Abstract: Acidic soils are widely distributed in South China,and their acidity is the major environmental stress factor limiting the growth of most crops.It is well known that soil Al solubilized at low pH is a main toxic factor for plant growth.Our study with three acidic soils showed that soil pH increased quickly,while soil exchangeable Al decreased sharply with the increasing concentrations of applied urea.The time-course experiment revealed that the increase of soil pH was short-lived,with a subsequently slow drop after reached its maximum. Urea fertilization caused a drastic change of soil pH during 2～4 weeks of the experimental period.There was a negative relationship between soil pH and soil exchangeable Al.Biological toxicity test demonstrated that applying urea to acidic soils could obviously decrease the aluminum toxicity of maize in a short-term period.

Key words: Rice, Evapotranspiration, Water balance, Soil water