NITROGEN-LOADING CAPACITIES OF SOME CLINOPTILOLITE-RICH ROCKS

Clinoptilolite-rich rocks (CRRs) from eight deposits in the western U.S. and one deposit in British Columbia were tested for their nitrogen-holding capacity using two commerical fertilizers. One fertilizer was a mixture of ammonium nitrate plus 10-34-0 (N-P-K) with a total of 18.5 weight percent N and 3.4 weight percent P2O5 ; the other solution was 57.1 weight percent ammonium nitrate. For the ammonium nitrate plus 10-34-0 treated CRRs, N capacities ranged from 1.8-3.9 weight percent nitrogen. For the CRRs exposed to the 57.1 weight percent ammonium nitrate solutions with 20 weight percent N, the CRRs held 4.7-8.4 weight percent nitrogen. Some of the N in these CRRs is held as ion-exchanged NH4 + ; the remainder is held as water-soluble N salts in the CRR pore space. The CRRs tested were high Ca + K ranging from 3.7 to 5.7 weight percent and with less than 2.5 weight percent Na. Sized 2.0-4.7 mm granules of CRRs were used. This study demonstrated CRR's N capacity for their potential uses in many agricultural applications. When used as a slow N-release fertilizer, non-point source surface and groundwater N pollution resulting from highly soluble synthetic fertilizers now used can be avoided. In present practices, the use of highly soluble synthetic N-fertilizers in sandy soils results in N losses as high as 35 to 50 percent; this results in serious nitrate groundwater pollution in many parts of the U. S. The use of an N-amended CRR for fertilizer would reduce the N pollution significantly and would also enhance the cation exchange capacity of the soil. INTRODUCTION The objective of this study was to examine the properties of nitrogen-and phosphorousamended clinoptilolite-rich rocks (CRRs) for agricultural (fertilizer) applications. This was done to develop a slow N-release fertilizer to mitigate non-point source N pollution of ground and surface water caused by widely-used, highly-soluble, synthetic fertilizer (Puckett, 1994). The use of CRRs as an NH4 + -exchanged medium to slowly release N for plant availability in the soil was first suggested almost two decades ago by Mumpton and Fishman (1977). The initial concept was that NH4 + exchanged into clinoptilolite would be protected from bacterial nitrification, but would be available for plants when exchanged out of clinoptilolite. In these earlier studies an emphasis was placed on the NH4 + -ion exchange capacities of both synthetic and natural clinoptilolites. For later studies by Loizidou and Townsend (1987), a maximum of about 2.7 weight percent of N (as NH4 +) was exchanged into a clinoptilolite of a CRR. Recent studies using granular (2.0-4.7 mm) CRRs exposed to diluted ammonium polyphosphate solutions have 37-88 weight percent N as exchanged NH4 + ; the remainder of the N is in pore space of CRRs as soluble N salts. Exposure of CRRs to 57 weight percent ammonium nitrate solutions made N-amended CRRs with as much as 8 weight percent total N, but only 25 percent of the total N was exchanged as NH4 + (Desborough, 1996). While the nonexchanged N in CRRs is water soluble, it is temporarily sequestered (in pore space of CRRs) and slowly released because of diffusion-rate limitations that are largely controlled by the CRR particle size. Presumably, the exchanged NH4 + in CRRs would dissolve more slowly than the water-soluble, or non-exchanged N salts which are precipitated in the pore space. However, this may only be true in the absence of diffusion-rate considerations, i.e., in very small CRR particles. It is assumed that larger N-amended CRR granules will release soluble, or nonexchanged N, more slowly than smaller particles. Because of the diffusion-rate aspect related to CRR particle size, a range of N-amended CRR granule size (2-4.7 mm) was used for the laboratory studies done here. The CRRs used for this study have relatively high concentrations of K plus Ca (e.g., 3.75.7 weight percent). CRRs with more that 2.5 weight percent Na were not used because high concentrations of exchangable Na are deleterious to plants of agronomic interest for human and animal nutrition (Nishita and others, 1968; Nishita and Haug, 1972). CRRs from deposits in eight western U.S. states and one deposit in British Columbia were chemically analyzed to assure that they could be used without adverse affects in the soil. One CRR from Oregon was determined to have potentially adverse acid-generating reactions in a prior laboratory study (Desborough, 1996) and was not used in the present study. LABORATORY METHODS All CRR samples were crushed and sized. The 10-4 mesh (2.0-4.7 mm) fraction was used for the nitrogen-capacity studies; a 20-50 g sample of this sized material was pulverized to powder (95 % < 0.1 mm) for X-ray diffraction and non-destructive chemical analysis. Mineralogy of the CRRs was determined by X-ray diffraction (table 1). Chemical composition of the CRRs given in Table 2 were obtained by X-ray fluorescence analysis using radioisotope excitation sources (55Fe, 109Cd, and 243Am) with a Li-drifted silicon detector and the computer program of Yager and Quick (1992). Because clinoptilolite weight varies with relative humidity, all CRR samples were dried at 60°C before weighing. Each group of experiments was done side-by-side, or simultaneously except for chemical analysis and pH measurements which were done serially. The pH measurements were done with a digital temperature-corrected meter. Table 1. Locality data, abbreviations, and diluent minerals in clinoptilolite-rich rocks used in the present study. Minerals determined by X-ray diffraction by G.A. Desborough and Richard Sheppard (U.S. Geological Survey). [Minerals listed in decreasing order of abundance; "clay" refers to layer silicates with a 10-angstrom spacing; tr. = trace] Locality and abbreviationDiluent minerals British Columbia Princeton-PBC Opal, feldspar, mordenite, quartz (tr.) Colorado Creede-CCO Opal, K-feldspar, plagioclase, quartz, clay Idaho Crisman Hill-CHI Opal, K-feldspar Preston-PMLI Opal, plagioclase PMUI Opal POLI Opal (tr.) POMI Opal PW1I Opal PW2LI Opal, clay PW2UI Opal PW3I Opal Montana Grasshopper Creek-GCM Opal, K-feldspar, plagioclase Nevada Pish Creek-PCWN K-feldspar, quartz, plagioclase New Mexico Winston-SNM Opal, clay (tr.) South Dakota Rocky Ford-SDK plagioclase, calcite, quartz, opal, K-feldspar, clay (tr.) Texas Tilden-ZBT Opal, clay, calcite Table 2. Chemical composition of selected elements in clinoptilolite-rich rocks (CRRs) determined by X-ray fluorescence analysis. (Analyst: G.A. Desborough) CRRK CaFi Cu Zn Rb SrZr~¥Nb Ce Nd La BaPb Weight percent Parts per million