Figure 22.2 Pore spaces in (a) concrete mix (b) and (c) sand mixes
The water-holding capacity of compost ingredients varies enormously. Peat is significantly better than most others. However, the importance of this depends on how the plants in the compost are to be irrigated. It is a major consideration if the plants are in small hanging baskets watered by hand and there are benefits in using absorbent polymers (see p390) that improve water-holding more than peat alone. Peat presents a problem if it dries out, because it does not rewet easily unless treated with wetters (see p390). On the other hand, if there is to be a constant supply of water through one of the many self-watering systems (see p350), this water-holding capacity is far less significant and the emphasis should be on choosing material that is stable and provides the right air-filled porosity.
Over the years growers have added a wide variety of materials, such as leaf mould, pine needles, spent hops, old mortar, crushed bricks, composted animal and plant residues, peat, sand and grit, to selected soils to produce a compost with suitable physical properties. To supplement the nutrient released from the materials in the compost, if any, various slow release organic manures or small dressings of powdered soluble inorganic fertilizers have been added to the mixtures to provide the necessary nutrition.
The correct physical and nutritional conditions are vital to successful growing in a restricted rooting volume. Significant developments occurred as a result of the work done in the 1930s at the John Innes Institute, where the importance of 'sterile' (pest and disease free), stable and uniform ingredients was demonstrated. The range of composts that resulted from this work established the methods of achieving uniform production and reliable results with a single potting mixture suitable for a wide range of plant species.
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