SPAD-CHLOROPHYLL RESPONSE TO NITROGEN FERTILIZATION AND EVALUATION OF NITROGEN STATUS IN DRYLAND AND IRRIGATED PUMPKINS*

Abstract

Field experiments were conducted on a dryland Flanagan loam and on an irrigated Plainfield sand to assess N status in processing pumpkins (Cucurbita moschata) using the Minolta SPAD-502 chlorophyll meter. At each location, five rates of nitrogen (N) (0, 84, 168, 252, and 336 kg ha−1) were applied to three pumpkin cultivars (hybrid ‘698’, hybrid ‘401’, and ‘Libby-Select’), with leaf SPAD (Soil Plant Analysis Development) readings and total N measured at anthesis, early-fruiting, and mid-fruiting stages. Fertilizer N requirements for 90% and 100% yield were estimated at 61 and 191 kg N ha−1 for dryland pumpkins and 148 and 245 kg N ha−1 for irrigated pumpkins, respectively. Cultivar and N rate affected leaf SPAD readings and N concentrations at both locations, with greatest effects on the irrigated sand. For both dryland and irrigated pumpkins, SPAD leaf readings at each sampling date were significantly correlated with both leaf N concentrations and fruit yield, although the relationships were markedly weaker on the dryland loam than on the irrigated sand. The results suggest that maximum or near-maximum yields of both dryland and irrigated pumpkins can be expected when leaf SPAD readings are ∼56.7–59.0 units at anthesis, ∼55.1–57.6 units at early-fruiting, and 52.2–54.3 units at mid-fruiting. The results also showed that while normalized leaf SPAD readings derived from high-N in-field reference plots can readily identify the critical threshold (i.e. 10% yield reduction) for N deficiency in pumpkins, they would be less precise predicting optimum N status (i.e. maximum yield). The potential usefulness for the SPAD 502 chlorophyll meter as a N management tool in estimating plant N status was demonstrated in irrigated pumpkins, and to a lesser degree in dryland production. However, because pumpkin N requirements could vary with cultivar and growing conditions, a range of SPAD values is proposed.