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U.S. Department of the lnterior U.S. Geological Survey

Controlled-Release Using

Introduction Controlled-release nitrogen The smaller the size the quicker the release. The relationship is shown by The ultimate fertilizer should be the following equation: wt. percent inexpensive and long lasting, should Urea is one of the most common N = 20 + 40 (log mean particle size in tend to increase soil fertility through nitrogen fertilizers. It is very soluble mm). repeated use, and should release in water, and can be leached through nutrients to plants as they are needed, the root zone. In addition, urea is Controlled-release phosphorous thereby eliminating pollution and converted into ammonium by an fertilizers inefficiencies. Whether or not such enzyme found in most soils. Soil systems will be realized will depend bacteria then convert these ammonium (H2PO4-) can be released to on agricultural research into ions into readily leachable nitrate ions. plants from phosphate rock (P-rock) technologies that are fine-tuned to Using zeolitic rocks in fertilizer can composed largely of the calcium ecologically sustainable methods. help prevent these nutrient losses. phosphate by mixing U.S. Geological Survey (USGS) the rock with having an preliminary experiments indicate that A controlled-release nitrogen (N) exchange such as ammonium. The the use of zeolites in agriculture is a fertilizer can be produced by heating approximate reaction in soil solution 0 promising avenue for such research. zeolite rock chips to about 400 C to is as follows: (P-rock) + (NH4-zeolite) drive out all zeolitic and pore water, = (Ca-zeolite) + (NH4+) +(H2PO4-). Background which is replaced with molten urea. The urea crystallizes at about 1320C. The zeolite takes up Ca2+ from the The use of soluble fertilizers can lead The rate of nitrogen release from the phosphate rock, thereby releasing both to water pollution and to wasted zeolitic rock is slowed in three ways: phosphate and ammonium ions. nutrients. Nitrogen, for example, can (1) by containing urea in the rock Unlike the leaching of very soluble leach into ground and surface waters, pores and zeolite crystals, thus phosphate fertilizers (for example, especially in sandy soils, and preventing the leaching of urea from superphosphate), the controlled- phosphate may become fixed and the root zone; (2) by slowing the release phosphate is released as a unavailable to plants, especially in conversion of urea by soil enzymes, result of a specific chemical reaction tropical soils. Zeolites, porous thus delaying the formation of in the soil. As phosphate is taken up with high cation-exchange ammonium ions; and (3) by taking up by plants or by soil fixation, the capacity, can help control the release ammonium ions onto exchange sites chemical reaction releases more of plant nutrients in agricultural in the zeolite, thus protecting them phosphate and ammonium in the systems. Zeolites also can free soluble from nitrifying bacteria. Potassium- attempt to reestablish equilibrium. nutrients already in soil for use by saturated zeolite prepared by the The rate of phosphate release is plants, and may improve soil fertility above method contained approx- controlled by varying the ratio of and water retention. Because zeolites imately 17 wt. percent elemental N. P-rock to zeolite. Phosphorus is also are common, they could be useful on The rate of N release can be controlled released from the rock by the lowering a large scale in agriculture. USGS by changing the size of the rock chips. of soil pH as ammonium ions are research uses zeolites in several ways converted to nitrate. to control the release of nutrients in soil. Controlled-release fertilizers were and K-, Ca-, and NH4-poor zeolites Information tested in greenhouse pot experiments can scavenge these ions from soil with sorghum-sudangrass using NH4- solutions and thereby limit plant For further information, contact: saturated zeolite (clinoptilolite) and P- growth when used in soils that am rock with a phosphate application rate deficient in these nutrients. These Dennis D. Eberl of 340 mg P per kg soil, and zeolite/P- negative results emphasize the need U.S. Geological Survey rock ratios ranging from 0 to 6. Total to use appropriate zeolites during 3215 Marine Street phosphate uptake and phosphate agricultural experimentation. Boulder, Colorado 80303 concentration measured for the grass 303-54 l-3028; Fax 303-447-2505 were related linearly to the zeolite/P- Selected references E-mail: [email protected] rock ratio, and yields summed over four cuttings were as much as four Barbarick, K.A., Eberl, D.D., and Lai, times larger than control experiments. T.M., 1988, Response of sorghum- sudangrass in soils amended with Release of trace nutrients phosphate rock and NH4exchanged zeolite (clinoptilolite): Technical Experiments indicate that zeolite in Bulletin TB88-1, Department of soil can aid in the release of some Agronomy, Colorado State University, trace nutrients and in their uptake by 62 p. plants. The release of phosphorus, potassium (K), (Mn), zinc -1991, Pine Ridge zeolite and (Zn), iron (Fe), and copper (Cu) was Fort Hall mill shale P effects on enhanced by the presence of zeolite in sorghum-sudangrass: Technical a neutral soil. The concentration of Cu Bulletin TB9l-2, Department of and Mn in sudangrass (in mg/kg) was Agronomy, Colorado State University, significantly related to the zeolite/P- 47 p. rock (x) in experimental systems that used two different NH4 saturated Barbarick, K.A., Lai,T.M., and clinoptilolites, two different soils, and Eberl, D.D., 1990, Exchange fertilizer two different forms of P-rock. For (phosphate rock plus ammonium- example, the relation for one system zeolite) effects on sorghum- was: Cu = 2.78 + 0.83x -0.06x2 (R2 = sudangrass: Soil Science Society of 0.94), and Mn = 40.03 + 18.49x + America Journal, v. 54, p. 911-916. 0 . 76x2 (R2 = 0.99). The mechanism for this effect may be similar to that Lai, T.M. and Eberl, D.D., 1986, discussed above: sparingly soluble Controlled and renewable release of minerals are dissolved as an effect of phosphorus in soils from mixtures of preferential exchange on the zeolite. phosphate rock and NH4exchanged Trace nutrients on the zeolite clinoptilolite: Zeolites, v. 6, p. 129- exchange sites are then freed for 132. uptake by plants. Pond, W.G. and Murnpton, EA., Potential harmful effects eds. 1984, Zeo-Agriculture: Use of natural zeolites in Agriculture and Zeolites can be harmful as well as Aquaculture: Westview Press, Boulder, helpful to plant growth. For example, Colorado, 296 p. zeolites with sodium as the chief exchange ion can be toxic to plants,

November 1993