Issue No. 23, Article 9/October 5, 2007
What Is the Nutrient Value of Corn Stover Removal?
In order to increase U.S. energy security, the Department of Energy Office of Biomass Program plans to increase the use of crop residue as a source of biomass for renewable fuel production. The initiative to utilize crop residue from crops, including corn, has stimulated interest in the removal of corn stover from the field. Many questions are being asked about the amount of stover that can be removed without adverse consequences to soil organic matter levels and to physical and chemical properties of the soil. Another important question: What is the nutrient value of corn stover? Several factors need to be taken into account to estimate total nutrient removal in corn stover and its value.
We are all familiar with the fact that removing grain also means removing nutrients from the soil. Since significant amounts of nutrients are not translocated to the grain but rather remain in the above-ground tissues of crops, removing residue from the field likewise means removing nutrients from the soil. (In this article, corn stover refers to all above-ground corn plant material except grain--leaves, stalks, shank, husks, and cobs.)
The basic steps I describe here are the ones to follow in order to determine the total amount of nutrient removal and the value of corn stover.
1. How much stover is produced in a corn field? The first step in determining total nutrients removed in stover is to calculate how much stover is produced. The amount produced is typically estimated from a harvest index (also known as a residue-to-corn grain ratio). The most widely used dry weight ratio is 1:1 residue to grain. Thus, to calculate the pounds of residue produced, multiply the grain yield (in bushels per acre) by 56 pounds. The value can then be divided by 2,000 to obtain the number of tons produced. Again, this is just an estimate; this calculation will tend to overestimate stover quantity in high-yield fields (>180 bushels per acre) and to underestimate in low-yield fields (<100 bushels per acre).
2. How much stover is actually being removed from the field? The second step is to determine how much of the total stover is being removed from the field. The most reliable determination method is direct measurement of the weight and water content of the residue being removed. Doing this would eliminate the first step and would give the most accurate information. But since this approach is not always feasible, the approximate removal amounts of the total stover need to be determined in relation to the harvest method. Data from Iowa State University has shown that shredding and raking stover removes 80% of the total; raking alone removes 65%, while collecting stover from the combine windrow removes 50%. Also, cutting height at the time of grain harvest influences how much stover can be removed. The higher the cutting height, the lower the amount of stover that can be removed. To obtain an estimate of the total amount of stover removed, multiply the estimated total stover produced (first step) by the percent removed by the method of harvest employed.
3. What is the nutrient content of stover? The third step in calculating the amount of nutrients removed when stover is taken from the field is to determine the stover's nutrient content. Again, the most accurate method is to collect representative samples from the already collected stover (bales or stacks) and analyze them for nutrients. If this is not an alternative, ballpark values can be used. However, the actual amount of nutrients present in the stover can vary significantly from a standard value dependent on several factors, including growing season conditions, hybrid, general fertility of the soil, and the time elapsed and amount and frequency of precipitation since the crop reached maturity and the time the stover was removed from the field. While phosphorus (P) in stover has low mobility because it is present in organic forms, potassium (K) is present in a highly soluble inorganic form. Thus, leaching from stover with rainfall is more pronounced for K than for P. At plant maturity, corn stover on average contains 7 pounds of P2O5 per ton and 30 pounds of K2O per ton. Due to the factors outlined above, there can be large variability in the actual amount of P and especially K in the stover. Phosphorus content typically varies between 5 and 8 pounds of P2O5 per ton, and potassium content can vary between 5 and 40 pounds of K2O per ton.
4. What is the estimated value of stover? Finally, to calculate an estimated stover value, multiply the amount of nutrients removed in stover (step 3) by the current price of each corresponding nutrient. It is important to recognize that I have discussed only P and K here because their removal directly impacts the corn-nutrient requirements that need to be included in fertilization plans for the following crop. Stover also includes other nutrients, such as N, Ca, Mg, S, and micronutrients, as well as organic carbon. The impacts of increased removal of these nutrients and of organic carbon due to stover removal are not as obvious in the short term as for P and K, but there are definitely consequences in the long term. While Ca, Mg, S, and micronutrients are not typically provided through fertilization in Illinois, greater removal can accelerate deficiency of these nutrients 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. Nitrogen reserves, as well as organic matter depletions, can lead to less crop availability of N through the process of mineralization (conversion of organic N to inorganic N). Diminishing organic carbon contents will also result in negative impacts on soil physical, chemical, and biological properties. Thus, all factors, including nutrient removal and soil resources, should be carefully considered when estimating the actual cost of stover removal.--Fabián Fernández