Effects of urea applications on bulk and rhizosphere soil properties in drip irrigated almonds

Abstract

High levels of urea applications in a drip irrigated almond orchard reduced soil and rhizosphere pH in the wetted soil volume to values as low as 3.8 to 4.0. This low pH is coincident with increased extractable Al and lowered exchangeable Ca, Mg, and K. The magnitude and in some instances the direction of these changes in the rhizosphere varied between the high and low rates of urea application. In the high N treatment rhizosphere samples had less extractable acidity than the bulk soil. In part, this was because they were less acidic, but in cases where the pH was similar, rhizosphere acidity was still lower than in bulk soil. Rhizosphere samples from the surface 10 cm had about twice the level of organic carbon as the bulk soil. In this field experiment, replications of the high N treatment varied considerably in pH. A higher pH in one replication appears to result from low subsoil permeability and wetter soil conditions, which limited nitrification. High levels of extractable NH₄ and Mn associated with the wetter location are consistent with the suggestion of restricted nitrification.