Last month, I discussed the basics of soil sampling. The take-home message was that we must accept, but try to reduce, some inherent variability and inaccuracies associated with the sampling process.

Unlike sampling, laboratory procedures have evolved to provide excellent soil testing accuracy. Quality control is constantly being maintained within and among soil-testing laboratories. Each laboratory maintains standard soils of known composition that are measured every time commercial soil samples are analyzed.

Soil-testing laboratories also participate in "blind" sample exchange programs. These programs evaluate laboratory accuracy by submitting anonymous known standards. Laboratories that provide inaccurate results risk losing accreditation status.

Given the standardization and accreditation discussed above, you would expect that soil-testing laboratories are identical. However, not all soil-testing laboratories use identical procedures when testing a given soil component. For example, the University of Missouri uses a salt solution to measure soil pH, whereas most other state and private laboratories use a water solution. A soil sample submitted to the University of Missouri laboratory will provide a different soil pH measurement than would be given for the same sample submitted elsewhere. Is this soil pH discrepancy among laboratories a major problem? Probably not, if you realize that each laboratory bases its soil pH correction liming recommendation upon the extraction that it uses.

Another discrepancy among soil-testing laboratories is the choice of measurement reporting units. One lab may use parts per million (ppm) while another may use pounds per acre, while a third may use percent by weight or volume. The average weight of an acre furrow slice (average soil sampling depth) is 2 million pounds. Therefore, one ppm is equivalent to 2 pounds per acre. A 25 ppm phosphorus measurement from one lab is equal to a 50 pound per acre phosphorus measurement from another. Similar to the liming discussion above, each laboratory has accounted for its measurement units within its soil-test recommendation equations.

Most soil-testing laboratories offer a basic soil test with options for additional analyses. Most basic soil tests include soil solution (active) and residual pH, organic matter, "available" phosphorus, and "exchangeable" potassium, calcium and magnesium. From these basic measurements, usable information like neutralizable acidity, cation exchange capacity and percent base saturation are calculated.

Please note the words "available" and "exchangeable." The amount of a given plant nutrient reported by a soil-testing laboratory is only a fraction of its total amount within a soil. Soil-testing procedures are designed to estimate plant nutrient availability. A large percent of the total nutrient within a soil is fixed or "tied up" as non-usable mineral or organic compounds. This is why soils often contain several times a crop's need for a given nutrient, but still require fertilizer inputs containing that nutrient.

Soil sulfate-sulfur, nitrate-nitrogen, ammonium-nitrogen and total nitrogen are commonly tested, especially when crop deficiencies or excesses are suspected. These nutrients are not included in basic soil tests because their soil activity is extremely complex, rapidly changing and very mobile. A laboratory analysis of these nutrients is a "snapshot in time" and does not fit into long-term soil management programs.

Another category of non-basic soil-test analyses would be crop micronutrients including zinc, iron, copper, manganese, chloride, boron and molybdenum. These nutrients are excluded from basic soil tests because they are required by crops in relatively small amounts and are generally supplied by the soil at sufficient levels for optimum crop production. I recommend testing for zinc, copper, iron, manganese and boron the first time a field is soil sampled. This initial test will reduce the potential for micronutrient

problems and help set a baseline for record keeping and nutrient tracking. I also recommend testing for micronutrients when unexplained poor crop performance or visual micronutrient crop deficiency symptoms occur.

Most soil-testing laboratories also offer a wide range of non-nutrient analyses like water holding capacity, bulk density, salt content, plant tissue content, pesticides, feeds, waste products, etc.

Make sure your soil-testing laboratory is accredited for your region. Non-accredited laboratories do not participate in regional sample exchanges and may use non-sanctioned analytical procedures. Select a soil-testing laboratory based upon a combination of service, price and personal comfort. When evaluating a laboratory, consider turn-around time, electronic support, agronomic support and soil-test recommendation philosophy. For consistency, use the same soil-testing laboratory over time. This will greatly increase your ability to maintain an efficient crop nutrient management plan.