Hybrid squash: Yield responses to potassium and phosphorus fertilisers at four sites of varying initial fertility

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Abstract
Hybrid squash (Cucurbita maxima L. cv. Delica) was grown at five levels of potassium (K) and five levels of phosphorus (P), at four sites differing in initial fertility at Pukekohe, New Zealand. K and P concentrations in soil and leaf samples taken at three stages of crop growth were measured. Total and exportable fruit yields were measured when the crop was mature. Exchangeable K and NaHCO3-soluble P levels in the soil increased approximately linearly with increasing inputs of K and P fertilisers respectively at each site. The effects of fertiliser treatments on tissue K and P concentrations were more pronounced in randomly sampled leaves than in young mature leaves and declined during crop growth. At harvest, the yield responses to P fertiliser increased from 14% to 82% with decreasing initial soil P fertility. The effects of the fertiliser treatments resulted primarily from effects on fruit number per plant. For K, relative yields (the yield for each treatment expressed as a proportion of the yield with nonlimiting soil fertility at each site) were related to soil exchangeable K or leaf K concentrations 14 days after crop emergence. Similarly for P, relative yields were related to NaHCO3-soluble P or leaf P concentrations 14 days after crop emergence. These relationships suggest that 90% of maximum yield for hybrid squash requires exchangeable K in the soil of 350 mg/kg and a leaf K concentration of 54 g/kg, NaHCO3-soluble P in the soil of 120 mg/kg and a leaf P concentration of 7 g/kg for crops on Patumahoe clay loam 14 days after emergence. Some implications of these results for fertiliser management for hybrid squash are discussed.