Container Gardening with Pumice

Billions of plants are produced and grown in shallow-drained container soils, including containers (pots, flats, planters, greenhouse benches), the shallowness induces capillary retention of too much water (high concentration) which displaces soil air and causes poor soil aeration.

The subsequent reduced oxygen supply to the roots often reduces root growth and function to the extent they are unable to absorb even the inadequate resident water supply. Natural soils are therefore generally unsuitable for direct use in shallow containers.

The obvious practical compensation for insufficient water content is increased irrigation frequency. Although the potentially available water supply is enhanced, inadequate aeration is only worse.

The correction of poor drainage usually involves a soil physical amendment to increase the proportion of large pores and ensure adequate drainage and aeration

A variety of materials are used for soil amendment, Pumice being one of them. The moisture characteristic of pumice is probably the single best assessment of a soil's physical suitability because it elucidates the nature (quantity and pore size distribution) of its porosity, thereby increasing water retention and aeration following irrigation and drainage.

Pumice is a mineral composed of aluminum silicate plus potassium and sodium oxides. Formed when a froth, caused by enormous gas pressure under a non-flowing magma, is violently ejected during a volcanic eruption. 

It rapidly super-cools (adiabatic cooling) at atmospheric pressure to a lightweight white or light grey porous solid. Sealed internal pore size distribution is usually more inconsistent and much broader than in perlite. Pumice is processed for horticultural use by crushing and grading. It is normally sterile (38), chemically inert, unaffected by pasteurization, and has a pH of 7.0 to 7.4