Instead of submitting a soil sample to the Clemson University Agricultural Service Lab, a farmer may sometimes choose to send his sample to a private soil test lab or to another state lab for analysis. Fertilizer dealers or consultants will also choos e to send soil samples to a private lab for analysis. Often when a farmer gets the results back from a lab other than the Agricultural Service Lab, he will try to figure out what the fertilizer recommendation would have been if he had sent the sample to the Clemson Lab. This could lead to significant errors.
When a lab receives a soil sample, they use an extractant solution to remove a portion of the available plant nutrients from the soil. The extractant containing the nutrients is then analyzed with laboratory equipment to determine how much of the vari ous elements are present. Unfortunately, there are several different accepted extractants used in the United States which extract different amounts of each nutrient. In our region, the most common extractants are Mehlich 1 and Mehlich 3. Some labs may additionally use a Bray extractant for phosphorus.
Results from any extractant can be used to make good fertilizer recommendations but you can't use the data from one extractant and apply recommendations which were calibrated for a different extractant. For example, the Clemson University soil test re commendations for phosphorus, potassium, calcium, magnesium, zinc, manganese, copper and boron are calibrated for extractions done with the Mehlich 1 extractant. A sample sent to the state lab in North Carolina will be extracted with the Mehlich 3 extrac tant and subsequently the Clemson University recommendations can't be used. It would be like mixing apples and oranges. So before trying to apply Clemson's soil test recommendations to soil test results from another lab, it is extremely important to det ermine which extractant they use.
If you know that the other lab uses the Mehlich 1 extractant as the Clemson lab, be aware that the other lab may use a scale different from Clemson's when rating the pounds-per-acre of extractable nutrients as high, medium or low. An extractable amoun t of nutrient that the Clemson lab labels as "high" might be labeled as "medium" by another lab. The "medium" soil test rating would call for more of that particular nutrient than would be recommended with a "high" soil test rating. In this case, if you want to use Clemson University's recommendations based on another lab's analytical report and you know that both labs are using the same soil extractant, the best way to make comparisons is to use the pounds per acre extractable value for each nutrient a nd convert it to a high, medium or low rating using the interpretation guidelines provided by Clemson University in Circular 476 "Lime and Fertilizer Recommendations Based on Soil Test Results". Then the Clemson recommendations can be properly used with the other lab's soil test results. The Clemson University county Extension agent can help you with this.
The easiest thing to do, of course, is to insure that the soil samples are sent to the Clemson University Lab through your local Extension office. You will receive quality analysis and Clemson's recommendations without the worry of future problems due to mixing apples and oranges.
See also "Is splitting soil samples between two labs a good method for comparing lab quality?"