SOYBEAN NODULATION AND NITROGEN-FIXATION ON SOIL AMENDED WITH PLANT RESIDUES

Residues from some tree species may contain allelopathic chemicals that have the potential to inhibit plant growth and symbiotic N-2-fixing microorganisms. Soybean [Glycine max (L.) Merr] was grown in pots to compare nodulation and N-2-fixation responses of the following soil amendments: control soil, leaf compost, red oak (Quercus rubra L.) leaves, sugar maple (Acer saccharztm Marsh) leaves, sycamore (Platanus occidentalis L.) leaves, black walnut (Juglans nigra L.) leaves, rye (Secale cereale L.) straw, and corn (Zea mays L.) stover. Freshly fallen leaves were collected from urban shade trees. Soil was amended with 20 g kg(-1) air-dried, ground plant materials. Nodulating and nonnodulating isolines of ''Clark'' soybean were grown to the R2 stage to determine N-2-fixation by the difference method. Although nodulation was not adversely affected, soybean grown on leaf-amended soil exhibited temporary N deficiency until nodulation. Nodule number was increased by more than 40% for soybean grown on amended soil, but nodule dry matter per plant generally was not changed compared with control soil. Nonnodulating plants were severely N deficient and stunted as a consequence of N immobilization. Nodulating soybean plants grown on leaf or crop residue amended soil were more dependent on symbiotically fixed N and had lower dry matter yields than the controls. When leaves were composted, the problem of N immobilization was avoided and dry matter yield was not reduced. No indication of an allelopathic inhibition on nodulation or N-2-fixation from heavy applications of oak, maple, sycamore, or walnut leaves to soil was observed.