Wheat harvest will soon be underway; we often get questions about the nutrient value of straw. The nutrient value of wheat straw is influenced by several factors including weather, variety, and cultural practices. Thus, the most accurate values require sending a sample of the straw to an analytical laboratory. However, “book values” can be used to estimate the nutrient values of wheat straw.
In previous newsletters, we reported that typically a ton of wheat straw would provide approximately 11 pounds of N, 3 pounds of P2O5, and 20 pounds of K2O. Michigan State University reports similar numbers for a ton of wheat straw: 13 pounds of N, 3.3 pounds of P2O5 and 23 pound of K2O. A 2013 analysis of wheat straw collected at the OARDC farm in Wooster contained 14-18 pounds of N, 3-4 pounds of P2O5, and 20-23 pounds of K2O. These values were across four wheat varieties and three spring nitrogen application rates (60, 90, and 120 lb N/acre). The 2013 values corresponded fairly well with the previously reported “book values.” Nitrogen values in 2013 were slightly greater than “book values” which may have been a result of wheat height/size. If plants are shorter/smaller, percentage nitrogen tends to be greater than taller/larger plants due to a dilution factor as the plant grows. Regardless, the 2013 analysis provides validity to the nutrient value of straw given in previous newsletters.
The nitrogen in wheat straw will not immediately be available for plant uptake. The nitrogen will need to be converted by microorganisms to ammonium and nitrate (a process called “mineralization”). Once the nitrogen is in the ammonium and/or nitrate form, it is available for plant uptake. The rate of which mineralization occurs depends on the amount of carbon and nitrogen in the straw (C:N ratio). The USDA reports a C:N ratio of 80:1 for wheat straw which means there are 80 units of carbon for every unit of nitrogen. Mineralization rapidly occurs when the C:N ratio is ≤ 20:1. At a C:N ratio of 80:1, mineralization will be much slower. (For comparison, corn stover is reported to have a C:N ratio of 57:1.) Rate of mineralization is also influenced by soil moisture and temperature. Since mineralization is a microbial-driven process, mineralization will be slowed (halted) in the winter when temperatures are cold. Thus, no N credit is given for wheat straw since it is not known when the N will mineralize and become available to the following crop.
Besides providing nutrients, straw has value as organic matter, but it is difficult to determine the dollar value for it. Removal of straw does lower soil potash levels. If straw was removed after heavy rainfall, some of the potash may have leached out of the straw, lowering the nutrient value of the straw. However, a soil test should be done to accurately estimate nutrient availability for future crops.