W.R. Carlile, I. Bedford
Calcined clay granules produced by the London Brick Company (Stewartby, Bedfordshire) contain high levels of both calcium (9% w/w CaO) and sulphur (3.4% w/w SO4). Addition of calcined clay to peat raises both the pH and conductivity of the medium. However, calcined clay has a higher bulk density than both peat and bark. Inclusion of calcined clay in container media increases the stability of containers, especially with tall shrubs and young trees.

Mixtures of calcined clay and peat were evaluated as container media by comparison with a grit containing medium. Seven nursery stock subjects grew well over a period of twelve months in media with up to 50% calcined clay. Nutrients were provided by slow release 'Osmocote' granules (18–11–10, 9 month release). Heathers (Erica spp) grew less well in media containing 35 and 50% calcined clay in a grit containing medium. This may be attributed to the high pH of media containing 35 and 50% calcined clay. However, Rhododendron spp grew exceptionally well in media containing calcined clay, even at 50% where the pH exceeded 6.5.

Nutrient concentrations, other than calcium, declined only slowly in all media during the growth period. Few, if any, deficiency symptoms were observed. Considerable nutrient depletion occurred in fallow pots, indicating that rainfall leaching represents a major mechanism of nutrient loss from substrates of container grown plants.

Carlile, W.R. and Bedford, I. (1988). PLANT GROWTH IN CONTAINER MEDIA AMENDED WITH CALCINED CLAY. Acta Hortic. 221, 117-132
DOI: 10.17660/ActaHortic.1988.221.9

Acta Horticulturae