Question

Discuss anion exchange in soil.

Answer 1

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Answer 2

Anion Exchange:

The process of anion exchange is similar to that of cation exchange. Under certain conditions hydrous oxides of iron and aluminium show evidence of having positive charges on their crystal surfaces.

The positive charge of colloids are due to addition of hydrogen (H+) in hydroxyl group (OH–) resulted in net positive charge (OH2+). This + charge will attract anions (—).

The capacity for holding anions increases with the increase in acidity. The lower the pH the greater is the adsorption.

All anions are not adsorbed equally readily. Some anions such as H2 PO4– are adsorbed very readily (quickly) at all pH values in the acid as well as alkaline range.

Cl– and SO4– ions are adsorbed slightly at low pH but none at neutral soil, while NO3– ions are not adsorbed at all.

Hence, at the pH commonly prevailing in cultivated soils—nitrate (NO3), chloride (Cl) and sulphate (SO4) ions are easily lost by leaching.

In general, the relative order of anion exchange is:

OH > H2PO4–>SO4–>NO3–