Ammonium Fixation in Sub-grade Decomposed Granite Substrates

Abstract

Granitic materials represent a common erosive substrate in California and much of the western United States. When granitic rocks weather, they disintegrate into coarse textured, non-cohesive substrates, known generally as decomposed granite (DG). Because of low moisture and N availability, revegetation of these substrates for erosion control is difficult. If nitrate based fertilizers are used, they can be rapidly leached, while NH ⁺₄ fertilizers may be sequestered mineralogically by interlayer fixation. In this study, we focus on the occurrence of NH ⁺₄ fixation on a decomposed granitic substrate and show that the fixation capacities of these sandy saprolites are significant despite analyses indicating that the samples are predominantly quartz, have low clay contents and have low cation exchange capacities (CEC). At field loading rates equivalent to less than 300 kg N ha⁻¹, 36–42% of added NH ⁺₄ may become unavailable to plants due to interlayer collapse and fixation for an unknown period of time. Ammonium fixation did not vary significantly in relation to substrate weathering class in these samples. Oriented X-ray diffraction analysis revealed the presence of vermiculite in particle size fractions from clay to very coarse sands. While other studies have identified silt as the most active fraction, the relative fixation capacity of these granite saprolites was greatest in the fine and very fine sand fractions when considered on a gram for gram basis of each individual particle size. We found that the extractant cation also influenced the measured levels of NH ⁺₄ fixation in these granite saprolites. At loading rates of 0–150 kg NH ⁺₄ ha⁻¹, extraction with KCl resulted in apparent NH ⁺₄ fixation capacities that were twice as great as those found with NaCl extractions when tested at low NH ⁺₄ concentrations and close to 35% greater at higher NH ⁺₄ amendment loading. Estimation of available ammonium in the decomposed granite using conventional KCl extraction methods appears to cause fixation, rather than extraction of at least part of the substrate’s NH ⁺₄ content.