PREDICTION OF NITROGEN-FERTILIZER REQUIREMENT IN COTTON USING PETIOLE AND SAP NITRATE

With the aim of refining methods of determining plant N status of cotton, we measured nitrate concentrations in petioles and xylem sap in field experiments over three, seasons. Main plots were rotation/tillage treatments which were split for fertilizer N. Petiole nitrate concentration was about 30 g N/kg at appearance of flower buds (squares). Rotation and N rate had highly significant effects on petiole nitrate concentration in every season. There was a linear decline in petiole nitrate with stage of development (measured in day degrees, base 12-degrees-C), particularly between 600 and 900 day degrees from sowing. The relationship between fertilizer N requirement and petiole nitrate concentration was exponential, with greatest precision when high fertilizer rates were required. The relationship between fertilizer N requirement and the rate of decline in petiole nitrate was linear, giving equal precision over the N fertilizer range. Optimum plant N status was represented by a petiole nitrate concentration of 21.5 g N/kg at 750 day degrees from sowing, or by a rate of decline in petiole nitrate of 0.0318 g N/kg/day degree. We conclude that the interpretation of petiole nitrate data could be improved by considering the rate of decline in petiole nitrate around flowering, particularly for sites where only moderate amounts of N are required. Concentrations of nitrate in xylem sap were highly correlated with petiole nitrate, but were more variable. For this reason, we concluded that petiole analysis was the superior diagnostic test.