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

In almond orchards which are fertilized and irrigated with drip system s, 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 site s. The effects of high NH4 concentration on nitrification, soil acidif ication, and exchangeable cation dynamics were studied in an incubatio n 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 20 00 mg N/kg treatment, nitrification activity nearly ceased and soil ac idification was reduced. Although nitrification activity was lower in the 500 mg N/kg treatment than in the control, after 10 days of incuba tion, nitrification activity lowered soil pH by 0.7 units. After the i nitial 10 days, soil pH was lowered to 4.8 and nitrification activity was depressed. Ammonium ions occupied about 20 and 36 % of the exchang e capacity in the 500 and 2000 mg N/kg treatments respectively, and ex changeable Ca, Mg, and K were significantly lowered. Extractable acidi ty was less than 0.1 cmol(c)/kg dry soil.