This article originally appeared in the November 2001 Newsletter of the Liverpool Branch

Part 1 - June 1998

Part 3 January 2002

On Base Fertilizers and Liquid Feeding, Part 2.

by Ray Allcock

1. The Fertilizer Controversy

During the last sixty years a great amount of research has been carried out in regard to the nutritional needs of plants in pots; nevertheless little or no agreement or consensus exists where cacti and succulents are concerned (cf. R. Mottram in BCSJ 12(1994) p.20), and many unfortunate mistakes continue to be made. If. Dear Reader, you are happy to engage in frequent repotting, and if all your plants always shine with health and vigour, then maybe you won't be too worried about this. Otherwise, please read on!

Part of the problem is that the natural growth rates of our plants are very slow, so that it can take years to properly assess the merits or demerits of any particular base fertilization or liquid feeding programme. Indeed, plants taken from a previously healthy and well nourished regime may continue to grow on their own internal reserves for a year or more, before showing any easily recognisable signs of drastic slow down, root trouble or mineral imbalance, excess or deficiency. One old gardener has somewhere been reported as putting it thus: “I don’t like cactuses. They take too long to die!”.

In most cases the long time periods requisite to the task already make it difficult and arduous to carry out scientifically controlled experiments. A further obstacle, uniquely singling out our plants from all other permanently green subjects, arises from the possibility that the treatment given in cultivation may involve periods of complete dryness at the root – an eventuality to which, in consequence of aeons of evolutionary development, our plants are naturally adapted; and which we should presumably be free to imitate and exploit in our culture.

In their natural habitats the drying out of the soil seldom does the plants and their roots any permanent harm, since the ionisable salts necessary to support plant life are spread only thinly throughout the inhabited soil volume. But in a pot the soil volume is small, and extra nutrients kindly supplied by the grower may consequently be distributed more densely within the soil mass. Then, as the soil dries out, so the ionic concentration in the remaining soil solution and around the root surfaces necessarily increases, with the effect that a substrate fertilized to an extent previously conducive to vigorous growth may now through lack of water expose the root to an agressive or even lethal ionic attack. For this reason already an experimental run of at least two years must be judged to be necessary, in order to adequately cover the two sides of a long dry season, and to find out whether the roots have survived it in a fully lively and undamaged condition.

Much has been written in the past, to the effect that it is desirable, as the weather gets warmer with the onset of Spring, to gradually resuscitate the roots of long-day cacti and succulents by frequent light sprayings with water. My own judgement on this is that it can be an unnecessary expenditure of effort, for although it may well be beneficial to an impatient psyche, the plants themselves are perfectly able to respond rapidly (and even spectacularly!) to a single generous watering, provided that this be postponed until a good growing temperature (10°C or more, depending on the species) prevails, and provided also that the roots have not been damaged over Winter by an excess of fertilizing salts in the dry soil!

Also contributory to the overall nutritional problem is the fact that the plants which we grow are of many sorts, differing greatly one from another both in their sensitivities to an excess of salts in the compost, and also in the nature of their various responses to such excess.

2. Observable Effects of Over- and Under-fertilization.

Following a move into an over-fertilized compost the leaves of some plants, especially amongst the more delicate Crassulaceae and the more soft-fleshed of the Mesembryanthemaceae, lose their turgidity, and growth slows down or stops. Sometimes this is for a while preceeded by the opposite phenomenon, in which the leaves become so turgid that they burst or burst off. Cacti can also burst, but this seldom occurs, and a more usual sign of trouble consists in a slow and insidious loss of turgidity – a state to which, in the absence of leaves, an associated dullness of the epidermis may still in some cases serve as a visible and recognisable indicator. But many of the tougher plants will struggle to resume growth with each successive watering, producing impoverished and perhaps deformed leaves or stems and (where cacti are concerned) an enfeebled and scanty spination. Rot may also eventually set in, due to general internal imbalance caused by ingestion of too much of salts and not enough of water. Others may exist in an apparently inactive state for years and years until one day they are found to be entirely dried up or, alternatively, to be full of wet rot inside. And even if they escape all these calamities many will suffer root damage or root death over the dry season, for the reason already mentioned in Section 1 above. Regrowth of the roots, even if successful, will deplete the reserves of a plant, and the concomitant delay will deny to it the growth and renewed energy which should accrue to it during the following wet season.

The effects of under-fertilization are usually much less dire, and consist principally in a restriction or absence of growth, poor spine development and poor spine colour, poor flowering and, in extreme cases, an epidermis which has become almost white or which has become suffused by 'autumnal colours' – red, orange, yellow, pink or purple. Underfertilized plants can usually be rescued and fully restored to health through consistently maintained liquid feeding, which is easier and less traumatic than repotting.

3. Observable Effects of Trace-Element Deficiency

There is a particular form of nutritional deficiency characterized by the development of uneven orangey-brown or light-brown necrotic scars near to the growing point, and in bad cases the death of the growing point. Within my own collection of plants those most evidently prone to this affliction are to be found within the genera Lobivia, Trichocereus, Rebutia and Sulcorebutia, though I have also seen signs of the described syndrome in other genera, including e.g. some Echinopsis, Coryphantha and Echinocereus. The syndrome has been described and well illustrated by K,-P. Gruber & A. Hazelgrove in BCSJ 1 (1983) pp 45-46 and by R. Mottram in BCSJ 4(1986) pp.9-13. It appears in alkaline inorganic composts, and is due to a trace element lock-up. Recurrence of the condition can be avoided by lowering the pH to some value below 6.5, the effect of which is to render the trace elements available by bringing them back into solution.

In the case of plants in soil-based or other strongly buffering media the lowering of the pH requires the administration of a considerable amount of acid, which can however be quickly yet safely supplied by waterings with solutions of aluminium sulphate (i.e. hydrangea colourant) – see our Branch Newsletter 1(1995) No.1.

The above mentioned pH-vulnerable plants and others like them are often called ‘lime-haters’ for reason that calcium carbonate (CaCO₃) is the most commonly encountered source of alkalinity. Calcium is however a necessary nutrient, and cacti could in that respect be truly described as ‘calcium lovers’. Their calcium requirements can be met by use of the neutral salt calcium sulphate in its hydrated form CaSO₄.2H₂O, the common name of which is gypsum. The unhydrated form is called Plaster of Paris; the use of which I would most certainly not recommend!

The ‘lime-haters’ are also somewhat liable to a more superficial form of epidermal necrosis, in which large areas of the. outermost layer dry out and become uniformly whitish or pale brown. This shallow necrosis seems to occur in conjunction with dryness and summer sunshine. It is not at all like sunburn however. It is often identified as due to red spider attack, but in spite of careful examinations I personally have never found any examples of the alleged offending Acaridae nor of their webs. I think it is much more plausible to suppose that this syndrome is just another manifestation of the pH-linked trace element lock-up.

Cacti endemic to limestone areas, such as Astrophytums, Strombocactus disciformis, etc. do not normally exhibit any symptoms of trace element deficiency and in fact grow quite well even under a mildly alkalinic (below 7.5, say) inorganic soil-based regime. Quite generally it is thought that limestone tolerant plants produce acidic root exudations which unlock the required trace elements. Consistent with this concept is the well-established fact that no plant will grow in a hydroponic set-up unless the nutrient solution is acidic. For it is obvious that under hydroponic conditions the root exudations will leak away as fast as they are produced.

4. Plants suitable for Fertilizer Trials

Any systematic attempt to place quantified limits on the amounts of fertilizer generally safe for our plants will advisedly use species of proven vulnerability and proven relative rapidity of response. Subjects suited to the purpose and easy also to propagate in large numbers could include such common items as Chamaecereus silvestrii, Rebutia minuscula, R. senilis, Aylostera albiflora, Setiechinopsis mirabilis, Notocactus ottonis, Schlumbergera rosea, Kalanchoe blossfeldiana, Crassula comptonii, C. pubescens v. higginsiana and many others, most of the softer leafy Mesembs, Aloe variegata, Stapelia variegata,….. I would also specially mention Dudleya farinosa and D. attenuata for a guaranteed manifestation of deformed growth, and Glottiphyllum uncatum for a guaranteed manifestation of necrotic patches in addition to the expected flaccidity and lack of growth.

On account of their especially tender and delicate root systems one could adjoin to this list a host of other sacrificial victims of great interest to the average collector, including such items as Astrophytum asterias, Obregonia, Austrocactus, Eriosyce, Homalocephala, Escobaria leei, E. nelliae, E. vivipara, Notocactus minimus, Frailea, Sulcorebutia, Pyrrhocactus…. but somehow I don't see any of these troublesome plants as popular candidates for comparative trials!

All the plants which I have listed as vulnerable are suggested by my own quite extensive experiences over the last twelve years. It seems to me that if plants known to be vulnerable nevertheless survive and grow well under some nutritional regime incorporating long dry periods, then so should others of a more robust constitution.

For lack of empty bench space and lack of hours in the day I have not been able to carry out statistically impeccable trials within individual species: nevertheless from observation with small numbers from very many different species certain overall inferences can confidently and even more reliably be made. Indeed it is only with the observation of many different species that one might hope to impose fixation or sharpness of definition upon the otherwise moveable and diffuse boundary between acceptably low and unacceptably high levels of fertilization.

5. Results of a Liquid-feeding Trial

My own trials expressive of the philosophy just indicated have for the most part been conducted within the context of a base-fertilized ‘improved’ inorganic loam-based mix of my own devising, in which the gravel or coarse sand more normal to the case is largely replaced by acidulated fragments of baked clay. I shall refer to this improved growth medium as The Mix. One of its virtues is that it allows smaller pots to be used, without thereby prejudicing the supply of water and nutrients. More details will be given in a sequel to the present article.

My first improved mix was devised in 1989, and for some two or three months gave results which I found to be quite amazingly good. But growth then slowed down considerably, due to a slow release of alkali from within the (at that time untreated) baked clay fragments, and later all the symptoms described in Section 3 manifested themselves in full measure.

The nature of this very puzzling failure was only identified in the Autumn of 1992 (subsequent to a lengthy conversation with Roy Mottram at the Chileans Meeting, in which alternative cultivational faults were one-by-one eliminated). It then took me to the beginning of 1994 to find out how and with what dosages of acid the faulty compost ingredients could be corrected, and a permanently acidic pH ensured (for details see my  article in our Branch Newsletter 2(1996) No.2 and my article in our Branch Newsletter 3(1997) No.1).

From the Spring of 1994 onward I repotted into The Mix as now corrected so many plants as I was able to deal with, so that by 1996 1 had a great assortment of plants growing well and ready for experimentation with liquid feeds (and also by 1997 a newly erected polytunnel to accommodate the associated areal expansion!).

During the greater part of the 1996 Spring and Summer I used a balanced soluble fertilizer (Chempak No.3: 20-20-20) at double strength (2 level teaspoons, i.e. 10ml, per 10 litre industrial bucket) for every watering. Spurred on by the results, which for the most part were very beneficial (see our Branch Newsletter 3(1997) No.1), and curious to see what might then ensue, I continued in 1997 with a high nitrogen soluble fertilizer (Chempak No.2: 25-15-15) at normal strength (1 level teaspoon per 10 litre bucket) at every watering. The results of this were very interesting. They are reported in our Branch Newsletter 4(1998) No.1. During 1998 I pressed on with normal strength feeds, using either high potash (Chempak No.4: 15-15-30) or low nitrogen (Chempak No.8:12.5-25-25) but not high nitrogen since this had been found to produce soft growth or excessively open growth in some of the plants. Some further comments regarding liquid feeding may be found in our Newsletter 4(1998) No.2.

The inception of the above-described 3-year stint of heavy feeding was in part prompted by the numerical and contextual vagueness of available information. I had to find out for myself what is possible and advantageous and what is dangerous, and all this specifically within a loam-based (and therefore strongly and permanently buffered) context. All advanced treatises on soils and horticultural practice affirm the virtues of loam as a preventative of feeding problems, in which respect it stands apart from all clay-free media.

The use of a doubled strength in the first of the three years was motivated by the presence in my collection, and growing under polythene, of various Brazilian Echinopsis and Echinopsis hybrids whose green colour seemed to be too light, by comparison with those grown by other enthusiasts. At that time I erroneously diagnosed a nitrogen deficiency of some severity. True enough, the balanced fertilizer which I so generously supplied did darken the green somewhat, but by no means to the extent which I expected. Only very much later did I accidentally discover that the green of these plants goes spontaneously very much darker following a move into half-shade or shade. Thus the light colour originally displayed was due to an excess of sunshine, and not to some imagined nitrogen deficiency.

By the end of the third year it had become more and more evident that the Crassulaceae and other soft succulents were in great trouble, lacking in turgidity and showing only a very stunted and often deformed growth or none whatsoever. Cuttings of plants taken in this state were often found to have lost the ability to make roots – in other words, they were in some way poisoned. A similar poisoning occurs when delicate cacti are grafted onto vigorous and overfed stocks – cuttings or offsets taken from the scion are unable to root, and have to be regrafted if they are to survive.

6. Interpretation of the Results

In order to assess the results of these three years of heavy feeding it is absolutely essential that I should mention that, like the majority of cactus growers, I keep my plants on fully waterproof level stagings, so that all run-off is retained and subsequently absorbed, I also lay over the staging a capillary matting, which acts beneficially to equalize the final water distribution, provided that there is enough run-off to properly wet it.

It is fully evident therefore that there cannot have been any fertilizer loss by leaching. This remark is important in regard to the accumulation of potassium and phosphorus. Nitrogenous compounds can however slowly decompose with subsequent evaporation of the decomposition products. I have not found any quantitative discussion of this, nor have I found anybody able to give me any answer to my queries. The most volatile of all the nitrogenous soluble fertilizers will be ammonium nitrate (NH₄NO₃) and carbamide ( (NH₂)₂CO, otherwise called urea). I obtained some of the first mentioned, and put a teaspoonful of it on a 3" plastic pot saucer in a shaded location in the greenhouse. After two full years it had all evaporated. In view of the long time intervals of interest to us this may perhaps suggest that nitrogen loss to the atmosphere cannot a priori be assumed to be negligible.

My experiment has proved for me that liquid feeding with modern soluble fertilizers within the context of a loam-based compost can be beneficial both in regard to the quantity and the quality of the growth, and that even at the excessively high dosages actually used. In view of the limited powers of assimilation of succulent plants the optimal dosages will be expected to be appreciably lower than those which I used, since the latter conform more to the requirements of ordinary fast-growing non-succulent plants.

It thus appears that liquid feeding with a loam-based compost carries with it no evident dangers, other than the possibility of a gradual long-term accumulation to excess. To that possibility the experiment gives valuable information, since I pushed it to the point where, with the Crassulas and other soft items, the signs of salinity stress became unambiguously apparent.

The maximum possible amount of the accumulation of unused salts can be estimated by reference to the strength and frequency of the feeding and the water retentivity of the compost. The significance of the estimated maximum amount may then be judged by comparing it with the amount of base fertilizer thought to be appropriate to the compost at the time of its mixing. Discussion of the base fertilizer requirement must therefore now take precedence. I hope to turn to this in the next Newsletter.

In regard to the pale Echinopses reported in Section 5 Roy Mottram has kindly supplied the following comments:

Your experience of sun causing damage is very familiar to me. The atmosphere here tends to be very clear, and damage caused by just one fierce day can be very distressing. Plants slow down and cease growth in hot sun, because the chlorophyll is damaged and ceases to perform its normal function of photosynthesis. Once a plant has been 'sunned' it is hard to get it going again, because it needs to make new growth to manufacture more food for itself - a chicken and egg paradox.

This year, I finally got around to putting up some permanent shading in my sales house. Previously I have used whitener, but heavy rain soon removes that.

(from a letter dated 21/4/01)

Excessive light can be especially damaging in combination with water shortage. Notice that I use the term light, not heat, as I believe that it is the shortwave element of light that causes damage to chlorophyll. I have experienced light bleaching as early as April, when the temperatures were still low. UV light is destructive to all kinds of organic life that is caught unawares without its natural barriers. Epidermal cells of plants lose calcium oxalate crystal protection at low light levels, and increase it with high light levels, but if the exposure to strong light is sudden, it may lead to necrosis.

Having put up a 50% shade cloth in my sales house this year, growth has been much stronger, and the plants have remained greener.

(from a letter dated 21/7/01)