In order to maximize the effectiveness of manure used to fertilize fields while minimizing the potential to pollute water supplies, it is necessary that the manure be spread at the right time. In addition, it has to be mixed with, and not left on top of the soil. The nutrients from manure spread in the spring and summer are likely to be absorbed by growing plants, minimizing the potential for contamination of the water supply. Manure that is mixed with the soil is subject to less run-off, further limiting the potential for a polluted water supply.