Wheat harvest is wrapping up and a lot of straw has been bailed this week. If you are bailing straw, remember to account for the additional fertilizer removal when planning for fertilizer needs next year. Each bushel of wheat removes .5 lbs of P2O5 and .25 lbs K2O. So 100 bushel/acre of wheat removes 50 lbs of P2O5 and 25 lbs K2O. The following article summarizes the value of the additional nutrients removed in the straw.
Before removing straw from the field, it is important to understand the nutrient value. Though we have seen some softening of the 2022 fertilizer prices, P and K fertilizer prices remain higher than normal. 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 value of wheat straw. In previous newsletters, we reported that typically a ton of wheat straw contains approximately 11 pounds of N, 3.7 pounds of P2O5, and 29 pounds of K2O. According to June 2023 fertilizer prices (Source: DTN Fertilizer Price Index: Ohio) and nutrient removal “book values”, one ton of wheat straw would remove N, P, K valuing approximately $25.13 ($16.55 of P2O5 & K2O).
Although N adds value, we do not give it an economic value in the form of fertilizer (as seen in Table 1). Within straw, N is in an organic form and will not immediately be available for plant uptake. The organic-N will need to be converted by microorganisms to ammonium-N (an inorganic form) before it is available for plant uptake – a process called mineralization. The rate of which mineralization occurs depends on the amount of carbon (C) and N 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 C for every unit of N. 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 (i.e., value) is given for wheat straw since it is not known when the N will mineralize and become available to the following crop.
In addition to N, removal of straw does lower soil K levels. If straw is removed after heavy rainfall, some of the K may have leached out of the straw, lowering the nutrient value. However, a soil test should be done to accurately estimate nutrient availability for future crops. Besides providing nutrients, straw has value as organic matter, but it is difficult to determine the dollar value for it.