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

Abstract

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.