Soil Testing Principles- Part IV

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,

December 2023—Part IV (Part 5 in a future Row Crops Newsletter)


Eight Soil Test Pointers for Texas Grain Sorghum:

  • 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?
  • Part IV: 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!


Continuing my series on testing, I am pleased to report the out-of-state labs (Part III) I contacted use calibrations that are generally specific to Texas.  They may even have Texas A&M data loaded in their data base.


Now for the basic ‘how to’ of soil sampling in the field.  Properly collected soil samples are the pre-cursor for accurate results—a representative sample—of your target soil zone.


Do you use good representative soil sampling and handling methods?


First, I acknowledge that most farmers do not soil sample regularly.  Many may test every 3 to 4 years.  Don’t have time.  It is too expensive.  (Research shows soil testing pays.)  The soil sample lab reports are hard to understand.  I suggest that soil testing is an example where ‘what you put into it is what you get out of it.’


The Texas A&M AgriLife soil test lab instructions for soil sampling on are on the back (page 2) of our standard soil test form.  Even if you send soil samples elsewhere, these instruction are a good review.  See  I will not repeat the information.  But keys include identifying areas of your field with different soil types, different landscape positions, and different uses.  Let each area be a soil sampling zone where you collect a composite sample.  Do not put your sample in a galvanized bucket.  You can air dry the samples before sending, but do not use heat.


What soil sampling depth should I use?


In the past this was rarely a question as everyone used 6” soil samples.  The labs all assumed you submitted a 6” soil sample.


But times have changed—because we understand soil sampling better now.  A 6” soil sample remains a common measure for P and other nutrients that concentrate near the surface.  Fertilizers you apply that are immobile (P, metallic micronutrients like iron, zinc, etc.) largely remain near the surface or no deeper than your plow later or depth of application if using a knife or coulter rig.


Nitrogen in its most common plant available form in the soil is mobile.  This nitrate nitrogen (NO3) can move downward in the soil with water.  Regional Texas soil sampling often reports be substantial soil nitrate-N below 6” depth (50 lbs. N per acre and more). This N is still readily available to plant roots to at least 24” depth.


Texas A&M AgriLife soil testing lab continues to base recommendations on soil test calibrations (Part II) for 6” soil samples.  Other labs may use 8” or 12” for all soil nutrient analyses.  I like that.  Until recently Texas A&M AgriLife soil testing lab used a “Profile Soil Nitrogen” sample form.  Only nitrate-N was assessed in a second sample below 6” that is paired with your routine 6” sample.  The paired soil sample below 6” goes to as much as 24” depth.  (This costs about 1/3 of a standard soil test.)


This gives a farmer a better reading on nitrogen status.  Nitrate-N is the most common form of N taken up by roots.  Any form of nitrogen is your most expensive fertilizer component of your cropping budget.  It represents the best opportunity to fine-tune your added N requirements a specific crop and yield goal.


If you are unsure about your 6” soil sampling depth, or want to learn more about sampling deeper for mobile N, check with your preferred lab.  You can use any soil sampling depth you want.  But make sure the lab knows what depth you routine soil analysis samples are from so they can adjust and make the proper recommendations.


Do fertilizer dealers collect and analyze your soil samples for free?


The dealer anticipates being your fertilizer supplier.  Dealer sampling (along with crop consultants) is how much soil is sampled.  It is convenient.  There are two things to remember:


  • Yes, there can be a financial conflict of interest of free soil testing and recommendations. The dealer hopes to sell you the fertilizer.  This is likely not a problem but be aware of it.
  • If a fertilizer dealer (or a crop consultant) conducts soil sampling and makes recommendations, get a copy of the reports. Keep them for 30 years.  If you have records 10 years and more, you can see if changes occurred in your soil over time.  Is potassium similar?  Is total nitrogen increased?  Has pH declined (more acid)?


A Lesson Learned—Get a copy of those soil test reports!


One reason I emphasize a farmer requesting copies of the soil sample reports from others?  About 2004 I was involved in a lawsuit in New Mexico on alfalfa.  The irrigated crop was growing poorly.  During legal discovery I found that the company collecting the soil, testing for nutrients, making recommendations, and selling fertilizer did not share the soil test reports with the farmer.  And the farmer did not ask for them.  After 5+ hours in a court deposition I was given soil sample reports from the field in question.  Sodium, sodium, sodium.  High levels!  I turned and asked the farmer if he had ever seen these soil sample reports.  “No.”  The farmer hopefully would have read them himself saw the problem with sodium.  (It came from the irrigation water.)  The company never mentioned it.


Both parties had made mistakes.  They settled out of court.

Comments are closed.