Within-Field Variation of Corn and Soybean Yield and Soil pH Responses to Lime Application in Iowa.

Soil acidification can be an important yield-limiting factor but little research has focused on within-field variation of soil pH and crop response to lime application. The objective of this study was to assess soil pH and grain yield response variability in Iowa fields managed with corn-soybean rotations and to identify optimal soil pH for these crops. Fourteen replicated strip trials were established from 2007 to 2009 using GPS, dense grid soil sampling (0.07-ha cells), harvest with 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 after each crop harvest. The trials were evaluated one, two, or three years according to the year of establishment. Within-field initial soil pH variation (CV) ranged from very low (3.28 %) to high (13.2 %) across fields. There were no large or consistent differences between corn and soybean response to lime. Grain yield increases due to lime application across soils with no calcareous subsoil within a 20-30-cm depth (several series of the groups Argiudolls, Argiaquolls, Hapludolls, Endoaquolls, Hapludalfs,  and Haplaquolls) were 7, 5, 4, 3, 1, and 0 % for pH ranges < 5, 5-5.4, 5.5-5.9, 6-6.4, 6.5-6.9 and > 7, respectively. With calcareous subsoil (series of the groups Hapludolls, Haplaquolls, and Endoaquolls), yield increases were small for soil pH < 5 and inconsistent for higher pH. An index that combined initial soil pH and buffer pH was a significantly better predictor of pH change due to lime application than either measurement alone. Inclusion of soil organic matter resulted in no significant improvement.