1. Soil and Water Sciences Div.; International Rice Research Inst. (IRRI); P.O. Box 3127, MCPO 1271 Makati City Philippines
2. Dep. of Soils; Punjab Agricultural Univ.; Ludhiana 141004 Punjab India
3. USDA-ARS; East Campus, Univ. of Nebraska Lincoln NE 68583
4. Texas A&M Univ., Texas Agricultural Experiment Station; RR 3, Box 219 Lubbock TX 79401