Role of fertilisers are as follows:
1. They provide macronutrients like N, P, K, S and Mg and micronutrients like Cu, B, Mn required for the proper growth and development of plants.
2. They are readily soluble in water and immediately absorbed by the plants.
3. They provide these nutrients in a specific quantity.
Role of irrigation are as follows:
1. It is applied in certain frequency and quantity for specific crops, e.g. Paddy needs more water.
2. It provides water supply to the plants which is very much essential for life processes, like photosynthesis.
3. It is applied in certain frequency and quantity for a specific type of soils, e.g. sandy soil needs more frequent irrigation.
The effect, when fertilisers are used in excess:
Excess of nitrogenous fertilisers from the soil is washed away into water bodies like a pond, encourages more algal growth and oxygen depletion. This is called eutrophication.
The effect when irrigation is used in excess:
Excess irrigation affects the respiration of roots if it is done before harvesting.