The study of cellular Biology Topics is essential to understanding the workings of all living organisms.
Why do we need to replenish the nutrients in soil?
The main nutrients required by the plants are nitrogen, phosphorus and potassium. These are called ‘mineral nutrients’ or just ‘minerals’. The plant nutrients like nitrogen, phosphorus and potassium are present in the soil naturally. When the plants are grown, they absorb the nutrients from the soil due to which the amount of plant nutrients in the soil goes on decreasing. And when the crop plants are grown in the same fields again and again, then the soil becomes deficient in plant nutrients like nitrogen, phosphorus and potassium. Due to this, the plant nutrients (or minerals) need to-be added from time to time to enrich the soil and restore its fertility. The plant nutrients (or minerals) are replenished (or put back) in the soil in the following two ways:
1. Nutrients are Replenished in the Soil by Adding Fertilisers and Manures
Fertilisers and manures contain plant nutrients (or minerals) such as nitrogen, phosphorus and potassium, etc. So, when fertilisers and manures are added to the soil in the fields, then the soil gets enriched with nutrients like nitrogen, phosphorus and potassium, etc. The crop plants can then grow well in this soil. Thus, plant nutrients are added in the cultivated fields in the form of fertilisers and manures so as to get good crops. In fact, many times we see the farmers spreading fertilisers and manures in the fields (see Figure).
The gardeners also put fertiliser and manure in the lawns and potted plants. This is done to provide essential nutrients for the growth of plants so that we get healthy plants. The two most common fertilisers which are used to provide plant nutrients (or minerals) in the fields are NPK and Urea. NPK fertiliser provides Nitrogen (N), Phosphorus (P) and Potassium (K) to the soil in the fields whereas urea provides only nitrogen.
Out of all the plant nutrients (or minerals), the crop plants grown in the fields require nitrogen in maximum amount to make the proteins. The crop plants take this nitrogen from the soil in the fields. So, after the harvest, the soil in the fields becomes deficient in nitrogen. This nitrogen deficient soil is incapable of growing another good crop. Now, one way to make this soil fertile again is to add nitrogen containing fertilisers. Another way is to grow leguminous crops in this soil. How the growing of leguminous crops enriches the soil with nitrogen will become clear from the following discussion.
2. Nitrogen Can be Replenished in the Soil by Growing Leguminous Crops
Though a lot of nitrogen gas is present in the air but the plants cannot use nitrogen in gaseous form. The plants need nitrogen in the form of water soluble compounds (such as nitrates). The plants such as gram (chana), peas, pulses (moong, etc.) and beans are called leguminous plants (or legumes). The leguminous plants have root nodules in them which contain Rhizobium bacteria (see Figure).
Rhizobium bacteria can convert nitrogen gas of air into nitrogen compounds (like nitrates). So, when a leguminous crop is grown in a field, the Rhizobium bacteria present in the root nodules of leguminous plants convert nitrogen gas of air into nitrogen compounds (like nitrates). Some of these nitrogen compounds are used by the leguminous plants for their own growth. The remaining nitrogen compounds made by Rhizobium bacteria mix with the soil in the field and enrich it. Thus, the soil in the fields gets enriched with nitrogen compounds in the natural way. The growing of leguminuous crops in the fields is of great importance to the farmers. This is because the farmers do not need to put nitrogen fertiliser in the fields in which leguminous crops have been grown earlier. This saves a lot of money.