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Managing at the critical level for P and K

By Greg LaBarge, Ohio State University Extension, Field Specialist, Agronomic Systems

The Tri-State Fertilizer Recommendations is the foundation of fertility management in Ohio. The build, maintenance, and drawdown approach of this bulletin rely on managing the soil test level around a critical level that provides enough nutrient to meet the immediate needs of the crop. This philosophy of management is used for two of our three macronutrients needed in crop production, phosphorus and potassium. The scheme works due to the immobile nature of these two nutrients in the soil and the method of uptake exhibited by plants.

It is important to fully understand how the bulletin co-editors defined the terms used in the Tri-State Recommendations as we put the principals into practice. The key term is “Critical Level.” Other parts of the recommendations hinge on defining this term.

As defined, the Critical Level is “…the soil test above which the soil can supply adequate quantities of nutrient to support optimum economic growth.” When soil test levels are above the critical level “…the soil is capable of supplying the nutrient required by the crop and no response to fertilizer would be expected.” Further when the soil test level is below the Critical Level “…the soil is not able to provide P and K requirements of the crop.” Below the critical level, we would not maximize yield in the current year of production without added fertilizer.

Research at universities across the Midwest has defined critical levels for phosphorus that fall into a similar range of 15 to 25 parts per million (30 to 50 pounds per acre). Ohio’s recommendations define the critical level for corn and soybean production at 15 parts per million (or 30 pounds per acre) elemental P. If alfalfa and wheat are in the rotation the critical level is 25 parts per million (or 50 Pounds per acre). For potassium, the range is dependent upon the CEC (Cation Exchange Capacity) of the soil type and ranges from 88 to 150 parts per million (or 176 to 300 pounds per acre).

A recent summary of soil test phosphorus levels was reported by the International Plant Nutrition Institute and is shown in Figure 1. The samples were combined from a number of laboratories across the Midwest servicing Ohio. Growers should take a look at their soil test results in relation to the critical level. If near the critical level, maintenance levels of fertility are recommended so soil test do not fall below the critical level with future crop removal. Soil tests substantially above the critical level do not have any benefit for crop production and can be drawn down.

For more information on fertility management for Ohio crop production refer to http://agcrops.osu.edu/specialists/fertility.

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