Average Soil Test
Humorist Evan Esar once wrote that “Statistics is the only science that enables different experts using the same figures to draw different conclusions.” What is the average soil test in Southeast Kansas? This simple question, like many simple questions, is in fact quite complex when statistics are involved.
Over the past year, I have had the opportunity to make fertility recommendations on just over 220 soil tests across the four counties (Crawford, Labette, Montgomery, and Wilson) the Wildcat District covers. The mass majority (nearly 200) came from Crawford and Montgomery so this data is definitely skewed toward them.
For the purposes of this data set, I have only included crop ground soil tests. If hay and pasture soil tests were included, I am quite confident the data would be considerably different.
The common soil test includes three major tests, Soil pH, Phosphate ppm, and Potash ppm. The K-State Soil Testing Lab uses the Mehlich-3 extraction method for extracting and measuring phosphate. Different labs may or may not use this method so comparing between labs may not be recommended.
The average phosphate level across the 220 samples was 29.4 parts per million (ppm). I must admit that I would have assumed a lower level than this without running the data. While the average is the most common measure of central tendency, the median (half of the measurements below and half above) can minimize the outliers in data. For phosphate, the median is 16.6 ppm, which would be much closer to my assumptions.
The highest sample I reviewed this year had 214 ppm phosphate while the lowest had only 1 ppm. If you break down the data into percentiles, the upper ten percent of the samples were at 69 ppm or above while the lowest ten percent were at 5.9 and below. For the major crops in our area, 20 ppm is the level in which no phosphorus fertilizer would be recommended on a sufficiency basis. There were 118 samples below 20 ppm which would have had some phosphorus fertilizer recommended.
For potassium, my assumptions were actually much closer to the results. The average across all samples was 146 ppm potash. Once again, the median was lower at 123 ppm and the range was quite broad with a maximum of 571 ppm and a minimum of 43 ppm.
For potash, the highest ten percent of the samples were at 255 ppm or greater while the lowest ten percent were 79 ppm or lower. The critical level for potassium fertilizer to be recommended on a sufficiency basis in our major crops is 130 ppm and 123 samples fell below that crucial amount.
Finally, pH was also quite interesting and my prior assumptions would be nearly accurate. The average and the median were quite close in for pH at 6.53 and 6.6 respectively. In addition, the range was wide with a maximum of 7.9 (which is extremely high for our area) and a minimum of 5.1.
The highest ten percent of pH readings were at 7.3 and above while the lowest ten percent fell below 5.69. In pH, the critical value is below 6.5 which would induce a lime recommendation. There were 91 samples which met this criteria.
To try to quantify the “average” soil test in Southeast Kansas, a number of statistical measures were applied. Feel free to draw your own conclusions. If you have any questions, please call me at the office (620) 724-8233, or e-mail me at jcoltrain@ksu.edu, or visit the Wildcat Extension District website at www.wildcatdistrict.ksu.edu.
Contact:
Josh Coltrain
Crop Production Agent
Wildcat Extension District
jcoltrain@ksu.edu
(620) 724-8233
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