AN EVALUATION OF VARIOUS UK METHODS FOR ESTIMATING AIR-FILLED POROSITY WITH SPECIFIC REFERENCE TO PEAT:CAPOGRO WOOL MIXTURES

At rates of up to 25%v CAPOGRO Water Absorbent (WA) wool had little effect on the Air-filled Porosity (AFP) of peat substrates and it may therefore be considered a peat substitute.
CAPOGRO Water Repellent (WR) wools at rates of 15%v to 30%v significantly reduced water capacity and increased the AFP of peat substrates.

Both the Wolverhampton method (as used by ADAS) and the BS4156:1990 method developed from it, under-estimated AFP, particularly where the hydrophobic wool was used.
This was due to a failure to achieve complete displacement of the air within the substrate during soaking, resulting in incomplete saturation.
Thus the volume of drainage water did not represent the total volume of drainable pores.

More reliable AFP estimates can be obtained using these methods by measurement of substrate volume (DV), wet weight (WW) and dry weight (WD) after drainage and the application of the formula:

AFP = 96.7 – [[100WW – 61.9WD]/DV]

The Levington Rapid Method, when supplemented by three overnight soakings, gave results similar to those calculated from data obtained via the BS4156:1990 method by application of the formula.

International Symposium on Horticultural Substrates other than Soil in situ

P.L. Waller, N.C. Bragg, S.K. Rodgers

https://doi.org/10.17660/ActaHortic.1993.342.20