Soil mixtures recommended for succulents are legion. One book tabulates 25 for cacti alone. AU of them are presumably based upon the author's intuition, because no experimental evidence is cited, and none in the least resembles the soil in which I have seen cacti growing in nature. The prospective newcomer may well close such a book indespairand settle fora less exacting hobby. In practice, however, most growers use a single compost throughout, with only minor modifications for epiphytes (which need an extra-rich medium) and rot-sensitive kinds (for which extra grit is added). Their success indicates that succulents by and large are not fussy: the basic needs for nitrogen (N), potassium (K), phosphorus (P) and lesser amounts of other minerals (trace elements) are much as in other plants. Thus we need consider only three points in determining the medium to use: (1) it must be porous and drain freely, to permit ample aeration to the roots; (2) it must supply a correct balance of nutrients in dilute solution; (3) it must support the
The choice today is between two types of more or less artificial media: a traditional loam-based compost, and an inert medium to which nutrients are added — the so-called soilless composts. Both have proved equally successful in the hands of different growers. Of the loam-based mixtures, the John Innes formulae remain unsurpassed after more than 40 years. A good loam (seven parts) is tempered by coarse sand (two parts) to improve drainage and peat (three parts) to add humus and water-retaining properties. A base fertilizer supplies N, P and K in slow-acting form, and contains enough lime (calcium hydroxide or carbonate) to ensure that the mix is just on the acid side (pH about 6.5) of neutral (pH is defined in the Glossary). Composts 1, 2 and 3 contain respectively one. two and three doses of the fertilizer, and a separate compost is available for seed raising. For rooting cuttings, a mixture of sifted peat
Three views ol the author's outdoor plantings Top right (6 12): Opuntia rastrera. white and red-llowered Crassula sarcocaulis and. in 1 between pink Ruschia uncinata; Agave lecheguilla at top right
Right (6.13): Lobivia silvestrii (better known as Chamaecereus silvestriij. the popular Peanut Cactus', is surprisingly hardy il it is grown in a sulticiently sheltered site
Below (6 14): A mantle ol snow protects plants Irom very low temperatures But the danger is rot Irom waterlogging afterwards This Agave coarctata survived many years without any torm ol protection
and coarse sand in equal parts is the norm. All the above materials are —or should be—sterile: only the loam requires special sterilization by heat or chemical means to kill unwanted pests and disease spores. English loam is rarely deficient in trace elements: where that is a problem, correctives must be added. These John Innes composts were developed primarily for non-succulent glasshouse crops, and for succulents cautious growers like to add extra grit to increase the drainage. Some sundriesmen offer a ready-made "cactus compost" adapted in this way. The greatest snag is the loam: an indefinable ingredient that varies from place to place, and is increasingly difficult to obtain in good form. Another problem is that peat is difficult to wet again once it becomes dry. A mixture of leaf mould and sand, favoured by some, may have the necessary nutrients, but varies from source to source, and needs sterilization
Because of these snags, many have gone over to loam-free formulae. The UC Soil Mix. developed by the University of California, uses sharp sand and peat moss with six added chemicals to provide food initially3. Levington Compost has a peat basis, without sand, and with enough added nutrients for 6 to 8 weeks of normal growth«. For either, subsequent watering includes dissolved nutrients, which for succulents should be high in potash and low in nitrogen. Commercial brands of tomato fertilizer are adequate, and can also be used for plants in loam composts when they are overdue for repotting. But never give fertilizer to dormant succulents, unrooted cuttings, or in an attempt to accelerate weakly specimens.
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