Soil acidification induced by leguminous crops

Solution culture and greenhouse studies have both clearly demonstrated the ability of legumes to acidify their rooting medium. Furthermore, research workers comparing the pH beneath undisturbed sites versus all-legume pastures or all-grass versus all-legume (or grass-legume) pastures have observed a lower soil pH under the leguminous pastures. The processes leading to legume-induced soil acidification are reviewed and discussed.
The growth of legumes which are fixing atmospheric N₂ involves the excess uptake of nutrient cations over anions from soil solution. This results in the net efflux of H₃O⁺ ions from plant roots into the rhizosphere.
When virgin lands are sown with legumes the accumulation of soil organic matter, with a consequent increase in cation exchange capacity and exchange acidity, is an important contributing factor to the long-term decline in surface soil (0–10 cm) pH. Nonetheless, such a phenomenon does not explain the decrease in pH below 10 cm soil depth nor the lower pH below leguminous than all-grass pastures. The efflux of H₃O⁺ ions from the legume roots may have an important effect on the soil pH under such conditions. The loss of symbiotically fixed N; from the system through leaching of NO₃⁻ - N may also contribute to soil acidification under leguminous pastures.