The nutrient value of wheat straw is influenced by several factors, including weather, variety, and cultural practices. Thus, the most accurate value requires sending a straw sample to an analytical laboratory. However, book values can be used to estimate the nutrient value of wheat straw (Table 6.7).
Wheat straw removes proximately 14 to 18 pounds of nitrogen per ton. However, 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 or nitrate form, it is available for plant uptake. The rate at 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-to-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 less than or equal to 20-to-1. At a C:N ratio of 80-to-1, mineralization will be much slower. For comparison, corn stover is reported to have a C:N ratio of 57-to-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 nitrogen credit is given for wheat straw because it is not known when the nitrogen will mineralize and become available to the following crop.
The economic value of the nutrients removed in wheat straw fluctuates with the price of P2O5 and K2O fertilizer. Table 6.8 shows the economic value of P2O5 and K2O at several fertilizer price points.
