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UW soil fertility specialist gives economic advice on fertility programs for cropland

“With fertilizer economics, you have to answer whether you can afford to fertilize, or afford not to fertilize,” says UW Soil Fertility Specialist Jay Norton.

“A lot of money is wasted on over-fertilizing, and there are some basic concepts producers should consider before investing in an expensive fertilizer program,” he continues.
Norton lists soil tests as a first step to make in deciding on a fertilizer program.

“Soil test results should be where every farmer starts, and the basis of economic decisions in regard to fertilizer. Many fertilizer companies will do the test for you, and then apply what you need. But, there’s an inherit conflict of interest in that. They may have the best of intentions, but its safer to send a sample to an independent lab, where there is no value attached to the sample for that lab,” explains Norton.

The law of the most limiting factor is another area Norton suggests producers look at prior to entering their fields.

“The most limiting factor will always control the yield. It can be nitrogen, phosphorus and potassium (NPK) fertilizers, soil nutrients, water or any number of other things, and yield won’t improve until you address that limiting factor. If it’s not NPK, regardless of how much fertilizer you apply, you won’t see improvement,” notes Norton.

“Fixing one problem often reveals another, and soil and climate factors can also have a large influence. Many times the worst soils don’t need fertilizers because that’s not their limiting factor,” continues Norton. “Sometimes it’s NPK, but sometimes it’s not, and it’s important to consider all potential limiting factors.”

He continues, saying that reaching an economic optimum rate of fertilizer application is also important.

“In soils with low nutrient levels, where you get a big response, this economic optimum rate is pretty narrow and you need to get the soil nutrient levels up so that soil is producing near maximum yield. Even if fertilizer is expensive that year, you’ll get a good response for it, and chances are you will see more economic benefit from applying.

“In soils with a higher residual nutrient supply, you get less response from applying fertilizer, or less bang for your buck. So, economic optimum depends more on the prices of crops and prices of fertilizer, and results in a wider range that will be economically optimal.

“If you’re spending part of your fertilizer budget on one nutrient you know is deficient, you want to spent most of those dollars on the areas of the farm where you will get the most response. Spend most on those areas that are less responsive, and some on the areas that are more responsive,” explains Norton.

He also suggests adding some of each nutrient (NPK) in short supply to make use of positive interactions

“If you’re allocating funds between two or more nutrients, keep in mind that nutrients interact, and you should spend some money on each nutrient your soil tests suggests is in short supply. In Wyoming soils, P and K are typically limiting, and they interact with each other,” says Norton.

He adds that his work and literature says N is N, and P is P, and that each nutrient should be purchased at the cheapest price per unit, then applied appropriately.

“There are quite a few slow-release, polymer-coated urease inhibitors, or liquid slow release forms of N, on the market now. I’m asked if those are worth it, and our work on N products suggests that it’s not worth it, and to stick with whatever is cheapest, and apply it correctly,” continues Norton.

For those producers who would like to see the impacts that variables in fertilizer costs and applications can have, Norton recommends visiting landresources.montana.edu/soilfertility.

“You can change the cost of fertilizer, play with the residual soil N, and look at impacts of different crop prices. Montana State has done a good job on the website,” he explains.

“The take-home is to use test results to cut back any nutrient that’s in excess, and save money that way, and to apply fertilizer where it will generate the most return, and to use nutrients in combination to make use of positive interactions,” concludes Norton.

Norton presented his information during the Fremont County Farm and Ranch Days in Riverton on Feb. 9. Heather Hamilton is editor of the Wyoming Livestock Roundup and can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..