The Iowa Environmental Protection Commission is concerned that adding manure to soybean crops causes an overflow of nitrogen to the fields and poses a water contamination risk. Some scientists say soybeans, unlike corn, don't need manure fertilizer because they can get nitrogen from the air and soil.
The state estimates that about half of livestock farmers in Iowa have indicated in their required manure-management plans that they might spread manure on soybean fields. The actual percentage at any time is probably 10 percent to 20 percent, state inspections show. Farm groups claim it could be as low as one percent.
The proposed rule would take effect over a three-year period, and would allow exceptions for times when heavy rains force changes in planting plans.