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When crops are harvested, nitrogen is removed from the soil (in the protein of the plant products). Adding nitrate fertilisers, manure or compost to the soil can replace soil nitrogen. This makes the soil fertile again, allows more plant growth to occur and increases the yield of the next crop.

Natural fertilisers

Manure and compost are examples of natural fertilisers. They release nitrates into the soil slowly, because they depend on the action of soil bacteria to break them down. The quantity of nitrate contained in natural fertilisers is relatively low, so large volumes of them may have to be added to the soil.