Comparison of Soil pH Measured In Water and Calcium Chloride to Predict Crop Response to Lime In Iowa.

There is little information in the Midwest about the convenience of using soil-water or soil-CaCl₂ suspensions for determining soil pH and predicting crop response to lime application. The objectives of this work were to compare these two methods in Iowa soils and to assess their value to predict corn and soybean response to lime application. Fourteen replicated strip trials were established from 2007 to 2009 using GPS, dense grid soil sampling (0.07-ha cells), grain yield monitors, and GIS. Treatments replicated 3 to 5 times at each field were a control and agricultural lime at 3.7 Mg ha^-1 of effective calcium carbonate equivalent. Soil samples were collected from a 15-cm depth before applying lime, and were analyzed for pH with using a 1:1 soil-water ratio and a 1:1 soil-0.01 mol L^-1 CaCl₂ ratio. These trials were evaluated during two years. Also, the temporal variation of both pH determination methods was evaluated in four sites by collecting soil samples during and 18-month period. The pH methods were linearly and highly correlated across sites (r² = 0.89). As expected, pH measured with CaCl₂ was lower than with water (0.49 pH units on average), and the difference decreased significantly (P < 0.05) with increasing soil electrical conductivity. Soil type or the cumulative precipitation one month before the soil sampling did not explain the pH differences. Both pH methods were equally correlated with crop response to lime application. In conclusion, the study showed no advantage for measuring pH in CaCl₂. The generally low salt concentration of Iowa soils may help explain this result.