By Greg LaBarge, CCA, Ohio State University Extension agronomist
The Fertilizer Institute periodically summarizes soil test results from private and public soil testing labs across North America. The summary is done every five years and provides an understanding of trends in soil test values. An overview of 2020 has just been added to the data. Table 1 shows Ohio soil test results since 2001. The number of samples processed is significant and has leveled off at 275,000 to 300,000 samples per year.
Table 1. Soil Test Numbers and Key Values for Ohio, 2001-2020. (Source: https://soiltest.tfi.org/)
|Year||Number of Soil samples||P-M3(ppm)||K-M3(ppm)||Soil pH||OM(%)|
Table 1 also provides us with a feel for trends in soil test values. Soil pH, an important indicator of solubility of soil nutrients, has held steady in the 6.3 to 6.4 range. Soil test potassium levels have dropped slightly to 153 ppm, which is mid-range of our Tri-State Fertilizer Recommendations maintenance range of 120 to170 ppm when CEC is > 5meq/100g of soil with our loam and clay soils.
Phosphorus is an essential nutrient for crop production and continues to be a focus in water quality improvement efforts. Soil test P (STP) levels have continued to decline since 2001, as shown in Table 1. However, the median STP is still between the critical level of 20 and the upper end of maintenance at 40 ppm.
The data can be broken out across soil test ranges for each summary year. This distribution of STP can be compared to critical agronomic and water quality thresholds. For example, Figure 2 shows the distribution for the three most recent years of 2010, 2015, and 2020. From an agronomic perspective, 38% of the 2020 samples were below the critical value of 20 ppm, and 29% were in the maintenance range. In those fields with STP results below the critical level, we would encourage the application of P for the upcoming crop to limit the risk of yield loss.
We can also look at the percentage of STP that might be a water quality concern. As soil test P-values increase, the potential for DRP losses from surface and tile discharges will also increase. Edge of field P loss concerns become more significant at around 100 ppm levels. In 2020, there were 9% of the samples above 100 ppm. From the water quality perspective, someone looking at this will observe that if high STP values are a problem, simply lower them. However, some perspective on the time it may take for lower soil test values is needed. Each year with no new P added and through crop removal, we only can hope to lower soil test values around 2-3 ppm. So going from 200 ppm to 100 ppm is estimated to take 33 to 50 years.
During the period it takes to reduce soil tests into the agronomic range, we need to consider conservation practices that will treat the water leaving the site. Wetlands and phosphorus removal structures are a couple of options. However, before using these types of capital intensive practices, it would be wise to measure the amount of P leaving a field site. You can find out more about treating high STP sites at https://go.osu.edu/highstp
Figure 1. Distribution of Soil Test P values in Ohio, 2010-2020. (Source: https://soiltest.tfi.org/)
This data is great to provide an overall perspective on regional trends. However, I would encourage you to summarize your farm to check your fertility program outcomes. First, check that all your fields are up-to-date with a current soil test done in the last two to four years. Then compare your recent soil test to the previous two or three samples, are the numbers trending as you would expect?