Maximizing crop yields, efficient use of N fertilizer, and minimizing nitrates in ground water depends on understanding the relationship between plant N uptake and the availability of soil mineral N, mineralized soil N, fertilizer N, and the availability of N from green manure. Interior Alaska soils remain relatively cold throughout the short summer, but long days and moderate air temperatures promote rapid plant growth taxing the soils' ability to supply N to the crop. This presentation combines information from several studies conducted on Volkmar silt loam near Delta Junction, Alaska, and relates the availability of various N sources to barley N uptake during the short growing season of interior Alaska. There is potential to grow about 3000 kg grain ha ^-1 yr^-1. To achieve this, barley will take up 100 kg N ha ^-1, of which 75% will occur by July and the remainder by August. There is about 10 kg N ha ^-1 as nitrates in the soil at spring planting, another 30 kg N will mineralize from the soil organic matter by the time of maximum N uptake. About 10 kg N will mineralize between crop maturity and fall freeze-up, which is subject to leaching during spring thaw. The crop must get 60 kg N ha ^-1 from external sources. Green manured fababeans were found to supply about 10 kg N ha ^-1 to the barley crop the following year. Therefore, 50 kg N ha ^-1 must be applied as fertilizer even following a green manure crop. About 25% of the N in green manured fababeans is mineralized the first year after incorporation. Continued use of green manure in rotation with barley is expected to increase the rate of soil N mineralization, but it will take several years to meet the demands of a barley crop.

Verlan L. Cochran
USDA-ARS, Northern Plains Soil and Water Research Center
P. O. Box 1109
Sidney, MT  59270
Phone:  (406) 482-2020
E-mail: vcochran@ars.usda.gov