Characterizing soil acidity in Coastal Plain soils

Abstract

Acidification of the soil due to agronomic practices, the most notable being the application of ammoniacal fertilizer, has long been identified as a source of crop yield decline, particularly in non-calcareous soils. In this project, mapping the soil pH buffer capacity, comparing a calculated titratable acidity plus extracted non-acid cations with a more conventional CEC method and quantifying the acidity produced per unit of ammoniacal-N oxidized to nitrate-N in Coastal Plain soils were the objectives. Soil pH buffering can be predicted from soil organic carbon and texture. Data of soil pH buffer slopes was regressed on soil organic C and clay using multiple linear regressions. The equation that best fit the data was applied in geographic information system (GIS) software to create maps of the soil pH buffer capacity for three fields in the Coastal Plain of GA. The sum of titratable acidity and exchangeable non-acid cations offers an alternative means of estimating CEC as compared to the method used by some soil test labs. However, more research is needed on this method, as it appears to underestimate titratable acidity. It may be necessary to do titrations with longer reaction times between additions, and/or to do titrations at the same soil : water ratio at which pH is determined. By titrating the soils, a more accurate portrayal of the exchangeable acidity present in the soil may be provided than the present method of using buffer pH. In the third study the nitrifying activity of the soils was low as was shown with the calculated % nitrified NH4- N. Although the soils had pH values of 5.6, 6.0, and 6.2, the soils were relatively low in organic carbon, and clay and were therefore poorly buffered. Application of the fertilizer to the poorly buffered soil could have lowered the pH and slowed the rate of nitrification.