SOIL ACIDIFICATION IN URINE-AFFECTED AND UREA-AFFECTED SOIL

Return of sheep urine (2.2 g N L-1) and urea solutions (0, 6, 9 and 12 g N L-1) to a red podzolic soil in pot experiments resulted in acidification following nitrification. The pH decrease in 0 to 10 an soil ranged from 0.2 to 0.8 units with the decrease being linearly related (r2 = 0.96, P < 0.01) to the NO3- produced which generally increased with N return. Acid subsurface layers developed in all urine/urea treatments within 6 to 10 weeks following urea-N return. During subsequent plant growth, between 0 and 78% of acid released from urine/urea solution treatments was neutralized. Neutralization was greatest at low rates of N return and when NO3- was not removed from the soil by leaching before plant uptake. The trend in the relationship between N uptake by plants and acid neutralization was not significant. As a result of plant growth in control pots, the surface soil pH decreased, while in the subsurface layers the pH increased. Based on the extent of acidification observed here, urinary return to grazed pastures could lead to a pH decrease in the surface 10 cm of soil of up to 0.5 units over a 10 year period.