Alfalffa,Medicago sativa L., in highly weathered,acid soils

Summary Alfalfa plants,Medicago sativa L., were selected from the Florida 66 cultivar for vigor in an acid (pH 4.4, Al≥.4 meq/100 g) and a limed, fertilized (pH 6.5, Al=0 meq/100 g, P and K added) Cecil topsoil. The selected plants were intermated by selection condition to achieve two germplasms, acid selected (A-1) and limed, fertile selected (L-1). ARhizobium meliloti strain (79-4s) was isolated from a high acetylene reducing nodule from a plant in a similar acid soil. The germplasms and the Rhizobium strain were then tested in greenhouse pots for agronomic performance under a variety of soil pH and fertility conditions. The 79-4s inoculum, as well as commercially prepared inoculum, gave better plant yield and acetylene reduction (N₂-fixation) at all harvests when compared to a sterile peat control, but the commercial inoculum was the best inoculum treatment. The A-1 germplasm produced higher shoot dry weight at the final harvest than did the L-1 germplasm at all soil pH’s when P and K were applied at the highest rates. The A-1 germplasm also had better root weight (mainly fibrous roots) and acetylene reduction in these soil conditions. The two germplasms appear to be genetically distinct and respond differently depending on soil pH and fertility conditions.