Soil-test biological activity with the flush of CO2: VII. Validating nitrogen needs for fall-stockpiled forage

Abstract

Nitrogen fertilizer is an expensive input, and its necessity in grazed tall fescue (Schedonorus arundinaceus) pastures can be questioned when sufficient soil N mineralization occurs. A soil-testing tool that could predict the need for N fertilizer inputs would be beneficial to producers to optimize profit and avoid environmental contamination. Thirty-seven on-farm trials were conducted in the Piedmont and Blue Ridge regions of Georgia, South Carolina, North Carolina, and Virginia in 2018 to evaluate fall-stockpiled tall fescue yield response to N and P fertilizer inputs. Carbon and N characteristics of the soil surface (0- to 10-cm depth) and surface residue varied among sites. Forage mass responses to P fertilizer input were below the economic threshold, regardless of Mehlich-III extractable P. Forage mass responses to N fertilizer input did not exceed a low cost/value threshold of 5 kg forage kg−1 N in 24 of the trials. Economically optimum N rate was greatest when soil N mineralization and soil-test biological activity (STBA) were at low levels. Results validated those of an earlier study on 55 fields, and taken together, suggest that N fertilizer for fall-stockpiled tall fescue could be as high as 80 kg N ha−1 on fields with very low STBA (<100 mg CO2–C kg−1 soil 3 d−1) and declining to nil with medium STBA (>250 mg CO2–C kg−1 soil 3 d−1), depending on cost/value threshold. Healthy soils with high STBA can be managed effectively without N fertilizer inputs to recycle nutrients and promote more sustainable agricultural systems.