Soil Testing Principles- Part 3

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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, ctrostle@ag.tamu.edu

Part III (Part 4 in a future ‘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?
  • Part III: 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?
  • Can soil test information be useful in buying and selling farm land? And if nutrient levels are high is this a possible depreciation tax consideration for the buyer?
  • Soil testing does not cost, it pays!

 

In Part I of this series I discussed the different objectives and philosophies that various soil test labs may have.  These include ‘crop nutrient requirement’ vs. ‘build-and-maintain’ residual soil fertility.  Each philosophy has merit.  Public and private soil test labs may approach these differently.

 

In Part II I asked what does each soil test lab base their recommendations on.  Hopefully, this is research field data that evaluates specific crop response to known or added nutrient levels.  This calibration research is most likely conducted by university agriculture programs.  So, what do private labs use?  At a minimum I hope they take research data into consideration.  It is possible a private lab may use the same data a university does.  (I would like that.  The data is available if desired.)

 

There is one major Texas caveat of basing crop nutrient recommendations on research data.  Lesser crops like sunflower, sesame, peas & beans, some forage crops, etc. have never been the subject of sufficient soil fertility research in Texas.  To deal with this Extension staff likely has enough knowledge of the crop that we can still estimate the likely nutrient requirement.  For example, we know about what the nitrogen nutrient requirement is for each 100 lbs./A yield of sunflower is.  So based on a yield goal and soil test data I could give a farmer a reasonable target for nitrogen applications.

 

Do you use a soil test lab that is out of state?

 

Surprisingly many farmers in Texas do use an out-of-state lab.  The ones I am aware of most are private labs in Kansas and Nebraska.  I am not sure about lab use in Arkansas or Louisiana among farmers and ranchers in east Texas.

 

This poses some immediate questions.

 

  • Is the out-of-state lab using a soil extraction and test method that is appropriate for your Texas soil type? Across the state we have acid soils in east Texas, heavy clays in the Gulf Coast Plain and the Blacklands, and sandy high-pH soils in the High Plains.  This has been a challenge even for Texas A&M’s soil test lab.  Our main extraction method used on Texas soils is “Mehlich-III” which is a compromise for various nutrients.  One example where it may be less than optimum is for phosphorus in high pH soils (TX Blacklands, West Texas including the High Plains).  In this case, if a farmer was interested mostly in P only, the Olsen Bicarbonate test is more accurate in assessing soil P nutrient availability—provided the crop-specific calibration data is available.
  • What data does the out-of-state lab use for soil fertility recommendations for Texas soils and the crops we grow? Nebraska grows lots of corn.  Is Nebraska calibration data appropriate to use on a field of corn grown in Texas?  What about soil samples for cotton production?  (There is no cotton production in Nebraska.)  Ideally, an out-of-state lab would load region- or state-specific, crop-specific information in their database.  Then recommendations are made based on zip code or a regional/state factor then the algorithm will pull up more specific regional or state recommendations for the client.

 

A Question to Ask

 

For soil test results, I stated in Parts I & II you are entitled to know what the results mean and what soil test philosophy is behind them.  Ask the lab.  But if you are also using an out-of-state lab, then you should also inquire about that possible difference in getting reliable recommendations.  The test value—provided the same soil test extraction method is used—is useful.  Then if you are aware, you can determine your own approximate crop nutrient requirement.  You can also look up the charts from Texas A&M’s soil test recommendations.  You can access these charts for nitrogen, phosphorus, and potassium at http://soiltesting.tamu.edu/webpages/recommendations.html

 

Texas A&M AgriLife is not recommending you avoid using an out-of-state soil test lab.  You may like the results you have received, the cost is reasonable, the turn-around time is fast.  Just be aware of what may—or may not—be behind the recommendations you receive.  I am taking this to heart myself and have sent several inquiries to learn how out-of-state labs make recommendations for crops in Texas soils.  The first lab that responded does use some Texas calibration data though for wheat they use Kansas and Oklahoma information (which is similar to Texas A&M’s recommendations).

 

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