Technical Update: Sample Preparation
Several of the Porometer users involved with peat or soil mix manufacturing have had questions about sample preparation. It seems several people are packing the samples with the moisture content that results directly from the production line. Some clarification is necessary to enable you to obtain the results desired.
The most important criteria in evaluating physical properties of substrates is to have a structure that does not change during testing. Consequently, shrinking and/or swelling of substrates during saturation or drainage causes erroneous readings.
If your substrate is swelling or shrinking more than 3 mm above or below the top of the aluminum cylinder during the test, the numbers are not valid.
Fortunately, controlling shrink/swell can be accomplished by using proper moisture content and packing to target bulk densities.
This test was designed to determine properties of STRUCTURE for components and mixes. There is NO structure in the bag or bale of product. Structure is created by the end user.
There are three steps to creating structure:
Only then is there a structure "set" for plant growth.
Growers and consumers can greatly alter the effective structure through handling and container selection. In order to evaluate the physical properties, we must "select" conditions for determining structure.
Here at the Horticultural Substrates Laboratory, we have tested thousands of samples including peats, barks, professional and consumer mixes. We have also evaluated many materials as perspective components. In all of our tests we try to evaluate structure as the plant would see it.
We use the following protocol for our testing:
The following table is offered as a guide. Your actual numbers may vary, according to your materials:
Check your manual for method of determining mass wetness.
While it is desirable to test pH and EC directly on freshly run product, it is not desirable to test structure that way. However, once you have established a proper test as described above, you can develop a quick, ON-LINE test for porosity, using samples directly from the production line. These numbers will not necessarily be the same as a proper Porometer test, but they can be repeatable and provide useful information.
For further questions, please contact: Bill Fonteno
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