Building Soils for Better Crops, Third Edition

Summary and Sources



There is true nutrient cycling on most farms as crop residues or manures produced by animals fed crops grown on the farm are returned to the soil. However, there are potentially large flows of nutrients onto and off of farms, and we are concerned about cases where the flows are unbalanced. The inflow occurs as commercial and organic fertilizers and amendments as well as animal feeds are imported onto the farm and in manures and composts brought from off the farm. Exports are mainly in the form of crops and animal products. In general, larger amounts of nutrients are exported off the farm in vegetation (grains, forages, vegetables, etc.) than in animal products. This happens because a high percent of the nutrients in the feeds pass through the animal and are available as manure. And relatively few nutrients are exported per acre in the form of milk, meat, wool, etc., compared to the amount exported from crop farms. Nutrient flows are of such great concern because as nutrient levels decline, the soil rapidly degrades. On the other hand, when nutrients build up on the farm, they tend to be more readily lost to the environment. Even midwestern U.S. cash grain farms that have balanced nutrient imports and exports lose nutrients. Nitrogen leaching losses from these farms are having negative environmental effects on the Mississippi River and Gulf of Mexico ecosystems.


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