Controlled-Release Fertilizers Using Zeolites
The U.S. Geological Survey (USGS) has experimented with zeolites to help control the release of fertilizer nutrients in soil. The use of soluble fertilizers can lead to water pollution and to wasted nutrients. Nitrogen, for example, can leach into ground and surface waters, especially in sandy soils, and phosphate may become fixed and unavailable to plants, especially in tropical soils. Zeolites are porous minerals with high cation-exchange capacity that can help control the release of plant nutrients in agricultural systems. Zeolites can free soluble plant nutrients already in soil, and may improve soil fertility and water retention. Because zeolites are common, these unique minerals could be useful on a large-scale in agriculture.
USGS research has experimented with zeolites applied to several different fertilizers including controlled-release nitrogen, controlled-release phosphorous fertilizers, and in the release of trace nutrients.
Controlled-Release Nitrogen Fertilizer
Urea is one of the most common nitrogen fertilizers. It is very soluble in water, and can be leached through the root zone. In addition, urea is converted into ammonium ions by an enzyme found in most soils. Soil bacteria then convert these ammonium ions into readily leachable nitrate ions. Using zeolitic rocks in fertilizer can help prevent these nutrient losses.
A controlled-release nitrogen (N) fertilizer can be produced by heating zeolite rock chips to about 400oC to drive out all zeolite and pore water, which is replaced with molten urea. The urea crystallizes at about 132oC. The rate of nitrogen release from the zeolitic rock is slowed in three ways: (1) by containing urea in the rock pores and zeolite crystals, thus preventing the leaching of urea from the root zone; (2) by slowing the conversion of urea by soil enzymes, thus delaying the formation of ammonium ions; and (3) by taking up ammonium ions onto exchange sites in the zeolite, thus protecting them from nitrifying bacteria. Potassium-saturated zeolite prepared by the above method contains approximately 17 wt. percent elemental N. The rate of N release can be controlled by changing the size of the rock chips.
Controlled-Release Phosphorous Fertilizers
Phosphate (H2PO4) can be released to plants from phosphate rock (P-rock) composed largely of the calcium phosphate mineral apatite by mixing the rock with zeolite having an exchange ion such as ammonium. The approximate reaction in soil solution is as follows: (P-rock) + (NH4-zeolite) = (Ca-zeolite) + (NH4+) + (H2PO4-).
The zeolite takes up Ca2+ from the phosphate rock, thereby releasing both phosphate and ammonium ions. Unlike the leaching of very soluble phosphate fertilizers (for example, super-phosphate), the controlled-release phosphate is released as a result of a specific chemical reaction in the soil. As phosphate is taken up by plants or by soil fixation, thechemical reaction releases more phosphate and ammonium in the attempt to reestablish equilibrium. The rate of phosphate release is controlled by varying the ratio of P-rock to zeolite. Phosphorus is also released from the rock by the lowering of soil pH as ammonium ions are converted to nitrate.
Controlled-release fertilizers were tested in greenhouse pot experiments with sorghum-sudangrass using NH4-saturated zeolite (clinoptilolite) and P-rock with a phosphate application rate of 340 mg P per kg soil, and zeolite/P-rock ratios ranging from 0 to 6. Total phosphate uptake and phosphate concentration measured for the grass were related linearly to the zeolite/P-rock ratio, and yields summed over four cuttings were as much as four times larger than control experiments.
Release of Trace Nutrients
Experiments indicate that zeolite in soil can aid in the release of some trace nutrients and in their uptake by plants. The release of phosphorus, potassium (K), manganese (Mn), zinc (Zn), iron (Fe), and copper (Cu) was enhanced by the presence of zeolite in a neutral soil. The concentration of Cu and Mn in sudangrass (in mg/kg) was significantly related to the zeolite/P-rock (x) in experimental systems that used two different NH4-saturated clinoptilolites, two different soils, and two different forms of P-rock.
Potential Harmful Effects
Zeolites can be harmful as well as helpful to plant growth. For example, zeolites with sodium as the chief exchange ion can be toxic to plants, and K-, Ca-, and NH4-poor zeoIites can scavenge these ions from soil solutions and thereby limit plant growth when used in soils that are deficient in these nutrients. These negative results emphasize the need to use appropriate zeolites during agricultural experimentation.
The U.S. Department of the Interior does not plan to pursue a patent on the use of zeolites with controlled-release fertilizers.
For More Information
For additional information about this research, please contact:
Dennis D. Eberl, U.S. Geological Survey