Optimizing Nitrogen Management on Organic and Biologically-Intensive Farms.

Organic farmers face a difficult problem in managing nitrogen: the magnitude and timing of nitrogen mineralization from organic matter cannot be easily predicted. Soil texture and management history both influence soil N-mineralization potential. Well-managed plant-soil systems are tightly coupled; they are balanced in terms of nitrogen availability and nitrogen uptake. A study was conducted with 5 organic vegetable producers across Washington State in 2016 and 2017. Nitrogen contribution from organic matter and the economic benefit of 5 different rates of fertilizer application were assessed with broccoli. Soil organic matter ranged from 2 to 7.7% and soil texture ranged from loamy sand to silt loam. Across all farm sites in 2016, broccoli head weight increased linearly with applications rates as large as 269 kg nitrogen ha^-1 applied in an 11-0-0 organic feather meal fertilizer. In 2017, an additional two rates of organic nitrogen fertilizer were added for the largest rate of 538 kg nitrogen ha^-1. The rate of yield increase with increasing nitrogen varied across sites and may be related to management history and soil nitrogen mineralization potential. The fertilizer value of nitrogen in soil organic matter was estimated to range from 4300-9800 dollars ha^-1. Through this ongoing research we will be able to place dollar values on the importance of optimizing soil nitrogen fertility. Growers will be able to more accurately estimate nitrogen mineralization from soil organic matter and adjust fertilizer applications up or down accordingly.