After reading this article you will learn about the forms and behaviour of various nutrients in soil:- 1. Nitrogen 2. Phosphorus 3. Potassium 4. Calcium and Magnesium 5. Sulphur 6. Micronutrients.
Nitrogen basically occurs in the soil in complex organic forms like proteins which are decomposed by proteinase enzymes produced by micro-organisms to amino acids that are further decomposed to ammonium compounds. Ammonium ions may replace basic cations from the clay and humic micelle.
Ammonium compounds are oxidized to nitrites by nitrosamines bacteria, which are further oxidized to nitrates by nitrobacteria. Plants take up nitrogen from the soil in the form of ammonium and nitrate ions. Ammonia is volatilized from hot, alkaline waterlogged soil when nitrates are reduced to nitrites, which are further reduced to oxides of nitrogen and nitrogen gas. Nitrates are leached down if added to paddy fields.
Phosphorus originates in the soil, both in the inorganic form i.e. apatite mineral, as well as in the organic form i.e. phytin, inositol, nucleic acids etc.
Microorganisms decompose these organic phosphorus compounds to release the phosphorus in the simple inorganic form i.e. orthophosphate, H2PO4 ions. Carbon dioxide is produced from the decomposition of organic matter and respiration of roots and micro-organisms.
It reacts with water to form carbonic acid, the reacts with tricalcium phosphate to form water soluble mono-calcium phosphate:
Water soluble mono-calcium phosphate reacts with aluminum and ferric ions in strongly acidic soil to form water insoluble aluminum and ferric hydroxyl phosphate.
Water soluble mono-calcium phosphate reacts with calcium ions and calcium carbonate to form insoluble tricalcium phosphate in strongly alkaline soils.
Potassium basically occurs in primary minerals like orthoclase, mica etc. in the soil. These mineral decompose to release the potassium contained them in the water soluble form.
When excessive amounts of potassium ions are present in the soil solution, they may be lost by leaching. Crops assimilate potassium in excessive amounts but the crop yield does not increase in proportion to the amount of potassium absorbed by the root. This is known as luxury consumption of potassium. Potassium ions are fixed by certain clays like illite.
4. Calcium and Magnesium:
Calcium and Magnesium occur in primary minerals like calcite, dolomite, hornblende etc. Which are decomposed by organic acids to release calcium and magnesium in the form of ions.
Usually calcium and magnesium ions are held by clay and humic micelle is exchangeable cations which are replaced by hydrogen ions as shown below:
These soluble basic bicarbonates are washed down by high rainfall; in humid region the above reversible reaction proceeds in the forward direction, which means hydrogen ions gradually replace calcium and magnesium ions form the clay and humic micelle. Ultimately soils become acidic in reaction.
Sulphur mainly occurs in the organic form i.e. sulphur containing proteins which are decomposed by micro-organisms to sulphides, that are further oxidized to sulphate. The soil solution contain HSO3—, SO3–, SO4— ion.
The micronutrients occur in primary minerals which decompose during the formation of soils resulting in the formation of oxide and sulphides and of iron, manganese and zinc.
Boron, molybdenum and chlorine occur as borate and borosilicate, molybdate and chloride respectively. Some of the micronutrients also occur in the organic form in the surface soil. Micronutrients usually occur in the soil solution in the ionic form.
All micronutrients except molybdenum are extremely soluble in a strongly acidic soil, and thus become toxic to crops. Boron has been leached form strongly acidic sandy soil.
When the pH is increased, Iron, manganese, zinc and copper are precipitated as hydroxides. So the availability of all micronutrients except molybdenum decrease, as the pH of the soil increases. Iron, manganese and copper exist in the lower valent form in poorly aerated waterlogged soils. They are oxidized to the higher valent form is well aerated soils.
The higher valent forms of micronutrients are less soluble than the lower valent forms, at the most common pH values of soil e.g. ferric hydroxide precipitates at a pH of 3.0 whereas ferrous hydroxide precipitates at a pH at 6.0.
Manganese and zinc also occur as integral parts of some silicate clays like those belonging to the smectite group. When large amounts of water soluble phosphate fertilizers are added to the soil, the crop takes up less iron and zinc and more molybdenum.
Iron, Manganese, zinc and copper combine with organic compounds to form chelates which protect them from certain harmful reactions, e.g. precipitation of water soluble phosphate by ferric ions. At the same time, they remain available for the growth of the crop.