Everything Old is New Again – Fertilizer Costs

Larry Oldham, Extension Soils Specialist
By Larry Oldham, Extension Soils Specialist March 27, 2015 13:19 Updated

It is not just a song, a new conversation about an old concept is happening in soil fertility management. I started working with soils/soil fertility/nutrient management/soil management 29 years ago this March at the West Kentucky Research and Extension Center in Princeton. Some things addressed then are being questioned now, but should they?

Plant nutrition in the soil environment is a dynamic process captured with soil testing. What we do with the results are soil test recommendations. When I started in 1986, a mentor in the profession, Dr. Lloyd Murdock, the UK soil fertility specialist, was working comparing different fertilizer recommendations developed from the same soil test results. A report is available at http://www2.ca.uky.edu/agc/pubs/agr/agr151/agr151.htm.

From Dr. Murdock’s publication (emphasis added):

“Five different philosophies or combinations of philosophies used in Kentucky to make fertilizer recommendations were evaluated and the results are summarized in . . . All recommendations resulted in excellent crop yields when the weather conditions were good. In almost all cases, there was no real difference in yields. However, there were always fairly large differences in the amount and kinds of fertilizer recommended. This resulted in large differences in the costs, with very high fertilizer costs giving no yield advantage. Soil tests taken a few years following use of the various recommendations indicated that surplus fertilizer was being stored in the soil. Fertilizer rates based on the crop sufficiency philosophy cost the least and produced equivalent yields compared to the more costly recommendations based on the other philosophies tested”. One of the other philosophies Dr. Murdock tested was fertilizing based on crop removal.

There is no doubt in the course of the past 29 years that genetics, water and pest management, and other factors resulted in unprecedented agronomic crop yields. This is documented in “Yield Gains in Major U.S. Field Crops”, a 2014Special Publication of the American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America. It is available through their respective websites.

So do the productivity gains negate soil testing and recommendations even if old? Remember that in soil fertility we have two nutrient aspects to consider: total uptake by a crop and removal from the field via harvest. Uptake is greater than removal because some amount of the nutrients used by plants during the growing cycle will recycle in straw, stalks, and decaying roots. A logical question is whether our soils can continue to sustain this level of production.

The first nutrients plants encounter and we measure with soil testing are “available nutrients”. The understanding of phosphorus and potassium uptake by plant roots increased exponentially in the past two decades, however the basic is still that the most available nutrients are those within the soil water (aka soil solution) in the rooting zone. This is a small amount at any point in time, a small fraction of the total needed to maximize plant productivity through the course of a growing season.

When depleted by plant uptake, the soil solution nutrients are replenished between 50 and 100 times, or possibly more during a growing season from the solid components of the soil through several mechanisms. These solid-phase nutrients comprise the majority of nutrients measured in common soil tests. Thus soil tested phosphorus and potassium are a combination of the solution and readily releasable nutrients. Soil testing DOES NOT measure all phosphorus and potassium actually present in a soil. While your soil test report may list 360 pounds per acre potash, the reality is many thousands pounds of potassium are present in that soil. It is the same for phosphate/phosphorus. The number on the report is catching lightening and putting it in a bottle. Over time, phosphorus and potassium in soil not measured via soil testing moves to forms which can be measured.

Dr. Bobby Golden of MSU has assessed various MSU soil fertility recommendations over the past several growing seasons throughout the Mississippi Delta using modern genetic varieties and production practices. Finding responsive sites has been challenging because it is naturally one of the most productive regions on the planet. In most trials, when there is a response to fertilization, it follows the prediction of the two soil test methods under evaluation.

Fertilizing based on crop use or removal ignores the capacity of the soil to provide nutrients. A few years after leaving the Western KY R and E Center, had the privilege of Dr. George Rehm, extension soil fertility specialist, as my co-major Ph.D. adviser at the University of Minnesota. George was a fantastic mentor who, while ‘retired’, works to this day. In September, 2014 he noted the following at http://agbuzz.com/ on the soil fertilizer costs versus return issue for the 2015 crop year (emphasis added):

“Various private soil testing laboratories may use other standards. There can be a substantial savings in money spent for fertilizer by using UNIVERSITY . . . standards which are based on considerable field research. Others do not have this research base. Don’t base fertilizer application rates on CROP REMOVAL. Following the crop removal concept will produce the most expensive fertilizer recommendations. The use of the crop removal concept ignores the importance of soil testing as a management tool and that’s just plain wrong. Let’s give the soil itself some credit for supplying some of the total amount of a given nutrient needed for crop production.”

More investigation is required on questions such as whether banding in the Mid-South increases phosphate fertilizer efficiency enough to reduce application rates as found in the upper Midwest, and whether soils are releasing sufficient potassium for modern varieties of cotton during the critical uptake period. However, crop removal based nutrient application rates were the most expensive alternative in 1986, and still are in 2015.

In summary, on our fertile soils, there is no reason to drop soil testing as the basis for your nutrient management. Consider the philosophy behind what recommendations you receive and whether the economics work for your situation. Ask questions.

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Larry Oldham, Extension Soils Specialist
By Larry Oldham, Extension Soils Specialist March 27, 2015 13:19 Updated
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