Urease-aktiwiteite in gronde van die sentrale besproeiingsgebiede in Suid-Afrika

Urease activity in soils from the central irrigation areas in South Africa. The objective of this study was to determine the range of urease activity in certain irrigated soils and the soil parameters which affect it. The course of hydrolysis was monitored in eight soils from the central irrigation areas after the application of urea at the equivalent of i 00 kg N ha-1 to both the top- and subsoils. During incubation at 30�C the urea nitrogen was measured at regular time intervals. These data were used to calculate the urease activity of the different soils. The urease activity in the topsoil varied between 1.23 and 7.37 mg N kg-1 soil hour-1 and in the subsoils between 0.80 and 11.10 mg N kg-1 soil hour-1. All the urea applied to the topsoils and subsoils was hydrolysed after 10-49 h and after 8-76 h, respectively. During these periods some leaching of urea may have occurred owing to excessive rain and l or irrigation. During the hydrolysis of urea the pH values of some soils increased by more than one unit indicating that large amounts of NH3 may form which can either volatilize or be phytotoxic to seed and seedlings. The danger of ammonia volatilization and toxicity was greatest about 10-43 h after urea application. The urease activity of these soils was best correlated with clay content, but also correlated well with organic carbon and total nitrogen content and with cation exchange capacity.