The question that is often asked is just how much nutrient is being removed with that baling of straw around the state?
From a pure fertilizer standpoint, wheat straw contains very little in terms of phosphorus (P2O5) but moderate amounts of nitrogen (N) and potassium (K2O), said Robert Mullen, director of agronomy, Potash Corp/PCS Sales. The actual amounts of N, P2O5, and K2O contained in a ton of wheat straw are 11, 3, and 20 pounds, respectively (or an analysis of 0.6-0.2-1 if it were printed on a fertilizer bag).
“These removal rates are based upon average nutrient removal estimates from various publications,” Mullen said.
Some estimates of nutrient removal can be based upon grain yield, but those estimates have an underlying assumption, he said. The Tri-State Fertilizer Recommendations have a K2O removal of 0.91 pounds per bushel of grain. This nutrient content estimate is likely based upon an assumption of a harvest index of 0.4. Harvest index is a ratio of the grain weight relative to the total amount of biomass produced. A harvest index of 0.4 indicates that 40% of the total biomass produced was harvested as grain, and the other 60% of the total biomass was straw.
“Considering current wheat varieties, harvest index is likely closer to 0.5, but harvest index is also very sensitive to environmental conditions,” Mullen said. “Tough late-season growing conditions can reduce harvest index. Have you ever heard the statement, ‘Ninety-bushel straw and 60-bushel grain yield?’”
How much is that straw worth from a nutrient perspective? Well, it obviously depends upon the current market value of nutrients. Using today’s prices, a pound of N, P2O5 and K2O costs 53 cents, 60 cents, and 50 cents, respectively. Thus, a ton of straw will contain $17.63 worth of nutrients, he said.
“Again, this number can be variable, but it gives you a starting point for your own economic analysis,” Mullen said.
Actual nutrient content can vary based upon environmental conditions during the growing season and soil nutrient supply, so if one really wants to know the actual value, straw analysis can be conducted by any lab that processes plant samples, he said.
The most precise way to determine how much fertilizer is being removed with the straw is to obtain the weight of the bales and extract a representative sample of the straw, said Fabian Fernandez, University of Illinois Extension specialist in soil fertility and plant nutrition. Using that sample, two values must be obtained from a testing lab: moisture content to determine the dry weight of the bale and nutrient content of the sample reported on a dry basis.
“This approach may not be practical or possible for all producers,” Fernandez said. “If that is the case, then it is possible to estimate the value of straw by following a few simple steps. But keep in mind that this second approach is an estimate and there can be substantial variability depending upon how the straw is processed.”
The first step in the calculation is to determine how much straw is being produced. The best estimates are obtained as 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, he said. In this step, remember that a bushel of wheat is 60 pounds, but there are 8.1 pounds of moisture (a 60-pound bushel of wheat contains 13.5% moisture). So a bushel of wheat is 51.9 pounds of dry grain. The dry grain yield in bushels per acre is multiplied by 51.9 and then divided by 2,000 to obtain the number of dry tons of straw produced.
The second step in the calculation is to determine how much straw is actually removed. Again, this is not an absolute value because it will depend on cutting height and how much of the cut straw is actually baled. In general, a good wheat crop yields about 2.5 to 2.8 tons of straw per acre on a dry matter basis, Fernandez said.
The third step in the calculation is to know the nutrient content of the straw. As a guide, a ton of wheat straw normally contains 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, allowing K to easily be leached out from the residue,” he said. “The time elapsed, the amount and the frequency of precipitation since the crop reached maturity and the time the straw is removed from the field will 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 contents in the straw are more or less stable because they are present in organic forms and require microbial decomposition of the straw before they can be leached out, Fernandez said.
The fourth and final step in the calculation is to determine the fertilizer value. This is done by using the above information and the current price of fertilizer to obtain a value of dollars per acre.
“Removal of major nutrients (N, P, K) is just one of the costs associated with straw removal, but there are other costs that might not be easily observed in the short term, but can have important impacts on future productivity of a field,” he said.
In addition to the major nutrients, wheat straw also contains calcium (Ca), magnesium (Mg), sulfur (S), and micronutrients. Greater removal of these nutrients by harvesting straw can accelerate depletion rates and result in deficiencies in the future, Fernandez said.
“Removal of basic cations, like K, Ca and Mg, can lead to an increase in the need to lime soils to maintain adequate pH levels,” he said. “Depletion of organic matter and N reserves can lead to less crop availability of N through the process of mineralization. Diminishing organic carbon contents can also result in negative impacts on soil conservation, and on soil physical, chemical and biological properties. It’s important to consider all of these factors when estimating the true cost of straw removal.”