absorbed by the growing plant.
FACTORS AFFECTING THE FERTILITY OF SOIL
The ability of soil to supply plant nutrients in the available forms is affected by many
physical, chemical and biological factors, including management practices.
Soil water
Plants tend to get their nutrients from the water in the soil. This water is usually referred to as the soil solution because it contains dissolved materials (cations and ions) as well as suspended colloids of clay and organic matter.
Soil solution does not contain sufficient nutrients at any one time to last the life of the plant.
It has to be replenished from the pool of nutrients adsorbed onto soil colloids (exchangeable nutrients) and those bound up in solid form as minerals or organic matter called the stable pool
Cation Exchange Capacity (CEC) and Base Saturation
Cation exchange capacity is generally defined as the ability of the soil to adsorb cations.
Many of these nutrients are absorbed in the form of cations. Most soils have at least some ability to hold onto these ions at negatively charged sites within the soil. The amount that they can hold is called the Cation Exchange Capacity. The cations are held to the edges of particles within the soil. This is referred to as adsorption. (Use magnets to demonstrate attraction of positive and negative.) The cations in the soil are divided into acids and bases.
The acids are predominantly hydrogen and aluminum. The bases are primarily calcium, magnesium, sodium, and potassium.
CEC is technically defined as the sum of exchangeable bases plus total soil acidity at a specific pH value, usually 7.0 or 8.0.
When acidity is expressed as salt-extractable acidity, the cation exchange capacity is called the effective cation exchange capacity (ECEC) because this is considered to be the CEC of the soil at the native pH value. It is usually expressed in centimoles of charge per kilogram of soil (cmol kg-1) or millimoles of charge per kilogram of soil.
Credit: Department of soil science (FUNAAB)
Credit
To be continued!