If you don't rotate crops, the soil in that field will inevitably begin to lose the nutrients plants need to grow. You can avoid this by sowing crops that increase organic matter and nitrogen in the soil.
Some farmers, for instance, sow legumes to take advantage of their symbiotic relationship with Rhizobia bacteria found in the soil. Why? When coming into contact with legume roots, Rhizobia bacteria form nodules where they convert nitrogen into ammonia, which the plant uses to grow. Likewise, crucifer green manures, such as white mustard, heal the soil by releasing substances that suppress the Rhizoctonia solani fungus.
Alternating between crops with different root systems also offers benefits. Plants with longer roots can get nutrients from deeper layers of the soil than those with a shorter root system. When you alternate between crops like this, you keep the soil healthy.
And yet, even farmers that know the benefits of crop rotation still sometimes don't do it. Say that the price for corn is high for several years in a row. The temptation arises to sow the fields with it nonstop to cash in while prices are high. In the long run, however, this practice leads to problems. Soil fertility decreases and it becomes extremely difficult to grow any crop with high yields.
