The high cost of phosphate, combined with recent environmental threats to the Chesapeake Bay and other environmentally sensitive areas of the Southeast, have put growers in a potentially tight situation for fertilizer.
Maryland recently enacted a bill to reduce the use of high phosphate fertilizers. The state's Chesapeake Bay Phosphorus Reduction Act of 2009 will ban the sale and use of fertilizer for home and lawn use with high levels of phosphoric acid throughout Maryland, except in special situations — agriculture being one such situation. The ban takes effect April 1, 2011.
Similar legislation is either pending or has been enacted in several other states. Though agriculture, via the Virginia Grain Producers Association and other farm groups, has worked closely with the Chesapeake Bay Commission, the message for farmers in the upper Southeast is clear — find a better way to apply phosphorous to crops.
The most common phosphate fertilizers are triple superphosphate (0–46–0), monoammonium phosphate (11–52–0), diammonium phosphate (18–46–0), and ammonium polyphosphate (10–34–0) liquid. The good news is all these materials are highly water soluble, which is bad news from an environmental standpoint.
The ammonium phosphates also are excellent nitrogen sources. Depending on price and availability, these are often the product of choice for farmers in the Southeast.
Monoammonium phosphate and ammonium polyphosphate, either alone or with some added potassium, makes excellent starter fertilizers because of their high P-to-N ratios, high water solubility, and low free ammonia. Again, making these fertilizers highly desirable for Southeastern crops.
Fertilizers used by farmers are broadly classified as either organic (composed of enriched organic plant or animal matter) or inorganic (composed of synthetic chemicals and/or minerals).
Chicken litter has been one of the top sources of organic fertilizer for many years in the Southeast. Researchers are now looking at ways to extract similar levels of N-P-K fertilizer from other livestock sources.
USDA Researcher Phil Bauer, who works with the agency’s office at the PeeDee Agricultural Research and Education Center near Florence, S.C., recently discussed one such process for extracting low phosphorous fertilizer from swine and poultry waste.
“Using new extraction technology developed by USDA scientists, farmers can take swine effluent from waste lagoons and chicken litter right on the farm and pull out the phosphorous,” Bauer says.
The phosphorous extraction process includes nitrification of wastewater and increasing the pH of the nitrified wastewater by adding an alkaline earth metal-containing compound. The presence of infectious microorganisms is reduced in the useable effluent. The precipitated phosphorus is recovered as calcium phosphate that can be exported from the farm and reused as fertilizer.
The method was tested with wastewater collected from nine hog lagoons in North Carolina. In all cases, the soluble P was effectively recovered as P-precipitate. Hog lagoons can be retrofitted at a relatively small cost and use the technology to precision control of the N:P ratio of the treated effluent to desired levels to match specific crop needs or to solve problems of P accumulation in the soil or remediation of contaminated spray fields.
Using the extraction process, Bauer says the phosphorous becomes phosphorous rock, which can be taken out of either swine effluent or chicken litter and made into phosphorous fertilizer. The chicken litter that remains looks just like any other chicken litter used for fertilizer by farmers, but it has much lower phosphorous content.
“Typically, chicken litter has a one to one ratio of nitrogen to phosphorous, but using the extraction process, most of the nitrogen is retained and virtually none of the phosphorous remains. Down the road, reducing the phosphorous content of litter in the Southeast, and swine-based fertilizer in other parts of the country, will be a significant advantage in environmentally sensitive areas,” Bauer says.
From swine effluent, Bauer says P concentrations of 11 percent can be obtaianed, but no nitrogen. Basically, we are getting 0-25-0 fertilizer. It’s concentrated enough to pull phosphorous out and ship it to areas that do need phosphorous, the USDA researcher contends.
Fertilizer from swine wastewater was formulated in large (2-4 mm) and small .05-1.0 mm) particles to simulate commercially available fertilizer particle size. In preliminary tests on cotton at the Pee Dee Research Station, the large particle size material appears to be much less water soluble than the smaller particle sizes. This could be valuable as a slow release fertilizer, Bauer says.
The solubility of phosphorus in fertilizer varies. The legal definition of available phosphorus in fertilizer is the sum of the phosphorus that is soluble in water, plus that which is soluble in a citrate solution. Regardless of the actual chemical form of the phosphorus, the analyses of phosphorus fertilizers are given as phosphate (P2O5). The water solubility of this phosphorus can vary from zero to 100 percent.
Generally, higher water solubility equates to a more effective phosphorus source. This is especially important for short-season, fast-growing crops, typical of the Southeast. It is also valuable for areas where less-than-optimum rates of phosphorus are applied to soils testing low in phosphorus.
In greenhouse tests with ryegrass at the Pee Dee Research Station, compared to triple superphosphate (0-45-0), phosphorous extracted from broiler litter was just as good as broiler litter from which the phosphorous was not extracted. Neither litter source was as good as the more soluble triple superphosphate.
Once field tested for crop results and operational at the farm level, the extraction process could be a valuable tool for Southeastern farmers in their quest to grow higher yielding crops with less negative impact on the environment.
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