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

More Observations on the Role of Nitrogen

by Ray Allcock.

In our Branch Newsletter of January 1997 I wrote about my feeding trial of 1996, and described the very gratifying benefits which accrued. Readers of that Newsletter may recall that early in the 1996 growing season I noticed that various specimens of Echinopsis eyriesii were looking rather yellowish and not showing much growth. These symptoms can indicate a shortage of nitrogen, and since I could not think of any other plausible explanation I reacted by feeding at a greatly increased rate throughout the rest of the season. During all that time I treated the whole collection to the same generous ration of 1 level teaspoon (i.e. 5ml) of Chempak No. 3 in every single gallon of water used. This is double the dose usual for pot plants, and since the NPK percentages in No. 3 stand equal at 20-20-20 it follows that the nitrogen component is proportionately quite substantial.

Prior to 1996 I had for many years been using the old-style Phostrogen (10-10-27) at the meagre rate of only one level teaspoon per 8 gallons. Thus the nitrogen supply in 1996 was boosted by a factor of 16 relative to what I had been using previously.

Today’s younger readers will probably wonder why it was that I used to be so mean in feeding my plants. And I used to wonder why it was that they grew so slowly, and erroneously attributed that to soil compaction and unavoidable neglect, and not to a lack of feed! But in this connection it must be borne in mind that the cactus books so eagerly read in my far-off youth were always full of dire warnings against feeding. These warnings were probably quite justified in their day, when the only feeds available were decoctions of unknown strength composed usually of fermented liquid animal manure, and when potting media contained large amounts (up to 30% or more by volume) of nitrogen-generating composted organic residues such as leaf mould, further supplemented by other slow-acting organic nitrogen sources such as bone meal.

There is a saying that old prejudices die hard, and so it was with me. And even today, in the most modern books, we still find similarly misleading advices, with no account taken of the fact that inorganic composts need far more nitrogenous feed than those which are genuinely organic as just described, nor of the fact that most present-day John Innes composts are very substantially lower in organic residues than those which used to be made using properly composted turves, and for which a programme of supplementary nitrogenous feeding was much less a necessity.

I can think back now to an interesting lecture given about 27 years ago by a then well-known NCSS member who had already been actively applying the science available at that time to explore the still rather novel possibilities afforded by Phostrogen and by liquid seaweed preparations. In his lecture we were advised to use Phostrogen in every watering but at ¼ strength. What did this lecturer mean? I took it that he meant ¼ of the strength recommended by the makers for use in every watering of ordinary non-succulent pot plants, and indeed thus 1 level teaspoon in 8 gallons. But it is perhaps not entirely impossible that what he meant was ¼ the strength recommended by the makers for occasional or sporadic feeds. This would have given 4 times as much, since the rate recommended for sporadic applications is 2 level teaspoons per single gallon. I have now been wondering what he did mean. Unfortunately and sadly we cannot now seek an answer directly. However, a perusal of an article on p.59 of the 1970 NCSS Journal makes me pretty confident that my understanding of his intentions was not inconsistent with his own written advice, for in that article he recommends 1 teaspoon in 4 gallons once per month, while I during subsequent years used 1 teaspoon in 8 gallons about once per fortnight, which amounts to the same thing.

Where agriculture and food production etc. are concerned I have long been and still am a convinced advocate and committed supporter of organic ideals. Nevertheless I am no purist, and where cacti and succulents are concerned it has now become my deliberate policy to cultivate inorganically (epiphytes perhaps excepted??), for the two obvious reasons that to do so gives one full control over the plants’ nutrition and absolves one from the laborious necessity otherwise to turn them out of their pots to replace exhausted compost.

A half-way approach wherein the feed is viewed only as a supplement to boost the fading nutritive performance of an organic potting medium is perforce less easily administered, since in any but the smallest collection some plants will have been repotted significantly earlier than others.

In an inorganic cultivation almost the whole of the necessary nitrogen has to be supplied artificially, since the pots contain virtually no decaying vegetable matter on which nitrogen-fixing bacteria might feed. My 1996 experiences convinced me that artificial feeding in conjunction with a loam-based compost of suitable pH (about 6.5) can give the good results that I had long wished to see, and this with the further practical benefit that the nutrition of the plants becomes less dependent on the maintenance of constant dampness, since the welfare of the bacterial population is no longer required. But one does not want to overfeed, since this might in the long run poison the plants. So the questions for me in 1997 were ‘Have I been using too much? How much should I use?’.

Now here we necessarily run up against the ages-old problem of conservation of fertility, that is, the problem of keeping the nitrogen in the soil. The operative facts are that nitrogen has an exceedingly complicated chemistry, in consequence of which any left unused will eventually escape from a damp compost under the guise of various gases or vapours (nitrogen dioxide, nitric oxide, nitrous oxide, nitrogen pentoxide, ammonia, nitric acid). The other principal nutrients (phosphorus, potassium and magnesium - P, K and Mg) are less fickle, and if not used will normally stay in the compost, where they might perhaps eventually build up to excess.

In view of this it seemed reasonable to me that while making some reduction in the nitrogen a bigger proportional reduction in the phosphorus and the potash might be advantageous also. Therefore, following another possibility also recommended in the Chempak Growing Guide for the earlier part of the season, I used in 1997 the high-nitrogen Chempak No. 2 at the suggested rate for cacti, i.e. 1 level teaspoon per 2 gallons in every watering. The relevant NPK analysis is here 25-15-15, so that the N was by this now reduced to five eighths of its 1996 amount and the other two to three eighths of their 1996 amounts. The solution taken up by the roots was thus weakened overall, but enhanced nevertheless in the proportion of N relative to P and K.

The results of this seemingly not very drastic modification proved to be quite unexpectedly interesting! All the plants grew with a vigour similar to that witnessed in 1996, and those in full sun produced growth of good quality, as before. But many of those in less favoured situations, such as those part-shaded by higher shelves for lack of alternative accommodation, now started to produce excessively open growth and to extend towards the light, and the more obviously so once the days started to shorten. This did not happen to any noticeable extent in 1996, even though every element was in that year supplied in greater amount!

It was thus confirmed, and properly brought home to me for the first time, that the proportions of P and K relative to N are rather crucial in regard to the quality of the growth produced under poor light conditions. To those more knowledgeable in matters horticultural this would in no way count as a new discovery! Far from it! And indeed the Chempak company itself produces a Cactus and Succulent Fertilizer specifically formulated for plants growing in poor light. Its NPK analysis is 8-34-32. But how instructive it is to experiment for oneself and to get a feeling for what is involved, rather than to try to build upon the considerable ambiguities, uncertainties and contradictions of received information!

At the beginning or middle of August I switched to the high-potash formulation Chempak No. 4 (15-15-30), in order hopefully to harden up the plants and to avoid the danger of soft growth, which was by this time starting to become apparent. However, this change was too late for the plants in poor light, and these became more evidently drawn and juicy during the succeeding months. But not all of them were so affected. Many cacti and succulents grow in the wild under bushes, and not out in the full sun, and these sorts stayed squat and firm - again a rather instructive result.

It may be that I could have done better to follow Chempak’s own recommendation for the mid-season switch, which is to go over to their low-nitrogen fertilizer No. 8 (12.5-25-25), or even their zero nitrogen formulation (0-10-10). On page 23 of their Guide it is stated that phosphorus combats the excessive use of nitrogen. On the other hand my gardening encyclopaedia says that potassium acts to offset the harmful effects of high nitrogen. Also one often hears that tomato fertilizer (i.e. high potash) is good for cacti. So I really don’t know what to make of all that. On one thing I am however quite resolved, and that is that I shan’t try No. 2 again on my plants, since I cannot guarantee good light for all of them. And I shall use up the remainder of the otherwise now unwanted No. 2 by mixing it half and half with No. 8 to provide an early-season feed very similar to No. 3.

Another reason for my using No. 4 in preference to No. 8 was that an excess of phosphate can lock up trace elements. The Guide asserts however that the amounts of phosphate in liquid feeds are too small to engender such an eventuality. But would that still be true over the long time scales which I have in mind? I cannot tell, but I feel inclined for 1998 to use either No. 4 or the rather similar new Phostrogen (14-10-27), which has almost the same potash:nitrogen ratio but appreciably less phosphorus, and which in its phosphorus :potash ratio is the same as its well respected precursor.

Finally, I must not forget to report on my winter-growing Mesembs. Because of the timing of their principal waterings these received mainly No. 4 in 1997, while in 1996 they were fed on No. 3, as was everything else. In both years they have done quite outstandingly well, far better than I ever used to achieve on the previous low feed regime, and with only a few genera showing any etiolation in spite of the poor light that I get late in the season under the degrading PVC cover of the greenhouse in which they are lodged.

All the findings reported here and in the mentioned article of January 1997 refer to plants potted in well-aerated loam-based media of pH about 6.5, and for the most part with no added organic ingredients of any sort, though in a few cases some peat is present within the original root ball. The cation exchange capacity of the media used is high, and quite comparable indeed to that of clay. There can be no guarantee that similar findings would accrue in chemically non-reactive media based on sand, gravel or cinders, Also I have some reason to suspect that the same feed rates would produce a softer growth in the context of soilless peat-based media, so some caution is indicated there.