Soil sampling this fall should not begin until we’ve had adequate rainfall and time. First, soils need adequate moisture to “soften” to collect a good soil core(s) to the proper depth. Second, soils need time for soil chemistry to rehydrate and balance back out. Extremely dry conditions will affect the soil test results. 

Sample Collection 

Collecting soil core samples is difficult when soils are hard and dry. No new news here. But when collecting in drier soils, the upper 1 to 2 inches of powder can compress and spill out of the soil probe. You now have an improper soil sample for testing because you no longer have a full intact soil core. 

Effects on Soil pH

Soil pH test is affected slightly and could be erroneously low by 0.1 – 0.5 pH units. To put this into perspective, a soil pH water test of 6.0 is borderline on needing a limestone application. But a soil pH water test of 5.5 would recommend 2.5 – 3.0 tons/acre of limestone. Most soil testing laboratories also provide the soil pH buffer test. This test is less affected by dry soil conditions and wouldn’t change the limestone recommendation appreciably.  While the Illinois Agronomy Handbook still uses the pH water test to drive limestone recommendations, many independent consultants and cooperatives are using soil pH buffer test to drive their limestone recommendations. 

Effects on Soil Test Phosphorus (P) 

Dry conditions can produce slightly low soil test P values, but not as low as one may think. Phosphorus chemistry in the soil is very complex. It can be bound with a whole host of other nutrients like calcium, magnesium, iron and zinc, as well as in the organic residues. Much of this is pH driven. For the most part, soil test P values may be 5-10% lower than expected. Even with a soil test P value of 50 lbs/acre, a 10% decrease to 45 lbs/acre is not going to change the P fertilizer recommendation that much. Currently, the price of our crop inputs and grain market prices will have a greater effect on recommendation rate than soil test values. 

Effects of Soil Test Potassium (K) 

Dry conditions can produce extremely low soil test K values, especially after an extended dry spell. I experienced this back in 2012. After harvest, we had enough rainfall to start soil sampling. I sampled a couple thousand acres, and when the results came back, all of the K values were in the range of 120 –150  lbs/acre. Previous soil testing events produced results that were in the 250 – 300 lbs/acre range.  

The first question out of my customer’s mouth was: “What did you do to my farm?!” I then made some phone calls.  My second or third call was to Dr. Robert Miller at Colorado State University. In a nutshell, he told me that after an extended droughty period, soil test K values can take anywhere from 3 to 24 months to balance back out in the soil and is largely dependent upon the amount and type of clay present in the soil. I then re-sampled my customer’s field the following year, at my expense. Those soil test K values had rebounded and they ranged from 240 – 280 lbs/acre. 

Here’s the chemistry and physics behind this. The clays in the Midwest are predominantly Montmorillonite, an shrinking/swelling clay type. When these soils are hydrated, the broken edges expand/swell and allow for K and Ammonium to move in and out of that expanded edge. That’s your Cation Exchange doing its job. As the soil dries out, these swollen edges start to close back and “trap” the K inside the edge of the clay particles. Typical air dried soil can contain 5-8% moisture, but after an extreme drought (and the heat we had), those soils may contain 1-3% moisture. When we are at that extreme, it takes plenty of moisture and time for the soils to balance back out. 

I’m going to chalk up my 2012 experience as a lesson learned. For the fall of 2025, you, me and the farmer will have to be patient to get quality soil samples and soil test data.  It is my opinion that soil sampling too soon after a drought is a waste of your Money and my time.