CROPPING SYSTEMS EFFECTS ON MYCORRHIZAL COLONIZATION, EARLY GROWTH, AND PHOSPHORUS UPTAKE OF CORN

A field study was established in 1986 on a Viborg silty clay loam (fine-silty, mixed, misic Pachic Haplustoll) soil in eastern South Dakota. The objectives were to quantify the influence of crop rotation, tillage, and residual P (254 kg P ha-1 applied in fall 1985) on the incidence of vesicular-arbuscular mycorrhizae (VAM) of corn (Zea mays L.) and to define the relationship between VAM colonization, early growth response to P, and early P uptake of corn. Plant and root samples were collected periodically from plots that varied in tillage and previous crop. Crop rotation and tillage influence the early growth and P uptake of corn. Large differences in early growth response to P were observed among cropping systems. Average relative growth response as compared with the check during both years ranged from 360% for the moldboard (MP) corn-fallow rotation to 7% for the ridge-plant (RP) corn-soybean (Glycine max [L.] Merr.) rotation. Early dry-matter production and P uptake in the check plots were highest in the RP corn-soybean system and lowest in the MP corn-fallow system. Generally, VAM colonization rates were significantly higher (P less-than-or-equal-to 0.10) in the RP systems than in the MP systems. Considerable reduction in VAM colonization rates were found with P fertilization (P less-than-or-equal-to 0.01) in all cropping systems. An inverse relationship was measured between VAM colonization and relative early growth response to P (Y = 647.0 - 49.4X + 0.97X2; R2 = 0.92; Y = growth response in percent, X = percent root length colonized). Considering early dry-matter production, P uptake, and mycorrhizal association the RP corn-soybean system appears to provide a good environment for P nutrition of corn during early vegetative growth.