The Importance of a Complete Soil Test

Is soil testing an exact science?  The answer is no.  However, from it we can evaluate the probability of seeing a crop yield response to additional fertilizer and/or look at nutrient relationships in order to see a particular nutrient(s) is negatively impacting the plant uptake of another nutrient(s).  Unfortunately, with the advent of grid-sampling the cost of soil sampling often becomes too expensive for producers.  So in order to continue to grid sample, they have to cut some sort of costs.  This is often achieved by running just pH, phosphorus and potassium, while calcium and magnesium are sometimes add-ons.  Yesterday, I was presented with an example where if I didn’t have a complete soil test in front of me, I never would have surmised what is going on.  This field was grid sampled, but only one area of the field was performing well (200 bu/a corn), versus the rest of the field which runs around 140 bu/a.  The area of the field performing well was the only area not receiving waste water from the town.  Below is a table looking at pH, P, and K for the field.



P (ppm)

K (ppm)

Mg (ppm)

Ca (ppm)

Good Area






Rest of Field Average







At first glance this all looks pretty good.  In fact, I would think the rest of the field would be doing just as well if not better given soil pH is located right in the “sweet spot” for good plant growth.  Phosphorus in the good area is sufficient, but double that for the rest of the field.  Potassium is low in both the good and poorer performing areas of the field.  Base saturation of calcium is 69% in the good area and 73% in the bad, while magnesium is running 7% in the good area and 10.5% in the poorer performing areas.  Cation exchange capacity is running right around 10 meq/100 grams of soil.  So what is the issue?  The poorer performing areas have better magnesium levels, soil pH right in the “sweet spot”, the same potassium level as the good performing area, similar CEC’s and calcium base saturation is still good at 73%.  It is not drainage because the problem is only in areas where they are applying the waste product.  It is not heavy metals, because the material is tested (EPA regulations) before applied, so what it is? 

Luckily the client did a complete soil test.  If we study all of the nutrients closely, including the micronutrients we can see that levels are the same across the entire field.  Elements such as potassium, boron, zinc, and copper would all rank very low in soil testing terms.  However, this is the same for both the good and bad areas.  So what has changed?  The issue is both calcium and phosphorus.  Both of these elements have increased from the applications of waste water.  Phosphorus has doubled (33 to 60 ppm), while calcium is 600 ppm (1200 pounds per acre) higher, which are causing imbalances in the soil.  Both calcium and phosphorus can greatly influence the uptake of other nutrients.  Calcium directly affects boron availability and will compete with potassium, zinc and copper for plant uptake.  Phosphorus will react with zinc and copper in the soil and form insoluble forms of zinc and copper phosphates, thus rendering them unavailable to the plant.   In addition, soil pH is a half a unit higher in the waste water areas and for those who don’t know it, zinc availability actually decreases 10 fold for each unit the pH increases.  Normally none of this would be an issue, but because potassium, zinc, copper, and boron levels are so low, it didn’t take much additional phosphorus and calcium for the soil to become unbalanced.  I would have never been able to make this determination without a complete soil test. 

Granted a complete soil test may be too expensive if you are doing grid sampling due to the sheer amount of samples needed.  However, why are you grid sampling?  Who is selling you on grid sampling?  What is your ultimate goal?  Is grid sampling the best approach?  Most of our consultants don’t use grid sampling, but use spatially directed zone sampling where layers of information are overlaid to create soil sampling zones. These zones are then adapted and changed over time as more layers of data are collected.  Just this past summer I attended a talk where a university researcher stood up and said that unless you are dong ¼ acre grids for grid sampling you are not getting good information (in my book no information is just as good as bad information)…most do 2.5 acre grids!!  Can you imagine the expense of ¼ acre grids?  This particular researcher said that in most cases it was better to take 1 whole field sample than to spend the money on grid sampling.  However, he did support the spatially directed zone sampling technique that most of our consultants employ as long as the zones were constantly be reassessed and changed if need be.  I guess the ultimate question that you need to ask is, “what are my grid sampling goals and is that paying off?”  I am a true believer in that if we are going to reach 300 bushel per acre corn and 90 bushel per acre soybeans on a consistent basis we are going to need more than pH, P, and K on a soil test.


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