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Fertilizer nutrient costs when harvesting biomass

By Don Reicosky

USDA-Agricultural Research Service Soils Lab, Morris

Alternative energy based on biomass has captured public attention, and considerable resources are being devoted to research, development, and implementation. There is potential for substantial environmental benefit, but also for unexpected expenditures for additional fertilizer. A part of the current research program at the Soil's Lab involves developing a spreadsheet calculator to estimate the nutrient loss with biomass removal to be used in fertilizer management and economic decisions. Fertilizer nutrient use and management in agricultural production systems will likely be impacted significantly within the next five years through biomass removal for both cellulosic and grain-based ethanol production. Any form of crop biomass removal for bioenergy will result in nutrient removal from the field and as a result impact nutrient cycling. A major challenge to the fertilizer industry and those conducting research on nutrient cycling will be the development of nutrient management approaches focused on ecological crop intensification where productivity is increased to meet growing demand and the environment is improved. This article will justify the development of the spreadsheet calculator and identify associated costs for fertilizer replacement in corn biomass removal cropping systems.

The most-commonly applied macronutrients are nitrogen (N), phosphorus (P), and potassium (K). Responses to all three elements were fairly widespread in the past, and it became customary to apply the three together. As a result, there are now many situations where plants do not respond to one or more of these fertilizer nutrients. Other plant-essential nutrients used in fairly large quantities are calcium (Ca), magnesium (Mg), and sulfur (S). However, fertilization with these nutrients is not usually necessary because the Ca and Mg contents of soil are generally sufficient for most plant species. Also, large quantities of Ca and Mg are supplied when acidic soil is limed with dolomite. Sulfur is usually present in sufficient quantities from the slow decomposition of soil organic matter, an important reason for not throwing out grass clippings and leaves.

Other cultural practices will likely be necessary on a site-specific basis since cropping systems vary in plant population, fertilization, pest management and tillage. Many growers are considering baling corn stalks and soybeans for hay. We need to remember that there is value to leaving the crop on the ground as fertilizer and organic matter. With fertilizer prices the highest they have ever been, what is the value of corn stalks and soybean residue? There are three aspects to consider: the nutrients in the stalks or residue, the availability of these nutrients to following crops and the cost of replacing the nutrients so yields of following crops are not reduced. And, it will be critical for the sustainability of the resulting modified system that the changes contribute positively to environmental impacts...that nitrate and phosphate losses to surface water and groundwater are reduced, soil erosion and soil loss from the field are lessened, nitrous oxide and ammonia emissions to the atmosphere are reduced, carbon is sequestered in the soil, and water is used efficiently.

One of the first steps is to determine the amount of the various nutrients removed from the field in the biomass. Because the large amount of corn stover, we will use it as an example to illustrate nutrients removed from the field both in the grain and stover. Traditional agriculture has understood that the nutrients removed in the grain must be replaced in order to maintain increased yields. Similarly, with biomass removal, there will be additional requirements for nutrients removed in the stover that must be replaced to maintain the nutrient balance required for maximum production. The nutrient content of this stover is difficult to predict for a specific site due to a wide range in nutrient concentrations reported in the literature. To illustrate the amount of macronutrients removed, I will draw on the data from Dr. Paul Fixen, Director of Research at the International Plant Nutrition Institute, which was recently published in Better Crops. Dr. Fixen's data represents the average of eight different estimates obtained from "typical" corn production locations in the Midwest Corn Belt. The pounds actual nutrients/dry ton of corn stover were 19 pounds. nitrogen (N)/dry ton, 5.7 pounds. P2O5  phosphorous (P)/dry ton, and 32 pounds potassium (K)/dry ton of corn stover. Similarly for harvesting corn grain, extrapolating his grain data to 200 bu/acre, the grain removes 180 pounds N/acre/year, 76 pounds P2O5/acre/year, and 55 pounds. K2O potassium (K)/acre/year. While these data showed some variation from location to location, they can give a reasonable first approximation of the current nutrient removal by corn grain and stover in the Midwest.

The first step is to estimate the corn stover yield associated with an assumed grain yield. Throughout these calculations we will assume 100 percent stover removal.  We can use the harvest index for corn, which is the pounds of grain divided by the sum of the pounds of grain and pounds of stover that allows us to estimate stover yield. The results of calculations for a 200 bu/acre grain yield (11,200 pounds grain/ac, with an assumed harvest index of 0.53 yields 9,932 pounds stover/acre.  Then using the nutrient concentrations in a dry ton of stover, we can estimate nutrient removal in the biomass. Results of these calculations for both the stover and the grain are summarized in figure 2. On a per acre basis, removing stover will remove actual 94.4 pounds N, 28.3 pounds P2O5 and 158.9 pounds. K2O that could be used to replenish the soil nutrients. The 9,932 pounds. stover/acre is about 8 bales at 1,200 pounds/bale. For each 1,200 pound bale, 11.4 pounds N, 3.4 pounds P2O5 and 19.2 pounds K2O are removed from the soil and eventually need to be replaced. These macronutrient removal values may be less than typical fertilizer recommendations and calculations suggest careful management decisions for maintaining the proper nutrient balance for sustained production without economic implications. The increased cost of fertilizer driving the economics will play a big role in those management decisions.

Similar calculations in figure 2 show a 200 bu/acre grain yield will remove 162 pounds N, 76 pounds P2O5, and 55 pounds of K2O. These amounts of nutrient removal have increased over the years with high fertilization rates required for higher yields and are approaching typical fertilizer recommendations for high-yielding corn. Adding the nutrients removed in the corn stover with the nutrients removed in the grain yields a total actual 256.4 lbs. N, 104.3 pounds P2O5 and 213.9 pounds K2O. This large amount of nutrients removed in the grain and stover reflects nutrient extraction from applied fertilizers, manure, and soil organic matter decomposition. The slow-release nature of nutrients from decomposing manure and soil organic matter will increase pressure on annual applications of synthetic fertilizers to maintain high yields.

The first question that arises is the economic consequence of maintaining this nutrient balance with biomass removal. How much will it cost to maintain soil nutrient levels? The increase in costs in fertilizer production is directly related to the cost increase of fossil fuels to make and transport fertilizers. Like any other market commodity, the fertilizer costs vary dramatically with the season and availability. As a result, costs of fertilizers are very dynamic and require strategic planning to purchase required fertilizers economically. Given this reality, it is very difficult to quantify the economic impact of replacing these nutrients associated with biomass removal. To give some idea of the current economic impact, fertilizer prices provided by Brian Kruize at the Morris Co-op on Feb. 7, were as follows: $570/ton urea (46-0-0), $610/ton for di-ammonium phosphate (18-46-0), and $540/ton for potash (0-0-60).  These values in tons calculate to $0.62/pound actual N, $0.47/pound actual P2O5 and $0.45/pound actual K2O. Multiplying the total nutrients removed in grain and stover by the cost per pound of N, P2O5, and K2O, yields a total of $298.17/acre for fertilizer replacement costs. This calculation accounts for 40 pounds N in the di-ammonium phosphate at a slightly lower cost than in the urea.

Looking at the stover removal only using the same calculation, the nutrient replacement cost would be $138.55/acre suggesting that the stover must be worth more than $138.55/acre for bioenergy. This value is for the replacement cost of N, P2O5, and K2O only and does not include other costs such as raking, baling and transporting the stover to the bioenergy plant. Also, this estimated replacement cost includes only N, P2O5, and K2O without consideration for any of the other macro- or micronutrients that may be necessary.  It is recognized that the total value for the fertilizer replacement will vary based on the price of the fertilizers and on the nutrient concentration for a specific field. The results do point out significant increase in fertilizer expense to replace nutrients removed with the grain and biomass. 

In summary, nutrient use and management will be significantly impacted in systems where biomass is removed for bioenergy. This includes grain-based ethanol and cellulosic ethanol production. We must conduct the research on nutrient management strategies to develop new approaches focused on increased crop intensity where productivity is increased to meet the growing demand for food, feed, fiber and fuel. This includes studying the carbon cycle and understanding how the carbon cycle interacts with all the many individual nutrient cycles. Management decisions associated with fertilizer recommendations have significant economic and environmental effects that need to be understood by society when food and fuel prices increase. Our concern at the Soils Lab is that we not make hasty decisions that will result in unintended consequences associated with biomass removal. Unfortunately, this will require some long-term research to get the answers. In the meantime, it is important that we use the best research information available to make nutrient management decisions that impact all of society through the food chain and the environment.