Productivity of Acidified Grassland Caused by Acidic Nitrogen Fertilizer and Aluminum Tolerance of Grasses and Legumes
Effect of nitrogen fertilizer application on yield of orchardgrass and soil solution composition of the soil was studied for a Tenpoku acid brown forest soil during a period of 10 years from 1974 to 1985. Soil solution pH of the surface soil in the grassland repeatedly treated with acidic fertilizer as ammonium chloride or ammonium sulfate decreased from 6.5 to 4.5 during a 2-year period. This acidification was caused by the decrease of the amount of cations with which anions such as NO3- or Cl- are associated. With the decrease of pH, a large amount of Al was dissolved in the soil solution and phosphorus uptake by grasses was also retarded. Differences in acid tolerance among several grass species were estimated in the order of orchardgrass > timothy Kentucky bluegrass > red clover > perennial ryegrass redtop > alfalfa > white clover. Acid tolerance was correlated with root growth, phosphorus uptake, and aluminum translocation to the shoots. Growth decline of grasses in acid soil was caused by the decrease in phosphorus uptake due to the suppression of root elongation by aluminum. Effects of lime application on grass growth, soil solution composition, and microbial activity were analyzed. The critical pH of the surface soil for which lime was needed was found to depend on the aluminum tolerance of grass species as follows: orchardgrass, 5.0; timothy and Kentucky bluegrass, 5.1; perennial ryegrass, red clover, and redtop, 5.2; alfalfa and white clover, 5.4.