Wednesday, February 27, 2008

Greenhouse and Nursery - Testing Media for Nutrients

There is often confusion on nutrient tests and pour through leachate tests for greenhouse and nursery media. The following is an article on the subject with a good link to a fact sheet on testing methods from the University of Connecticut and University of Massachusetts.

Many growers are performing their own soil tests for pH and soluble salts on-site. Others are sending samples to different labs for their soil tests and some do a little of both. Often growers end up with a confusing collection of tests done different ways with widely different “numbers”.

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. The number representing the level of soluble salts from a soil test using a 1:2 dilution method are going to mean something different than results from saturated media extract (SME) or leachate pourthru. For example, 2.6 would be “extreme” (too high) for the 1:2 method, “normal” for SME, and “low” for leachate pourthru. Always use the interpretative data for the specific soil testing method used or otherwise you could make an incorrect interpretation of the results. The differences in pH results between the tests is much smaller.

The major difference between the current methods of greenhouse media testing is the way plant-available nutrients and soluble salts are “extracted” from the media samples for analysis. Differences between the actual methods of pH, soluble salts, and nutrient analysis and the laboratory equipment used to do the analyses are not as important as how the plant-available nutrients are extracted from the sample.

When people think of “soil testing”, they might imagine some procedure which analyses the solids in a mix for pH, nutrients, and soluble salts. However soil samples themselves are not actually analyzed during a soil test, but rather plant-available nutrients are pulled out or “extracted” from the sample using an “extracting solution”.

For more information on the three different methods for testing soil , proper sampling and a comparison table with soluable salt levels for each of the methods, see the fact sheet:
Current Methods of Greenhouse Media Soil Testing and How They Differ

From the New England Greenhouse Update Site

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