WWW.WAS.ORG • WORLD AQUACULTURE • MARCH 2014 43 Fertilizers can be applied on underwater platforms (about 45 to 60 cm below water surface) to avoid contact with the soil (Lawrence 1954). Although only one or two platforms are needed in most ponds, this practice has not been adopted widely. Research on fluid fertilizers (Metzger and Boyd 1980) revealed that these products could greatly increase the effectiveness of pond fertilization because phosphorus could be dissolved completely in the water. Of course, fluid fertilizers have a specific gravity of about 1.4 and, if poured directly into the water, they sink to the bottom without mixing in the water. In the experiments, fluid fertilizer was diluted 1:10 with water and splashed over pond surfaces. A phosphorus rate of 4.5 kg P2O5/ha using liquid fertilizer gave similar fish production as achieved in ponds to which twice as much phosphorus was applied in granular fertilizer broadcast over pond surfaces. Fluid fertilizers with nutrient grades of 10-34-0 and 13-38-0 have become popular for sportfish pond fertilization. Fluid fertilizers are more expensive than granular ones. A less expensive way to assure the dissolution of fertilizer nutrients in pond water is to make a slurry of granular fertilizer in water (1:10 ratio) and apply the slurry over pond surfaces. This method is not much more time-consuming than using liquid fertilizers and it is equally effective (Boyd and Hollerman 1981). However, it is interesting that the use of liquid fertilizers has been widely adopted by pond owners while few have selected the less expensive option of pre-dissolving granular fertilizer. Another innovation in improving fertilizer solubility is to finely pulverize fertilizer compounds to make an “instantlysoluble” product that will dissolve before settling to the bottom when broadcast over pond surfaces. When used at equivalent P2O5 rates, an “instantly-soluble” product with a grade of 10-52-4 was as effective as fluid fertilizer with a grade of 10-34-0 (Rushton and Boyd 2000). A particularly effective “instantly-soluble” fertilizer (Tepe and Boyd 2001) with a grade of 8-24-15 was made from potassium phosphate and sodium nitrate, but it was quite expensive and the company that supported the research apparently did not try to market the product. In an experiment on controlled-release fertilizers, a coated, commercial product with a grade of 13-13-13 was applied to ponds once in the spring; it maintained a fairly good plankton bloom throughout the growing season and fish production was acceptable (Kastner and Boyd 1996). However, the high cost of controlledrelease fertilizers has prevented their adoption as pond fertilizers. Recent renovation of research ponds at Auburn University included removal of sediment. The newly-renovated pond bottoms had soil properties similar to the original bottoms (Yuvanatemiya and Boyd 2006). This allowed re-evaluation of pond fertilization rates. The optimum nitrogen and phosphorus application rate using pre-dissolved granular fertilizers in renovated ponds was 6 kg N and 3 kg P2O5/ha per application (Wudtisin and Boyd 2005, Boyd et al. 2008). Nitrogen (from ammonium nitrate) application rates of 8 kg/ha and above reduced sunfish production by inhibiting reproduction through ammonia toxicity (Boyd et al. 2008). Potassium application did not improve fish production in ponds fertilized with nitrogen and phosphorus (Viriyatum and Boyd 2011). The natural potassium concentration in pond waters of this study was around 2 mg/L and potassium fertilization might be beneficial in waters with lower potassium concentrations. In summary, the optimum fertilizer rate for new or previously unfertilized ponds is 6 kg N and 3 kg P2O5/ha per application. However, after a period of regular fertilization with nitrogen and phosphorus – probably about 5 years – the nitrogen fertilization rate can be reduced or halted entirely. Factors Affecting the Effectiveness of Pond Fertilization The findings discussed above are based on studies conducted in ponds at a specific location and can be applied with most confidence to ponds in areas with climatic, pedologic and water quality characteristics similar to those of the study area. Information on factors affecting liming and fertilization rates will be provided to assist in adjusting the findings reported above for different conditions. Low Alkalinity. The most common reason that ponds do not respond as expected to fertilizer application is acidic bottom soil and low alkalinity water. The total alkalinity of pond waters should be checked periodically. If alkalinity is below 40 or 50 mg/L, a bottom soil sample should be collected and sent to a soil testing laboratory to determine how much liming material to apply. Alternatively a pond can be treated with 1 to 2 t/ha of agricultural limestone, the alkalinity checked after 2 or 3 months, and more agricultural limestone applied if necessary. Ponds should be limed in the late fall or early winter so that the liming material has time to neutralize acidic bottom soil and increase alkalinity and hardness of the water before fertilization begins in the spring. Agricultural limestone initially removes phosphorus from the water, and when applied during the growing season, it may lessen the effectiveness of fertilization long enough to cause water to clear, allowing aquatic macrophytes to grow. There are some areas in the lowland, coastal areas of several southern states where acid-sulfate soils occur. These soils contain iron pyrite that oxidizes when exposed to the air, releasing sulfuric acid and causing very low pH. Such soils contain at least 0.75 percent total sulfur and total sulfur concentrations up to 5.5 percent have been reported (Boyd 1990). An acid-sulfate soil also can be identified by drying a small sample for several weeks and measuring pH in a 1:1 soil-water mixture. The pH will be below 4 and often as low as 1 or 2 in a sample of acid-sulfate soil. More than 25 t/ha of agricultural limestone is often needed to completely neutralize acidity in acid-sulfate soils. A better solution than applying such huge amounts of liming material is to cover areas of acid-sulfate soil with a layer of better quality soil during construction. If the problem soil can be prevented from having contact with air and oxygenated water, the iron pyrite in it will not oxidize. Moreover, if acid-sulfate soil on watersheds can be covered with non-acid-sulfate soil, the acidity resulting from oxidization of pyrite in the pond bottom usually can be controlled by normal liming. Turbidity from Suspended Soil Particles. Turbidity from suspended soil particles reduces light penetration into ponds and thereby lessens phytoplankton productivity. The only permanent solution to chronic turbidity problems is to control erosion on pond watersheds, and this usually requires re-vegetation of denuded areas. Organic materials such as barnyard manure, fresh-cut (CONTINUED ON PAGE 44)
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