Stand Dynamics and Yield Components of Alfalfa as Affected by Phosphorus Fertility

Stand maintenance of alfalfa (Medicago sativa L.) relies on the persistence of the original plants. Yield (yield per plant, yield per shoot) and stand components (plants per square meter, shoots per square meter, shoots per plant) were studied in two experiments designed to determine yield response to P on a low P soil (Windthorst fine sandy loam; fine, mixed, thermic, Udic Paleustalfs) near Stephenville, TX. Three preplant incorporated rates of P (0, 29, and 59 kg ha - 1) and five annual surface broadcast rates of P (0, 7, 15, 29, 59 kg ha - 1) were applied to two field experiments established in September 1988 and 1989. The experimental design was a split plot in four randomized blocks. Whole plots were incorporated P rates and subplots were annual broadcast P rates. Plants and shoots per square meter were counted 4 and 8 wk after planting in 1988 and 1989, at each harvest in 1989 and 1990, and at the beginning and end of the season in 1991. Relative distribution of root mass to a 90-cm depth was determined in the fall of 1990 and 1991 for selected treatments. Phosphorus incorporated before planting increased alfalfa dry matter (DM) yields more efficiently than did broadcast P. Plant and shoot densities did not differ among treatments; thus, low yields at low P levels were a function of reduced yield per shoot. Delayed application of P on fall-planted alfalfa did not affect plant density. Under 0-P, root mass was concentrated in the upper 20 cm of soil, whereas under adequate to high P, more root mass was distributed throughout the upper 40 to 50 cm.