Evaluating Soil N Tests for Winter Wheat N Management in the Southeastern USA.

The development of an efficient and accurate method to predict in-season N requirements for winter wheat in the Southeastern USA is needed given the economic and environmental concerns surrounding N fertilizer use. The Amino Sugar Nitrogen Test (ASNT) which determines a potentially mineralizable soil N fraction shows promise for making in season N predictions. Other soil N tests which detect NO₃-N and NH₄-N have had little success when used alone to predict N rates for crops in humid, high rainfall environments. The objective of this research is to develop an in-season fertilizer N recommendation system using different soil N tests for winter wheat in the humid southeastern USA.

Field experiments were conducted under conventional and no-till conditions during 2005-2008 at 60 sites across North Carolina. A randomized complete block design with 5 replications was used with N rate as the main plot treatment. There were 7 fertilizer N treatments applied in Spring at GS-30 ranging from to 0 to 168 kg ha^-1. Soil samples were collected in mid-Winter at GS-25 to three depth increments (0-10 cm, 0-20 cm and 0-30 cm). These samples were analyzed for amino sugar N (ASN), nitrate, ammonium and soil organic matter content. ASN analysis was conducted using a modified procedure described by Williams et al. (2007). Site information such as soil series, map unit, drainage class and previous crop rotation was gathered at each site. At harvest grain and straw yield data were collected and grain and straw N analysis were conducted in order to ascertain crop N uptake. This data was then analyzed in SAS.

ASN and grain yield at optimum N rate were correlated. Soil mineral nitrogen needs to be accounted for in fertilizer N prediction models for sites where N carryover is present. The ASNT may be a useful tool to aid N management decisions for winter wheat in the Southeastern USA.