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Fluid Journal : Fall 2010
P and K fertility programs by different institutions/individuals as it is a difference in 'philosophy'-- a particular approach to risk management and/or past experience among those developing fertility recommendation programs. Which is correct? This is a relevant question, but there is not a simple, clear-cut answer. There are many nutrient management programs that vary between a strict sufficiency approach and a strict build-maintenance approach. The advantages of P and K management programs that are closer to a nutrient sufficiency approach include the fact that 1) P and K applications are minimized at soil test values less than the established critical value and 2) that the risk of not obtaining a profitable response to the last increment of applied P and/or K in the year of application is also minimized (not eliminated, but minimized). Situations when the sufficiency approach makes the most sense include 1) limited resources available to invest in a particular year, 2) expected short land tenure situations, and 3) the relatively few soils with a very high capacity to quickly convert relatively soluble/exchangeable forms of P and K to forms that are largely unavailable for crop uptake in a given year. Disadvantages of this approach include 1) need for frequent, precise, and accurate soil testing, 2) very good knowledge of optimum application rates each year is required, and 3) the risk of P or K limiting crop growth and long- term crop productivity and profitability is greater. Build-maintenance. The advantages of programs closer to a strict build- maintenance approach include 1) greatly reducing risk that P or K will limit crop growth and long-term productivity/ profitability, 2) reducing need for frequent soil testing, 3) allowing for timely planting and management of fertilization over time, and 4) increasing future flexibility in the overall fertility program. However, this increased flexibility and risk reduction may require a greater investment in fertilizer initially to build soil test values to the maintenance range. In the long term, however, both nutrient sufficiency and build-maintenance rates eventually 'tend to' stabilize at rates equal to the amounts of P and K removed in the harvested portions of the crop. Individualize programs. Both of these nutrient recommendation approaches and management strategies specific to each approach are appropriate for individual farmers, individual fields, and for specific conditions in any given year. Regardless of what State a farmer operates in or consideration of an individual producer's attitude concerning risk, their viewpoint in making long-term investments in soil fertility, expected land tenure, and other farmer-specific objectives should be used to develop individualized P and K fertility management programs. In the past, the risk and benefits of various approaches generally have not been well communicated to farmers and crop advisors. Any of the discussed approaches may be 'right' for a given situation or any might be 'wrong.' Fertilizer a substitute? One of the assumptions that most P and/or K recommendations are based on is the premise that fresh fertilizer applications to low-testing soils will fully substitute for the fertility provided by high-testing soils. In other words, it is generally assumed that maximum yields can be obtained either by building up soil test P and K levels to 'high' values or by applying enough nutrients to soils testing 'low.' Research has shown that this may not always be the case. For example, several Canadian studies with small grains clearly demonstrated that annual applications of row-applied P to low-testing soils never did equal the yields of wheat and barley grown on high-testing soils. Long-term studies at the Rothamsted Experiment station in the United Kingdom also found that P fertilized crops on low-testing soils did not equal those on high-testing soils. Penalty severe. Recent studies with corn and soybeans have shown similar results. Table 2 presents the summarized results of a three-year University of Minnesota research study that included both low (6-9 ppm Bray P1) and high- testing soils (20-27 ppm Bray P1). In this study, P was applied only to the corn crop in the corn/soybean rotation. On the high-testing soil, corn yields averaged 192 bu/A over the three years with no response to freshly applied P. On the low-testing soil, which had been mined by ten years of either corn for grain, corn silage or soybeans with no P added, there was a modest response to the applied P but yields only averaged 167 bu/A even at the higher 50-lb P2O5/A rate. For soybeans, there was a small response to residual P applications to corn measured on the low P soil with no response on the high P soil, but yields averaged 49 bu/A on the high-testing soils and only 37 bu/A on the low-testing soils. Clearly, there was an advantage to both corn and soybeans for having a high P soil test as compared to a low test. The yield advantage across all eight treatments averaged 30 bu/A (18%) for corn and 12 bu/A (32%) for soybeans. Moreover, the economic penalty associated with the low P-testing field was severe even when P was applied at nutrient sufficiency rates recommended by the University. Interesting questions. The results from this and other studies do raise some interesting researchable questions. Under what conditions would applied fertilizer be expected to fully substitute for low soil fertility? How does subsoil P and K fertility enter into this discussion? How do soil/environmental conditions interact to affect the effectiveness of fertilizer applications vs. soil fertility (e.g., temperature, moisture, etc.)? Are current university nutrient recommendations based on data from older, lower-yielding sites appropriate for very high-yielding production systems where daily nutrient demand and annual drawdown from the rooting profile can be substantial? The previously discussed research suggests that current soil-test-based fertilizer recommendations are not always adequate for obtaining very high yields. These and other questions identify numerous and valuable opportunities for 1) additional research to continue improving crop production efficiencies, 2) achieving the very high yield potential of current high-yielding hybrids/varieties, and 3) achieving the promise of future genetic advances. 11 The Fluid Journal Fall 2010 Dr. Leikam is President of the Fluid Fertilizer Foundation, Dr. Randall is soil scientist/professor at the Southern Research and Outreach Center, University of Minnesota, and Dr. Mallarino is Professor, Soil Fertility and Nutrient Management, Department of Agronomy, Iowa State University.
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