Effect Of Nitrogen and Sulfur Rates and Timing On The Strength and Decomposition Of Wheat Residue.

Application of nitrogen (N) fertilizer as urea ammonium nitrate (UAN) and N plus sulfur (S) fertilizer as ammonium thiosulfate (ATS) in a mist on the residue to stimulate microbial activity and subsequent decomposition of the residue is often debated for its potential to solve problems establishing a good plant stand in high residue situations. We conducted wheat straw decomposition field experiments under different fertilizer rates and combinations at three locations in western Kansas in 2011 and 2012.  UAN was applied at rates of 0, 11.2, 22.4, 33.6, and 67.3 kg N/ha. ATS was applied at rates of 16.8 and 33.6 kg S/ha, which also contained 7.7 and 15.5 kg/ha N, respectively. There were a total of 13 treatments with four replications. Liquid fertilizer was applied to the residue with a flat fan spray tip onto 37.2 square meter plots. Treatments were applied in November 2011 and February 2012. Wheat straw was collected in late spring of 2012 and 2013 to determine residue mass and strength. A double shear box apparatus instrumented with a load measured the shear stress required to cut wheat straw. Twenty-five wheat straws from each plot were tested. A photomicrography technique and SigmaScan software were used to measure the cross-sectional area of wheat straw after shearing. Results showed the timing of application did not affect wheat straw strength parameters. The wheat straw from plots sprayed with a mix of UAN and ATS in fall 2011 had significantly lower shear stress at two sites out of three. Also, the UAN with 11.2 kg N/ha rate applied in 2012 spring decreased wheat residue shear stress and specific energy at two sites (p<0.05). Liquid fertilizers decreased straw strength relative to the no-fertilizer control, thus indicating potential benefits of liquid fertilizers to speed decomposition in heavy residue situations.