Received for publication 12 Oct. 1984. Paper no. 830, Dept. of Vegetable Crops, Cornell Univ. The authors gratefully acknowledge the technical support of Barbara A. Kempter. This paper was part of the program of the Center for Root-Soil Research. The cost of publishing this paper was defrayed in part by the payment of page charges. Under postal regulations, this paper therefore must be hereby marked advertisement solely to indicate this fact.