Issue No. 17, Article 5/July 18, 2008
Plant-Nutrient Value of Wheat Straw
The wheat crop this year will be remembered positively by many in Illinois. The harvest was late, but at this point most fields have been harvested. Now the order of the day is what to do with the straw before starting a double-crop. The office of the Biomass Program of the United States Department of Energy plans to increase the use of crop residue for generating renewable fuel to reduce reliance on foreign oil. This initiative to use crop residue from crops has stimulated interest in baling this year's wheat straw. I have observed wheat bales in several fields over the past week, and several people have called wondering what the fertilizer value of wheat straw is.
Just as with harvested wheat grain, straw removal represents an export of nutrients from the field. In an article I wrote last year in the Bulletin (issue no. 23; October 5, 2007), I mentioned a few steps needed to calculate nutrient removal when corn stover is taken out of the field. Those same steps can be used to calculate removal of nutrients when wheat straw is baled.
The first step in the calculation is to determine how much straw is being produced. Estimates are typically a function of plant height and grain yield. These estimates vary somewhat, but a typical value is close to 1 pound of straw per pound of grain.
The second step is to determine how much straw is actually removed. This will be a function of cutting height and how much of the straw lying on the ground is actually baled. Generally speaking, straw yields of a good wheat crop are no more than 2.5 to 2.8 tons per acre (dry matter bases).
The third step is to know the nutrient content of the straw. A ton of wheat straw typically has 9 to 12 pounds of nitrogen (N), 3 to 4 pounds of phosphorous (P) in the form of P2O5, and 25 to 45 pounds of potassium (K) in the form of K2O. The large variability in K content arises from the fact that K is not incorporated into organic compounds and thus can be easily leached out of the residue. The time elapsed and the amount and frequency of precipitation since the crop reached maturity together with the time the straw is removed from the field impact how much K there is. Typically, K leaching occurs rapidly. It is not unusual to see a decrease in K content of as much as 80% after the straw has been rained on a few times. On the other hand, N and P content in the straw are more or less stable because they are present in organic forms and require microbial decomposition before they can be leached out.
The values presented in steps 1 to 3 are estimates, of course, but there could be substantial variation depending on how the straw is processed. The best way to determine more precisely how much fertilizer is being removed with the straw is to obtain the weight of the bales and to extract a representative sample of the straw once it is baled to determine nutrient content.
The final step is to calculate the fertilizer value. This is done by using the information above and the current price of fertilizer to arrive at a value of dollars per acre. It is important to keep in mind that there are other costs associated with straw removal that are not likely to be readily seen in the near future. Additional crop nutrients, such as calcium, magnesium, sulfur, and micronutrients, are being removed when wheat straw is taken out of the field. In Illinois these nutrients are typically at sufficiency levels, so it is not necessary to supply them through fertilization. Greater removal of these nutrients by removing straw residue can accelerate a deficiency in the soil. Removal of basic cations (such as K, Ca, and Mg) can lead to an increase in the need to lime soils to maintain adequate pH levels. Depletion of organic matter and N reserves can lead to less crop availability of N through the process of mineralization (conversion of organic N to inorganic N). Diminishing organic carbon contents can also result in negative impacts on soil physical, chemical, and biological properties and conservation of one of the most important assets of Illinois: the soil. So carefully consider all factors when estimating the true cost of straw removal. --Fabián G. Fernández