Returning Hungry
By Lyndon Brush

Conservation Reserve Program land being brought into grain production will need some fine-tuning in soil nutrients

Over the years, calls have come in regarding CRP ground and the new soil tests results from these fields. With more CRP returning to row-crop production, these calls have become common Most often, the comment from land owners is, “How can the soil test be so low in phosphorus and/or potassium when it was fertilized to soil test when it went into CRP production 10 or 20 years ago?”

To address that situation, first consider that the field was fertilized only the year it was established in CRP with only the nutrients required to get the grass (cool-season or warm-season) established. A low-testing soil in phosphorus and/or potassium requires fertilizer to get the new crop growing (let’s call that the maintenance factor) plus a certain portion of fertilizer to start to raise the soil to an optimum nutrient level (let’s call that the build factor). The build factor, depending on what you specify on the soil test, can require a year and up to 8 years to achieve optimum level. Most soil testing labs use a default build period of 4 to 8 years. Thus, a certain amount of fertilizer has to be applied every year— even if no crop is ever taken off—to raise the soil test. Could you do it all in a one-year build? Yes. But because of the cost, which would also be incurred in that one year, most landowners want to spread that expense over many years.

Then the second most common question is, “The lime recommendation does not look that bad. How can that be?”

Lime recommendations are given on soil-test reports as a one-time correction until the next soil test is taken, which should be every 3 to 4 years. If lime was needed to establish the CRP grass, all of it was applied prior to establishment. Thus, if lime was applied to these CRP fields to raise the soil pH and now, 20 years later, a soil test is taken, technically, the pH should be higher than it was. However, soil tests taken 20 years ago that did not call for any lime (because the pH was just high enough to get cool season grasses established) may now be on the low side of the optimum range, depending on which crop you want to grow.

The bottom line is simple: These calls don’t surprise agronomists because, typically, there was just enough fertility applied for one season. To establish CRP, the government required a soil test and the recommendation from it had to be followed to establish the grass. The recommendation didn’t address adding additional fertilizer to build the soil phosphorus or potassium levels to an optimum level. Now, 20 years later, it should be easy to understand why fertility levels are where they are, especially if fields were released for emergency hay cuttings. It will take significant phosphorus and/or potassium to bring these fields up to an optimum crop-growing fertility level.

Lyndon Brush is a staff agronomist for MFA Incorporated.

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