13. How many cmolykg of exchangeable acidity are in a soil that has a CEC of 20

Question

13. How many cmolykg of exchangeable acidity are in a soil that has a CEC of 20 cmoldkg, and 14. Give quantitative definitions of saline and sodic soils in terms of the electrical conductivity of 15. What would be the SAR of a soil which had 30 meq/l of Nar; 6 meg/l of Ca". and 2 meg/l of 16. Explain how soil productivity is limited by: (a) soil salinity and (b) soil sodicity which has an acidic (H+ and Al…) cation saturation percentage of 25%? the saturation extract; and the sodium adsorption ratio (SAR). Mg"* in the saturation extract? Would the soil be sodic or non-sodic?

Explanation / Answer

13. Cation Exchange Capacity (CEC) is given as 20 cmolc/kg. This means there are a total of 20 centimoles of charge per kg of soil. Also given is the cation saturation percentage which is 25%. So, the exchangeable acidity (EA) in the soil, i.e. the replaceable cations in raw soil, can be computed as:

CSP = (EA/CEC)*100

25 = (EA/20)*100

Thus, the exchangeable acidity (EA) in the soil is 5 cmolc/kg.

14. The electrical conductivity of saline soils is atleast 4 milli-mhos per cm, whereas for sodic soils it never exceeds 4 mmhos/cm. The sodium adsorption ratio, i.e the percentage of exchangeable sodium, is less than 13 for saline soil, and it is greater than 13 for sodic soil.

15. Given that Na+ = 30 milli-equivalents per litre, Ca2+ = 6 meq/l, Mg2+ = 2 meq/l

    Sodium Adsorption Ratio (SAR) is calculated using the relation:

SAR = [Na2+] / ( ( [Ca2+] + [Mg2+] ) / 2)1/2

       =30 / (8 / 2)1/2

       = 15

Since the SAR is greater than 13, the soil would be sodic in nature.

16. The ability of soil to aid the growth of plants, i.e. the productivity, depends on the salinity of soil water. The higher the amount of salt in the soil solution, the lesser will be the capacity of plant to squeeze out water from the soil. Conversely, the lesser the amount of salt in the soil solution, the plant will easily obtain water from soil.

The productivity of soil is also affected by the presence of sodium ions. The Na+ cations adsorbed on the soil particle surface affects the structure of soil. The higher the amount of Na+ adsorbed on soil particles, lesser will be the productivity as the particles would disperse away from each other. This is because of the fact that sodium cations adsorbed excessively on anions of clay disturbs the binding force between soil clay particles. High sodicity, thus, makes the soil infertile and prone to erosion.

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