Comparing Aluminosilicates Adsorption Capacity of Nitrogen, Phosphorus and Potassium.

Aluminosilicates with negative charges provide an exchangeable site for positive cations such as K, Ca and Mg, and positively charged groups such as water and ammonia. Due the weak attraction absorbed cations can be easily replaced using the standard ion exchange techniques, making these natural minerals good ion exchangers and their use may improve the fertilizers use efficiency by controlling nutrient retention and release. A laboratory study was undertaken to determine the adsorption capacity of nitrogen, phosphorus and potassium by the three natural aluminosilicates. Four experiments were carried out in laboratory for nitrogen and phosphorus/potassium adsorption study in a totally randomized experimental design with three replications. Adequate period for adsorption study were determined with two experiments where 1 g of clinoptilolite was shacked in 50 mL solutions of 0.001 mol L^-1   (NH₄)₂SO₄ and 0.001 mol L^-1 K(H₂PO₄) for 0, 0.5, 1, 2, 4, 5, 8, 10 and 24 hours. Nitrogen adsorption were determined by shaking 1 g of clinoptilolite, zeolite and bentonite aluminosilicates at 5, 10, 20, 40, 60, 80 and 100 mg N L^-1 in 50 mL solution of (NH₄)₂SO₄ at water, 0.1 mol L^-1 NaCl and 0.1 mol L^-1 KCl, respectively, for a total of 30 min. Phosphorus and K adsorption were determined by shaking 1 g of clinoptilolite, stilbite and bentonite aluminosilicates at 0, 5, 10, 20, 40, 60, 80 and 100 mg L^-1 of P an K in 50 mL solution of K(H₂PO₄) for a total of 30 min. After centrifugation N and P concentrations in supernatants were measured by spectrophotometry with flow injection analysis (FIA), and K concentrations with flame photometry. Maximum adsorptions were obtained with 30 min of shacking indicating ion exchange phenomena for N and K. Clinoptilolite adsorbed almost 100% of N and K in solution, bentonite 50% and zeolite 25%. Results showed N and K adsorption order was clinoptilolite, bentonite and stilbite. There was no difference on P adsorption, which was lower than 10% of total P in solution for all aluminosilicates. These results also indicate that aluminosilicates minerals probably are able to improve the efficiency of N and K through the control of ions retention and, therefore, enhancing plants absorption ability.