By George Silva
The International Plant Nutrition Institute (IPNI) has released the 2015 report of soil test levels in North America. The report summarizes data from roughly 7.2 million soil samples submitted by 61 university and private laboratories. The summary provides a snapshot of soil test levels in 2015 and a comparison to three previous reports. Soil tests indicate the relative capacity of soil to provide nutrients to plants.
Table 1. The median soil P, K, Mg, S and pH in Michigan 2001 - 2015 |
|---|
Year * | Bray P1 (ppm) | K (ppm) | Mg (ppm) | S (ppm) | Soil pH |
|---|
2001 | 50 | 128 | - | - | 6.5 |
2005 | 49 | 149 | 225 | 4.0 | 6.7 |
2010 | 42 | 131 | 189 | 6.0 | 6.7 |
2015 | 37 | 129 | 196 | 5.0 | 6.7 |
* The data is based on 67,927 samples in 2001; 98,297 in 2005; 198,915 in 2010 and 269,045 in 2015.
The Michigan data for median phosphorus (P), potassium (K), magnesium (Mg), sulfur (S) and soil pH are summarized in Table 1. The median P levels declined by 5 ppm in 2015 compared to 2010. Since soil P levels are highly buffered, this decline is significant. The median K declined by 2 ppm. Soil pH was unchanged.
This report, Soil Test Levels in North America 2015, is available at the IPNI website. It also shows the frequency distributions of soil Mg, S, Zn and Cl levels. Of particular interest was the increase in frequency of soils testing low for S which is consistent with reports of increasing S deficiency in crops. Most scientists do not consider soil tests for S to be of good diagnostic value, rather tissue tests are preferred.
There was a substantial increase in soil samples tested in 2015 compared to 2010. It is encouraging that agricultural producers are increasingly relying on soil testing as a guide to nutrient management decisions. Most university fertilizer recommendations are based on agronomic, economic and environmental considerations.