Leaf Indices to Quantify Nitrogen in Cotton

Acala cotton (Gossypium hirsutum, var. Maxxa) was planted into a split-plot factorial design field trial on the Shafter Research and Extension Center, Shafter, CA. The two main treatments were optimum irrigation (Io) and stressed irrigation (Is) containing five levels of soil nitrogen availability (-100, -50, optimum, +50, and +100 lbs N/ac). The objective of this field trial was to determine if nitrogen levels could be estimated from narrow-band spectra and vegetation indices derived from leaf reflectance measurements, and to determine if water stress caused by deficit irrigation influences prediction accuracy. Leaf reflectance in the visible bands increased slightly as nitrogen availability decreased. Leaf reflectance in the nearinfrared region increased slightly as nitrogen availability increased. Water stress decreased the range in leaf reflectance among nitrogen levels compared to optimum irrigation. The vegetation indices GDVI and MCARI, and others utilizing the green band, regressed very well to nitrogen availability. Minolta Chlorophyll SPAD 502 meter readings had high correlation to nitrogen treatments in both optimum and stressed irrigation plots.