Nitrogen treatments (0–134 kg N/ha) were applied on a sandy loam over 11 growing seasons to establish critical soil and tissue N levels, and to evaluate the effects of seasonal precipitation on annual variability in response of ‘Painter’ and ‘Centennial’ sweet potatoes [Ipomoea batatas (L.) Lam.] to applied N. Critical concentrations of soil NO3-N at 29 days after transplanting (DAT) and laminal N at 72 DAT were determined to be 37 μg N/g soil and 5% (dry weight), respectively. Correlation between soil NO3-N at 29 DAT and leaf N at 72 DAT was highly significant. Multiple regression analyses predicted a quadratic relationship between root yield and applied N and predicted a yield maximum at 88 kg N/ha. Inclusion of annual precipitation during the fallow season (1 Oct. – 31 May) in the N model significantly improved the prediction of response to applied N. Fallow season precipitation provided an index of N carry-over in sandy loam soils and could be used to improve estimates of N fertilizer requirements for sweet potatoes.
Bush-type snap beans (Phaseolus vulgaris L.) were seeded by a no-tillage method into standing wheat (Triticum aestivum L.) stubble of 8, 15, 23, 30, and 38 cm in height to evaluate the effects of stubble height on pod mechanical harvest efficiency, plant morphology, and shoot component yield. Basal internode elongation, stem plus leaf yields, pod yields, efficiency of mechanical pod harvest (MH), and height of basal pod set were related in a positive linear or curvilinear fashion to wheat stubble height. Quantity of pods missed during MH was related negatively to height of basal pod set. Harvest efficiency was maximized with 15-30-cm stubble heights, and these notillage systems yielded MH pod levels that equaled or exceeded those of a conventional tillage (plow, disk 2 times) system. Superior MH efficiency was attributed to increased basal internode length and mechanical support of the shoots by the wheat stubble.