Effect of Fertilisers on Soils and Crops

After reading this article you will learn about the effect of fertilisers on soils and crops.

When water soluble fertilisers are added to the soil, the soluble salt content of the soil is increased. The salt index of a fertiliser is a measure of the tendency of the fertiliser to increase the soluble salt content of the soil.

The salt indexes per unit of plant nutrient of a few common fertilisers are given below:

Thus we find that the continuous application of a heavy dose of nitrogenous fertiliser, especially sodium nitrate, potassium nitrate, ammonium sulphate and ammonium nitrate highly increases the soluble salt content of the soil.

The continuous application of potassic fertilisers also increases the soluble salt content of the soil, but this increase is much less than that caused by the application of nitrogenous fertilisers. The continuous application of phosphatic fertilisers increases the soluble salt content of the soil by a negligible amount. The salt index of nitrogenous fertiliser per unit of nitrogen decreases when their nitrogen content increases.

Whenever ammoniacal fertilisers are applied to the soil, ammonium ions replace the basic cations, especially calcium from the clay and humic micelle as shown below supposing the clay and Humic micelle have absorbed calcium magnesium, potassium, sodium and hydrogen ions in the ratio of 50: 20: 5: 5: 20.

Calcium sulphate is gradually dissolved in rainwater in the humid region.

The above reversible reaction proceeds in the forward direction which means that basic cation are gradually replaced from the micelle by ammonium ions which are then oxidized to nitrous, which is further oxidized to nitrate as shown below:

When urea is added to the soil, it is converted to ammonium carbonate, which; oxidized to nitrous acid that, in turn, is oxidized to nitric acid.

This ammonium carbonate then may react with the clay and humic micelle and ammonium ions gradually replace the basic cations from them, as shown below:

The ammonium ions of ammonium carbonate and the clay and humic micelle are then oxidized to nitrous acid that is further oxidized to nitric acid as shown below:

Ultimately the clay and humic micelle is saturated with hydrogen ions and nitric acid is also formed. So if a heavy dose of ammoniacal and amide fertilizer (urea) are continuously applied to the soil, they become acidic in reaction. In a similar way, if sodium nitrate is continuously applied to soil, sodium ions gradually replace other cations as shown in the following equation, and the soils become alkaline in reaction.

Tall varieties of wheat and paddy lodge i.e., fall down, if a heavy dressing of nitrogenous fertilisers is applied to them. The dwarf varieties of wheat have responded more to the increasing doses of nitrogenous and phosphatic fertilisers than the tall varieties.

For example, the yields of tall varieties of wheat are increased when the dose of nitrogenous fertilisers are increased to up to 80 kg nitrogen per hectare, whereas the yields of dwarf varieties of wheat continued to increase when the dose of nitrogenous fertilisers are increased up to 120 kg nitrogen per hectare.

The applications of nitrogenous, phosphatic fertilisers have increased the yields of the high yielding varieties of wheat. Tall varieties of paddy have responded to up to 60 kg of nitrogen per hectare whereas the dwarf varieties have responded to up to 120 kg of nitrogen per hectare.

Paddy usually responds to the application of phosphatic fertilisers much less than to nitrogenous fertilisers. But dwarf varieties of paddy may respond to phosphatic fertilisers. High yielding paddy varieties yield more if nitrogenous, phosphatic and potassic fertilisers are applied to them.

Yields of hybrid maize have increased much more as a result of application of nitrogenous and phosphatic fertilisers than the yields of local maize. Hybrid maize has responded to the application of potassic fertilisers to potassium deficient soils.

Pulses like pigeon pea (Cajanuscajan) and green gram (phseolus aureus) have responded only to the application of phosphatic fertilisers. Lentil (Lens esculenta) and pea (Pisum sativam) have responded to the application of nitrogenous and phosphatic fertilisers whereas Gram (Cicer arietinum) has responded to the application of phosphatic fertilisers.

The yields of oilseeds (Brassica sp) have increased as a consequence of the application of nitrogenous fertilisers only. The application of phosphatic fertilisers to oil seeds increases their oil content. A balanced application of Nitrogenous, phosphatic and potassic fertilisers increase the yields of potato and other vegetables. Phosphorus and potassium are essential for improving their quality.

It has been reported by Chowdhury and Bains (1967) from the Indian Agricultural Research Institute, Delhi that the protein content of wheat grain and straw increased when increasing doses of nitrogenous fertilisers are applied to them. Similar results have been obtained for other crops.

The phosphorus, thiamine and riboflavin content of wheat have also increased as a consequence of application of nitrogenous, phosphatic and potassic fertilisers, at the same place. If excessive amounts of nitrate fertilisers are applied to fodder crops, they may contain toxic amounts of nitrates.