FORAGE PRODUCTION ON ANDIC SOILS

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Abstract
Ash-influenced soils in northern Idaho have large P-fixing capacities. Consequently, alfalfa (Medicago sativa L.) production has traditionally been poor without large additions of P. The objectives of this study were to determine the P fertilizer requirements and potential use of alfalfa and birdsfoot trefoil (Lotus cornicu-latus L.) as forage legumes on an ash-influenced soil. A split-plot experimental design was established on a Mission silt loam (coarsesilty, mixed, frigid Andic Fragiochrept) at the Univeristy of Idaho Research and Extension Center in Sandpoint, Idaho, in 1981. Forages served as main plots, while P rates of 0, 40, 80, and 120 kg/ha/yr served as subplots in 1981, 1982, and 1983. Yield, plant tissue P, and sodium-acetate (NaOAc)-available soil P were the parameters evaluated. Birdsfoot trefoil and alfalfa responded to increasing rates of fertilizer P as maximum yields of 11 487 and 6825 kg/ha, respectively, resulted from the highest P fertilization rate (120 kg P/ha/yr). Forty kilograms of P/ha applied to birdsfoot trefoil and alfalfa increased yield 169 and 131%, respectively, compared with yields of legumes with no P application in 1982. At high P rates, birdsfoot trefoil had the highest forage yields in 1982 and 1983. Forage yields of alfalfa and birdsfoot trefoil were greater in 1983 than 1982. Sodium-acetate-available P was found to be a better indicator of alfalfa (r2 = 0.87) and birds-foot trefoil (r2 = 0.89) yield than plant tissue P. © Williams & Wilkins 1986. All Rights Reserved.
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