DRY-MATTER AND NITROGEN ACCUMULATIONS IN DETERMINATE SOYBEAN GROWN ONLOW-NITROGEN SOILS OF THE SOUTHEASTERN UNITED-STATES

Reliable assessments of erosion potential, N fertilization need, and n itrogen (N) non-point pollution potential for soybean [Glycine max (L. ) Merr.] cropping systems require accurate estimates of soybean dry ma tter and N accumulations. The objective of this field study was to det ermine dry matter and N accumulation in soybean during the growing sea son and at harvest in samples large enough to reduce sample variation and increase the confidence in measured values. A split-plot design wa s used with cultivar (Braxton, Coker 338, and Davis) as the main plot treatment and sampling date as the split-plot treatment. Each split-pl ot contained eight rows 4.6 m in length on 0.75 m spacing. The seed we re sown in a Norfolk loamy sand (fine-loamy, siliceous, thermic, Typic Paleudult) on May 18 at the rate of 33 seeds/m. Water was applied by use of an overhead irrigation gun. Plant samples were collected from 2 0 m2 of the six center rows on 89, 115, and 138 days after planting as well as at seed harvest. Fallen plant material (crop litter) was coll ected from each plot at each sampling date. Intact plant samples, crop litter, and soil samples were analyzed for total Kjeldahl N. The mean seed yield was 2.01 Mg/ha; the mean maximum dry matter accumulation f or intact shoots plus crop litter was 10.2 Mg/ha, and the coefficients of variation were <10%. The actual harvest index (seed yield/total dr y matter accumulation) ranged from 0.19 to 0.28, and the mean maximum N accumulation was 293 kg/ha. These accumulations are greater than tho se reported for indeterminate soybean grown on high-N soils in the mid western United States, and they clearly show that determinate soybean grown in the southeastern United States accumulate substantial amounts of dry matter and N.