Greenhouse - Methods for Greenhouse Media Testing

Many growers are experimenting with various ways of performing their own soil tests for pH and soluble salts on-site. Others are sending samples to different labs in search of timely and inexpensive media testing. Often growers end up with a confusing collection of tests done different ways with widely different "numbers". It is important to know what test is being done and the associated numbers for making decisions. The following is information on current testing methods being used.

There are three commonly used methods of testing soilless media based on the use of water as an extracting solution, they are: saturated media extract, 1:2 dilution method, and leachate pourthru.

Saturated media extract (SME).

SME is currently "the" method of testing soilless greenhouse media and it is almost universally done by commercial and university labs, including the UD Soil Testing Lab. In this test a paste is made using soil and water and then the liquid portion (the extract) is separated from the solid portion for pH, soluble salt, and nutrient analysis. Special skills and laboratory equipment are required to perform this test. SME is probably not suitable for a grower to use unless the greenhouse operation is large enough to support a lab, a technically trained person is hired to carry out the tests, and there is a committment to frequent testing and tracking of the results.

1:2 dilution method.

This method has been used for many years and has good interpretative data to back it up. In this test an air-dried sample of soil media and water are mixed together in the volume ratio of 1 part soil to 2 parts water (e.g., using a measuring cup, 1 fl. oz. of soil + 2 fl. oz. of water). The liquid extract is then separated from the solids using laboratory grade filter paper or a common coffee filter. The extract is then ready for analysis. This is a very easy test to master and quite suitable for on-site greenhouse testing of pH and soluble salt using the so-called pH and EC "pens" available from greenhouse suppliers. In my opinion the 1:2 method is a very good choice for occasional pH and soluble salts testing by growers on-site.

Leachate PourThru.

This is the newest way of testing soilless media and its apparent simplicity has caught the interest of many growers. One of the major advantages to leachate pourthru is that there is no media sampling or preparation. Unlike SME and 1:2 methods, plants do not have to be sacrificed or disturbed for testing because the extract is the leachate collected from the container during routine irrigation. The leachate can be analyzed on-site using the pH and EC pens or it can be sent to a commercial laboratory for a complete nutrient analysis.

Leachate pourthru is best used for continuous monitoring and graphical tracking of pH and soluble salts. To make this method work best an irrigation and leachate protocol must be established and carefully followed when sampling takes place. Based on my observations, leachate pourthru is not a good choice for casual checks (use 1:2 method for this). Some growers like to "whip out" the old pH or EC pen now and then and check the drainage from some pots. Unfortunately, with casual use like this, the "numbers" are often quite variable, inconclusive, and probably unreliable. A North Carolina State University Fact Sheet tells exactly how to correctly use the leachate pourthru method. http://www.ces.ncsu.edu/depts/hort/floriculture/crop/crop_PTS.htm

Information adapted from "Current Methods of Greenhouse Media Testing and How They Differ" by Douglas Cox, Plant and Soil Sciences, University of Massachusetts