Relationship of Exchangeable Acidity to Yield and Chemical Composition of Alfalfa1

Abstract This study was undertaken to help clarify the soil acidity‐plant response relationship in western Oregon in an effort to improve the efficiency in the use of lime and fertilizer. The objective of the study was to relate the lime and P response of ‘Du Puits’ alfalfa (Medicago sativa L.) to selected measures of soil exchange acidity and exchangeable Al and Mn. Extractable acidity in soil samples from the A horizon of six representative soil series ranged from 0.3 to 8.1 meq/100 g of soil with a range of 0.2 to 7.9 meq/100 g exchangeable Al and somewhat lower amounts of Mn in the extracts. Responses of alfalfa to applied lime and P treatment combinations in greenhouse experiments were recorded for five of the six soil series studied. Maximum yield response was obtained with applications of either 4 or 8 meq of lime per 100 g of soil which largely eliminated KCl exchangeable acidity and Al at approximately pH 6.0 or higher. There was evidence of lime × P interactions on those soils that were both strongly acid and low in P with lime treatments increasing the availability of soil P. Lime application decreased tissue concentrations of Al and Mn, but increased plant Ca and P. It was concluded that the presence of exchange acidity and Al in the soils was indicative of a potential yield response to lime. The relatively large liming increments used in the study were not conducive to evaluating the precise amount of lime needed to react with a specified amount of exchange acidity or Al. The greatest benefit was derived from liming increments sufficient to neutralize the exchange acidity.