Fall season is when Iowa corn and soybean plants mature and die, and harvest begins.
You may have noticed corn and soybean fields getting green; what you see are cover crops farmers plant after harvest. Why would a farmer do this? The answer is tied to what good Iowa soil does: The organic matter in soil mineralizes nitrogen when it warms in the spring, and the crop in the field uses this nitrogen to produce yield. Although corn and beans have died and have no use for nitrogen, the soil still is warm and producing nitrogen. So the cover crop planted in the fall will use the nitrogen in the soil and keep nitrogen in the plant so fall rains will not leach it into the ground water.
Likewise, in the spring, as the soil makes nitrogen before the corn and soybeans are planted, a cover crop starts growing early in the spring and can use the nitrogen. In the spring at planting time the cover crop is killed, now the nitrogen in the dead cover crop will be released as the plant decomposes and can be used by the corn and beans.