Nitrogen retention and movement in a variable charge
Arcelay, Carmen Lis
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A study was conducted to evaluate the effect of lime application and frequency of N fertigation on NH4 + and NO3 - concentrations on Coto clay. Two field experiments were established at Isabela, PR, one in March, 2002, and another in February, 2003. No significant differences were observed between lime treatments or between fertigation treatments in the field. Despite the presence of high point of zero charge iron oxides, other low point of zero charge minerals, along with organic matter content, resulted in a low point of zero salt effect in the samples. The occurrence of phosphate and sulfate may have influenced the lack of nitrate retention capacity for this soil. Results coincide with the batch studies that indicate nitrate repulsion occurs in this soil. Negative adsorption isotherms were constructed under three pH levels and nine NO3 - solution concentrations. These show that anion repulsion causes negative adsorption and that NO3 - may move faster than water in Coto clay. As part of the field experiment, two methods were used to estimate nitrogen leaching. A percolation analysis suggested losses by leaching in the order of 26% of the total N applied as fertilizer the first year, and 15% the second year. A nitrogen balance approach doubled the estimated N losses from the percolation method. Since the percolation did not account for nitrogen leached under unsaturated flow conditions, and the N balance method did not account for N uptake by plant roots or fruit, the actual N leached was probably between the two estimates. Considering the high permeability of Coto clay, potential nitrate leaching must be considered when establishing a management plan for this soil.