Nitrogen on Peanuts

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Peanut Response to Inoculation and Ammonium Sulfate Rate in North Carolina.

M. CARROLL*, T. BRITTON, C. FOUNTAIN, M. PARRISH, D.L. JORDAN, and P.D. JOHNSON. North Carolina Cooperative Extension Service, Raleigh, NC 27695.

Adequate nitrogen (N) fixation by peanut is essential for optimum yield. Approximately 75% of growers apply Bradyrhizobia inoculant to peanut. In 35 replicated trials in North Carolina from 1999-2014, applying in-furrow liquid or granular inoculant increased yield from 3,507 lbs/acre to 5,072 lbs/acre in new peanut fields and 4,256 lbs/acre to 4,454 lbs/acre in fields with a previous history of peanut. Economic return was determined as the product of pod yield and price less a production cost of $916/acre without inoculant and $924/acre with inoculant. The increase in economic value from inoculation ($8/acre) in new ground at peanut prices of $355/ton, $425/ton, and $535/ton was $269/acre, $324/acre, and $410/acre, respectively. In rotated ground with a previous history of peanut, the increase in economic value from inoculation was $27/acre, $34/acre, and $45/acre at these respective peanut prices. In a second experiment, ten replicated trials were conducted from 2007-2014 in fields without a history of peanut production or fields not rotated to peanut in recent memory to determine peanut response to N rate. Economic return based on peanut prices described previously was determined to reflect cost of ammonium sulfate (AMS) priced at $0.28/lb. Ammonium sulfate was applied at rates of 285, 428, 571, and 714 lbs/acre corresponding to N rates of 60, 90, 120, and 150 lbs/acre in one application 45-60 days after planting when canopy foliage began to express N deficiency. A no-inoculant/no-AMS control was included along with in-furrow application of inoculant without AMS. In 5 of 10 trials, AMS applied did not increase pod yield or affect economic return compared with non-treated peanut regardless of peanut price (p > 0.05). In these fields N deficiency was observed but was not extensive based on visible assessments of the canopy compared with inoculated peanut. In 3 trials a linear increase in yield and a linear increase in economic return for the 3 pricing structures were observed as the rate of AMS was increased. In 2 additional trials a linear increase in yield was observed across AMS rates for yield while a quadratic response was noted for economic return across pricing structures. Peanut yield following inoculation in absence of AMS equaled or exceeded yield when inoculant was not included regardless of AMS rate. In the trials where a significant increase in yield was noted as AMS rate increased, visible symptoms of N deficiency were severe early in the season and in some cases excessive rainfall occurred during the month following application. Collectively, results from these experiments demonstrate the economic value of inoculation in both new ground and fields with a previous history of peanut. In these respective types of fields, at $535/ton increases in economic return were approximately 50-fold and 5-fold over inoculation cost of $8/acre. Determining the most effective rate of AMS to correct a visible N deficiency was more difficult. Due to variation in response and unknowns relative to rainfall after application and plant available N in soil, the current recommendation in North Carolina is to apply AMS at 500 lbs/acre when peanut foliage expresses N deficiency and nodulation is non-existent or very poor. This recommendation is conservative in most instances but may be excessive or limiting under some conditions.

Article first appeared as North Carolina Peanut Note (PNNC-2015-014)