Occasionally, a farmer may want to compare the soil-test results from two separate labs. The farmer collects a soil sample from a few spots in the field, mixes the samples together in a bucket, and sends a portion of the soil to two different labs. When samples are split between labs in this manner, there are several reasons why the soil-test results are likely to be quite different.

It's difficult to mix soil samples to make them completely uniform.

Every year, several labs from the Southeastern and Mid-Atlantic States participate in a regional soil-test exchange involving approximately seven state-supported labs and five privately owned labs. Before the samples are distributed to these labs for analysis, the bulk sample is first completely air-dried to ensure thorough mixing. Then it's run through a grinder that breaks up the clods and sieves out any rocks or organic debris. Finally, the sample is mixed in a large container for several minutes.

When the individual samples are removed, care is taken to keep the larger soil particles in the container from settling. A number of samples are then removed and analyzed by one lab to ensure that the mixing was thorough.

Professional technicians take these steps to make the sample as uniform as possible. This detailed procedure is a far cry from taking two samples of soil simply mixed in a bucket. Mixing soil that is either moist or too dry is difficult, and studies show that samples split in this manner will be significantly different even when measured by the same lab.

Soil-test labs use different extractants.

After the exchange samples are analyzed by the various laboratories, the results are grouped by the type of soil extractant used for a proper comparison. Due to differences in lab techniques, even the results from samples tested by laboratories that use the same extractant solution will usually vary by a few pounds per acre. Differing test results are normal -- and expected. The following table illustrates that matching tests rarely produce the same results.

Soil phosphorus (in pounds per acre) reported by six regional state labs using the Mehlich 1 extractant for the 1997 sample exchange.

Sample No.	Lab A	Lab B	Lab C	Lab D	Lab E	Lab F	Average
1	35	21	38	32	32	24	30
2	32	16	45	32	35	22	30
3	58	45	51	59	56	34	51

Soil-test labs use different extractable nutrient rating systems.

The rating system used to indicate whether an extracted level of a nutrient is high, medium, or low will differ from one lab to another. Even when two labs extract the same amount of a nutrient using the same extractant, one lab may call that amount medium, while another lab indicates that the level is high. This is called "soil test calibration" and should be based on extensive and dependable research with similar soils and crops.

Soil-test labs make different recommendations.

The fertilizer recommendations may also vary for each lab. The recommendations are ideally based on Experiment Station information specific to a particular region's climate and soils. If they don't have data from their own Experiment Station for a particular crop, some labs will use data from adjacent states. Then too, some private labs use data from universities.

It's easy to see how a farmer can get different results from a soil sample sent to two separate labs. The results may differ because the sample wasn't mixed properly. Perhaps the labs used different extractants, different rating systems, or follow different guide-lines to make their recommendations. With only two sets of results for comparison, it's extremely difficult to know which is best to follow. Many farmers will choose the lab result that shows what they'd like to see, even though it may not be the most appropriate for their situation.

It's best to stick with one laboratory and develop a long-term relationship with their services. If you have any questions about a sample, the lab should be willing to re-test it.

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