Inadequate N supply can limit yield of cotton (Gossypium hirsutum L.), while too much N can cause excessive vegetative growth and delayed maturity. Reflectance sensors offer the potential to diagnose N need and to translate this diagnosis into a variable‐rate application of N in real time. Our objective was to calibrate canopy reflectance sensors to predict economically optimal N rate (EONR) in support of variable‐rate fertilization. Nitrogen rate experiments were conducted on three soils in 2006 and 2007. Reflectance was measured with three sensors (Crop Circle, GreenSeeker, and Cropscan) at three growth stages (early square, mid‐square, and early flower) and at three heights above the canopy (25, 50, and 100 cm). Economically optimal N rate ranged from 0 to 220 kg N ha−1, suggesting that the need to diagnose EONR is great. Relative reflectance for all three sensors was weakly related to EONR at the early square stage but was related more strongly at the mid‐square and early flower stages. Regression equations were not significantly different between mid‐square and early flower stages, suggesting that a single equation could be used to translate reflectance measurements to N rates over this period. Relationship to EONR was best for all sensors when placed 50 cm above the canopy. For these stages and height, all three sensors were similarly related to EONR (r2 = 0.59 to 0.62). These relationships could feasibly support successful variable‐rate N applications to cotton.
The question of boron mobility and re-use in planits has not been adequately investigated. It is often dismissed with the statenment that a continuous supply of boron is necessary during growth because boron cannot be stored and re-use(l. However, there is evidence that boron is not entirely immobile, although re-use has not been (lefinitely establishe(l. Eaton, McCallum, and Mlayhugh (3) showed that, under conditions of excess boron, stone fruit trees translocate boron to the fruit an(d that higlh concentrations can accumulate there -while only moderate concentrations remain in the leaves. Scott and Schrader (8) showed that the concentration of boron in older leaves of the grape plant (lecrease(l after the source in the root media hadl been removed. They On the (late designatedl for Period 1 the nutrient solution was (lumped an(l the gravel wvas flushed four timles with (listillech water and nutrient without boron then provided. At the end of each of 12 assigned intervals, the plants of a particular group were cut at the cotyledonary no(le and the fresh weight was taken. The leaves w\ere then strippedl from the stem and numbered consecuttively from the base of the plant. Leaves
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.