Ornamental Plants -- Annual Reports and Research Reviews 1998

Special Circular 165-99

Important Considerations When Changing to a New Soil Testing Laboratory

Mary Ann Rose

Summary

With the closing of The Ohio State Univer-sity's Research-Extension Analytical Laboratory (REAL), former clients will have to find new providers of soil, growing media, plant tissue, and water analyses. This article is a guide to some of the differences in analytical procedures and methods of reporting that may be encountered when switching to a new lab.

Introduction

After many years of serving Ohio's agricultural industry, the Research-Extension Analytical Laboratory (REAL) at Ohio State University closed at the end of 1998. The REAL lab had provided testing services for soil, media, plant tissue, feed, manure, and compost.

The most common tests used by the nursery industry included the soil, soilless mix, plant tissue, and water analyses. Growers and landscapers who find themselves switching from one testing laboratory to another can expect to see differences in the way results are expressed (and interpreted) and perhaps in the types of tests that are used. It is possible that test results from a new lab may require an entirely different set of interpretive guidelines. A testing lab should provide appropriate guidelines with its test results, although the guidelines may not be specifically geared to nursery crops. Your Ohio State University Extension agent also can help interpret results if you switch to a new lab. This article is a guide to the types of changes that may be encountered when switching labs.

Testing Procedures

The Mineral Soil Test

The standard soil test provided by the REAL laboratory included a lime recommendation (if needed); available phosphorus (P); exchangeable calcium (Ca), magnesium (Mg), and potassium (K); base saturation for Ca, Mg, and K; and the cation exchange capacity (CEC). Most labs in the Midwest are probably using the same or similar tests for these values; however, the units in which they express the values may vary.

For example, the REAL lab had recently switched over to using ppm (parts per million) instead of lb./A (pounds per acre) to express available P and exchangeable Ca, Mg, and K. If you change from the REAL lab to a commercial lab using lb./A, expect to see these soil test values double, since 1 ppm = 2 lb./A. A third unit that is sometimes used to express exchangeable Ca, Mg, and K is milliequivalents per 100 grams (meq/100g). Somewhat more complex computations are required to convert meq/100g to lb./A.

Soil soluble salts (EC) is a special soil test that is useful if over-fertilization or deicing salt problems are suspected. It is particularly confusing to compare this test between labs, because EC may be expressed several different ways. The REAL lab used mhos x 10^-5/cm (for the soil test, but not for the soilless test). Most labs use millimhos/cm. The difference is a factor of 100. For example, 100 mhos x 10^-5/cm = 1 millimhos/cm.

The Soilless Mix Test

The soilless test is the appropriate choice for most container media used today, even if they contain a small percentage of soil. Soilless tests provide pH, nitrate-N, soluble salts (EC), P, K, Ca, Mg, and some micro-elements, typically iron (Fe), copper (Cu), zinc (Zn), and manganese (Mn). Labs may also provide ammonium-N, chloride, fluoride, sulfate-S, boron, molybdenum, and sodium as part of a standard test package.

Most labs are using the saturated medium extract procedure (SME), although some may be using a 1:2 soil-water dilution procedure. The same sample extracted by the SME method can be expected to have nutrient values approximately double those from the 1:2 dilution method, because less diluent (water) is used in the SME. With one exception, results from different labs that use the same dilution procedure (either SME or 1:2) should be very similar. Micronutrients can vary, however, because some labs (including the REAL lab) use a chelating agent (DTPA) when extracting media for micronutrients. Use of the chelating agent results in higher micronutrient test values than the water-only extraction procedure, and different interpretive guidelines should be used.

Plant Tissue Tests

A plant tissue test (most commonly leaves) reflects the elemental content of the tissue. The whole tissue, not an extract, is tested; thus results will not vary among labs because of different extraction procedures. Labs may use different types of equipment to perform the analyses, but the same sample submitted to different labs should nonetheless produce very similar results.

Nitrogen content of plant tissue can vary somewhat between labs because of two fundamentally different methods of determination; however, even in this case, the difference is not more than a few tenths of a percent. Tissue concentrations of macro-elements (N, P, K, Ca, and Mg) are usually expressed as percent by dry weight of tissue, and the microelements (Fe, Cu, Mn, Zn, B, and so forth) are expressed in ppm.

Water Tests

An irrigation water analysis usually provides all or most of the following -- pH, alkalinity, soluble salts, nitrate-N, ammonium-N, P, K, Ca, Mg, Fe, sulfate-S, Mn, Zn, Cu, B, Cl, F. As with plant tissue-testing, results from lab to lab should be similar, because no extraction process is used. All elements are expressed in ppm. Alkalinity is the only parameter in a water test that may be expressed differently between laboratories. The REAL lab expressed alkalinity as ppm of calcium carbonate, whereas some labs use ppm bicarbonate. To convert from one to the other:

1.2 x ppm CaCO₃ = ppm HCO₃^-

Testing Laboratories

Table 1 contains a list of laboratories that perform one or more types of analytical procedures. This list is offered as a reference to growers and landscapers, but no endorsement of any lab is implied.

When choosing a lab, considerations include not only cost, but also turnaround time, lab reputation, and attention to accuracy and precision. Granted, there is no easy way for a prospective client to judge the latter characteristics. But if lab representatives are able to intelligently discuss testing procedures and offer interpretive advice, this is a good sign.