Nitrogen and Phosphorus Fertilizer Effects on Nodulation, Yield, and Quality of Newly Seeded Alfalfa.

Alfalfa (Medicago sativa) producers 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 quantify N and P fertilizer effect on the quantity of nodules per plant, the density of nodulation, and forage yield and quality in alfalfa. Soil samples were taken in the spring of 2012-14 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 inch strip randomly within the plot area and counting the nodules on the roots.  Nodule samples were taken at 2, 3, 4, 5, 6 and 8, 11, and 14 wks after emergence.  Alfalfa was harvested at pre-bloom stage by cutting 1.25 m swath 6 m 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 influence nodulation. The study is in the third year of a four-year study to examine the long-term affect of N fertilization on alfalfa nodulation, yield, and forage quality.