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Balancing Yield Goals with Fertilizer Inputs for 2011 [PDF version] Troy Bauder – Extension Water Quality Specialist Higher than normal crop commodity prices combined with good yields from 2010 are benefiting Colorado agricultural producers. Outlooks for next year’s prices are also positive compared to long-term averages and these prices may entice producers to edge yield goals upward when determining fertilizer application rates. However, this spring’s fertilizer and other input costs are also trending much higher than previous years (Figure 1). Thus, maximizing the efficiency of applied fertilizer is especially important to achieve desired yield goals and ensure that fertilizer inputs are cost effective.
Figure 1. Average retail fertilizer prices paid for three products. 2011 is approximate. Source for 1990 - 2008 costs, USDA/NASS. Fortunately, crop producers today have many tools available to improve fertilizer use efficiency that were unavailable in the past. These techniques have been defined in various terms, but have traditionally been called best management practices (BMPs). Recently, nutrient BMPs have been re-defined as 4R nutrient stewardship by the International Plant Nutrient Institute (IPNI). The four ‘R’s in their terminology refers to applying fertilizer nutrients in the Right amount for the crop yield goal, at the Right time in the growing season, in the Right place and using the Right nutrient source. Together, these management practices give producers the best possible economic yield response for their fertilizer dollar. Information on this stewardship program can be found at: http://www.ipni.net/4r. For this article, we are going to focus on the first ‘R’ in the program, determining the Right amount. Advances in precision agriculture, such as crop sensors and variable rate technology, are offering new tools for determining the right place and time for fertilizer application. However, the first approach for determining the right amount of fertilizer to apply is an annual soil analyses and an appropriate yield goal for each field. Soil testing is particularly critical when determining nitrogen (N) fertilizer application rates since N is a dynamic soil nutrient and required by crops in the greatest amount. In the western U.S., most public and private laboratories assess soil N status by measuring nitrate-nitrogen (NO3-N), which will be available for crop uptake. If the depth of the soil sample is known, the amount of nitrate-nitrogen in the soil can be converted to pounds per acre by multiplying the concentration by a conversion factor (typically 3.6 for a one-foot soil sample). Yield goals are used to determine the total amount of N required for the desired production and the recommendation is adjusted for the soil test level, organic matter (O.M.) content and other potential N credits. Nitrogen credits that should be considered to adjust a recommendation include irrigation water nitrate, previous legume crop, manure or biosolids applications within the past three years and last year’s sugar beet tops. Make sure your laboratory, consultant and/or fertilizer dealer has all of your field history regarding crop rotation, past amendments and irrigation water quality so they can provide you with the most accurate fertilizer suggestion. Colorado State University fertilizer suggestions for many crops are available at: http://www.ext.colostate.edu/pubs/pubs.html#ag . In contrast to nitrogen, soil phosphorus (P) availability is typically assessed differently than N. This is because the amount of soluble, plant available P is small compared to the total amount in the soil and this P fraction is not a good predictor to yield response. Therefore, soil testing laboratories use an extraction index that dissolves a portion of the total P that may become available during the growing season. Soil test P levels from these extraction methods have been calibrated to predict the probability of a yield response through replicated field trials. So, while a laboratory may provide an actual recommendation in pounds per acre, it is also important to pay attention to the relative soil test P level typically provided as very low to very high. As a soil test level increases in these categories, the probability of a yield response to added P decreases. Thus, even a soil testing in the low range has a small probability that additional P fertilizer will not result in a yield response. Conversely, a field with a high soil test P level may respond to additional P fertilizer given favorable growing conditions for higher yield. One ‘R’ that the 4R stewardship program does not address is choosing the Right analytical lab. Good laboratory methodology is critical to achieving a sound recommendation. Fortunately, many laboratories have voluntarily chosen to participate in the North American Proficiency Testing (NAPT) Program (a program of the Soil Science Society of America). This program assists soil, plant and water testing laboratories in their performance through inter-laboratory sample exchanges and a statistical evaluation of the analytical data. To find out whether your laboratory participates, search: http://www.naptprogram.org/about/participants. Finally, knowing the soil fertility philosophy utilized by your laboratory is also important in interpreting their recommendation. Ask your laboratory whether the recommendation is based upon replacing the nutrients taken up by a crop to meet your yield goal. This replacement approach is most often recommended by university soil scientists. Other approaches include building up soil fertility levels or basing fertilizer recommendations on crop requirements to maximize yield. The last two approaches result in higher fertilizer recommendations that can lead to a buildup of nitrogen and phosphorus in the soil and potential pollution of water sources and increase your costs. Finally, remember that a fertilizer recommendation from a laboratory or consultant is the starting place for a determining the best fertilizer rate, not the final word. Crop producers have the most knowledge regarding their field’s yield potential, cropping and amendment history, and response to fertilizer application and should adjust recommendations according site and growing conditions. Additional Resources: Best Management Practices for Phosphorus Fertilization XCM 175- http://www.ext.colostate.edu/pubs/crops/XCM175.pdf Best Management Practices for Nitrogen Fertilization XCM172 http://wsprod.colostate.edu/cwis435/WQ/documents/factsheets/xcm172BMP_Nitrogen.pdf Nitrogen and Irrigation Management (Fact sheet 0.514)- http://www.ext.colostate.edu/PUBS/CROPS/00514.pdf Agronomy News - Nitrogen Fertilization, Vol. 24, Issue 1. http://www.extsoilcrop.colostate.edu/Newsletters/documents/2004_nitrogen.pdf N Use Efficiency vs. High Corn Yields Troy Bauder |
