In-Season Nitrogen and Phosphorus Availablity From Fall Compost Application in Spring Wheat.

Current interests in carbon sequestration, organic farming, and alternative fertilizer requires information on nitrogen (N) mineralization rates of compost under irrigated conditions in Southeast Idaho.  The objectives of this study were to examine the effect of different compost and fertilizers rates on nitrogen (N) and phosphorus (P) availability for a spring wheat crop on small.  Compost was applied and incorporated in the fall before spring planting at rates of 0, 22, 34, 45, 67, and 90 Mg ha^-1.   Compost treatments were split into four sub-plots which included a control, N fertilizer only (135 kg ha^-1), P fertilizer only (90 kg ha^-1), and N and P fertilizer treatments.  Results from 2009 did not include N only and P only fertilizer treatments.  Results from 2009 showed that NO₃-N and Olsen’s P were not different among compost only treatments, and fall and spring NO₃-N levels for the plots were not different.  The average fall P level across compost treatments was 107 kg ha^-1 which was higher than spring P levels (59 kg ha^-1), but the spring and fall control P levels (63 and 67 kg ha^-1, respectively) were not different suggesting compost application increased P fertility.  The compost plus fertilizer and fertilizer only treatments had the highest yield (6.4 and 6.2 Mg ha^-1, respectively), and the 22 Mg ha^-1 treatment had the lowest yield (3.0 Mg ha^-1).  Compost only treatments were related to yield.  The control treatment yield was 3.7 Mg ha^-1 which was higher than 90 Mg ha^-1 treatment (3.0 Mg ha^-1) suggesting the compost may have caused denitrification.  Soil temperatures during the summer of 2009 were between 3 and 7° C lower than the 14 year average.  The cooler temperatures may have slowed N and P mineralization.  Soil test and yield results taken during the 2010 growing season are not yet available.  It is hypothesized that a combination of compost and N fertilizer will be the best yielding treatments.  In conclusion, compost had little effect on N fertility in spring soft white wheat in southeast Idaho which could be the result of cooler soil temperatures and volatilization of N during the composting process (increased C:N ratio potentially causing denitrification).  Phosphorus levels increased when large amounts of compost (45 kg ha^-1) were applied, and these levels may be adequate for optimal yield (but may not be economically feasible).