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close this bookOrganic and Compost-Based Growing Media for Tree Seedling Nurseries (World Bank, 1995)
close this folderPotting media characteristics & components
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Components of potting media

Potting medias are either composed of a single substrate (unmixed material from a single source) or, more likely, are mixtures of various organic and/or mineral components. Mixtures of various components with complementary physical and chemical properties will produce superior potting medias. Individual potting media components mentioned below are discussed in more detail in Annex I.

Organic Components

The desirable characteristics of organic components used in growing media are as follows:

- a large proportion of micropores to improve water-holding capacity,
- a good texture which resists compaction,
- a relatively high CEC to help retain nutrients; and
- low weight (bulk density) to facilitate transport and handling.

Sugarcane wastes, coconut husk fiber, and rice hulls (Annex IV), peat moss, sawdust and tree bark (Annex V), are organic components which are commonly used because they have the desirable characteristics either before or after composting. Most organic materials (with the exception of peat moss and rice hulls) benefit from composting prior to their use. Composting improves their physical properties and balances the ratio of carbon to nitrogen in the material.

Inorganic Components

Inorganic components are included in potting media to improve the physical characteristics by improving drainage and aeration by increasing the macropores. In some cases the inorganic components are very light weight, such as vermiculite, perlite, pumice. and styrofoam. Sand, however, which is one of the more commonly used components, adds considerable weight to a mixture. If possible, the use of sand should be avoided in order to hold down transport costs.