1. Plant and Soil Sci. Dep. Oklahoma State Univ. 368 Agricultural Hall Stillwater OK 74078
2. Soil Science Dep. Luiz de Queiroz College of Agric., Univ. of Sao Paulo Piracicaba SP Brazil 13418-900
3. Brazilian Agric. Res. Corp. Natl. Soybean Center 86001-970 Londrina PR Brazil