NCSU Horticulture Substrates Lab Diagnostic Services

Technical Update: Sample Preparation

Background | Procedure | Mass Wetness vs Moisture Content | MW, MC, & Gallons of Water in a Substrate | Mass Wetness Guide |


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:

  • Mixing of components
  • Placing the substrate in the final flat, pot, basket, or other container
  • Watering-in the substrate and allowing for initial settling.

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.

Procedure Top

We use the following protocol for our testing:

  1. Determine mass wetness (moisture content) of sample by drying. This can be accomplished with conventional drying oven, forced-air drying oven, microwave oven, or moisture determining balance. This can take from 15 minutes to 24 hours, depending on sample and technique used.
  2. Calculate the target mass wetness, add moisture, blend in, seal in plastic bag and let equilibrate over-night. This is a critical step. If we try to pack our sample right after moisture addition, the substrate may be "tacky" or sticky. If we equilibrate it over night, this tackiness is gone.
  3. Pack sample to target bulk density. If we have run test on the product before, we match mass wetness and bulk density for comparison.

Mass Wetness vs Moisture Content Top

Mass Wetness


  • Moisture Content is the weight of the water divided by the wet weight of the sample
    • (Wet weight - Dry weight) / Wet weight
  • Mass Wetness is the weight of water divided by the dry weight of the sample
    • (Wet weight - Dry weight) / Dry weight

Mass Wetness, Moisture Content, and Gallons of Water in a Peatlite Mix Top

Mass Wettness
Moisture Content
Water in a Peatlite Mix
% Moisture
Gallons/ Cu Yd

Mass Wetness Guide Top

The following table is offered as a guide. Your actual numbers may vary, according to your materials:


Mass Wetness
g water / g substrate
Moisture Content
% wt
Bulk Density
g / cc

Sphagnum peat

3.0 - 5.0
75 - 82
0.06 - 0.10

Pine bark


Pine bark fines

Commercial Mixes,
70 - 75

Retail Mixes

1.0 - 2.5
50 - 70
0.10 - 0.40

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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Maintained by Bill Fonteno

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