AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Spring 1997, No. 8
In 1972, total annual phosphorus loading into Lake Erie was estimated at 27,000 tons. Sixty-two percent of this loading came from point sources, such as municipal sewage discharge. By 1985, controls had reduced the proportion from point sources to only 24 percent of the total loading. Control of non-point sources has been slower, but recent studies by Dr. David Baker of Heidelberg College in Ohio have shown substantial reductions in agricultural source loading as well, during the years between 1975 and 1990.
Increased use of conservation tillage and changes in phosphorus fertilizer use have both contributed to the decrease in agricultural source loading. Crop producers have reduced rates of application of phosphorus fertilizers and are more likely to apply the fertilizer in a band instead of broadcasting. Total use of fertilizer phosphorus in the Great Lakes basin declined by 35 percent between 1981 and 1991. Yet the current use of fertilizer is substantial, and essential to the high productivity of Great Lakes basin crops.
Zebra mussels invaded Lake Erie in the late 1980s. Before that time, Lake Erie had already shown remarkable improvements in response to the phosphorus loading abatement programs. The zebra mussels have apparently helped to clear the water and to re-establish rooted aquatic plants. The full impact of zebra mussels on nutrient cycling is not well understood. Consequently, the extent of phosphorus load reduction from agriculture that is needed to optimize water quality in Lake Erie is less certain now than twenty-five years ago. Successful control of phosphorus in Lake Erie may have resulted in phosphorus impoverishment in Lake Ontario, relative to the productivity it enjoyed when Lake Erie was suffering from too much phosphorus.
Although agricultural lands contribute to phosphorus loading, some of the best quality streams in Ohio are located in predominantly agricultural areas. Urbanization is increasingly being viewed as a more serious threat to stream biota than is agriculture.
Particulate forms of phosphorus, associated with soil sediment particles, have been the main concern in most abatement programs. Increasingly, with conservation tillage, where animal manures are applied, and in hay or pasture systems, soluble forms of phosphorus are of concern. Conservation tillage reduces the loss of particulate phosphorus but increases the loss of soluble phosphorus.
Over the past twenty-five years, agricultural productivity in the Great Lakes basin has not declined. Rather, it has improved, demonstrating that wise use of both commercial and manure sources of nutrients is compatible with desirable levels of water quality. The evidence on the large scale indicates that a healthy nutrient supply for growing plants is compatible with water quality.
The connection of the agricultural system with the Great Lakes ecology is a complex one, and much remains to be discovered. Yet, the improvement in the health of both systems over the past twenty-five years can be considered a success story.