This tip was provided by:
Calvin Trostle, Extension Agronomy, Lubbock, 806-746-6101, firstname.lastname@example.org
Nitrogen Fertility Guidelines for Grain Sorghum
In our four years of compiling Sorghum Tips, we should have addressed basic N requirements for grain sorghum long ago. From the Sorghum Tips archive at http://texassorghum.org/sorghum-tips, in 2012 we did discuss 1) fully crediting 100% of your soil-test nitrate-N down to 24” and the use of Texas A&M AgriLife’s Profile Soil Sample Information Form (Nov. 7, or page 7 of archive), and 2) the full N credit you can take for any nitrate-N in your irrigation water (Dec. 4, or page 6 of archive).
But what about the straight-up, season-long N requirement for grain sorghum?
Texas A&M AgriLife’s long-time fertility goal based on soil fertility research is tied to your yield goal:
2.0 lbs. of actual N per 100 lbs. of yield goal
This is a helpful rule of thumb—for planning purposes—for your N requirement. Therefore, if you have a 5,000 lbs./acre yield goal, then the N requirement is 100 lbs. of N per acre, e.g.:
2.0 lbs. N/cwt. X 50 cwts. = 100 lbs. N/acre
This is not the fertilizer N requirement as there are credits you may claim against the total required N. In Texas A&M AgriLife these recommendations include:
- Soil nitrate-N in your surface soil sample (usually 0-6”, sometimes 0-8”). If you have 7 ppm nitrate-N in the top 6”, that is equivalent to 14 lbs. N per acre. (Each 6” deep layer of soil is about 2,000,000 lbs. of soil; thus for each 1 ppm nitrate-N, you have 2 lbs. of nitrate-N.)
- Sub-soil nitrate N (if you have the data, see #1 above). For each 6” of soil below the six-inch depth, 1 ppm nitrate-N equals 2 lbs. of nitrogen per acre, a credit against the full N requirement. For example, you have a sub-soil test sample of 6-18” deep at 4 ppm nitrate-N, then the credit determined, calculated as two 6-inch layers, is:
2 soil layers over an acre X 4 N (from soil test ppm) X 2 (ppm conversion factor) = 16 lbs. N/acre
- If you irrigate, you could also take a credit for any nitrate-N in the irrigation water (see #2 above).
- Are there other possible N credits? In Texas we don’t claim any, but a farmer sending a Kansas soil sample to Kansas State’s soil test lab will answer questions for soil texture, previous crop (if legume), and soil organic matter. Each of these will slightly alter a KSU recommendation.
The full example—
For the 5,000 lbs./A yield goal…
Fertilizer N to apply = N requirement – soil test N (0-6”) – subsoil test N (below 6”)
70 lbs. N/acre = 100 lbs. N/acre – 14 lbs. N/acre – 16 lbs. N/acre
What this example does not address—
This is the N requirement. It does not address timing (most N should be on by growing point differentiation about 5 weeks after planting), fertilizer placement, or N use efficiency. I will address these in a future Sorghum Tip. In the interim, refer to the N fertility section of your region’s USCP pocket grain sorghum production guides for Texas at http://sorghumcheckoff.com/for-farmer/production-tools/ ).