By: Dustin Sawyer, MSc

A little rain has begun to return to Wisconsin in the past weeks, easing the minds of drought-stressed farmers. This may not be enough, though, to float the Midwest out of its drought condition. As of July 25, the U.S. Drought Monitor reported much of the region to be in some level of drought, with some areas in the severe or extreme categories. While crop yield is the immediate concern, such conditions can continue to have an impact after the harvest.

It’s been eleven years since the Midwest experienced a significant drought, and while many of us would like to forget about the 2012 growing season, there were valuable nuggets of information that we should learn from. The 2012 post-harvest soil sampling season was as weird as the weather that preceded it. Agronomists and farmers across the Midwest were perplexed trying to reconcile their soil test potassium and pH levels with previous sampling data. Prolonged drought actually effects soil test results, and that’s something to keep in mind as we move into this year's post-harvest sampling. Soil moisture, or lack thereof, can impact the results of soil pH and potassium tests, though the mechanism is not the same for each.

Soil pH

The lab method for measuring soil pH is deceptively simple; the soil is dried and ground, then mixed with equal parts of ultrapure water. The resulting slurry is allowed to sit and equilibrate for ten minutes, then a pH electrode is jabbed in and the reading is recorded. The pH electrode is really a marvel of electrical engineering and it’s a shame to dumb it down to this, but it’s fundamentally a super sensitive volt meter. Free hydrogen floating around in the slurry alters the slurry’s electrical conductivity, and the meter measures that alteration. A low pH means there is a lot of free hydrogen and conductivity is relatively high while a high pH means there is less free hydrogen and conductivity is relatively low.

There’s something besides free hydrogen that alters the electrical conductivity of the soil slurry, though, and that’s salt. Guess what happens when the soil dries out? Water from deep in the soil profile is wicked to the soil surface and evaporates, leaving salt behind. That’s right - drought increases the salt content in the soil surface and the increased salt content increases the soil conductivity, which is misread by the pH probe as excess free hydrogen. In simple terms, salty soils give a false-low pH reading. There is one caveat to this. Salt only impacts the soil pH measure that uses ultrapure water in the slurry. Some states have inherently salty soils already so the pH analysis procedure in those states is to add a high background level of calcium salt to the slurry, effectively masking the impact of the drought. If you’re not sure which method your lab uses, take a look at the report. It should say something to the effect of “water pH” or “salt pH.” In this example, “salt pH” would be unaffected whereas “water pH” would be skewed low in a drought year. 

Potassium

The reason that drought affects soil test potassium is a little less clear, but it is real. There’s a theory, and to understand it, let's revisit how soil nutrient extraction works. The mineral portion of soil is made of… well, minerals (and rocks, which are made of minerals), and that poses an obstacle to measuring soil fertility. In most cases, if we want to know the mineral content of something, we just boil it in acid until it’s all dissolved and in liquid form, then measure the mineral content of the liquid. This is known as “total mineral content.” The difficulty with soil analysis is that we want to target only the minerals that are available to plants, and the potassium (for example) that’s locked away in a grain of sand isn’t available. Only the ionic form of potassium, K+, can be accessed by plants. If we were to use the “throw it in acid and cook it” approach, we would find a lot of potassium, but we wouldn’t know how much of that a plant can actually access, so we need to differentiate between “total” and “plant-available” mineral content.

The obvious solution to this puzzle is to target only the mineral forms that are available to plants. And as such, we need to make assumptions. This is why there are so many different methods of soil analysis like Bray-1, Bray-2, Mehlich III, Olsen, yadda yadda yadda. Different researchers had different approaches. These various soil test methods have one thing in common, though, in that they target the cation exchange sites of the soil and attempt to displace the minerals that are held there. This is where the drought conditions come into play. Potassium exists in three pools within the soil: solution, exchangeable, and fixed. As long as there is water in the soil, fixed potassium can become exchangeable, exchangeable potassium can become solution potassium, and the plant can take up the solution potassium. In fact, it’s the plant’s removal of the solution potassium that drives this whole process. As long as there is a plant sucking up potassium at one end, and a source of potassium at the other end, potassium will flow to the plant. The removal of water from the system shuts this pathway down and either the plant removes all of the plant-available potassium, or the potassium becomes fixed, or both. The result is that the soil test extracts less potassium than it would if there had been normal soil moisture levels.

Understanding the mechanisms that drive lower soil pH and potassium in droughty soil is key to understanding why the opposite is not true. Really wet soil will NOT yield higher pH or potassium values. It’s also the key to understanding why these phenomena are the result of a true drought - a prolonged period of abnormally low rainfall. It takes time to build up salts and deplete the exchangeable potassium pool. Seasonal dryness doesn’t have the power or duration to cause these changes.

So what should a person do in a drought year? The best advice is to hold off on soil sampling until moisture levels return to normal. If fall samples can be pushed back to spring, do it. If sampling is a must, be sure to look at the results with field history in mind. If the pH or K look abnormally low, lean on the historical data and put less weight on the current samples.