Nitrogen Fertilization of Peanut to Rescue an Inoculant Failure.

Peanut (Arachis hypogaea L.) is leguminous and fixes N when Bradyrhizobia infect the root.  However, improper handling of inoculants or adverse conditions can reduce or negate efficacy.  In the event of an inoculant failure, re-application is not an option and surface applications of fertilizers are the primary options for supplementing the plant’s N needs.  An experiment was conducted in Tifton, GA in 2010 (one location) and 2011 (two locations) on land with low native rhizobia populations (minimum of 15 years since last peanut planting) to simulate an inoculant failure.  In addition to a commercial liquid inoculant control, all other treatments did not include an inoculant but had soil applications of ammonium sulfate at 0, 67, 135, 202 kg N ha^-1 at first bloom, or split applications of either 67 or 135 kg N ha^-1 at first bloom followed by an additional 67 kg N ha^-1 at early pod fill.  In all trial replicates, the untreated plots had from 31-71% lower nodule weight than inoculated plants.  When N fertilizer was applied in-season, nodulation was further reduced.  With 67 or 135 kg N ha^-1 (whether single application or split), a reduction in nodule weight in the range of 9-67% occurred in all instances compared to the untreated plots.   At 202 kg N ha^-1 (single application or split), the reduction in nodule weight was at least 55% in five out of six instances compared to the untreated.  There were few statistical differences in yield, but inclusion of inoculant provided positive net revenue over the untreated in two of the three site-year locations.  Also, there were no instances where N fertilizer provided an economic advantage compared to the untreated plots.  Based on these results, rescuing an inoculant failure with N fertilizer is not cost-effective, although more research is needed to evaluate additional management conditions.