Effect of N Fertilizer On Nodulation, Yield, and Quality of Alfalfa.

Alfalfa producer traditionally have used fertilizer that contained only phosphorus (P) such as triple super phosphate (TSP), but the cost of TSP is greater than other P fertilizers such as monoammonium phosphate (MAP).  Some producers are concerned that using P fertilizers that contain some nitrogen (N) could delay or reduce rhizobia nodulation.  The objective of this study is to determine how nitrogen (N) and phosphorus (P) fertilizers affect root development, nodulation of rhizobia, yield, and feed quality of alfalfa (Medicago sativa). Healthy root development and nodulation is essential for alfalfa to obtain sufficient N for optimal yields and quality.  The objective of this study is to quantify the affect of N and P fertilizer on the quantity of nodules per plant, the density of nodulation, and forage yield and quality. Soil samples were taken in the spring of 2012 to determine initial fertility levels.  Fertilizer was applied pre-plant and treatments were a control, 112 kg P ha^-1 only, 22 kg N ha^-1 only, 44 kg N ha^-1 only, and 5 treatments with 112 kg P and 11, 22, 33, 56, 82 kg N ha^-1. Roots for nodule counts were obtained by removing plants from a 12 inch strip randomly within the plot area and counting the nodules on the roots.  Nodule samples were taken at 4, 6, and 10 wks after planting.  Alfalfa was harvested at pre-bloom stage by cutting 5 ft swath 20 ft long and avoiding nodule sampling areas. Initially analysis of nodules counts showed no differences among treatments at 4 and 6 wks after planting.  These results suggest the fertilizer has no affect on nodulation during the initial months of alfalfa growth.  The study is on going, and week 10 nodule counts, yield, and forage quality have yet to be measured.  The study is in the first year of a four-year study to examine the long-term affect of N fertilization on alfalfa nodulation, yield, and forage quality.