This article originally appeared in the January 1995 Newsletter of the Liverpool Branch

Acid Salts and Cactus Cultivation

by Ray Allcock

Part 1 - Aluminium Sulphate

At the Branch meetings I have spoken twice about my compost experiments, and so it would seem to be a good idea to set down in print some of my spoken recommendations. As explained later in the present note, there is a definite tendency for ordinary potting media, whether of peat-based or John Innes types to lose their acidity and become alkaline under the dry conditions natural to a cactus house. The symptoms then induced in the plants include a slow-down of growth, a lack of turgidity and a dull epidermis and, as the condition progresses, distorted or spindly growth, feeble and colourless spination, brown scarring near the growing point (usually erroneously blamed on red spider), browning all over with small seedlings, die-back, root rot, neck rot, sudden collapse, or lingering death. Among the first plants to suffer are such allegedly 'easy' acid-lovers as Echinopsis eyriesii and many of its relatives, Chamaecereus, Setiechinopsis, Trichocereus in general, Rebutias minuscula, senilis and xanthocarpa and various others including most Sulcorebutias, Zygocactus and Schlumbergera and many of the other epiphytes and, from North America, all Ferocacti and most of the green Echinocerei. If these useful indicators are growing well for you then it is unlikely that you have any serious alkalinity problem - your compost is maintaining a sufficient level of acidity.

If however you think that you may possibly have a problem then I would recommend that you try out the effect of a full watering of the whole collection during the growing season, using water dosed with the acid salt sulphate of aluminium ( i.e. hydrangea colourant) at the rate of 1 level teaspoon of the white crystalline powder per gallon. This will produce almost overnight a remarkable increase of turgidity and a visible improvement in the state of vitality of any rooted plant struggling against insufficiently acid condition, and will be followed by a temporary surge of growth.

If the results of this initial trial are positive, then my recommendation is to continue watering for the rest of the growing season using half the above dosage once per week (or equivalently adjusted for other time intervals). You may be very agreeably surprised!
The above dosages are small, and in most cases altogether without any risk of any adverse effects. Over a whole season the cumulative effect on any John Innes compost or any standard (i.e. lime-containing) soilless compost will be to reduce the pH by at most half a unit, or at most one unit if a 50:50 mix with horticultural sand or grit has been used. All cacti and succulents grow well throughout the pH range 4.5 - 6.5, with 5.5 the optimal value; and from this well-established information in conjunction with the figures given below we may readily assess the safety of the recommended procedure. The recommendations are not appropriate to plants potted in chemically inert hydroponic aggregates (which suffer greatly when innocently introduced into a more conventional milieu) but, laying these aside for immediate repotting, the only foreseeable danger lies with ericaceous composts, which could by the end of the season become rather too acid for most desert cacti and succulents.

It is important that I should emphasise that the unpleasing symptoms described in the first paragraph need not necessarily be indicative of alkalinity. For they are in fact symptoms of malnutrition, and malnutrition can be induced not only by the wrong pH but also, even under a regular and reasonable feeding regime, by nutrition-inhibiting physical and electrochemical deficiencies in the compost. Therefore a third recommendation is in order, to wit, that if the initial trial with aluminium sulphate produces no positive results, then at the next watering the regular feed (normally given at some fractional strength in every watering) should be boosted to double the full strength, and thereafter to full strength for so long as this appears to be necessary (pending transfer to a more effective compost).

Finally, some brief explanations. Aluminium sulphate is so gentle in its action that individual doses up to ten times more concentrated than those recommended can safely be given, although in point of fact the repeated small doses as suggested will be more effective in securing a sustained and steady growth. All of the doses mentioned, both those recommended and those ten times stronger, deliver the acidifying sulphate at a pH of about 4, if measured after a time lapse sufficient for full dissolution to occur. To deliver the sulphate content of the one-teaspoon dose by means of one dose of sulphuric acid would require the latter at a lethal pH of about 2.5, and similarly for the acidifying effects of any of the other strong acids mentioned in the more recent cactus literature!

All commonly used commercial composts contain peat which, provided it is kept permanently damp and aerated, decomposes through bacterial action and thereby continually replenishes the supply of humic acids, as these in turn decompose into simpler non-acidic constituents. The problem for cactus growers is that under dry conditions the bacteria may either go dormant or die altogether, whereupon the pH soon rises to alkaline values (i.e. above 7) in any of the standard lime-containing composts, and to somewhere in the range 6.5-7 in the case of the lime-free (ericaceous) composts. When the bacteria are fully active the pH values of properly prepared commercial composts settle to the following values: J.I. #1, 5.5; J.I. #2, 6; J.I. #3, 6.5; standard soilless, 5.5-6, depending on the brand; soilless for trees and shrubs, 6-6.5, depending on the brand; soilless ericaceous 4.5-5, depending on the brand. There exists also a soil-based ericaceous compost (the so-called John Innes A-formulation), but I have yet to find this on sale anywhere. The A-formulation is free of lime, which by itself might be expected to bring its pH down to about 5. In addition, however, it has sulphur added and this, if bacterial action establishes, gradually converts over a period of months into sulphuric acid which acidulates the clay component, yielding eventually a lower value which I would guess to be about 4.5.