NCERT Class 7 Science Textbook for Blind Students made Screen readable by Professor T K Bansal.

Have you seen farmers spreading manure or fertilizers in the fields, or gardeners using them in lawns or in pots? Do you know why this is done?

We have learnt that plants absorb minerals and nutrients from the soil. So, their amounts in the soil keep on declining. Fertilizers and manures contain nutrients such as nitrogen, potassium, phosphorous, etc. These nutrients need to be added from time to time to enrich the soil. We can grow plants and keep them healthy if we can fulfil the nutrient requirement of plants.

Usually, crop plants absorb a lot of nitrogen and the soil becomes deficient in nitrogen. We have learnt that though nitrogen gas is available in plenty in the air, plants cannot use it in the manner they can use carbon dioxide. They need nitrogen in a soluble form. The bacterium called Rhizobium can take atmospheric nitrogen and convert it into a usable form. But Rhizobium cannot make its own food. So it often lives in the roots of gram, peas, moong, beans and other legumes and provides them with nitrogen. In return, the plants provide food and shelter to the bacteria. They, thus, have a symbiotic relationship. This association is of great significance to the farmers. They can reduce the use of nitrogenous fertilizer where leguminous plants are grown. Most of the pulses (dals) are obtained from leguminous plants.

In this chapter, you learnt that most of the plants are autotrophs. Only a few plants are parasitic or saprotrophic. They derive nutrition from other organisms. All animals are categorized as heterotrophs since they depend on plants and other animals for food. Can we say that insectivorous plants are partial heterotrophs?