North American agriculture relies heavily on soil testing to assess soil fertility and guide nutrient management decisions. Use of soil testing has increased substantially since 2005, a positive sign of increasing farm-level management intensity. This report documents important variability and changes in soil fertility levels. The 2020 survey captured information on 7.7 million soil samples. Including previous summaries, this assessment of trends from 2001 to 2020 included data from about 25 million samples. It is probably the most comprehensive evaluation of soil fertility ever conducted in North America.
Phosphorus. In 2020, 46 percent of soils tested below critical in P, an increase of 5 percent since 2001. Compared to 2015, increased needs for P showed up mainly in southeastern and southern states. Much of the Northern Great Plains and Texas showed fewer soils testing below critical P levels in 2020, although the overall need for P there remains high.
Potassium. In 2020, 44 percent of soils tested below critical in K, an increase of 4 percent since 2001. Compared to 2015, needs for K increased sharply in Wisconsin, Vermont, and Tennessee and also in several western and Great Plains states including Washington, South Dakota, Kansas, Colorado, and Oklahoma.
Soil Acidity. From 2001 to 2020, about 2 percent more soil tests fell into the range where crop growth and nutrient availability are greatest: pH 6.1 to 7.5. The relative frequency of samples with pH below 6 also increased by about 2 percent.
Magnesium. Data from 2005 to 2020 indicate that the few soils testing low in Mg are becoming even more rare. The number of samples being analyzed for Mg increased over 3-fold from 2005 to 2020.
Sulfur. From 2005 to 2020, more samples tested lower in S – a trend consistent with lower deposition of sulfate from the atmosphere. Sulfur soil tests are not well correlated to probabilities of yield response, so agronomic interpretations are unclear.
Zinc. Comparing 2020 to 2010, about 6 percent more samples test low in Zn. Although local interpretation is required, data from 2020 indicate many states and provinces may require Zn fertilization.
Chloride. The Northern Great Plains has a high frequency of soils with low levels of Cl-. The percentage of samples in the lowest category declined by about 20 percent from 2010 to 2015, but increased by 13 percent from 2015 to 2020.
Soil Organic Matter. Included for the first time in 2020, soil organic matter shows a median level of 2.9 percent over 4.3 million samples. A general assessment indicates soil organic matter increases from south to north, with differences by local soil parent material and management.