Influence of pH on Yield and N and P Nutrition of Alfalfa Grown on an Andic Mission Silt Loam 1

Yields of alfalfa (Medicago sativa) average only 7500 kg/ha on ash‐influenced Mission silt loam (coarse‐silty, mixed, frigid Andic Fragiochrepts) in northern Idaho. This relatively poor growth is commonly attributed to the chemical characteristics of these andic soils. The objectives of this study were to: (1) determine the effect of soil pH on alfalfa establishment and growth, (2) N and P uptake, (3) Rhizobium meliloti survival, and (4) P availability in a Mission soil under greenhouse conditions. Aluminum sulfate and finely ground calcium carbonate were used to adjust soil pH. Soil pH values were 4.8, 5.1, 5.6, 6.2, 6.4, 6.9, 7.0, and 7.4. Alfalfa was grown in 150‐mm pots in the greenhouse and harvested five times at approximately 20‐day intervals. Yield was measured and plant material was analyzed for total N and P. Rhizobium meliloti populations were monitored using the most probable number (MPN) technique and available soil P was determined using sodium acetate extractant. Manipulation of pH in the Mission soil was found to have a significant effect on alfalfa growth, tissue N and P concentration, survival of R. meliloti in the soil, and on sodium acetate extractable soil P. In general, tested parameters increased with increasing pH. Greatest forage yield occurred at a pH of 7.4. The andic nature of the Mission soil requires a pH adjustment above pH 6.6 for acceptable alfalfa establishment and yields, probably because of the relatively high degree of Al saturation that occurred at pH values below 6.6. This is in sharp contrast to non‐andic northern Idaho soils where alfalfa yields are usually not adversely affected above pH 5.6.