Nitrogen and Phosphorus Fertilizer Effects 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 nitrogen (N) could delay or reduce rhizobia nodulation.  The objective of this study is to determine how N and P fertilizers affect root development, nodulation of rhizobia, yield, and feed quality of new seeded 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 and 2013 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 plants from within the plot area and counting the nodules on the roots.  Nodule samples were taken at 4, 5, 6, 7, 8 and 10, 13, and 16 wks after planting.  Alfalfa was harvested at pre-bloom stage by cutting 5 ft swath 20 ft long and avoiding nodule sampling areas. The first year of the study showed that P treatments had a greater influence on nodulation than did N treatments.  Treatments containing both N and P had higher yields than the control.  Results showed that N had no influence on yield or relative feed quality but it may delay nodulation.