MANAGING PHOSPHORUS IN MANURE:
CAUSES, CONCERNS AND BMPS

T. C. Daniel and P. A. Moore, Jr.
College of Agricultural, Food and Life Sciences
Department of Agronomy
University of Arkansas
Fayetteville, Ar 72701

BACKGROUND

By design confined animal feeding operations are very efficient in producing protein and manure and can account for a major source of some states' income. For example, in 1995 the sale of poultry products in Arkansas, Georgia, Maryland, and Alabama accounted for 47, 42, 54, and 59%, respectively, of the total cash receipts from agricultural products. Arkansas is the leading broiler producer in the US, followed by Georgia and Alabama. Over a billion broilers were produced in Arkansas in 1995 along with approximately a million tons of broiler litter.

Most of the broiler litter produced in Arkansas and other states is land applied, serving as an excellent source of fertilizer for permanent pastures such as fescue. Our studies have shown that when applied at the proper rate (2.5 tons/ac), transport of manure nitrogen (nitrate) through the soil profile is minimized. Phosphorus in the runoff, however, poses a potential problem because most of it is in the soluble form and readily available for growth of aquatic weeds and algae. Further, Arkansas research has shown that this source of runoff phosphorus can be reduced by 85% through the addition of manure amendments such as alum (aluminum sulfate) to the litter. The benefits of alum use will be presented at this conference by P. A. Moore, Jr., et al.

Today's confined animal operations are quite a departure from the diversified family-farm enterprises of only a few decades ago. More animals (and manure) are economically produced on less land, leading to surpluses in on-farm nutrients due to their import in off-farm feed. This is especially true for phosphorus. For example, manure is applied at rates to meet the nitrogen needs of the crop and, unfortunately, crop removal of phosphorus is relatively small. In Arkansas, broiler litter is approximately 3.5% nitrogen and 1.8% phosphorus and is usually applied to fescue at a rate of 2.5 tons/ac, resulting in the addition of 175 and 90 lbs/ac of nitrogen and phosphorus, respectively. Allowing for some volatilization, sufficient nitrogen is available in this application to produce about 3 tons/ac of fescue forage; however, about three times too much phosphorus is added.

ELEVATED SOIL TEST PHOSPHORUS LEVELS

Long-term use of such practices has elevated the soil phosphorus levels in many states well beyond the range considered optimum for crop growth. Because of this, many state soil testing laboratories now report that most of the soils tested contain phosphorus levels in the high or above range. This is especially true in those states where confined animal operations are popular. Because phosphorus is the nutrient limiting plant growth in most of the nation's surface waters and because a direct relationship exists between the level of phosphorus in the soil and that contained in the runoff water, several states have attempted to establish cut-off or threshold levels of phosphorus that will ensure crop production but will not result in eutrophic runoff. Generally, one value is selected and applied uniformly to all soils of the state. For example, the threshold level for soil test phosphorus in Texas is 200 ppm or 400 lbs/ac (Bray 1 or Texas A&M extractant). Once that level is reached phosphorus additions cannot exceed that removed in the crop. Other states use a phased approach, whereby as the level of phosphorus increases in the soil due to phosphorus additions, recommendations for limiting phosphorus input become more conservative.

Using a soil test level that produces a concentration of 1 ppm phosphorus in the runoff water, we determined the threshold level of soil test phosphorus for five soils widely used for manure disposal in Arkansas. As expected, threshold values varied with the soil series. On some soils, the threshold value of soil test phosphorus was twice as high as the other soils to produce the 1 ppm concentration of phosphorus in the runoff.

BEST MANAGEMENT PRACTICES

Traditional practices such as proper application rate and buffer strips are proven methods for minimizing phosphorus in runoff water. More innovative approaches such as alum addition to manure are being evaluated in some places and adopted in others. A very new and promising approach to phosphorus management in the manure involves manipulating the phosphorus content and availability in the feed. The phytase enzyme, when added to the feed, increases the availability of the organic phosphorus in the feed, thus reducing the phosphorus content in the manure. Another exciting approach being pursued by a major company is the development of a corn variety low in phytic acid. Altering the phytic acid content genetically or adding phytase to the feed, alone or in combination, has the potential to improve feed efficiency for monogastric animals and reduce phosphorus loss to the environment.

In watersheds where the aquatic resource is known to be limiting in phosphorus, resource managers must begin to develop manure application strategies based on phosphorus as well as nitrogen. When the level of soil phosphorus is relatively low, application rates should be based on nitrogen, as the soil phosphorus approaches the threshold level, application strategies for limiting phosphorus inputs should be phased in. Field research has shown that threshold values for individual soils can and should be developed. This technology, however, needs to be tailored for broader use and acceptance by state resource management agencies.

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