Sorghum Tips

Nitrogen Fertility Guidelines for Grain Sorghum

This tip was provided by:

Calvin Trostle, Extension Agronomy, Lubbock, 806-746-6101, ctrostle@ag.tamu.edu

Statewide

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/ ).