As crops grow, they remove nutrients from the soil; however, some crops return certain nutrients over the seasons and between species. Unlike a natural ecosystem, in which there is a balanced exchange of nutrients between old growth and new growth, a crop system is totally dependent on the farmer's techniques. Farmers have three alternatives for keeping their fields productive. They can add natural or chemical fertilizers; they can let the field remain fallow, or uncropped, for one or more growing seasons; or they can rotate their crops on a regular basis.
Crop rotation requires a knowledge of how different crops interact with the soil. One of the most fundamental and earliest known facets of crop rotation was the use of legumes, which include peas, beans, and lentils. Legumes, as well as other crops like oats, return vital nitrogen to the soil. The Greeks and Romans rotated crops on a three-field system as early as 200 b.c., but knowledge of that practice, like so much other technical expertise, was lost with the decline of Classical civilization. Europe's rediscovery of crop rotation was in the form of the four-course Norfolk system in which four fields were alternately planted in wheat or rye, root crops, legumes, and nitrogen-rich clover. This system provided a balanced nutrient exchange.
It also provided the people with a more diverse diet. Farm animals were allowed to graze in the clover, further enriching the soil with their droppings. The Norfolk system was responsible in part for the use of horses as draft animals in Europe and provided horses with oats. An improved horse collar and the iron-edged moldboard plow, gave Europe the means to feed its growing population and assert its influence over the rest of the world.
It was not until the 1880s that Herman Hellriegel (1831-1895), a German chemist, learned that legumes extracted nitrogen from the atmosphere and introduced it into the soil. Prior to Hellriegel's discovery, farmers were simply aware that rotating legumes with other crops increased production and diminished the level of plant diseases. During the mid-twentieth century, chemical fertilizers became more readily available, and farmers, intent on mass production and increased profits, shifted from crop rotation to single-crop production. Wheat, corn, and soybeans became the major cash crops. Over time, constant application of fertilizers, both natural and man-made, along with pesticides and herbicides, led to pollution of the soil and streams. Intensive agriculture also depletes the water table and damages the soil structure. More recently, although agribusiness still utilizes fertilizers, there has been a return to crop rotation and other natural farming techniques in conjunction with fertilizer usage.
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