Crops get their nutrients or food from the soil.
The bigger the crop, the more nutrients are removed. Well-structured soils that are rich in nutrients are said to be highly fertile. Given sufficient light, warmth and water, they will produce abundant crops. However, the nutrients removed by crops or feeding animals must be replaced or soil fertility will be run down. Nutrients can be replaced by returning to the soil, plant, animal or human waste, such as sewage sludge, or by adding mineral fertilisers.