Soil pH Change Over Time as Affected by Sources and Rates of Limestone.

Soil acidification is an important problem in Iowa’s agriculture but uncertainty exists concerning the pH change over time after applying different limestone sources and application rates. The objective of this study was to study the rate of soil pH increase due to the application of finely ground pure calcium carbonate, calcitic limestone, and dolomitic limestone in different Iowa soils. Four replicated plot trials (4 m² plot^-1) were established in acid soils (Typic Hapludolls of pH 5.4 to 5.7) in Spring 2009. The amendments were incorporated into the soil by disking, and soybean was planted after 2 to 3 weeks. Application rates were 0, 4.5, 9, 13.5, and 22.5 Mg ha^-1 calcium carbonate equivalent (CCE). Soil samples were collected from a 15-cm depth eight times during a period of 14 months since lime application (also from a 15-30-cm depth for the seventh date). The soil pH increase over time for the three lime sources was curvilinear with decreasing increments to a maximum that was reached about 100 days after liming. However, the early increases were greater for pure calcium carbonate than for the calcitic and dolomitic limestones. Soybean yield responses were observed at three of the trials. There was no significant yield difference among lime sources and no rate by source interaction. The source and time of reaction in the soil are not major issues for commonly used agricultural limestones in Iowa soils.