Revegetation on disturbed, low organic matter content, decomposed granite (DG) substrates are limited by low plant-available moisture and nitrogen. Data from a single DG site in northern California, USA, showed that a significant fraction of the ammonium from fertilizers or organic matter mineralization was fixed into silicate interlayer positions. To evaluate the broader relevance of NH4+ fixation, the NH4+ fixation capacities of 11 other drastically disturbed DG substrates throughout California were evaluated. The fixation capacities of the substrates were quite varied and increased as added NH4+ application levels increased (124–1,670 kg NH4+ ha−1). When amended with 124 kg NH4+ ha−1, 7 of the 11 substrates fixed between 14 and 78% of the added NH4+. Analysis of particle size fractions of a typical material indicated that the very fine sand fraction had the highest fixation capacity and the clays and very coarse sands had the lowest, on a gravimetric basis. The overall fixation capacities showed no significant relation to potential predictive characteristics, including extractable K+, NH4+, or total N levels. Three methods of cation exchange capacity (CEC) measurement were tested for their ability to predict NH4+ fixation. The Ba method which utilizes an indicator cation that is not subject to interlayer fixation was not a reliable indicator of NH4+ fixation. The NH4+ method had the strongest relation to NH4+ fixation in the DG materials. The difference between the measured CEC of the NH4+ method and the Ba method was found to be most predictive of NH4+ fixation.
Keywords: Ammonium fixation - Decomposed granite - Nitrogen availability - Cation exchange complex