Rotational cropping sequence affects nitrogen fertilizer requirements in processing pumpkins (Cucurbita moschata)

Field experiments were conducted over a 5-year period (1994-98) to determine the effect of various cropping systems (rotations) on fertilizer N requirements in processing pumpkins [Cucurbita moschata (Duchesne ex Lam.) Duchesne ex Poir.] on medium- to fine-textured soil. Treatments consisted of a factorial combination of five N fertilization rates (0, 56, 112, 168, 224 kg.ha(-1) N) and four pumpkin cropping systems: 1) pumpkins following corn (Zea mays L.); 2) pumpkins following soybeans [Glycine max (L.) Merrill]; 3) pumpkins following 2-years corn; and 4) pumpkins following fallow ground. Cropping systems were chronologically and spatially arranged in two complete cycles, with pumpkin studies taking place in 1996 and 1998. Averaged over the two studies, the optimal N fertilization rate for highest total weight of ripe fruit following soybeans was estimated at 109 kg.ha(-1) N, compared to 128 kg.ha(-1) N following fallow ground, even though yields were similar, suggesting a soybean N-credit of 19 kg.ha(-1) N. Concurrently, the N fertilizer rate for highest total ripe fruit weight following corn was estimated at 151 kg.ha(-1) N, and 178 kg.ha(-1) following 2-years corn, indicating a negative rotation effect on pumpkin N requirements of 23 and 50 kg.ha(-1) N, respectively. Minimum N fertilizer requirements, the N fertilizer rate associated with a ripe fruit yield of 50 t.ha(-1), were calculated at 45, 37, 69, and 47 kg.ha(-1) N in the respective cropping systems. Negative effects from excessive N fertilization were greater in pumpkins following soybeans than in pumpkins following corn or 2-years corn, with reductions in total ripe fruit weight of 21%, 9%, and 3%, respectively, at the highest N rate. A critical level for preplant soil NO3-N of 17.6 mg.kg(-1) was identified above which there was little or no pumpkin yield response to N fertilization.