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Fluid Journal : Fall 2010
availability, an individual farmer's long- term approach to managing risk and land investment, crop fertilizer prices, and other factors are not estimated by soil testing but they generally influence crop nutrient rate decisions. While plant-available nitrate and/ or ammonium nitrogen (N) soil testing historically has been used for N recommendations in lower rainfall areas, such as the Great Plains and other western states, N soil testing has generally not been used in more humid regions such as the Corn Belt and southeastern states. Higher rainfall in these areas causes much more weather-induced variability in inorganic soil N supplies and much less reliability in assessing available N supply to the growing crop. P, K interpretation Nutrient recommendations. As cropping systems change with the increased adoption of reduced and no- till systems, it is possible that nutrient recommendations may also need to change as compared to those developed with past conventional, aggressive Summary: There are several logical and appropriate approaches to managing phosphorus (P) and potassium (K) fertility. Within the bounds of environmental stewardship, it should be up to the individual producers to determine the appropriate fertility approach suitable for their production system. Nutrient sufficiency programs generally minimize fertility inputs in the early years but have increased risk of P or K limiting crop growth and long-term profitability. Build/maintenance programs may cost more in the initial years if soil tests must be built up, but they generally provide for maximum yield and long-term profitability while increasing fertilizer management flexibility in the coming years. In addition, an individual producer's attitude toward managing risk, the producer's long-term viewpoint in making investments in soil fertility, expected land tenure, and other farmer-specific goals and objectives should be incorporated into the decision-making process for determining the P and K fertility management program that best suits an individual producer's needs. To continue to increase crop yields in the future, it is important to note that research has shown that annual fertilizer applications may not fully substitute for high P and K soil fertility. Highest crop yields are often associated with soil tests greater than the established critical value. There may be a severe economic penalty associated with low P or K soil tests even when fertilizer is applied-- especially in years/situations with high-yield potential. Farmer-speciﬁc goals should be incorporated into the decision-making process. A Further Look into Fertilizer Recommendation Adequacy Regarding Phosphorus and Potassium The Fluid Journal • Ofﬁcial Journal of the Fluid Fertilizer Foundation • Fall 2010 • Vol. 18, No. 4, Issue #70 Drs. Dale Leikam, Gyles Randall, and Antonio Mallarino Profitable crop production requires adequate crop nutrition and there are few fields that do not require the addition of supplemental crop nutrients. As a result, there has been much investment in time, expertise, and money devoted to developing reliable soil tests that are well correlated to crop nutrient uptake and crop yield response. Once a reliable soil test is developed, the test is then calibrated to estimate the nutrient application rate required for optimum crop growth at various soil test levels. Historically, the soil test value and crop to be grown have been the main, and often only, factors used in making nutrient rate recommendations--although there are sometimes adjustments made for factors such as expected crop yield, soil type, and/or soil association. However, there are many other factors that affect crop growth, nutrient availability, nutrient uptake, and crop production efficiency that need to be taken into consideration in order to arrive at a nutrient management program that best fits a specific field. The cultural and tillage system used, planting dates, soil/ environmental condition, equipment tillage. Additionally, as crop yields continue to increase year after year, the overall amounts of crop nutrients required and rate of crop nutrient uptake are also increasing. As yields continue to climb, farmers need to consider the total amount of nutrients required by these higher yielding crops and the daily nutrient requirements, especially at critical stages of crop development. Table 1 presents the very large total nutrient uptake and daily nutrient requirements of high-yielding corn and soybeans in a Rutgers University study. Since most P and K moves to the root surface across only very short distances by diffusion, questions sometimes arise about the adequacy of many current crop nutrient recommendations developed at much lower yield levels than are currently obtained by top producers. Soil tests for P and K do not directly tell how much of a nutrient is available to a crop--nor do they accurately predict precisely how much of a nutrient to apply to a specific field situation. Instead, what soil tests do much better is estimate the soil's relative ability to supply a nutrient to a growing crop. This provides an index Fall 2010 The Fluid Journal 08
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