Using a Simple Rhizotron for Observing Fertilizer Affects On Root Development.

Alfalfa producers are concerned with the effect of N fertilizer on Rhizobia nodulation in newly seeded alfalfa. It has been a common held belief that adding N to alfalfa will inhibit or delay nodule development. The objective of this study was to determine the effect of N and P fertilization on rhizobia nodulation in alfalfa. Using simple rhizotron scanners, nodulation of Rhizobia was determined and measured for different rates of fertilizer applied to newly planted alfalfa.  Fertilizer treatments consisted of N, P, and N+P at rates of 22, 44, 66, and 88 kg ha^-1. Rhizotron scans were performed daily for 28 days following planting with scan resolution set at 1200 dpi. Statistical analysis was performed using ANOVA and an LSD means test.  The 22 kg P ha^-1 treatment had more nodules (7) when compared to all other treatments. The means separation test showed that the 44, 66, 88 kg P ha^-1, the 22, 44 kg N+P ha^-1, and the control treatments were not different, and the mean number of nodules for these treatments was 2 to 3 nodules per plant. It was observed that as the P fertilizer rate increased the mean nodule formation decreased.  The treatments containing only N fertilizer had no nodules except for one plant in the 88 kg N ha^-1 treatment that had two nodules. These results suggest that some P fertilization improves Rhizobia nodulation in newly seeded alfalfa.  Additionally, application of N with P fertilization may delay nodulation whereas applying only N fertilizer may delay or inhibit nodulation. These initial results are limited to only one trial, but the research is on going and additional replications will be performed.