Agronomists and growers need to be aware soil test levels for phosphorus and potassium are declining, as today’s high yields are drawing down more than is being supplied.

With the assistance and cooperation of numerous private and public soil testing laboratories, the International Plant Nutrition Institute (IPNI) periodically summarizes soil test levels in North America (NA). Soil tests indicate the relative capacity of soil to provide nutrients to plants. The latest summary was for over 725,000 soil samples collected in 2015, including fields from Illinois.

Therefore, this summary can be viewed as an indicator of the nutrient-supplying capacity or fertility of soils in NA. This is the tenth summary completed by IPNI or its predecessor, the Potash & Phosphate Institute (PPI), with the first summary dating back to the late 1960s (Nelson, 1980). The summary offers a snapshot view of soil test levels in 2010, but also provides a comparison to the previous two summaries which were completed in 2001 and 2005 (Potash & Phosphate Institute, 2001; 2005). Since the 2010 summary is the third one in which laboratories were asked to contribute complete frequency distributions of soil test results, temporal trends in soil test level distributions can be viewed for states and provinces.

Important to appropriate use of this report is recognition that nutrient management should occur on a site-specific basis where management objectives and the needs of individual fields and, in many cases areas within fields, are recognized. Therefore, a general soil test summary like this one cannot reflect the specific needs of individual farms or fields. Its value lies in calling attention to broad nutrient needs, trends, and challenges, and in motivating educational and action programs that are in turn relevant to growers and their advisers.

The specific details of the Illinois portion of the latest IPNI Soil Test Summary are presented below.


Chart 1: Changes in Illinois phosphorus levels

The data in Chart 1 show that there has been an increase in low P1 soil test levels in Illinois over the period from 2001 to 2015, and a 16% decrease in soil tests above 50 ppm. The combination of reduced fertilizer use, increased P removal in higher crop yields, and economic pressures and environmental concerns, have all contributed to the changes.

As Chart #2 shows, about 18% of the soil tests are still in the category above 50 ppm. Categories between 30 and 50 have all decreased, indicating a trend toward less P available to crops. But the soil tests below 30 ppm have all increased substantially, with all of these below the critical level for optimum crop yields.


Chart 2: Frequency distribution of phosphorus soil test levels.

Illinois Potassium Soil Test Summary

The data in the Chart 3 summary show a decrease in high K tests and increases in the categories between 120 and 200 ppm, all of which are in the range of supplying insufficient K to crops.


Chart 3: Changes in Illinois potassium levels.

The data in Chart #4 show that K soil tests have declined in the last 15 years—with a much smaller number of samples testing above 200 ppm and a large increase in the number of samples testing in the categories below 200 ppm. The dramatic shift to a large percentage of the samples testing below the critical levels for K means that much of the Illinois corn and soybean crops are inadequately supplied with potassium to support optimum yields.


Chart 4: Frequency distribution of potassium soil test levels.

Even at today’s lower crop prices, this lost yield potential translates to significant economic losses to Illinois farmers, even though their yields are increasing. In fact, the increasing yields are a significant contribution to the lower soil tests.

Illinois pH Soil Test Summary

Another important concern from the soil test summary data is soil pH. Here, Illinois farmers are doing a much better job of maintaining pH levels in the range for optimum crop production. The data in Chart 5 show clustering of about 65% of the samples in the range of pH = 5.6 to 7.0, or that the majority of the soils are in the pH range for top productivity. The greatest increase in the last 15 years is in the categories from pH = 6.1 to 6.5, indicating an awareness of the need to maintain pH and the action that farmers are taking to do so.


Chart 5: Changes in Illinois pH levels.

The data in Chart 6 show the frequency distribution of the pH values across Illinois soil tests. The charges in distribution of different pH levels shows a trend toward more acidic soils, with about 25% falling into ranges that are too acidic for most crops. That means paying more attention to soil pH and liming.


Chart 6: Frequency distribution of soil pH.

Harold Reetz is a Certified Crop Adviser working as an agronomy consultant in Monticello, Illinois. His areas of expertise include high-yield crop production, precision farming technology, 4R plant nutrient management systems and conservation systems. 

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About the Author: Harold Reetz

Harold Reetz is a Certified Crop Adviser working as an agronomy consultant in Monticello, Illinois. His areas of expertise include high-yield crop production, precision farming technology, 4R plant nutrient management systems and conservation systems.