Soil acidification caused by four commercial nitrogen fertilizer solutions and subsequent soil pH rebound

Soil acidification resulting from nitrification of ammonium- and urea-containing nitrogen (N) fertilizers is a management problem in tree fruit orchards, particularly where fertigation through drip irrigation systems is practiced. Anecdotal field evidence suggested that fertilizer-induced acidification may be transitory because soil pH will increase, or "rebound," during the following winter. We conducted a laboratory experiment to determine the influence of N fertilizer solution source [calcium nitrate (CN), calcium-ammonium nitrate (CAN), ammonium nitrate (AN), and urea-ammonium nitrate (UAN)] and rate (0, 100, 300, and 500 mg total N L-1 in fertigation solution) on the pH of five Washington orchard soils, using a procedure to simulate two fertigation events followed by a water-only leaching event. Soil pH was measured five weeks after each event. The relative ranking of the acidification potential of the four fertilizers was consistent with their published values: CN<CAN<AN=UAN. Although CN is usually stated as having a net basic soil reaction, the CN product had no or a slightly acidifying effect, consistent with the presence of the small amount of NH4 in the commercial formulation. At each simulated management stage, soil pH tended to decrease with increasing N rate for each N source. Leaching the fertilizer-acidified soils caused pH to increase, with one-third of all the plus-N treatments returning to the zero-N treatment pH. The results of the current experiment suggest a possible mechanism explaining the field observations of soil pH rebound. Furthermore, the substantial changes in soil pH induced by N fertilization or leaching suggest that the timing of soil pH assessment should be determined by its temporal relation to specific farming practices.