Mission silt loams (coarse-silty, mixed, frigid Andic Fragiochrepts) and other similar, ash-influenced soils in northern Idaho have large P-fixing capacities. Consequently, forage legume production has traditionally been poor without large additions of P. The objectives of this study were to evaluate the P-fertilizer requirements and potential use of red and white clover as forage legumes on an ash-influenced soil.
A split-plot experimental design was established on a Mission soil at the University of Idaho Research and Extension Center at 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, 1983, and 1984. Yield, plant tissue P, and sodium acetate (NaOAc)-available soil P were the parameters evaluated.
Significant, positive relationships between P application rate and white clover yields were observed in 1982 (r2 = 0.76), 1983 (r2 = 0.65), and 1984 (r2 = 0.81). Conversely red clover did not respond to increasing P applications. Consequently, if the situation exists where P cannot be applied to these soils, red clover should be the forage legume of choice. This lack of response to applied P by red clover may be attributed to established mycorrhizal associations. Red clover was also able to reseed itself and persist at the study location with greater vigor than the white clover. Phosphorous-fertilization rates were related to both tissue and soil P levels.
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