Soil Test Methods

Soil test methods used for vegetable production are designed to provide a measurement of lime requirement and nutrient availability. Soil testing labs use different methods and it is best to select a lab using analytical methods appropriate for New England soils that provide soil test interpretations based on field correlation and calibration under local conditions. A list of New England soil test laboratories is provided at the end of this section. In New England, routine soil analysis typically includes a measure of extractable phosphorus (P), potassium (K), calcium (Ca), and magnesium (Mg), plus several of the micronutrients (e.g., boron, manganese, zinc, copper, and iron). In addition to routine soil analysis, other tests and analytical services are available including: soil organic matter, soluble salts (conductivity), Pre-sidedress Soil Nitrate Test (PSNT), plant tissue analysis, manure and compost analysis, and irrigation water testing.

The appropriate soil test methods for a given region are selected based on soil characteristics and climate. Different analytical procedures can result in vastly different results. This is especially true of the different extraction solutions used to estimate a soil's nutrient supply. The two extraction solutions used in New England are the weak acid modified Morgan extract and the strong acid Mehlich 3 extract. These are both universal extraction procedures, meaning they are used to determine all major nutrients and many of the micronutrients simultaneously. Most New England Land Grant University labs (ME, VT, MA, and CT) use the weak acid modified Morgan extract. University of New Hampshire uses the strong acid Mehlich 3 extraction solution. A saturated media test using water as an extractant is used to measure nutrient availability in greenhouse potting media and in some cases, high tunnel soils. See Soil Testing Labs in New England (page 12) for contact information.

The quantity of extractable nutrients may be reported in different units. Some labs report the concentration of nutrients in ppm (parts per million) which is equivalent to mg/kg (weight basis) or mg/dm3 (volume basis). Other labs report nutrient values in lb/acre (weight per area basis). Nutrient concentration in ppm can be converted to lb/acre by multiplying by 2. This is based on the assumption that there are approximately 2,000,000 lb of soil in the surface 6 inches of one acre. Differences in reporting units are important to be aware of, but they are of little consequence to our interpretation of soil test results. Soil test results only provide an index of nutrient supply that must be interpreted based on correlation and calibration data developed by nutrient response research in local soils. It is important to understand what the nutrient interpretations mean.