Home » Nutrient application guidelines for field, vegetable, and fruit crops in Wisconsin (A2809) » Soil Test Procedures
Soil Test Procedures
Soil Test Procedures
What can I test soil for?
The routine soil testing program for laboratories using the Wisconsin soil test recommendation program includes soil pH, organic matter content, lime requirement (buffer pH), and extractable phosphorus (P) and potassium (K). In addition, special tests may be requested for nitrate-nitrogen, calcium, magnesium, sulfur, boron, manganese, and zinc. Soil tests for copper, iron, molybdenum, and chlorine have not been calibrated to crop response in Wisconsin; these nutrients are rarely deficient in Wisconsin soils.
Several other tests can be performed on request. These tests include physical analysis for particle size distribution (% sand, % silt, % clay), exchangeable sodium, soluble salts, total nitrogen, inorganic nitrogen, total organic carbon, and heavy metals (arsenic, cadmium, chromium, cobalt, copper, iron, lead, manganese, molybdenum, nickel, selenium, zinc).
Where can I test soils in Wisconsin?
In Wisconsin, a soil testing laboratory must be certified by the Wisconsin Department of Agriculture, Trade, and Consumer Protection (WDATCP) if results are to be used in nutrient management planning or related to any government cost-sharing program.
A current list of the Wisconsin certified laboratories can be found here.
Table 3.1 briefly describes the procedures used for each soil test performed at University of Wisconsin laboratories and other WDATCP-approved laboratories.
Table 3.1 – Analytical procedures for soil tests performed at University of Wisconsin laboratories and Wisconsin DATCP-approved private laboratories
| Soil Test | Procedurea |
|---|---|
| Soil pH | Prepare a 1:1 soil to water mixture and measure the pH with a glass electrode. |
| Buffer pH (BpH) | Prepare a 1:1:1 soil to water to Sikora buffer mixture and measure the pH with a glass electrode. |
| Phosphorus (P) | Extract with Bray 1, develop color, and measure colorimetrically using a spectrophotometer. |
| Potassium (K) | Extract with Bray 1 and measure with atomic absorption, flame photometer, or ICP-OES. |
| Organic matter (OM) | Loss of weight on ignition at 360°C for 2 hours. OM = 0.07 + 0.89 (LOI)b |
| Calcium (Ca), magnesium (Mg), sodium (Na) | Extract with neutral 1 Nc ammonium acetate and measure with atomic absorption, flame photometer, or ICP-OES. |
| Sulfur (S) | Extract with 500 ppm phosphorus in acetic acid, develop turbidity, and measure with a photoelectric nephelometer. |
| Boron (B) | Extract with hot water, develop color, and measure colorimetrically using a spectrophotometer. |
| Manganese (Mn) | Extract with 0.1 N phosphoric acid and measure by atomic absorption or ICP-OES. |
| Zinc (Zn) | Extract with 0.1 N hydrochloric acid and measure by atomic absorption or ICP-OES. |
| Nitrate-nitrogen (NO₃-N) | Extract soil with 2 N KCl and analyze colorimetrically using a spectrophotometer. |
| Physical analysis (% sand, silt, clay) | Prepare 50 or 100 g soil with dispersing solution and measure with hydrometer. |
| Soluble salts | Prepare 1:2 soil to water mixture and measure with conductivity bridge |
How to account for soil test result variability
To maintain certification in Wisconsin, a WDATCP-certified laboratory is required to meet specific analytical quality standards. However, with any soil test there is a level of inherent variability that can be expected both within a lab and between certified laboratories. The variability within a lab should be lower than between labs.
In general, soil pH and Sikora buffer pH results should be within 0.2 pH units when the results of two laboratories are compared. Soil test levels for P and K should be within 10% of the “true value.” For example, a soil with 20 ppm P should test in the range of 18–22 ppm when run by different certified labs, and a soil with 100 ppm K should test in the range of 90–110 ppm.
Understanding cation exchange capacity
If exchangeable calcium (Ca) and magnesium (Mg) are run on a sample along with the routine analysis, an estimated cation exchange capacity (CEC) will be calculated and reported with the other soil test results. The estimated CEC is calculated from the soil test levels for Ca, Mg, and K using the following equation, and the results are reported in cmolc/kg, which is equivalent to meq/100g of soil.
Estimated CEC =
(ppm Ca/200 + ppm Mg/122 + ppm K/391) x
(5 grams/wt of soil in 5-gram scoop)
Sample density is used in the equation to estimate CEC because soil density varies with soil texture and CEC is strongly related to soil texture. Sample density is the weight of oven-dried soil in a 5-gram scoop, which has a volume of approximately 4.25 cubic centimeters. This value is expressed as grams per cubic centimeter (g/cm3) and is provided on the soil test report. Sample density is listed on the soil test report, but is only used in the estimation of CEC.
Soil test values for P and K are interpreted from very low to excessively high. The category is based on the soil test value in combination with the crop demand level. The probability of a yield response to applied nutrients is much greater for the very low (VL) and low (L) categories than for the high (H), very high (VH), and excessively high (EH) categories. Probability of a response to fertilizer applied at each soil test category is described in Table 3.2.
Table 3.2 – Codes and descriptions of soil test interpretation categories
| Soil Test Interpretation Category Name | Soil Test Interpretation Category Symbol | Description | Probability of a yield increase to applied nutrients (%) |
|---|---|---|---|
| Very Low | VL | Substantial quantities of nutrients are required to optimize crop yield. Buildup should occur over a 4- to 8-year period. Response to secondary or micronutrients is likely or possible for high or medium demanding crops, respectively. | >90 |
| Low | L | Somewhat more nutrients than those removed by crop harvest are required. Response to secondary or micronutrients is possible for high demanding crops, but unlikely for medium or low demanding crops. | 60-90 |
| Optimum | O | This is economically and environmentally the most desirable soil test category. Yields are optimized at nutrient additions approximately equal to amounts removed in the harvested portion of the crop. Response to secondary or micronutrients is unlikely regardless of crop demand level. | 30-60 |
| High | H | Some nutrients are required, and returns are optimized at rates equal to about one-half of nutrient removal by the crop. | 5-30 |
| Very High | VH | Used only for potassium. Soil tests are above the optimum range and gradual drawdown is recommended. Approximately one-fourth of nutrient removal is recommended. | 2-5 |
| Excessively High | EH | No fertilizer is recommended for most soils since the soil test level will remain in the non-responsive range for at least two to three years. On medium- and fine-textured soils, a small amount of starter fertilizer is advised for some crops (for more detail, see Chapter 10: Starter fertilizers). | <2 |
Chapter Authors and Editors
Natasha Rayne, Ph.D.
Assistant Professor and Extension Specialist – Soil Fertility and Nutrient Management
Manure placement, timing, and nitrogen credits; organic soil amendments and nutrient cycling; climate-smart and site-specific nitrogen management; improvement of nitrogen use efficiency in cereal crop production.
Hava Blair, Ph.D.
Researcher, Soil Scientist
Developing and implementing soil fertility, erosion, and nutrient loss models in SnapPlus
Last Updated: June 18, 2024