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Soil analysis is a tool we use to help evaluate soil phosphorus status, identify deficient situations, and help determine if phosphate fertilizer is needed or not. There are several laboratory methods used to determine a value that we call “plant available” phosphorus. This laboratory value provides a way to estimate the soil’s ability to supply phosphorus to a crop over the next growing season.
Each laboratory method extracts different amounts of soil mineral and organic phosphorus, depending on soil pH, parent minerals, soil texture, and other factors. The chemical characteristics of different phosphorus extractants are discussed in Crop File 1.01.231 “Soil test phosphorus methods.”
Comparing results from two different laboratories can be confusing, especially if they use different methods. Even if the two soil samples have identical ability to supply phosphorus, the numbers reported by the laboratory may be very different due to the extraction method or the units used in the laboratory report.
Soil phosphorus is found in various combinations with other elements, including calcium, aluminum, iron, and magnesium. Phosphorus can also be supplied to the growing crop by breakdown of various organic compounds, ranging from plant residues to humic materials. Many of these mineral and organic phosphorus compounds are important to plant nutrition, but they each differ in their solubility and their contribution to the total crop phosphorus requirement.
Soil analysis is used to estimate the “phosphorus supplying power” of the particular soil as represented by the sample being tested. Soils are analyzed by mixing a small amount of soil with a certain extracting solution and shaking for a specific time. The solution is removed from the soil by filtering, then the phosphorus concentration is determined by developing a color (colorimetric) or by light emission (ICP).
An extracting solution dissolves or removes a small fraction of the total mineral or organic phosphorus from the sample. The amount of phosphorus removed by the extracting solution is not considered an actual quantity of soil phosphorus, like “X pounds of phosphate per acre.” The concentration of phosphorus removed by the extractant during a few short minutes in the laboratory is related to the rate that particular soil releases phosphorus to the plant root system over an entire growing season.
Common phosphorus extractant formulas include acids, bases, chelates, or a combination. Distilled water or bottled cola could be used as phosphorus extractants, but the results would not provide useful information to identify deficiencies or the need for additional fertilizer. Certain methods are better adapted for certain soil conditions, geographic regions, or plant species.
A good extractant can be used to separate soil samples into “low”, “medium”, “optimum” and “high” classes, allowing us to rank them according to their individual ability to supply phosphorus to the crop over the next growing season. These rankings are then used to help predict the overall need for additional phosphate fertilizer.
If the concentration is in the “low” range, extra phosphate is needed because the soil is not expected to meet the crop’s needs, resulting in reduced growth and lower yields. If the concentration is in the “high” range, the soil is expected to meet the crop needs. Additional phosphate fertilizer is very unlikely to improve growth or yields, but may be applied to maintain the soil test in the optimum range.
The actual range of numbers obtained by the laboratory instrument will vary between soil test methods. A harsh extractant will dissolve more phosphorus than a mild extractant, so may have a wider range of numbers for interpretation. For example, the “very low” to “very high” range might be from “1 ppm” to “50 ppm”. A milder extractant used on the same soil sample might give us a range from “1 ppm" to “15 ppm” to separate the “very low” soils from “very high” soils.
Laboratories may report results differently. Servi-Tech and most other laboratories report soil phosphorus in “parts per million (ppm),” because we measure soils volumetrically - using standard soil scoops.
Some laboratories report “milligrams per kilogram (mg/kg)” if samples are weighed before analysis. The units “mg/kg” and “ppm” are considered equivalent.
A few laboratories report results as “pounds per acre (lb/ac)” even though soils are measured volumetrically. This is based on an assumption that the soil sample represents an “acre-furrow slice” or a “plow layer” of a standard 6b-inch depth. “lb/ac” is calculated by simply multiplying the “ppm” by 2. This may also be reported as “parts per two million (pp2m)”.
A result reported as “10 ppm P” is equivalent to a result reported as “10 mg/kg”, “20 lb P/ac” or “20 pp2m”.
Servi-Tech Laboratories adopted Mehlich-3, colorimetric phosphorus as the standard, default method in 1993, switching from the Mehlich-2 method we adopted in 1978. Other methods, like Bray-1 and Olsen sodium bicarbonate, are available on request.
There have been many comparisons used to correlate results between different extractants. The correlations from research studies often differ because of differences between geography, soils, or instrumentation.
The table below provides general correlations between methods, based on our internal research studies.
These correlations may differ from those obtained by others. Remember that these are calculated values, not exact correlations. They can differ depending on the characteristics of an individual sample, but can be used for general comparison of soil test results.
¶ Table 1. Calculated Comparisons, Soil Test Phosphorus Methods* |
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| Mehlich-3, colorimetric ppm P |
Olsen sodium bicarbonate, colorimetric ppm P |
Bray-1, colorimetric ppm P |
Mehlich-3, ICP ppm P |
| ————— | ———— calculated equivalent ppm P and common range (in parentheses) ———— | ||
| 2 | 2 (1–2) | 2 (2–3) | 3 (3–4) |
| 5 | 3 (2–3) | 5 (4–5) | 7 (6–7) |
| 10 | 5 (4–5) | 9 (8–10) | 12 (11–13) |
| 15 | 7 (6–7) | 13 (12–15) | 18 (16–19) |
| 20 | 8 (8–9) | 18 (17–19) | 23 (22–24) |
| 25 | 10 (9–11) | 22 (21–23) | 28 (27–30) |
| 30 | 12 (11–14) | 26 (26–28) | 34 (32–36) |
| 35 | 14 (13–16) | 31 (29–32) | 39 (37–41) |
| 40 | 16 (14–18) | 35 (33–37) | 45 (43–47) |
| 45 | 18 (16–20) | 40 (38–42) | 50 (48–53) |
| 50 | 20 (18–22) | 44 (42–46) | 56 (53–58) |
| 55 | 22 (20–24) | 48 (46–51) | 61 (58–64) |
| 60 | 24 (21–26) | 53 (50–55) | 67 (63–70) |
| 65 | 26 (23–28) | 57 (54–60) | 72 (68–76) |
| 70 | 27 (25–30) | 62 (58–65) | 77 (74–81) |
| 75 | 29 (26–32) | 66 (63–69) | 83 (79–87) |
| 80 | 31 (28–34) | 70 (67–74) | 88 (84–93) |
| 85 | 33 (30–37) | 75 (71–78) | 94 (89–99) |
| 90 | 35 (32–39) | 79 (75–83) | 99 (94–104) |
| 95 | 37 (33–41) | 83 (79–88) | 105 (99–110) |
| 100 | 39 (35–43) | 88 (83–92) | 11 (105–116) |
¶ ReferencesVocasek, F. 1997. “Correlation of the Mehlich-3 extraction with other soil test methods on High Plains soils”. Agronomy 615, Soil Problems. Kansas State Univ. 16 pg. and Vocasek, F. 1996. “Correlation of Olsen bicarbonate and Mehlich-3 phosphorus methods on High Plains soils”. Proc. of 1996 Agricultural Laboratory Analysis Workshop, Downtown Holiday Inn, Denver, Colo. 17-18 October 1996. 7 pg |
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