EFFECT OF HIGH AMMONIUM LEVELS ON NITRIFICATION, SOIL ACIDIFICATION, AND EXCHANGEABLE CATION DYNAMICS

In almond orchards which are fertilized and irrigated with drip systems, fertilizers are applied to a relatively small soil volumes several times during the growing season. Where NH4-based fertilizers are used, high NH4 levels are anticipated in soil solution and on exchange sites. The effects of high NH4 concentration on nitrification, soil acidification, and exchangeable cation dynamics were studied in an incubation experiment where 500 and 2000 mg N/kg soil were added as (NH4)2SO4. After incubation for 25 days with added (NH4)2SO4, nitrifying bacteria were lower than the populations at the start of experiment. In the 2000 mg N/kg treatment, nitrification activity nearly ceased and soil acidification was reduced. Although nitrification activity was lower in the 500 mg N/kg treatment than in the control, after 10 days of incubation, nitrification activity lowered soil pH by 0.7 units. After the initial 10 days, soil pH was lowered to 4.8 and nitrification activity was depressed. Ammonium ions occupied about 20 and 36 % of the exchange capacity in the 500 and 2000 mg N/kg treatments respectively, and exchangeable Ca, Mg, and K were significantly lowered. Extractable acidity was less than 0.1 cmol(c)/kg dry soil.