Soil Testing Principles–Part 2 of 8, Texas A&M AgriLife

This item is adapted from an AgriLife submission to Texas Grain Sorghum Association’s “Sorghum Insider”

Calvin Trostle, Ph.D., Professor & Extension Agronomist, TAMU Dept. of Soil & Crop Sciences, Lubbock, (806) 746-6101,

Part II (Part 3 in the next Texas Row Crops Newsletter)


Eight Soil Test Pointers for Texas Crops:

  • Part I: What is your soil test lab’s philosophy of nutrient provision?
  • Part II: What does your soil test lab base fertilizer recommendations on?
  • Do you use a soil test lab that is out of state?
  • Do you use good representative soil sampling methods? What depth should I sample? Does a fertilizer dealer collect and analyze your soil samples for free?
  • How do I read soil test lab results?
  • How long should I keep my soil sample reports?
  • Soil testing does not cost, it pays!


In the first edition of this series (March), I discussed the two main philosophies of soil testing.  Namely, these are ‘crop requirement’ and ‘build and maintain.’  The former seeks to provide which nutrient and how much the current or planned future crop requires.  This begins with a soil test.  The latter philosophy seeks to establish a higher level of residual soil fertility.  This may help a farmer make a higher yield when conditions are especially favorable.  Each philosophy has its merits.


The Crop Requirement approach is common among universities.  Build & Maintain may be more common in industry.  I encourage you as a farmer to understand which philosophy your test results and the subsequent recommendations reflect.  Ask.


What are your soil test recommendations based on?


Most likely anyone would answer (and hope) “research!”  This is our expectation.


Most university soil test labs, at least for major and moderate acre crops in their state, have many years and locations (including different soil types) of soil fertility research and crop nutrient response.  To do this correctly, scientists must measure existing individual soil nutrient forms (to determine what is readily available, perhaps only slowly available, and unavailable) then create a range of available nutrients by adding targeted amounts of fertilizer.  (Researchers must ensure added nutrients are incorporated into the root zone.  For example, if you surface-apply relatively insoluble phosphate fertilizer but do not incorporate the P, it is likely not available to the crop hence there is little to no response.)


For grain sorghum, cotton, or corn, added fertilizer N could provide fertilizer N + available soil N equal to 0, 40, 80, 120, 160, and 200 lbs. of actual N per acre.  This applied fertilizer N must reach the crop root zone.  Harvest yield is measured.  Then a nutrient response curve of applied/available nutrient vs. yield is made.  When this is repeated over numerous sites and years, composite data is added to the soil test lab database to produce a calibration curve for each specific nutrient and crop.


This information becomes the basis for a crop nutrient requirement and recommended fertilizer addition that reflects existing readily available nutrients (the soil test) and your projected yield goal.


Do private soil testing labs also use soil test and crop yield response for calibration?


I have learned most private labs have chosen some type of research-based data to load in their algorithm for fertilizer additions for a specific crop, its nutrient response, and yield goal.  It is quite possible they may use the exact same data the public soil test lab uses in their state.  It is also possible a private lab may make a different recommendation for your crop and yield goal based on the exact same soil test data.  (Remember the two different philosophies discussed in Part I.)


A private lab likely does not have the means to conduct this research themselves like a university and its agricultural experiment station can do.  But the data from university research is publicly available (if not published then on request), which a private soil test lab could load in their database—provided they use the same standard soil test methods.


What about fertilizer recommendations for lesser agronomic crops?


Minor crops like sunflower, sesame, guar, some forage grasses, alfalfa, etc. probably have not been subjected to soil fertility research.  It is expensive to conduct field calibration trials over several locations, conditions, and years.  At Texas A&M’s soil test lab, this is true for these noted crops  You can see examples in the Texas A&M Soil Testing Lab’s charts at  In this case for N, P2O5, and K2O (potash) there is a linear line of nutrient recommendation based strictly on soil test results. Yield is not factored in.  This means a farmer with yield goal, for example, of 2,500 lbs./A sunflower would get the same soil test lab N recommendation from AgriLife as a 1,000 lbs./A crop.


You know that does not make sense and is not correct.


But Texas A&M does not have the data to develop calibration curve for lesser crops.  This is not an issue for major crops in Texas (calibrations have been conducted and recommendations are based on a yield goal).  Sunflower states like North Dakota, South Dakota, and Kansas have this data.  So, if you had a soil sample submitted from a field in those states, there would be a research-based recommendation for sunflower.  In my opinion, this is where Texas A&M could consider including Kansas sunflower calibrations in the Texas A&M database.  Though imperfect, it would be better than getting a generic recommendation that does not meet the standard of calibration.  In fact, in this case perhaps you would be better to send a soil sample for sunflower to Kansas State University instead of Texas A&M.


A Question to Ask


You as a farmer are entitled to ask:  “What are my soil test recommendations based on?”  Is it university or other research measuring crop response to known levels of nutrient?  Or perhaps the lab applies a general number based on typical crop requirement that is not actually validated through research.  You as a farmer most likely would choose your fertilizer application recommendations based on research.  Learn if this is the case.  If not, then you may wish to consider another soil test lab/fertilizer dealer.


If your desired crop is like sunflower or sesame noted above (charts posted in 2012), it would be good to ask the Texas A&M Soil Test Lab if they are now using calibration data.

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