Begin the Nutrient Management Season the Right Way

Larry Oldham, Extension Soils Specialist
By Larry Oldham, Extension Soils Specialist March 3, 2011 11:26 Updated

The original Best Management Practice (BMP) for managing nutrients to maximize profitability and minimize potential environmental issues is soil testing.

This time of year is always exciting for Mississippi farmers; we’re past the winter (maybe. . .) and tractors are beginning to move. Land rentals, crop decisions, tillage systems, equipment decisions, variety selections, weed protection plans, insect management programs, water management plans, and hopefully soil fertility/nutrient management plans are finalized and being implemented. The adjective ‘hopefully’ is used for the nutrient component purposely because I learned last week of a crop operation that tested their soil for the first time last fall in over ten years! Crop production fields should be sampled at least on a three year cycle especially with current fertilizer nutrient costs,

Soil testing provides information on nutrients in the soil available for plant uptake in the current growing season. Soil testing programs 1) accurately determine the nutrient status of a soil; 2) convey the seriousness of any nutrient deficiency or excess; 3) form the basis for fertilization management (including lime programs); and 4) allow economic assessment of the options.

But you say that you farm in the Delta, where historically the soils are high in phosphate and potash, and furthermore often do not require lime. Therefore the only nutrient to be managed is nitrogen because this is warm and humid Mississippi. In many cases this may be true, but you do not know without a soil test.

In 2008, in a project funded by the Mississippi Soybean Promotion Board, former MSU Agronomist Dr. Dan Poston and other workers surveyed 56 soybean fields along the major highways in the Delta at about 10 mile intervals. They recorded the apparent management practices at each site and took soil samples to be analyzed by the MSU Extension Service Soil Testing Laboratory. Ten locations had a lime requirement reported, eight needed phosphate, four needed potash, one needed lime and potash, and one location needed phosphate, potash, and lime. Overall, at least one amendment was recommended for 29% of the fields. To trigger an amendment recommendation in the MSU ES system, the soil test had to be medium, low, or very low for phosphate or potash.

Interestingly, a similar project conducted in June and July, 1964 tested ten different soil series at 352 sites that had active cotton or soybean crops in ten counties in the Delta region. Lime was recommended on eight of the ten soils using the lime requirement procedures of the era. Phosphorus and potassium were of concern on sandy loam and silt loam soils.

The previous Mississippi Crop Situation entry on micronutrients for Mississippi crop production noted the paramount role pH plays in determining their plant availability in soils. Of all soil samples processed by the MSU laboratory from the Delta region between 1989 and 2009, 36% had pH values less than 5.9.

The point of the above is that without soil sampling and testing, we do not have an accurate beginning for the nutrient management program. If you are in the group not requiring an amendment, you have documentation. If you require an amendment, the process offers a beginning for the fertility program development.

Fertility program development is another way of saying that different soil testing services may provide different levels of fertilizer recommendations. The MSU Extension Service primarily uses the ‘feed the plant’ concept rather than ‘feed the soil’ as do most public laboratories (i.e. usually Extension Service operated). Many other laboratories utilize the ‘feed the soil’ concept for their recommendation basis. This concept is used only in some situations with very low phosphate soil levels by the MSU ES program.

‘Feed the plant’, the term used by one of the people who trained me, is another term for the maintenance concept of providing sufficient nutrients for the growing crop to complete its growth cycle. ‘Feed the soil’ is called buildup within the soil testing community, and provides nutrients at levels for the current year, plus extra. As through the dual use within the MSU ES system, there can be roles for both. However, with the current volatility in energy and fertilizer markets, prices for ‘feed the soil’ based recommendations may be ‘eye-opening’. This has sparked several inquiries in the past few weeks, so be sure to verify the philosophy behind the fertilizer recommendations for your farm.

Some say that fertility management should provide nutrients at replacement levels for crop removal. This can be informative; however it ignores the role of soil in providing nutrients through the growing season. Phosphorus, potassium, calcium, and magnesium are released from soil solids over the year; nitrogen and sulfur are mineralized from soil organic matter. Moreover, recent research in the Midwest has shown that fertilizing at nutrient replacement levels does not increase yields over the maintenance philosophy but increases costs with recent higher fertilizer prices.

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Larry Oldham, Extension Soils Specialist
By Larry Oldham, Extension Soils Specialist March 3, 2011 11:26 Updated
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