The formulation of a compost for rooting cuttings really only requires two considerations: the retention of sufficient moisture to help prevent desiccation of the cutting, and the provision of an aerating agent so that air can always circulate within the medium.
Conventionally, peat has been used as the water-retentive component and, although various peats are available, sphagnum moss peat is best as it retains a good structure for a long period. For most reliable use and to achieve uniformity it should be riddled through a ^ in sieve.
Sand is used as the aerating agent, and it also allows adequate drainage—peat by itself tending to become waterlogged. In horticultural parlance sand usually means grit, and for these purposes a washed and crushed lime-free grit providing a particle size of between £in and in across is the most desirable. The particles should also be "sharp", that is they should not be rounded but have points and corners and thus be irregular in shape.
Although these two components provide the basic compost they can be substituted with such items as sedge peat, well-weathered sawdust, perlite, vermiculite and graded coal dust—in fact by any material that has suitable physical properties, and that is chemically inactive and biologically more or less sterile.
Cuttings composts are usually formulated by evenly mixing equal parts by volume of peat and grit, although it is often difficult to assess how much sieved peat there is in a particular mix. In the end there is no substitute to determining the "feel" of the compost and whether it has the right properties.
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