Dr. Kevin Bronson
Soil Fertility Research
Texas A&M University Research and Extension Center
The following information provides instructions on the use of the Cotton Nitrogen Fertilizer Calculator. The Calculator is based upon the latest soil fertility research conducted in the Texas High Plains region. NOTE THAT THIS CALCULATOR HAS NOT BEEN EVALUATED OUTSIDE OF THE TEXAS HIGH PLAINS REGION.
Please carefully consider the instructions below in order to better understand the use of the Calculator.
Explanation of the Fields
- Yield goal
Enter a realistic yield goal, e.g. 1 bale/ac for dryland, 2 bales/ac for center-pivots, 2.5 or 3.0 bales/ac for subsurface drip.
- Soil test
We cannot over-emphasize the importance of getting your soil tested every winter or early spring. Multiple sub-samples should be taken from each field and composited so that one sample is sent to a commercial soil testing laboratory for each field. Soil samples to be tested for nitrogen need to be taken with a shovel or soil probe to a depth of 24 inches. Try to keep the amounts of sampled soil the same for the upper and lower parts of your 24 inch sample. All other nutrients beside nitrogen require a normal 6 inch sample. We recommend a separate 24 inch sample that you tell the soil test lab is only for nitrate-nitrogen. The Nitrogen Fertilizer Calculator will still generate a nitrogen recommendation if you only take a 6 inch sample, but because there is almost always more nitrogen below 6 inches, your nitrogen recommendation will be higher than necessary and cost you money.
- Soil texture
If you can see and feel sand grains in a bit of your soil after wetting, and you cannot make a “ribbon” your soil is sandy (texture 1 in calculator). If you can make a strong ribbon, than your soil is clayey (texture 3). The in-between texture is a loam (texture 2).
- Irrigation water analysis
We recommend you get your irrigation water analyzed for nitrate-nitrogen as well. This does not have to be repeated every year, but maybe every third year.
- Irrigation amount
Give your best estimate in acre-inches of how much you think you will apply. Do not include pre-watering. Input a zero (0) in this cell for dryland cotton.
- Manure applications
Manure should also be analyzed at a commercial laboratory for nitrogen (and phosphorus). Commercial applicators should do this for you and already know the lb nitrogen per ton of manure or compost.
Understanding the Results
The number on the top right of the Nitrogen Fertilizer Calculator is the total (seasonal) nitrogen fertilizer amount needed in lb/ac. For dryland, this amount can be applied with a ground rig all in one dose, shortly after stand establishment. For center-pivots, the second line of results indicates the number of 30 lb nitrogen/ac doses. Typical 1000 gallon tanks used to inject 32-0-0 (urea-ammonium nitrate solution) through a 120 acre center-pivot will provide about 30 lb nitrogen/ac. For subsurface drip systems, the Nitrogen Fertilizer Calculator provides a daily injection rate in lb nitrogen/ac for mid-June to mid-August. Dividing this number by 3.5 (lb nitrogen/gallon of 32-0-0) gives the gallons of 32-0-0/ac required to inject each day. For drip irrigation systems, be sure to turn off the fertilizer injection pump 1 hour before turning off the irrigation for the night.