South Florida Beef - Forage Program

"Article of the Month"
January 2003


Judicious Use of Sewage Sludge for Pasture Production is a Win-Win Situation

Dr. Martin Adjei
UF/IFAS Range Cattle REC


Scientists from the University of Florida's Institute of Food and Agricultural Sciences have concluded that sewage sludge has the potential to boost forage production and mitigate growing scarcity of landfill areas in the less industrialized subtropical and tropical world.

Using organic wastes as fertilizer is not new. Before the 1940s, when synthetic nitrogen fertilizer became widely available, animal manure and human waste were the primary amendments for improving crop yields around the world. Crop fertilization with organic waste has received renewed interest as municipalities face increasing disposal problems. A survey by the US Environmental Protection Agency (USEPA) showed that the amount of sewage sludge generated in the USA increased from 8.5 metric tons in 1990 to over 12 metric tons per year in 2000 as a result of increased population and advanced sewage treatment processes. Prohibition of waste dumping in streams and oceans, diminishing landfill space, skyrocketing landfill costs, and concerns over air pollution from incineration of waste have contributed to a strong public interest in finding alternative, environmentally sound solutions to waste disposal.

In Florida, where the human population increases by 0.25 million new residents annually, about 70% of sewage sludge is land-applied, 13% is land-filled and 7% is incinerated. There is very little crop production information and environmental impact assessment to support such practices. A study partly funded by the Florida Department of Agriculture and Consumer Services was conducted from 1997 to 2000 at Ona to compare the effects of ordinary liquid sludge, lime-treated liquid and cake sludges, and regular ammonium nitrate fertilizer. Each was applied to supply 80 and 160 pounds of N per acre, yearly, to a bahiagrass crop. Yield, protein, mineral content, and digestibility of forage were studied and published in the November-December issue of Agronomy Journal. Forage uptake of heavy metals, and accumulation of heavy metals and nutrients in soil and groundwater were also evaluated.

UF/IFAS scientists Martin B. Adjei and Jack E. Rechcigl determined that forage production was similar for synthetic fertilizer and the liquid sludges, but 30% lower for the cake sludge due to the latter's lower nitrogen availability. In a very dry year, the water in liquid sludge enabled nutrients to reach the crop's rooting zone more efficiently than synthetic fertilizer. Because of a rapid N release, forage protein content was better with ammonium nitrate fertilizer only in early spring. After that, there were no differences in protein or digestibility between sludge and fertilizer. By contrast, phosphorus, calcium, iron, and copper contents in forage were better with sludge treatments

"We also observed negligible traces of heavy metals in the crop, groundwater, and soil, regardless of how forage was fertilized" Adjei said. "This was not surprising considering the fact that concentrations of metals in sludge we used were far below legal limits. This is partly because Florida has relatively little industrial base and partly because of successful efforts in preventing industrial sources of metals from contaminating sewage". Considering that only 50% of the 2.5 million acres of bahiagrass pastures are fertilized with synthetic nitrogen in Florida, yearly, lime-stabilized liquid sludge, has the potential to boost forage production because it is low in pathogens, inexpensive and environmentally safe. But it must be processed and applied according to USEPA guidelines and careful attention must be paid to over-application from defective meters. Also the P loading factor form sludge must be considered in ecologically sensitive areas such as the Okeechobee Basin.


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