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Effect of Summer 1998 Drought on Soil pH

Crop and Soil Environmental News, October 1998

Steve Donohue
Extension Specialist, Soil Testing and Plant Analysis

Soil pH levels tend to run somewhat lower in the fall than in the spring. This is fairly consistent from year to year with pH levels 0.1 to 0.3 pH units lower in the fall. (Because of this, we have always recommended that farmers not switch back and forth between fall and spring for sampling; rather, we encourage them to pick one time of year and stay with it.) In a dry or drought year, with fewer nutrients being taken up by the crop and also less downward nutrient leaching into lower soil profiles, the higher fertilizer salt concentration will result in a pH level about 0.5 units lower in the fall than the following spring. This is a "temporary" pH depression, and limestone is not needed to correct this - the winter (leaching) rains will take care of the situation. However, it should be noted that fall soil tests may call for somewhat more lime than normal, perhaps 0.5 - 1.0 tons/acre. To address this problem, two things can be done. First, if one has been keeping good records on amounts and time of previous lime applications for his fields, this past treatment history should be considered before making a greater-than-normal lime application. Second, the field can be tested for soluble salts, which is actually a direct reflection of fertilizer salts still remaining in the soil. If the salt level is greater than 100 ppm, one can expect a soil pH depression of about 0.5 units or a recommendation that is perhaps 0.5 to 1.0 tons/acre on the high side, and make the appropriate adjustment. This difference in pH between fall and spring is normally more pronounced on sandy soils which have a lower buffering capacity.

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