Suppose your soils consultant is a glutton for punishment, and figured out how to get your soil samples pulled even though soils were bone dry and rock hard although the way into mid-fall. The numbers come back and you set down to compare them to the two previous sets of samples pulled on a particular field. They were likely pulled either three or four years apart.
The first thing you may notice is that the pH values on this year's samples pulled this fall under dry conditions may be somewhat lower. In fact, Betsy Bower, an Indiana certified crops advisor and agronomist for Ceres Solutions, Terre Haute, says that the difference could be significant. She's seen readings as much as 0.5 points lower. That means if a soil tested 6.0 before and you've taken steps to maintain it at 6.0, it could test as low as 5.5 right now. That's critical because a reading that much lower would indicate a need for more lime. However, Bower indicates that the lime index shouldn't be affected.
Trying to do some sort of adjustment on the results over all really isn't practical, she says. For example, maybe you're looking for a magic formula that you n apply across the results and make them comparable and fair for comparison purposes to samples pulled under more normal conditions either four, six or eight years ago. "It's just not that simple," she insists.
She isn't aware of any literature or scientific studies that show that phosphorus levels would be affected by pulling samples under very dry conditions. So you wouldn't expect much if any difference in that category.
Potassium is a different story. Because of how it's structured and how it fits into soil clay layers physically within the profile, potassium can be harder to draw out into a soil sample. The net result is that potassium will tend to test lower if soils maples are pulled under dry conditions. Estimating how much lower it will be is again a pretty dicey game.
Put it all together and that's why Bower and the Ceres staff suggest sampling at other times instead of forging ahead now might be the better solution. If you delay it until spring, they can do it for you, assuming conditions are suitable. But once you sample in the spring, then consider sampling in the spring each time from then on, she notes.