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

Authors: Franzluebbers, Alan; POORE, MATTHEW - North Carolina State University

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/28/2020
Publication Date: 5/27/2020
Citation: Franzluebbers, A.J., Poore, M.H. 2020. Soil-test biological activity with the flush of CO2: VII. Validating nitrogen needs for fall-stockpiled forage. Agronomy Journal. 112:2240-2255.

Interpretive Summary: Soil testing for nitrogen availability is now possible with a rapid and reliable indicator that assesses soil biological activity. A scientist from USDA-Agricultural Research Service in Raleigh NC collaborated with a ruminant livestock specialist from North Carolina State University to evaluate fall-stockpiled tall fescue productivity responses to nitrogen fertilizer inputs at 37 field sites in Georgia, South Carolina, North Carolina, and Virginia. More than 60% of field sites had no economic gain from nitrogen fertilization. None of the sites responded significantly to phosphorus fertilizer inputs. Soil organic carbon and nitrogen fractions were predictive of the extent of yield response to nitrogen fertilizer. Soil nitrogen mineralization varied widely among sites and was predictive of the need for nitrogen fertilizer if in short supply. As a quick and reliable alternative, soil-test biological activity was also predictive of the need for nitrogen fertilizer. This study demonstrated that cattlemen can increase profit by adopting good grazing management principles and adjusting nitrogen fertilizer inputs in response to conserved nutrient cycling in the biologically active organic fraction.

Technical Abstract: Nitrogen fertilizer is an expensive management input, and its necessity in grazed tall fescue (Schedonorus arundinaceus) pastures can be questioned when much of the N in pastures is recycled. A soil-testing tool that could predict the need for N fertilizer inputs would be beneficial. We conducted a series of 37 yield response trials to N and P fertilizer inputs on farms in the Piedmont and Blue Ridge regions of Georgia, South Carolina, North Carolina, and Virginia in fall of 2018. Soil-surface characteristics (0-10-cm depth) and surface residue C and N varied widely among sites, as expected. Yield responses to P fertilizer input were below economic threshold, irrespective of Mehlich-III-extractable P. Yield responses to N fertilizer input did not exceed low cost-to-value threshold of 5 kg forage/kg 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. Yield responses to N fertilizer were greater in the >10-cm than the 5-10-cm harvest layer. Results validated those of an earlier study on 55 fields, and taken together suggest that N fertilizer for fall-stockpiled tall fescue should be as high as 80 kg N/ha on fields with very low STBA (<100 mg CO2-C/kg soil/3 d) and declining to nil with medium STBA (>250 mg CO2-C/kg soil/3 d), depending on cost-to-value threshold. Healthy soils with high STBA can be managed effectively without N fertilizer inputs to recycle nutrients and promote more sustainable agricultural systems.