This article originally appeared in the March 2000 Newsletter of the Liverpool Branch

Of cactaceous calamities, chemiculture, coir and calcium

by Ray Allcock

I have always found a great fascination in raising cacti and succulents from seed, and watching the embryonic plants gradually develop through various intermediate stages until finally they assume the adult form. Many years ago I habitually used a mix of finely sieved loam, crumbly leaf mould and coarse sand, usually in more or less equal parts by volume, and with very small amounts of bone meal and potash thrown in. Growth was rather slow, but the only real problem evident was that the emergent seedlings were liable to damp off.

Later, and spurred on by the increasing acceptance of lightweight composts such as U. C. Soilless and Levington, I concluded that the damping off problem indicated a need for some mix lighter and more ‘fluffy’ than that which I had been using (i.e. a mix with a higher air-filled porosity). To this end I tried a mix of equal parts of finely sieved loam, fine but winnowed peat, seed-grade vermiculite and coarse sand; to be followed, after the first spines were well formed, by liquid feeding. This compost proved indeed to be a good one, giving virtually no damping off and a growth substantially faster than that which had attended any of my previous efforts. But, as alas is so often the case when we try to improve upon Nature’s own methods, a number of unexpected and extremely puzzling troubles later ensued.

Contributory to these troubles, or at least to the particular forms which they took, was the fact that my earlier experiences had taught me to wait for two or three years or more before potting up my seedlings. By that time I would in the old days have potfuls of robust and well-hardened and easy-to-handle little plants. But with the new compost growth in many cases slowed down and stopped in the second or third year, and ugly scarring developed at or near the apex of the plants, with most subsequently dying. I had never seen anything like this before, and it had me completely mystified. Eventually the problem was traced to a catastrophic rise in the pH. The batch of vermiculite which I had used proved to be the villain of the piece, and should never have found its way onto the horticultural market. Straight from the bag it gave a pH of 9 whereas the sorts used for horticulture should not register above 8. By the time all this got sorted out I had in desperation gone back to some of the old books and to an important article by Dr. Buxbaum in NCSJ9(1954)36, wherein the use of crushed brick is recommended, but without any indication of the variability of its properties in regard to the pH and the powerfully adverse or powerfully beneficial chemical effects that may thus be encountered. Well, it is often said that a little knowledge is a dangerous thing, and so it proved for me in that connection also!

But let us move on to the next disasters! One is always advised never to let the seed compost dry out, and during the first season it is of course wise to follow this advice rather carefully indeed. But one can be careless or forgetful, and can thus come to realise that once a cactus seedling has put down a proper root it can survive drought quite well provided it is shaded from the direct light of the sun, and provided one hasn’t used too much fertilizer. But in the latter respect I naively followed the makers’ instructions, and thereby used far far too much! Whole potfuls of two- or three-year seedlings would either dry up all at once from below and then die, or would all at once go brown all over and then dry up. At the time I had never heard of or read of such a phenomenon and I misinterpreted it as a manifestation of some sort of bacterial infection. So I kept the plants drier and devised drier mixes, and so made the problem even worse!

After several more years, when all this seemed at length to have been painfully sorted out, there arose yet a third sort of problem equally vexatious, and doubtless contributory in some measure to those already described. It arose from the peat. This commodity, when allowed to dry out, has a propensity to become unwettable. It is thought that this property arises from a mono-molecular layer of grease. Not all peats are equally greasy. Some are far worse than others. Moreover the loam which I was using at the time was an organically rich topsoil and thereby also greasy. It is better to go deeper. The late Geoff. Williams, a professional gardener and a great stalwart of our Branch prior to his premature death in the year 1975, used to recommend that one take one’s loam from that depth where the dark colour of the topmost layers is just beginning to fade towards the lighter colour of the subsoil. I have never had any wettability problems with loam dug out according to this advice. But that was a long time ago, and they say that novelty is the spice of life. It thus happened that in spite of due attention to watering (most of the time!) I started to find pots of two- or three-year seedlings that always looked thirsty and unhappy. Eventually I discovered, on depotting to try to find out whether the plants had lost their roots, that even under my regime of alternating bottom soaks and generous top waterings there had developed in the pots a curious sandwich effect, with the top 3mm and the bottom cm sopping wet, and in between these two wet layers a dry and unwettable 3cm of compost. I was unable to satisfactorily deal with this situation even with the help of detergents. There were not many survivors from that sowing!

It was therefore with more than a mere disinterested curiosity that at this juncture I decided to try coir as a peat substitute. It is indeed a remarkable material – it loves water, and accepts it with enthusiasm even when completely dry. Also it gives an acid reaction, about pH 5.5 – 6.0.

Just for fun and to make a first acquaintance, so to speak, I tried a few spare seeds in unfertilized coir taken neat, i.e. with no additives of any sort, other than dissolved salts carried in by the fertilizer-enriched irrigation water. As it turned out, it so happened that the seeds of two of the four species which I tried were duds. Those of the other two (Gasteria armstrongii and Gymnocalycium friedrichii) germinated well and grew on rapidly. But their development has shown up yet another problem, suggesting a warning to any grower relying solely on soluble fertilizers for plant nutrition.

The soluble fertilizers most often recommended by cactus addicts appear to be various of the Chempak range and Phostrogen. In the case of Chempak the packets make no mention whatever of the element calcium, and it was Chempak alone that I had in use during the three seasons of growth pertinent to the mentioned two species. I did employ tap water for some of the waterings however (a hot tap is rather useful when warm water is needed), and of course our tap water here in Liverpool is laced with a small amount of calcium hydroxide, to prevent contamination from lead pipes. Nevertheless the three-year Gasterias, although at first sight appearing to be very nice little plants, have now developed the classic symptoms of calcium deficiency – most of the roots have now died and those which are still alive have now gone to a rich brown colour instead of the normal healthy white. With the Gymnocalyciums it is not so clear-cut – they grew so vigorously that some were eventually pushed upward till their roots were quite clear of the compost, necessitating that all be potted up. Those which had not suffered this premature and drastic uprooting have now established themselves in their new and more conventional compost, but most of the others have just shrivelled away, suggesting that their roots were irremediably damaged by their earlier unfortunate experience. But we cannot legitimately trace a connection to any surmised calcium deficiency, because the root system of this species is already known to be very vulnerable and easy to damage.

The Phostrogen company supply better information. In its original 10-10-27 formulation their product contained 3.6% by weight of the element calcium, which made it suitable for most hydroponic applications. In the current 14-10-27 formulation the calcium content is however only 0.71%, and although this is probably capable to support cactus life unassisted it is undoubtedly much below optimal, except perhaps for the epiphytes and certain saxicoles.

It is however easier to add than to subtract, and one simple way to provide more calcium without upsetting the pH or other chemical properties is to add a little gypsum (hydrated calcium sulphate) powder to the irrigation water at the same time that one adds fertilizer. The horticultural grade of gypsum should be used because although it does contain some impurity in the form of insoluble grains of sand it is free of alkali, whereas builders’ plaster is not. For many desert cacti a feed containing equal amounts by weight of calcium and nitrogen might perhaps be judged to be suitable. Such a feed may be approximated by using 1 volume of gypsum powder to every 2 volumes of Phostrogen powder. There do not appear to be any counter-indications.

Most of my own plants are potted in media containing neutralized brick fragments (see Newsletter 2.2). These fragments already contain far more gypsum than can dissolve into the soil solution (the solubility of calcium sulphate increases with temperature but even at 20°C it amounts only to 2.4g per litre of water). The plants appear to show considerable benefit from this ready supply of assimilable calcium, both in general robustness of their tissues and in the quality of the spination. Chempak, from whom the horticultural grade of gypsum may be obtained, give a blanket recommendation of 8oz/bushel (6.24g/l) for its use as a calcium supplement in the mixing of soil-based composts, which is somewhat more even than I estimate as the content of my own potting mixes.

In February 1999 I started a serious trial run with coir, sowing in a mix composed of 6 parts of unfertilized coir and one part each of seed-grade low-alkali vermiculite, seed-grade perlite, fine-grade Biosorb/Ultrasorb, and winnowed siftings from the previously mentioned neutralized gypsum–rich brick. I thus used a bit of everything that happened to be at hand at the time and suitable, other than loam. The latter I purposely missed out, partly because of the wettability problem and partly just to see what would happen in a medium completely devoid of dust-sized components.

The water acceptance of this mix has remained at all times excellent, and the pH has stayed steady at 6.5 over the whole twelve months. This I find quite remarkable, for it represents the maintenance of a pH balance between a mineral-based alkali (namely the vermiculite) and an organically produced acidity dependent presumably upon some sort of bacterial action. It clearly suggests that in the context of coir, with its very mild natural acidity (pH 5.5 – 6.0), gypsum should be chosen as the principal source of calcium, whereas of course in most peat composts chalk is used.

The seedlings have for the most part done rather well so far, but from memories of previous years I have a distinct and indelible impression that a greater uniformity as between prompt and delayed germinators and a higher quality and amount of growth overall could have ensued if some loam had also been incorporated. As it actually was, such seed as germinated late made surprisingly little progress. Also, and again presumably because of the absence of loam, certain small-seeded and delicate sorts (Aztekium and Strombocactus) failed to establish after germination, and faded out after a few weeks, and Ancistrocacti also just faded away. Curiously, the small-seeded Parodias (subgenus Microspermia Fric) established well, reflecting I suppose the mineralogical and environmental disparities as between their Argentinian habitats and those in Mexico and Texas. So, for my millenium year trial, I shall put in some loam harvested with Geoff’s precepts in mind, and see what change that brings about. I am always looking for one mixture that will suit everything – maybe the goal is now in sight?!?!

As base fertilizer I used Vitax Q4 at the very low rate of 0.4ml/l of compost (approximately ½ oz/bushel: equivalent JI No. 0.2), and the first watering was done with Chempak No. 3 at the standard rate of 0.5ml/l of water. From then on no more fertilizer was applied until the first baby spines were widely in evidence, which was about 14 days into May. At that stage I judged it to be appropriate to start to feed, and so from then until early October I did all the irrigation with the new Phostrogen at quarter strength (0.125ml/l), but with crystal tops to cut down evaporation until well on in the season for most sorts, and for some sorts still even now. This sparing feed has proved to be quite enough even for the most vigorous sorts, such as Trichocereus. Maybe it is more than enough. The danger, as already mentioned, is that the seedlings are easily slowed down or killed by fertilizer in excess, but so far all seems to be in good order.

A feeding programme cannot be meaningfully assessed in the absence of any reference to the amount of leaching. Recommendations given on packets of fertilizer are therefore for our purposes misleading and inappropriate, for we deal with very long periods of time and small or more often zero leaching losses. The danger of accumulation to excess is therefore to be taken very seriously indeed. In the present instance I judge the leaching losses to be negligible and I reckon that each watering conveys to a pot the fertilizer content of water of amount equal to 1/5 of the pot volume. I reckon also that the amount of fertilizer so far incorporated into the bodies of these tiny plants must be negligible. On this calculational basis I reckon that the present fertilizer content of the seed compost may in a rough measure equate to that of a compost base-fertilized by Phostrogen to the amount 0.5ml/l, which corresponds approximately to JI No. 0.5. This value may be quite high enough, bearing in mind that we wish them to emerge undamaged from possible droughts, and bearing in mind also that the move out of the sowing pots will be done into JI No. 1. Therefore it seems to me that I should now carry on just with water, until some slow-down of growth or some deficiency symptom indicates otherwise.

Some readers not aware of the present state of horticultural and agricultural glasshouse technology may think that all this is getting a bit too complicated for comfort. I would like to be able to agree with them. After all, we grow our plants for pleasure and relaxation, not complication. I know of at least two proven ways to minimize complication. One way is to revert to quasi-organic methods, as exemplified in my first paragraph. Another way, now by far the more popular, is to prick out early and repot often, and thus to dispose of all problems of physical structure and chemical imbalance along with the discarded compost. Some very magnificent plants are produced in this way, but it is labour and materials intensive. Everyone to their own taste!

Maybe there are significant variations of practice to consider. Our Branch has several very capable seedling growers among its membership – it could be interesting and helpful to hear more about their various and in some instances very different methods and preferences.