1. Potash and Phosphate Inst. 2423 Rogers Key San Antonio TX 78258
2. Potash and Phosphate Inst. 655 Engineering Dr., Suite 110 Norcross GA 30092
3. Rothamsted Research Harpenden AL5 2JQ England
4. North Carolina State Univ. Dep. of Soil Sci. Box 7619 Raleigh NC 27695‐7619