This article originally appeared in the January 1997 Newsletter of the Liverpool Branch

Some Observations concerning Nitrogen Starvation

by Ray Allcock.

The nutrition of plants is an exceedingly complex matter, dependent upon many and varied factors, so that it is difficult to write about it in such manner as to eliminate the possibility of misinterpretation or ambiguity. Yet it is just this very complexity that makes it all the more needful to try to write something that is both brief and meaningful. In the present note I propose to pen a few useful words about the nutrition of cacti and succulents in regard to the nitrogenous part of their diet - that component which, if supplied by a synthetic fertilizer, is represented by the letter N of the NPK analysis now mandatory on the supplier’s bag or bottle.

Thoughts about the nutrition of cacti and succulents in pots have changed, and developed along various differing lines, since I first became interested in the hobby 53 years ago. In those days the World was still at war, and recipes for the formulation of soluble complete fertilizers suited to the cultivation of plants in pots had still to be developed and tried out. The nutritive needs of such plants, in so far as they were adequately met, were accommodated by enriching the best available garden loam through incorporation of substantial amounts of organic material (leaf mould, peat, fully fermented manure, bone meal, hoof and horn meal) to which might be added some sulphate of potash perhaps, and were maintained by giving a complete change of compost to the whole root system every one or two years.

All the mentioned organic materials slowly decay through bacterial action, and thereby continually release fresh supplies of nitrogenous compounds, some of which are immediately accessible to the roots. The use of these materials was recommended by Borg and by Haselton and indeed by most authorities, yet Vera Higgins eschewed even their use, and maintained that loam alone, in conjunction with a change of all the compost every two years, could satisfy all the nutritive needs of the plants. Properly prepared loam does of course contain a small amount of immediately accessible nitrogen and also a small amount of partially decayed nitrogen-generating organic residues, and maybe she helped the plants to find what was needed, by providing them with generously sized pots. Impurities in the Croydon water supply could perhaps have also been a factor contributory to the undoubted health of her plants.

After the War opinions and predilections about cactus composts proliferated greatly, and especially in regard to formulations deemed by their inventors to be uniquely appropriate to the imagined particular needs of various particular genera. All authors recognized however that the composts used had to be porous to the air and readily permeable to water; nevertheless composts mixed according to the recipes of those days tend to be somewhat fine structured, and often therefore become compactified, clogged and dusty within a few years. The consequent poor aeration and poor water conduction properties were in those days somewhat mitigated in practice by the use of porous clay pots (the walls of which exerted a powerful attraction on the feeder roots!), and would of course be to a large extent avoided if the already-stipulated frequent and drastic repottings were actually to be undertaken and accomplished. It is however quite impractical, without an army of assistant gardeners, to carry out the recommended frequent repotting of a large collection, and therein lay and still does lie a real problem!

In a rather ambiguous present-day parlance the epithets “well-drained” or “free-draining” are applied to any well-aerated compost which readily accepts and carries away any water poured onto it. But in those earlier days very little attention was given to the difficult problem of how to realize this happy condition in any long-lasting way, while at the same time a great and nonsensical fuss was made about the so-called “drainage”, a word which at that time was applied not to the compost, but to the rubble which was to be placed at the bottom of the pot beneath it. An amount so excessive as to fill the whole of the bottom third of the pot was often reckoned to be utterly essential to the continuing good health of the compost above it!

When suitable soluble complete fertilizers appeared on the scene in the mid nineteen fifties it was widely anticipated that frequent changes of the soil would thereby no longer be necessary. The fulfilment of that expectation was however in practice always less than complete, due to the problem of soil compaction. At best the compaction of the potting medium leads to a slow-down of growth and at worst, if combined with a lack of care in watering, to a loss of the roots, in consequence of which a fatal infection may sometimes enter and travel up into the neck and body of the plant.

At the time of my retirement ten years ago I resolved to tackle the problem of compaction, which by then was only too evident throughout the whole of my collection. After a very promising start I ran however into several incidental but catastrophic difficulties of a chemical nature, which arose because some of the absorptive ingredients which I used contain vast reservoirs of alkalinity, which slowly leaks out and over a period of weeks or months (depending on the details of the mix) raises the pH to an unacceptably high level. I have already described the devastating effects of alkalinity in our Newsletter No. 1.1 of 1995, and have given there some well-tried and safe recommendations suited to the lowering of the pH of implanted composts. Also, in our Newsletter No. 2.2 of 1996, I have given very precise and tested recommendations as to how the problem of compaction can be permanently overcome, and in the pertinent article and in another article in the same issue have given away a few findings concerning an effective pretreatment of one of the previously offending absorptive ingredients. I fully intend to write again concerning the uses of acid salts in pretreatments; meanwhile it must suffice for the purposes of the present note to say that I now have an inorganic soil-based compost fully reliable as regards the long-term maintenance of its porosity, permeability, capillarity and pH. Plants which I have moved into it are now growing very much to my satisfaction, and better even than they did in those very far-off days when, with a small yet rapidly expanding collection, it was possible for an all-too-brief period to give at least some effect to the afore-described very demanding organic principles. It should be possible now therefore to pot the plants on as they grow, without any damaging and time-absorbing disturbance of the roots. That at least is the hope, and so it remains only to establish more carefully the actual nutritional requirements.

It is of course very well known that an excess of nitrogenous material can induce a soft and watery growth, and that potash encourages flowering and fruiting. With this and various received advices in mind I have for many years been sparing with the fertilizer, and have unquestioningly limited my unfortunate plants to a rather meagre ration, comprised of Phostrogen (10-10-27) at a rate of 1 level teaspoon in every 8 gallons of water used, which is only ¼ of the rate which the makers recommend for pot plants. I also carried on at this rate when I switched to my present inorganic soil-based formulation, and I further primed the latter by adding 10ml of Phostrogen to every 5l of the mix, which is just a little under ½ of the rate recommended for the mixing of composts.

Quite early in the 1996 season I noticed that several specimens of Echinopsis eyriesii were looking rather yellowish all over and not growing very much. Since everything else had now been put right the explanation had to be found in some mineral deficiency, and from the gardening encyclopaedias it seemed that a lack of nitrogen was implicated. It could not be a lack of magnesium, since that leads to a patchy yellowing. Moreover magnesium deficiencies are not very likely when the pH lies between 6 and 6.5, as was the case here. Nor, and for the same reason, could it be a lack of iron, and the more so because I had in fact used sulphate of iron to effect the pretreatments. I decided therefore to switch to a formulation with a higher nitrogen content, and for this, still erring perhaps on the side of caution, I chose the Chempak balanced fertilizer No. 3 (20-20-20) rather than the high nitrogen formulation No. 2 (25-15-15). In going over to Chempak I was also very much motivated by the consideration valid then, but not now, that the Phostrogen formulation at that time made no mention of the essential trace elements boron, copper, molybdenum and zinc, whereas the Chempak packets explicitly mentioned these elements.

With a little trepidation I decided moreover that since there was a deficiency I would start with double the strength recommended by the makers as being most appropriate for pot plants, i.e., I would start at a strength of 1 level teaspoon per gallon of water, and decide later how to continue. I need not have worried. The plants obviously benefitted, becoming very turgid and greener. One or two were so taken by surprise by this unexpected bounty that they swelled up and burst! Reassured by these positive indications I carried on the same treatment. I found that the plants soon adapted to the new regime, continuing to go greener and producing a more robust and quicker growth, with large and very remarkably colourful spines.

I continued in this way throughout the whole of the summer, and with all of my extensive and varied collection of summer-growing cacti and succulents. With the exception of a few epiphytic cacti which I had moved into a peat-based ericaceous compost no soft growth was induced. Also the Echinopses and all my other South American cacti carried on growing right through the summer heat. Winter-growing succulents have also benefitted, except that some of the Senecios and Crassulas have become too lanky.

Flowering seemed to proceed much as usual, except in the case of a large bedded-in Trichocereus, which from its size and vigour was obviously getting enough nitrogen from somewhere long before this experiment started. This plant grew even more vigorously than before, but produced only 1/3 of its usual quota of blooms.

One conclusion which is quite unavoidable is this: that ¼ strength old-style Phostrogen, used with rain water when available, and otherwise with our fairly pure Liverpool tap water, is inadequate to support a satisfactory growth of potted cacti and succulents unless supplemented by substantial amounts of organic materials in the compost. It is not however very easy to do fully scientific investigations in a congested greenhouse, nor is it an attractive proposition to deny to part of one's precious collection a treatment which is obviously proving highly beneficial! I would therefore freely admit that while the results now reported suggest that the 16-fold increase in the nitrogen supply was the principal operative factor, they do not prove that it was so.

I had however in the house two pallid and sickly-looking Clivias of the same clone, potted years ago in the same compost and in the same pots and living on the same window sill. For years I had watered them with the same quarter-strength solution, conveniently taken indeed direct from my big tank in the greenhouse. During the whole of the summer of 1996 I now watered one of the two exclusively with a quite strong solution of nitrate of soda (in the region of about half a level teaspoon per gallon). It flowered normally and gradually became deep green and happy-looking, though still by far inferior to a magnificent specimen of the same clone grown by one of my daughters, who has a great liking for plants provided only that they are non-succulent, and whose mind has in consequence never been filled by silly cacticultural theories and inhibitions! To the other Clivia I denied completely all trace of fertilizer, and instead entertained myself by irrigating it with weak solutions of a commercially supplied liquid humus. It too flowered normally, but by the end of the summer its foliage was still more pallid and part-shrivelled than it was at the beginning. Life is short and life is busy, so for me this little Clivia experiment clinches the matter. Any who still doubt that nitrogen was the main factor in the principal experiment will have to seek elsewhere for further proof or disproof!

From what I have described it seems clear that the diagnosis of nitrogen deficiency within a collection is a simple enough matter, once one knows which plants are suited to serve as reliable indicators of this condition. Amongst the cacti there are some, including Echinopsis eyriesii, Echinopsis multiplex, Chamaecereus silvestrii, Echinocereus pentalophus, Echinocereus blanckii, Cereus peruvianus, and doubtless various others, which show an obvious over-all yellowing even under mild conditions of nitrogen starvation, a dogged determination not to die no matter how hungry they get, and a rapid and obvious over-all greening when the nitrogen deficiency is rectified. Such plants are excellent indicators of this specific condition, whereas of course the mere presence or absence of adequate growth is by no means specific of the underlying cause. Among the other succulents there are some green plants which also give a similarly pronounced colour signal - these include Sedum praealtum, the lighter-coloured forms of Crassula lycopodioides, and some of the green Aeoniums. There are also various subtle indications, having to do with turgidity, gloss, leaf size, etc. But I would urge any serious readers to experiment for themselves with some of the indicator plants, and thus to see for themselves just what is involved, rather than be subject to the limitations of verbal description.

Not all cacti and succulents respond to chronic nitrogen deficiency by changing their colour. All however sink sooner or later into a state of torpor and eventually root failure, from which in serious cases they may re-awaken and try to re-establish only years later, if at all. In the case of the Crassulaceae and other leaf succulents the same happens, but usually on a shorter time scale, and often the plants become completely unable to make the necessary new set of roots. These observations provide for me a very plausible explanation of the oft-reported phenomenon of randomly distributed long periods of zero growth.

Agricultural scientists have long recognised that the principal key to fertility of the soil lies in the presence and conservation within it of nitrogen. It has been established in more recent times that this element is taken up by plant roots in at least three different forms, of which two are ionic, the third being neutral. One of the ionic forms is the nitrate anion NO₃- and the other is the ammonium cation NH₄+. It is also now established that these two ionic forms are accepted directly and with comparable facility by the roots, and that once the nitrate gets into the plant it gets reduced to ammonia by enzymatic reactions.

One still reads in many places the false information that plant roots take up nitrogen only in the form of nitrate. This misconception arose because the bacteria in the soil solution rapidly oxidise ammonium to nitrate. In point of fact however the possibility of such oxidation is in part pre-empted in well-buffered (plenty of clay and/or humus) soils by the processes of cation exchange, whereby ammonium cations watered in or otherwise supplied attach themselves to the cation exchange complex (i.e., the clay and the humus), displacing the more loosely attached calcium cations. In this captured state the ammonium remains accessible to the roots, but is at the same time protected against short-term evaporation (as ammonia) and leaching, and also against oxidation to nitrate by the soil bacteria. This is important to soil fertility because nitrogen in the nitrate form is lost by leaching, by evaporation, and by spontaneous and/or bacterially induced chemical decomposition with evaporation of the decomposition products.

In soluble fertilizers nitrogen is incorporated in three different principal forms, namely as nitric nitrogen (i.e., nitrate), as ammoniacal nitrogen, and as ureic nitrogen. The last-named refers to the chemical compound urea, otherwise known as carbamide, whose chemical formula is (NH₂)₂CO. In this third form nitrogen is taken up directly not only by the roots but also, if applied to them, by the leaves as well. Urea in the soil and not taken up directly is converted into ammonium carbonate (NH₄)₂CO₃ by the soil bacteria, which cleverly rearrange the atoms of the urea molecule and incorporate into it 2H₂O.

The above-described chemical realities make it altogether evident that the ammoniacal and ureic forms can play a particularly valuable role in absorbent and well-buffered composts, and the more so if the watering is infrequent and the regime on the dry side (as in my cultivation). It was therefore with some interest that I learned that in the new Phostrogen (14-10-27) the nitric nitrogen has been left at 8 (as before), the ammoniacal nitrogen has been increased from 2 to 3.5, and the ureic nitrogen from 0 to 2.5. In the same way the information on Chempak packets, that the ureic content in the balanced fertilizer No. 3 (20-20-20) stands at 10, and that in the high nitrogen fertilizer No. 2 (25-15-15) at 19, has now acquired for me a quite new significance.

I have read somewhere that an excess of nitrate is unfavourable to the preservation of the crumb structure of loam, so that on this account it may perhaps be appropriate now to investigate the growth of the reinvigorated plants at a somewhat lower dosage. The possibility of an excessive build-up of salts may also be mooted, although I have no evidence at present to suggest any threat of this sort of danger. Perhaps during 1997 I shall work at only the recommended strength, rather than the doubled strength used in the resuscitative year 1996.

I am somewhat spoilt for choice in so far as concerns the actual formulation which I shall use. On one point however I am now quite resolved, namely, that I shall not be using the low nitrogen formulation Chempak No.8 (12.5-25-25) but rather something with more nitrogen and, because a phosphate build-up can lock up trace elements, less phosphorus. These considerations suggest the high potash formulation No. 4 (15-15-30) or the new Phostrogen (14-10-27) which is evidently somewhat similar but lower in phosphate. The Chempak Growing Guide recommends for cacti either the high nitrogen No. 2 or the balanced No. 3 during the greater part of the season, with a switch to the low nitrogen No. 8 later on to ripen the growth. It must be borne in mind in all cases that the effect of fertilizers on cacti and succulents depends not only upon the formula and the dosage, but also on the frequency and amount of watering, the amount of light and heat, the permeability and retentivity of the compost, the pH, etc. Recommendations based on good growth in one milieu may produce soft growth in another, and should not therefore be accepted uncritically, and without appropriate experimentation and thoughtful observation.

MID-CENTURY REFERENCES
Borg, J., Cacti, Macmillan 1951.
Haselton, S. E., Cacti for the Amateur, Abbey Garden Press 1938.
Higgins, V. & Marrable, H. T., Cactus Growing for Beginners, Blandford 1943.

TECHNICAL WORKS CONSULTED
Richardson, W. D., The Chempak Growing Guide.
Yagodin, B. A. (ed.), Agricultural Chemistry (transl. V. G. Vopyon), Mir 1984.
Foth, H. D. & Ellis, B. G., Soil Fertility, Wiley 1988.