Helping farmers and crop consultants manage nitrogen (N) levels in cotton plants and adjust applications to maximize lint yield is the purpose of a University of Arizona research project under way.

Project leader Sam Wang, cropping systems specialist at the UA’s Maricopa Agricultural Center in Maricopa, Ariz., hopes to develop guidelines to help farmers make the best N application decisions in upland cotton.

“Nitrogen is the most important nutrient input in Arizona cotton production,” Wang said. “Too little N in the plant reduces yield potential. Too much N can cause excessive vegetative growth resulting in reduced bolls and lint yield.”

Wang shared first-year N project findings during the Beltwide Cotton Agronomy and Physiology Conference in Orlando, Fla., in January.

In cotton, leaf petiole analysis is recommended by university researchers to manage N fertilizer input. For years, farmers and crop consultants have gathered cotton leaf petioles to determine crop N status. The samples were sent to laboratories with current costs at $10 to $20 per analysis.  The drawback has been the turnaround time of one to two weeks.

Today, the SPAD meter is evolving as a more popular and timely N measurement tool across a variety of crops. In Arizona, farmers primarily use the SPAD meter to measure N in wheat to gain premium protein content. The device’s popularity is gaining traction in cotton.

A SPAD meter is expensive in the $2,000 and $2,500 range. The device is a one-time investment and of growing importance for farmers with more crop acres needing immediate results.

“Producers need to know the crop N status to make the best informed N application decisions for the long-term yield potential of the crop,” Wang said.

Generally, farmers and consultants gain N information based on SPAD readings of the 4th leaf from the top (the most recent fully-expanded leaf) and from their experience over the years in N management to determine if the plant N amount is low or high.

“Some individuals may think a certain reading is too high and some think the number is too low based on their experience,” Wang said. “In reality, they are lacking a scientific guideline. We’re working to provide producers with that valuable information.”

An accurate guideline gets even tougher when different cotton varieties are considered. The SPAD reading can vary up to 10 units between varieties since varieties have different levels of leaf greenness.

Last year, Wang conducted research into SPAD use and leaf N measurement in four popular Arizona upland varieties – Stoneville 4288B2F and 4498B2F plus Deltapine’s 164B2RF and 1044B2RF. The green leaf color varied widely across the varieties. Stoneville’s 4288B2F had the darkest leaf color. Deltapine’s 1044B2RF had the lightest green color.

The tests were conducted with five different nitrogen rates.

Nitrogen rates

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