NCSU Horticulture Substrates Lab
 

General Description | Cost | Procedures | Calculations | Reference | Porometer Data Guidelines | Mass Wetness Update |

General Description

The NCSU Porometer was designed to provide rapid, consistent, and reliable testing of substrates. It was developed in consultation with the professional substrate manufacturers and the National Bark & Soil Producers Association. Data developed from this equipment has been approved for use in registering potting soils in the state of Georgia.

The main part is an aluminum base plate having an inner and outer plate with 8 holes in each. The plates fit together as one unit and can be rotated so that the holes are aligned in the open position to allow drainage through the plates.

Also, the plates can be rotated such that the holes are covered in the closed position to prevent any seepage of water through the plates.

Porometer

The second part is a 3 inch aluminum core that is packed with a substrate sample and then placed onto the base plate.

Porometer 2

The third part is a funnel in which the base plate with the core is placed for saturation and drainage of the sample.

Porometer 3

The NCSU Porometer is also available on a limited basis for purchase by soil mix manufacturers and researchers of horticultural substrates. We do not keep a large supply on hand, but make the porometers when demand is sufficient.

The porometers are sold in three-porometer sets. Each set contains three of the following:

  • Baseplate
  • 3" aluminum core
  • Drying lid
  • Saturation funnel
  • Support ring
  • Rubber stopper

Each set also contains one Packing Kit which includes:

  • Top packing cylinder (6")
  • Bottom packing cylinder (3")
  • Bottom lid
  • Clamping assembly (2)
  • Substrate leveling spatula
  • Substrate prep & storage pan w/lid

Each set also includesone Core Extraction Device and one Instruction Manual

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Cost

Cost may vary as prices for aluminum, and labor change.

ITEM

PRICE

Porometer Set

$600.00

Additional porometer set of 3

$450.00

Additional Packing Kit

$150.00

Additional cores w/lid

$20.00

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Procedures

  1. Each cylinder is first properly packed to a known bulk density at a specific moisture content.
  2. The base plate is attached to the cylinder. The unit is placed into the funnel and the uniti checked to make sure the plate is in the open position.
  3. Next the funnels are filled with distilled water in a step-wise fashion to prevent air entrapment. Once completely filled, the unit is allowed to equilibrate for 15 minutes (or 1 hour).
  4. The base plate is carefully closed by rotating the unit by hand. The water outside the core is allowed to drain from funnels.
  5. A graduated cylinder is placed under thefunnel. The base plate is rotated into the open position, and the core allowed to drain for 60 minutes.
  6. Water remaining in the sample is determined gravimetrically. Samples should be dried at 105° C for 24 hours.

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Calculations

Total Porosity = [(Wet weight - Dry weight + Drainage) / 347.5] X 100

Water Holding Capacity = [(Wet weight - Dry weight) / 347.5] X 100

Air Space = (Total Porosity - Container Capacity) = (Drainage / 347.5) X 100

Bulk density = (dry weight / core volume)

  • Db (g/cc) = Dry weight / 347.5
  • Db (lbs/cu ft) = (Dry weight / 347.5) X 62.427961

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Reference

Fonteno, W.C. and T.E. Bilderback. 1993. Impact of hydrogel on physical properties of coarse-structured horticultural substrates. J. Amer. Soc. Hort. Sci. 118(2): 217-222.

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General Guidelines*

*This is a compilation of over 2,000 samples tested in our laboratory using the NCSU Porometer. These data are offered as guidelines, NOT STANDARDS, for these substrates. There are no standards; and it is our position that standards for these materials are inappropriate. However, it is useful to have numbers as guides.

The values for total porosity, air space, bulk density, percent moisture, and mass wetness are not averages but those that we expect to see from that particular substrate. The Max/Min values are the total range we have found for that material - not "acceptable" ranges.

Substrate

Total Porosity

Air Space
Bulk Density
Mass Wetness

Median

Max/Min

Median

Max/Min

Median

Median

Vermiculite

78-80

85/74

6 - 10

26/4

0.16-.018

1.5-2.0

Perlite

68

82/65

28-32

46/14

0.15-0.17

1

Bark

75-80

83/73

19-24

41/6

0.2

1.5

Bark Fines

75

78/73

16

20/10

0.25

1.5

Peat

89-94

97/87

20 - 12

27/6

.06-.10

3.0-5.0

Compost (yard waste)

83-87

88/80

7 - 18

44/3

.23-.28

1.0-1.5

Rockwool

90

94/87

20

25/19

0.2

1.0-1.5

Peat:Vermiculite

88

90/82

9

13/6

0.14

2.0-3.0

Peat:Perlite

78

82/76

16

27/14

0.12

1.5-2.5

Sand

38

38/36

3

4/2

1.4

0.1

Mineral Soil

50

-

3

-

1.2

0.05-0.17

Bark:Sand

76

-

10

-

0.46

1.9

3Bark:1Sand: 1Peat

71

-

5

-

0.5

0.45

3Soil: 1Bark

54

-

4

-

1.08

0.14

3Bark: 1Peat

81

-

16

-

0.17

1.58

50 Compost:50 sawdust

86-84

-

13-11

-

0.26

1.25

Bark Ash

80-68

-

21-5

-

0.10-.038

0.56-4.01

Commercial Peatlite Mixes

87

90/84

9

13/9

0.14

2.5

Retail Mixes

82-86

89/77

9 - 12

17/3

0.10-4.0

1-2.5

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Maintained by Bill Fonteno


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