Soil Testing Principles–Part 1 of 7, Texas A&M AgriLife

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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

March 6, 2023

 

Part I (Part 2 in the next Texas Row Crops Newsletter)

 

Seven Soil Test Pointers for Texas Crops:

  • Part I: What is your soil test lab’s philosophy of nutrient provision?
  • 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!

 

Best soil testing principles are founded on calibrations of crop yield response to available residual and added fertilizer.  I will expand on this more in Part II.  But how individuals and fertilizer companies apply this information may vary.  There are two main philosophies of soil testing.  It is to your benefit to understand each.

 

  • Crop Nutrient Requirement (CNR). The goal is to provide the needed nutrients for your crop this year to meet a projected yield goal.  It seeks to minimize the potential for overfertilization beyond what is needed.  This could be a lower cost approach.  If cropping conditions are exceptionally favorable for higher-than-expected yields, then CNR could be limiting.  A farmer recognizing higher than expected yield potential may choose to increase mid-season nutrient applications prior to key crop-specific growth stages where yield potential is determined. (For grain sorghum or wheat this would be growing point differentiation.  Small amounts of later N, e.g. boot stage, might help finish a crop out if yields are much higher than expected and crop N status is low.  This could be determined by light color or tissue testing of the crop and for nitrogen older leaves more yellow than normal.)
  • Build and Maintain (B&M). The goal of this approach is to work over time to raise soil nutrient levels and availability to a higher level to ensure crop nutrient yields are met.  Strong residual fertility is rightly associated with good crop yields.  But the potential benefits may not be obvious.  Also, higher levels of soil nutrient status could lead to leaching losses of too-high mobile nutrients, especially nitrogen.  Fertilizer applications are higher during the build phase.  Once the desired soil nutrient status is achieved, then ongoing fertilizer applications should in theory return to levels similar to CNR.

 

Is one soil testing philosophy better?

 

Not necessarily.  What are your goals?  These two soil test philosophies may complement each other.  Each may be a better choice in certain circumstances.  You might have a crop consultant that takes a different approach then the soil test lab you use and the lab’s recommendations.  You may change your philosophy depending on the price of fertilizer.  For example, in the 2022 Texas cropping season, N & P fertilizers were at record high prices, often 80% more than in 2021.  A producer seeking to reduce potential expense might have taken a minimalist approach to how much money he or she would spend.  This could involve moving away from B&M to providing what you project the crop would need in 2022.  If you built up strong residual fertility, you might decide to “borrow” from that residual fertility while fertilizer prices are so high.  Then you may rebuild back to the desired fertility level in the near future when fertilizer prices are hopefully lower.

 

As noted above, it is best you understand each of the two above philosophies.  Which do you prefer?  The choice is yours.  Choose what you are most comfortable with and what best helps you meet your goals of profitability.

 

Soil test philosophy contrasts?—University & Private Labs

 

Typically, most university labs favor the Crop Nutrient Requirement (CNR) approach.  That is, supply what the projected crop needs for this year’s expected weather and yield goal.  This would likely be lower cost than Build-and-Maintain, especially in the years leading up to the desired soil residual nutrient levels.  But remember, it depends on what your goals are.  CNR is more likely to avoid excess nutrient applications.  But the argument could also be that B&M would be able to deliver the extra needed nutrients for higher yield if you are have a strong year for yield potential due to favorable weather and good rains.  It is possible when typical grain yield potential is increased by 20 to 25% due to good growing conditions, that CNR nutrient status might become limiting.

 

A Question to Ask

 

For soil test results, you are entitled to know what the results mean and what soil test philosophy is behind them.  Ask the lab.  Also, if you have a crop consultant or fertilizer dealer ask them as well.  The awareness of these two contrasting philosophies is for your benefit as you make management decisions for your crops’ nutrient needs.

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