Wang took SPAD readings at pinhead square, first bloom, and peak bloom so the SPAD readings could be correlated with leaf petiole NO3-N concentration. He took a SPAD reading from many leaves on the cotton main stem.

“The close correlation with leaf petiole NO3-N concentration at first bloom was obtained from the differential between SPAD readings taken on the 6th and 4th leaves taken from the same plant,” Wang said.

Using the differential minimizes leaf color difference among cotton varieties so the guidelines could be used without considering the leaf color of the specific variety.

“If the differential is less than 7 units then the plant probably needs more nitrogen,” Wang said. “If the N difference is higher than 7 than the plant N level is probably OK.”

In uniform fields, Wang recommends taking samples from 30 plants across the field to gain an accurate per-field N reading. Measurements require about 15 to 20 minutes per field.

Wang’s one-year trial findings are preliminary. He will conduct another trial this year and possibly again in 2013. More conclusive findings and suggested recommendations will be made available then.

Wang believes the principle behind the procedures is applicable across the U.S. Cotton Belt. Actual numbers in the guideline could vary due to growth stages and local growing conditions.

He says the bottom line is improved N management and increased lint yield potential.

“This information can help producers apply nitrogen to cotton when it’s needed or delay N applications when the crop has sufficient N,” Wang said.

UA graduate student Ruth Asiimwe is assisting in the N project.

For more information, contact Wang at (520) 381-2259 or by e-mail at samwang@ag.arizona.edu.

In 2011, Arizona farmers produced the top upland lint yields in the nation with 1,548-pounds-per-acre including 800,000 bales from an estimated 248,000 acres. The previous Arizona (and U.S. record) was set a year earlier at 1,517 pounds per acre.

cblake@farmpress.com