This article originally appeared in the June 1998 Newsletter of the Liverpool Branch

Part 2 - November 2001

Part 3 January 2002

On Base Fertilizers and Liquid Feeding

by Ray Allcock

1 General Concepts: Buffering and Overfeeding

Fertilizers - which one, how much, how often, and when? Every cactus enthusiast must ask such questions but, as Jim Mercer pointed out in our March 1996 issue, straight answers are not forthcoming. And that situation is not unique just to cacti. On a recent visit to a garden centre I found that different makes of fertilizer, although intended for the same named plant types (whether that be for tomatoes, for vegetables, for conifers and heathers, or whatever) nevertheless show widely differing relative proportions of the principal ingredients N, P and K.

I am in no doubt that all these varied formulations have their own particular merits within the contexts of the plants for which they are intended, yet advices on the packet, to suggest the stage of growth and the growing conditions under which the given formulation might be uniquely appropriate, were not to be found, except in the case of one manufacturer who offers several alternative NPK ratios.

Some of the considerations which could be relevant to an informed choice might include the age of the plant - thus established conifers will need a relative N proportion lower than that optimal for rapidly growing youngsters - and the nature of the substrate - thus a tomato growing in peat should benefit from a proportion of P higher than that suited to a plant growing in soil, since soil already contains quite a lot of slowly accessible P in the form of insoluble phosphates of calcium. The amount of light should also be considered - the quality of the growth of plants in poor light can be substantially improved by giving extra K, which will also increase their chances of flowering and fruiting.

Considerations such as these are not however easy to dig out from the technical literature and are very difficult indeed to assess in regard to their various quantitative aspects. Most of us therefore, at least at the beginning, will be inclined to buy something claimed to be suitable and then to follow the maker’s instructions and hope for the best!

In an attempt to get somewhat beyond this primitive stage, and to gather such special items of information as might be peculiarly relevant to the cultivation of cacti and succulents, I have been perusing an interesting research monograph by A. C. Bunt, entitled ‘Modern Potting Composts’, and I look forward also to finding the time to study its more - up- to - date sequel, which is entitled ‘Media and Mixes for Container-grown Plants’. My reading thus far has amply confirmed for me two fundamental concepts, on which one can anchor a rational approach to the nutrition of potted plants. The first is that pot plants (because of the limited root space) should, for the best possible results, be very generously fed. To this end, and appropriately to its own constitution, the potting compost will be primed with a base fertilizer in amount sufficient to produce strong and vigorous growth in that compost and also, in order then to maintain the supply of readily accessible nutrients at an optimal level, the irrigation water will often contain a suitably judged feed of soluble fertilizer. The second fundamental concept runs contrarily to this, and consists in the fact that too much fertilizer will impede growth, and may even kill the plant.

To put the danger thus indicated into a sharper focus I can report that the experimental results contained in the book indicate that a mere doubling of the amount of the base fertilizer in the compost can be all that stands between an optimal performance and a damaging or even a lethal excess. We learn also that some sorts of plants, including lettuce and certain epiphytes, saxicoles, etc., are particularly sensitive to high concentrations of the nutrient salts, and can take no more than half or thereabouts of the base fertilizer optimal for others. In my own experience I find the same sort of thing in regard to the soft succulents (Crassulaceae, Mesembryanthemaceae, Stapeliae, etc.) and now use for them a somewhat weaker compost (for a rough orientation, think of J.I. No. 1 plus sand as opposed to J.I. No. 2 plus sand). I suspect that the epiphytic cacti and some of the saxicoles such as Frailea may also be a bit sensitive to an excess of base fertilizer.

Several times in the book we encounter the message that loam-based composts are much less liable to produce problems of under-feeding or over-feeding than are soilless composts. This message is especially relevant to the cultivation of cacti and all other drought-resistant plants. Loam (because of its high ion-exchange and adsorption properties) provides plenty of places for the dissolved salts to attach themselves and thus to go out of solution as the soil water evaporates, later to come back into solution again if more fresh water is supplied. In this way loam and other similarly-acting materials (baked clay, vermiculite, charcoal, shale) protect the roots of the plant against the adverse short-term changes both of the salt concentration and of the pH, which would otherwise occur through evaporation.

The stabilization of either of either of the two said attributes of the soil solution is called buffering the loam serves to buffer the salinity and the pH, and so also do the other ingredients mentioned. The buffering effects are particularly important for cacti and succulents, since with these plants a growth approximating to that produced in habitat can only be obtained by allowing evaporation to proceed to a point beyond that which would be tolerable for ordinary plants potted in the same soil mix. There is also the practical aspect - obviously it is very convenient to have the plants adequately nourished yet at the same time able to withstand droughty conditions without suffering root damage.

In a conventional soilless compost almost all of the buffering action comes from the humic acids contained within the peat. However, unlike loam etc., the humic acids are chemically unstable in the presence of air and water, and gradually decompose into simpler and non-buffering constituents. They must therefore be continually replenished if a peat-based compost is not to fail in its buffering action. Also, because of the incorporated calcium carbonate, any loss of humic acid is accompanied by a rise in the pH, since the solid matter of the peat is itself neither acidific nor alkalific.

In the cultivation of ordinary plants the peat is necessarily kept constantly moist. The moisture enables soil bacteria to flourish, feeding upon the peat, and thereby producing all the time fresh supplies of humic acids as a by-product of their metabolic activity. This is how a peat-based compost operates. It follows that a careful watering regime adapted to bacterial needs and favouring continuation of bacterial activity is essential to the successful cultivation of cacti and succulents in such composts. Most of us know of skilful amateurs and professionals who produce healthy and vigorous plants in peat. But anyone who has either mismanaged peat-grown plants himself or has acquired such plants from other neglected or otherwise mismanaged collections will have seen numerous cases wherein almost all of the root system is dead and denatured, and simply drops off. This necrosis of the root system does not occur in soil-based composts, except of course in extreme cases of wrong pH or chronic starvation of the necessary nutrients or gross over-watering.

Although the indicated problem of bacterial dependency becomes most apparent in regard to plants growing under dry conditions it is in fact also of some significance even for the cultivation of ordinary (i.e. non-xerophytic) plants. Horticultural researchers have sometimes suggested that the buffering properties of the peat might be improved by mixing in with it a smallish amount of powdered clay, to supplement the humic acids. This has been proved experimentally to be beneficial and in some countries has become almost a standard practice, though it seems not as yet to have gained much popularity in Great Britain.

In regard to cactus cultivation a better alternative, giving a more favourable compost structure, would undoubtedly be to use granules of baked expanded clay, obtainable in this country under the name ‘Biosorb’, or in a more costly version under the name ‘Seramis’. These products are admittedly quite expensive, but since they act very powerfully as buffers a little can go quite a long way in this suggested application - thus with Biosorb the advice given is to inmix an amount between 10 and 25%. (More details concerning these useful products may be found in our March 1996 Newsletter). Yet another possibility which has emerged in recent years is to water the plants with a solution of liquid humus, at about one teaspoonful to two gallons, or whatever the makers recommend for ordinary plants. Strange though such a prophylactic might at first appear when mentioned in the context of desert plants, the fact is that it is perfectly safe; and according to the logic already given it ought to prove to be positively beneficial, though I cannot yet personally vouch for this last point.

Mention has already been made, following Bunt, that the margins between an optimal application of fertilizer and an over-application are tightly drawn. The cultivator seeking the best possible vigorous and beautiful growth will certainly be concerned to supply plenty of everything necessary thereto, and will do well therefore to be informed of and to have in mind the symptoms exhibited by plants struggling to cope with an excess. In the case of ordinary non-succulent plants the first manifestations of the so-called salinity stress are quite definitive. To describe them I can hardly do better than to quote from a remarkably informative and enlightening gardening manual by Rice & Rice, entitled ‘Practical Horticulture’ , and written for a college subsidiary course in the U.S.A. The relevant passage, itself indicative of the lucid style of the whole book, goes as follows:

“Overfertilization pulls water out of the roots and creates, in effect, a drought condition. This leads to wilting, brown leaf tips, and stunted growth. But, unlike a drought-stricken plant, an overfertilized plant will not immediately recover when watered, since the cause will still be present. The only cure is to remove the excess salts as quickly as possible. This is done by leaching. Large amounts of water should be poured through the medium to flush the fertilizer salts out.”

In this passage we see both an explanation of what happens and a description of four of its characteristic and visible symptoms, to which in extreme cases (for the most part induced in practice by lack of water) Bunt adds the total death of the roots.

Where cacti and succulents are the sufferers the incipient condition can be less dramatic, and its identification correspondingly less certain, due to the ability of our plants to stay alive during long periods of water shortage and to struggle to make a bit of growth every time that the salinity of the substrate is temporarily lowered by a good watering. In consequence of this ability cacti and succulents implanted in excessively fertilized or poorly buffered mediums can be expected in some cases to show a sporadic and irregularly stunted or attenuated and meagre-looking top growth and a dull epidermis. An undesirable condition just like this overtook several bought--in plants which, for reasons of easier water management, I transferred in early 1996 from a stony and waterless substrate to my own water-absorbent potting medium (March 1996 Newsletter). Some of these plants have still not established and some have dried up. I can only conclude that under their previous ownership the plants had undergone a long period without feed and had thus used up their inner reserves of nutrients. For obviously, just as a starving man cannot cope with a large meal, so also if the concentration of salts in a plant’s sap is low, then that plant may be unable- to overcome the contrary osmotic pull of a new and adequately fertilized compost. I have in fact had this sort of trouble from at least three different suppliers. Plants in a J.I. type of compost give me no trouble. These I accept as they are and treat as the rest of the collection, i.e., with feed-enriched but rather infrequent waterings. But in the case of plants in stony compost I now do my best to keep them well watered and gently fed and so to build them up to cope later with a transfer to a more rewarding substrate. Meanwhile there remains constantly the danger of sudden root death, due to dryness in conjunction with the presence of appreciable amounts of nutrient salts left over from the previous liquid feeds and not adequately buffered. An alternative treatment, perhaps safer in the long run, might be to repot straight away into a compost with reduced base fertilizer content, e.g., J.I. No. 1. But whatever one does it is well to bear in mind that with cacti the time scale for growth is long, so that even when a previously run down plant appears to be re-established, re-rooted and growing one should not assume that its sap concentration has yet built up to normal levels. The fact is that such plants may suffer a sudden root death even many months after their apparent recovery, as I know to my cost.

If my interpretation of this anecdotal evidence is correct (and I have no reason at all to suppose otherwise) then a switch from a stony and poorly buffered medium to a more conventional compost base-fertilized to a quite normal and acceptable strength (and in fact in the present case quite comparable in its buffering and nutritional properties to straight J.I. No. 2) can give great trouble when used for a plant suffering from chronic starvation. The astute reader will not fail to notice that at this point we have an entangling of two opposite syndromes: on the one hand that of overfertilization and on the other that of underfertilization. The underlying cause of this entanglement is of course that it is not possible to base-fertilize an unbuffered medium, since such a medium is unable to take the nutrient salts out of solution. A fuller orientation in regard to this last point will be provided in the second part of this account, where quantitative data will be given.

It is important to distinguish between overfeeding, with its attendant problems of salinity, stunted growth, etc., and unbalanced feeding, such as may arise, especially in a poor light and damp conditions, if the proportion of N is too high relative to K. The application of too much N leads to a bloated and watery growth, to splits in the epidermis, and even, in the case of Echeverias etc., to the leaves bursting themselves off from the stems. Contrary to this, an Echeveria suffering from overfeeding reveals its situation by a lack of turgidity, a lack of growth, and maybe also a death of the growing point. In the case of cacti showing the mentioned stunted growth the growing point may also die, sometimes even many months after an apparent recovery of turgidity.

The recovery of turgidity in a plant which has been transferred to a compost too saline for it in its present starved condition seems to be very much a matter of chance. If recovery does occur it happens rather suddenly. Indeed it looks to me as though it is a matter of tipping the balance between the salinity of the sap and the salinity of the compost — once the balance goes the right way the plant starts to take in water and with it the salts needed to pull in more water. To indicate the chancy nature of the process I could refer, for example, to two clumps of Frailea pumila, started six seasons ago as single heads from the same parent clump and subsequently treated identically. One has now burgeoned out into .a gold-spined bright green floriferous clump that completely fills and indeed spills over from its 2” pot. The other remains brown-looking and flaccid, dull and unappealing and covering less than a single square inch.

I used to have trouble through alkalinity of my compost ingredients and through feeding too little, and I still have problems through wilfully growing more plants than I can reasonably manage! On all these counts I have had to deal with and to try to re-establish a large number of chronically starved plants. But I will not bore the reader with further anecdotal evidence, since what I have already written serves sufficiently to illustrate all that I have observed both in my own collection and in others.

There remains yet a rather important practical point, concerning plants which have been acquired from other growers, and which may be potted in a substrate not adapted to one’s own cultivational methods. For me this usually means plants in peat or plants in a clay-deficient mineral medium. For those in peat my first instinct is to remove the same and replace by my own compost, but this is not possible without extensive root damage in some cases. These I do not disturb, but instead simply pot on into a larger container, filling in now with my own compost, but using always a pot saucer underneath the pot in order to ensure that the peat ball at the centre gets its share of the water. This trick has worked very well for me. For those in an unbuffered mineral medium a more subtle approach could be useful because, as will be evident from the preceding discussion, everything now depends on the state of the cell sap. It is however an observable fact that a plant which has in its own recent history been well nourished will grow when fed only on water, while one whose reserves are already used up is incapable of any growth of any sort, and will eventually die. If one is lucky one’s new plant will be displaying a vigour characteristic of the former condition; in any other case it will presumably be advisable to proceed as suggested 5 paragraphs back. I am not aware of any other visible manifestations of a run-down condition, which if available would enable a more sure diagnosis to be made.

In the preceeding discussion I have introduced not only certain matters of practice but also a number of important theoretical concepts, sufficient to enable us not only to apprehend the practical benefits of buffering, but also to understand, in a general way, the actual mode of its operation. Primed for nutrients by a base fertilizer, and for pH by calcium carbonate (or more rarely by sulphates of aluminium or of iron or by yellow sulphur powder), the buffering medium becomes a plentiful reservoir of nutritive ions and pH-regulatory ions. At any instant of time nearly all of these ions are attached to the buffer (in the case of the nutrient ions at least 98% of them) and are thus out of solution. But some are in process of detaching and passing into the surrounding soil water and, if a steady state has established, an equal number are in process of being recaptured. In this way the buffer automatically regulates the ionic content of any water with which it is in contact, and if everything is working properly, imparts to it in the steady state a fertilizing strength and a pH the same as those of a proprietary liquid feed at the normal strength appropriate to the type of plant being grown.

Lack of space (and maybe also .the reader’s patience!) preclude further details for the present, but in the next issue I plan to return to the matter and to give there some quantitative information.