AGRONOMIC NEWS ITEMS
From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335
Winter 1999, No. 8
SOIL TESTS DOCUMENT PHOSPHORUS AND CROP NEEDS
A crop’s growth requirement for phosphorus and/or potassium does not change with the crop’s market price or its level of worldwide supply. Such marketing factors can change rapidly. They are seldom within our control and most often at the mercy of weather conditions or the demand for food and feed in other regions of the world. Thus, a commitment to planting a crop becomes a commitment to providing the essential ingredients for high yield crop production. One such ingredient is an adequate and balanced crop nutrition program.
Soil testing provides valuable decision making information essential for good nutrient management. Specifically, it gives the best possible measure of the level of acidity and a prediction of availability of essential nutrients such as potassium and phosphorus. Equally valuable is the measure of change in fertility level over time by comparing soil test values from the same field year after year. A vital component of high yield production is to maintain a highly productive and fertile soil.
A recent summary of soil test results for agronomic crops on a state by state basis indicates that 26 to 75 percent of the soils in the Southeast are inadequate in phosphorus and 23 to 74 percent inadequate in potassium for optimum crop production. The survey evaluated 1.8 million soil samples collected across the U.S. in the fall of 1996 and spring of 1997 and analyzed by private and university labs. Surveys are no substitute for quality sampling on a site-specific field by field basis. Yet, this survey clearly reveals that phosphorus and potassium are limiting factors in too many fields and that soil testing is an essential tool for nutrient management planning and the determination of crop fertilizer needs.
Some believe that soil testing is utilized more in high management cropping systems, and that the survey values might not adequately measure the sector of untested, low fertility, low-yield fields.
What information of value might be derived from such a summary? Consider the following observations about the southeastern U.S. as they relate to phosphorus and potassium nutrient management for optimum crop production, profitability and protection of the environment.
- Over one-third of the fields sampled are in need of lime. This practice is essential for optimum nutrient availability and use efficiency by crops.
- Phosphorus shortage is most striking in Georgia and Alabama and less of a problem in the poultry/livestock states of Delaware and North Carolina. Concerns with phosphorus and the environmental must be considered. However, equally important to farmers is the need to (1) supply adequate phosphorus to responsive soils to achieve target crop production goals, (2) improve nitrogen use effectiveness, and (3) generate soil protecting crop residues.
- Potassium fertility seems to be lowest where high potassium-requiring agronomic crops are grown—mostly on low exchange capacity, coastal plain soils. The small nutrient reservoir with such soils favors best management practices such as multiple application of nitrogen, potassium and other nutrients subject to leaching.
- The large percentage of medium to low testing soils suggests significant loss in yield and income potential throughout the region. Such soils provide little opportunity for farmers to benefit from an exceptional growing season, or from stress protection offered by good plant nutrition, or from improvements in technology and/or interacting production practices.
For more information, contact Dr. Cliff S. Snyder, Midsouth Director, PPI, P.O. Drawer 2440, Conway, AR 72033-2440. Phone: (501) 336-8110. E-mail: firstname.lastname@example.org.
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