The soil is known to be the chemical laboratory of nature in which various chemical decomposition and reactions taking place in a hidden manner. The major chemical properties which affect plant growth are discussed below in brief:
- Cation Exchange Capacity
- Soil Acidity
- Silica-sexquioxide Ratio
1. Cation Exchange Capacity
The total capacity of soil for holding cations and is usually expressed in terms of milli-equivalents per 100gms of oven dry soil. An equivalent is the quantity chemically equal to one gm (No. of hydrogen atoms=6.02×1023) of hydrogen and so a milli-equivalent is equal to 0.001gm of hydrogen.
Soil has both positive and negative charge, yet negative charge is of much greater magnitude, which has capacity to absorb positively charged ions from the soil solution. The cations absorbed are Ca, Mg, K, Na and H. The exchangeable cations of Ca, Mg, K, Na are all associated with compounds which are more basic than acid in reaction. So, these are referred to as exchangeable bases, while H as exchangeable acid.
Therefore, cation exchange is the process by which a colloidal nucleus absorbs certain cationsfrom the soil solution and gives up other cations held by it in equivalent quantities. Symbolically , CEC always stands for cationexchange capacity.
Importance
- CEC is a sort of store house of mineral nutrients, which determine total amount of nutrients available to plants
- CEC determines the rate of fertilizers to be applied to nursery and plantation soils. Eg. In fine-textured soils, where CEC is high, large amounts of fertilizers can be applied
- It determines the degree to which exchange is saturated with bases as compared with hydrogen. If H-ions predominate, the soil becomes acidic. But if bases predominate, the soil may be neutral to alkaline.
- It helps in the choices of species for successful plantation
- It helps in proper and complete soil utilization
2. Soil Acidity
Acidity of a soil solution is due to an excess of hydrogen ions over hydroxyl ions. But if hydroxyl ions are in excess over hydrogen ions, the solution becomes alkaline. Pure water contains equal amount of hydrogen and hydroxyl ions and is, therefore, said to neutral. Those concentration of ions are indicated by PH. PH of 7 indicates neutrality, higher values indicate alkalinity and lower values acidity. PH value does not remain constant throughout the soil profile. It usually varies from horizon to horizon.
Importance of PH value
PH value has a profound influence on the distribution and growth of forest trees directly as well as indirectly through its influence on the availability of nutrients, activities of soil organisms, physical properties of soil as given below:
- PH value indicates what species would be successful in a particular soil, eg. Sal grows best in soils with PH 4.5 to 5.5; Teak occurs best in PH 6.5 to 7.6
- PH value affects the availability of nutrients. Increase in PH results in increased availability of calcium. Nitrogen is best available when PHis between 6 to 8
- value determines the degree of maturity of soil and the stage of development in plant succession. Soil acidity is associated with maturity of soil and climax vegetation, whereas basic soils are indicative of immaturity.
3. Silica-sesquioxide Ratio
As a result of loss of silica by leaching, sesquioxides get accumulated in the soil profile. The relation between silica and sesquioxides of soil, is known as silica-sesquioxide ratio. Increase in sesquioxides results in decrease in cation exchange capacity and moisture retentivity of soil.
Soil Organic Matter
Organic fraction of the soil, which includes plant and animal residues at various stages of decomposition. It is commonly determined as those organic materials which accompany the soil when put through a 2mm sieve.
Litter
The uppermost layer of organic debris (dead vegetable matter) on a forest floor, freshly fallen or only slightly decomposed and consisting chiefly of leaves but also including bark fragments, twigs, etc. Since it lies on the soil as the topmost layer, it constitutes A00horizon of the soil profile. All the fallen leaves, twigs, litter, etc are termed as liiter.
Humus
The more or less decomposed organic matter of the soil which may form a separate layer or be intimately mixed with the mineral particles. There different kinds of humus. They are sour humus, raw humus, mild humus and leaf mould.
Importance of humus
- Humus get mixed with soil, it improves its structure increasing their water holding capacity and rate of percolation of moisture
- It makes sandy soils more cohesive, thereby improving their structure
- It makes clayey soil more permeable to air and water
- It is a great source of plant nutrients
- Its addition to the soil increases cation exchange capacity
Factors affecting formation of humus
- Nature of forest, its composition, age, etc
- Climatic factors
- Soil organisms and other animal life
- Nature of soil
- Lopping, removal of leaf litter by human beings or its destruction by burning
- Micro-climate
