Sorghum Tips

Agricultural Trends toward Standard Deep Soil Sampling for Soil Testing

This tip was provided by:

Calvin Trostle, Extension Agronomy, Lubbock, 806-746-6101,


Agricultural Trends toward Standard Deep Soil Sampling for Soil Testing

In the Jan. 20, 2017 Sorghum Tip I discussed “Still ‘Free’ Nitrogen for Grain Sorghum Production:  Revisiting Soil Profile N.”  There I noted the merits of deeper soil sampling below 6” in particular to assess subsoil nitrate-N.  In a Texas A&M AgriLife soil test this subsoil nitrate-N is fully credited 100% to crop nitrogen requirements (and research shows it should be).  This involves soil sampling down to 24”, which is difficult to do on many Texas soils (so sample as deep as you can, down to 24”).  Nevertheless, there is increasing recognition of the value of subsoil nitrate for crops.  You can review the January report at

Dr. Ron Schnell and I were both attended the American Society of Agronomy/Soil Science Society of America national meetings in Tampa, FL earlier this week.  I noticed increased discussion in several presentations about better fitting the amount of fertilizer additions to projected crop requirements.  This was noted on corn, wheat, and cotton, but the same principles apply to grain sorghum.  One component of this was the use of deeper soil sampling for N, which when credited to crop nutrient requirement will reduce fertilizer N costs.

Furthermore, though there is currently no decision about changing soil test procedures in Texas within our Texas A&M Soil Testing lab, several major agricultural states including Kansas and North Dakota have moved their standard soil sampling procedures to 24”, or as deep as you can get below the traditional 6” soil sample (maximum 24”).  North Dakota State Univ. takes this even further for deep rooted crops like sunflower, especially for fields that have not been in sunflower before; they recommend a second soil sample of 24-48”.  This second sample, like in Texas with the Profile Soil Sample Information Form ( which Dr. Schnell also discussed in relation to soil sampling on fields affected by Hurricane Harvey in his Sept. 29 Sorghum Tip) is analyzed for nitrate-N only.

The bottom line is as we strive for efficiency in input costs, including nitrogen on grain sorghum—or any crop you grow in Texas—subsoil nitrate-N is an asset to be factored into your cropping decisions.  It should represent a net cost savings to most producers.

Do you have your soil tests run at a lab far from where you farm?

There are many soil test labs that producers can choose from. If your fertilizer dealer collects your samples, you may not even know who tests your soil.  Ask.  If you have your soils tested far from home, including out of state, here are some considerations I encourage you to ask the soil lab:

  • Is the method of soil testing extraction procedure appropriate for my soil?—In Texas the Mehlich III test is used to remove nutrients from your soil sample. This method is a compromise between different tests that would be used for acidic soils (east Texas) vs. alkaline soils (Blacklands, High Plains, much of South Texas).
  • What do you base your soil recommendations on?—A lab in Nebraska, Illinois, or Kansas may use a different algorithm to determine what recommendation you receive. I can’t assure you that the basis for their recommendations is appropriate for your soil or crop.  Would you be concerned getting soil test recommendations for your cotton crop from a lab in Nebraska where cotton is not grown?   In another state they may have a different yield response curve for nitrogen in sorghum and other crops, one that is not a good fit for your Texas farm.  Texas A&M AgriLife soil test recommendations are derived from tests over 40+ years for a particular crop in Texas soils.
  • If you have a fertilizer dealer or consultant that tests your soils for you and makes recommendations, A) ask for a copy of all soil test reports—you are entitled to them, B) inquire about the basis for the recommendations for fertilizer addition.