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St. Louis Post-Dispatch

Lee Enterprises

Tuesday, November 4, 2008 10:42 PM CST
Identifying the value of a soil test

By SHARON KUHNS

For 2009, approximately 25 percent of your corn crop will be required to pay for fertilizer. Yes, just fertilizer, according to Mike Roegge, University of Illinois Extension Crops Systems educator. And that’s just the amount of nitrogen, phosphorus and potassium that will be removed by the crop (no buildup). Those of you who have recent soil tests will be much more able to determine how cost effective the P and K you’ll be applying will be.

In other words, are you applying P and K based upon what you’ve always done, or are you applying (or not applying) because your soil tests tell you to? The only way to know how much fertilizer your crop needs is through a soil test.

Now more than any other time in history, the soil test is a valuable tool. And while it may not be as accurate as we’d like, it is accurate enough, especially when you’re looking at spending $250 or more per acre for fertilizer.

No doubt producers will be looking closely at their soils fertility level before spending these kinds of dollars. And asking the question: At what point do soils not respond to additional nutrients?

The Agronomy Handbook suggests P levels of 40 pounds per acre on soils testing high in available P, which our soils do. Can you grow a corn crop at lower levels? Of course. The question is how low can you go before you begin to sacrifice yield. The 40 pound goal is for a cropping system that includes corn, soybean, wheat and hay. For those fields in which corn and soybean are the only crops grown, research shows optimal yields at a P test of 30 pounds per acre.

Likewise, the Agronomy Handbook suggests a K level of 300 pounds per acre for those soils with higher (12 or more meq/100g soil) cation exchange capacity, and 260 for those soils with lower capacity. Generally speaking, lighter colored timber soils would be considered lower exchange soils, while darker, higher organic matter soils would be considered higher exchange soils. So potassium targets will vary by field, based upon cation exchange capacity. A good soil test will tell you that.

Bottom line, if you don’t have a recent soil test in which to guide you as you apply nutrients, you’re likely going to lose money. Either by over applying (which you’ll be applying excessive nutrients) or by under applying (which could short you on yield). That $5 or $6 soil test is the cheapest cost you’ll have this year.

Sharon Kuhns is county director for University of Illinois Extension in Coles County.

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