Dr. B. Werminghausen
In the last ten years plastic foams have been used in increasing amounts for horticultural soils, supplementing the traditional organic composts, clay, sand and peat. In Germany, expanded polystyrene flakes (Styromull) and ureaformaldehyde foams (Hygromull) have acquired an established place in horticultural practice.

The manufacture of Styromull arose from the necessity of usefully disposing of waste arising from the production and processing of expandable polystyrene beads.
Formula available in full text only

The flakes are odourless, chemically neutral and have no adverse effects on plants. The material does not rot and is extremely light, one cubic metre weighing only 15 to 20 kg. It does not absorb water and holds very little moisture on its surface. Therefore it improves soil drainage and aeration and aids permanent loosening of the soil.

The permeability of loosely packed Styromull can be expressed by Darcy's Law: the flow of water through a layer of thickness d and area A under a pressure equivalent to a head of water of height, h is Q = K (hA/d).

If centimetre-gram-second units are used, the numerical value of K varies between 1. 8 and 0. 1 as the degree of compression of the material varies between 0% and 50%. The permeability of flakes thus corresponds to that of fine gravel or sand with particle sizes between 3 and 0. 6 mm.

The lightening effect of Styromull in soil has both advantages and disadvantages. The improvement in watering, higher temperatures around the plant roots, improved nutrient uptake and reduced transport, weight are among the important advantages. On the other hand, Styromull reduces the nutrient content and the water-retaining capacity of the substrate, and it must therefore be watered and fertilised more frequently. Styromull does not withstand high temperatures and certain organic solvents, so that the flakes may collapse and become ineffective on steam sterilisation or disinfecting with substances such as chloropicrin, dichoropropene dichlorepropane or methyl bromide.

The water-retaining capacity of pot plants can however be improved considerably by admixture of Hygromull. Hygromull is a synthetic foam

Werminghausen, Dr. B. (1972). THE APPLICATION OF SYNTHETIC MEDIA IN PLANT PROPAGATION. Acta Hortic. 26, 159-164
DOI: 10.17660/ActaHortic.1972.26.19

Acta Horticulturae