Lubbock, Texas
May 12, 2003

What do you get if you combine the accuracy of a soil test with the utility of a desktop calculator? If you have Internet access and a current version of Microsoft Excel software, you can use the Cotton
Nitrogen Fertilizer Calculator – now online at http://lubbock.tamu.edu under the "What's New" heading.

The calculator is a basic spreadsheet that allows you to enter up to seven production variables to generate a total recommended nitrogen fertilizer application rate. Along with this rate, users can get
recommendations for applying liquid nitrogen in split applications through center pivot sprinklers, or as daily injections of liquid nitrogen in a sub-surface drip irrigation system.

Kevin Bronson, Texas Agricultural Experiment Station soil scientist, and Randy Boman, Texas Cooperative Extension cotton agronomist, made the calculator available to producers and others through the web site of the Texas A&M System's Agricultural Research and Extension Center at Lubbock.

The calculator comes with a set of instructions for using it accurately. "One of the factors we can plug into this calculator is a projected yield goal for dryland or irrigated acreage. We have to use a realistic
yield goal, however," Boman said. "Beyond that, the number of factors we use to generate a recommended nitrogen application rate depends on our production system.

"A producer with a very basic dryland system, for example, will probably enter a yield goal, followed by a soil-test N value, and a soil texture value – three factors altogether. A producer with an irrigated system could add in a N value for their irrigation water, and the inches of irrigation water they expect to apply – five factors. Either producer could also plug in values for soil amendments such as manure or compost, and the N found in those amendments."

A dryland cotton producer could apply the total nitrogen fertilizer recommendation generated by the calculator (expressed as pounds of N per acre) with a ground rig in one single dose, shortly after a stand is established.

Irrigated producers should apply one of the calculator's recommended split applications at first square, another at first bloom, another at mid-bloom, and the final split application at peak bloom. A 120-acre
center pivot sprinkler using a typical 1,000-gallon chemical tank to inject a 32-0-0 ammonium nitrate solution will provide about 30 lbs. of nitrogen per acre in each of these split applications, Boman noted.

For sub-surface drip irrigation, the calculator generates a daily injection rate (lbs. per acre) for mid-June through mid-August. "We have to divide that number by 3.5 to get the gallons of liquid 32-0-0 to inject each day, because there are 3.5 lbs. of nitrogen in each gallon of 32-0-0," the agronomist added. "With drip systems, its important to remember to turn off the injection pump one hour before turning the irrigation off. This flushes the fertilizer solution out of the drip irrigation lines."

Boman and Bronson also recommend having a soil test run each year, in winter or early spring, and pulling multiple samples to composite into one representative sample for each field. The multiple samples are best taken with a shovel or soil probe, to a depth of 24 inches. It's also a good idea to keep the amount of soil the same for the upper and lower portions of each 24-inch deep sample.

The instruction sheet that accompanies the cotton nitrogen fertility calculator should answer most questions about how to use this online production tool, but producers can call Boman at the Lubbock center (806) 746-6101 for additional assistance.

Writer: Tim McAlavy, (806) 746-6101, t-mcalavy@tamu.edu
Contact: Randy Boman, Kevin Bronson, (806) 746-6101; r-boman@tamu.edu or k-bronson@tamu.edu