The Nursery (Part 6: Watering, fertiliser and other additives)

As with all things related to cacti you can make it as complicated or straight forward as you like, and there is no guarantee that overly complicating things lead to better results. On the other had, not caring about such things as the pH-level of your water or the effects of the fertiliser you use will likely have a detrimental effect on your plants.

The below paragraphs on watering is about seedlings, but the other parts of this post is just as valid for adult plants.


Watering when it comes to seedlings isn’t really as complicated as it sometimes can be when they’re all grown up. As adults they may have wildly differing requirements when it comes to watering, but at this stage they all have much the same needs. Young seedlings which have just come out of their enclosed humid atmospheres like their soil to still be constantly moist and never to dry out completely. This doesn’t mean the soil should be dripping wet other than when you water them, but rather that they grow best if there is always some moisture present in the soil. How much moisture is a matter of experience and the species in question.

Most species aren’t very demanding and at least for a beginner it’s recommended to start out with some of the easier species so as not to lose heart. I remember sowing a tray full of Ariocarpus and Pediocactus among others when I was around 13-14 years old and after being very happy to see lots of seedlings appear, it was just as disheartening and demoralising to see them all die within a month, some from fungi, some from rot and some from drying out.

Astrophytum myriostigma var. nudum at three months old. At this point the plants are grown
enough that the soil should almost dry completely out between waterings. If it does dry completely
out it is not really a problem at this point either. The more difficult species should probably be allowed
to have their soil dry out between waterings at this stage.

Generally speaking, the first three months of a seedling’s life are the most difficult. It’s at this stage they’re most at risk of fungal attacks and drying out. If you can keep them in an enclosed humid atmosphere for this entire time without any problems with fungi or too moist conditions, all the better. Often this isn’t possible though, and you may often have to remove your plants from humid conditions sooner rather than later. When this happens you are immediately risking the seedlings drying out, so extra care needs to be taken at this point to ensure that they don’t.

The same Astrophytum myriostigma var. nudum as in the above photo, now five months old.
At this stage the plants can be treated almost as adults, and the soil allowed to dry out between
every watering.

Once the plants are about three months old and/or show clear signs of more “adult” growth (i.e. spines, body shape, colouring) it’s usually time to allow the soil to dry out more between waterings. This will accustom the plants to slightly drier conditions and begin hardening them. Again, this is in many ways a matter of experience and takes a bit of time to get right. Depending on the species, how and where you grow them, and the level of experience you have, I’d say it’s better to err on the side of caution and water less rather than more. Even though most seedlings who may be rot prone in adulthood aren’t as rot prone yet, it’s still easy to over-water many species at this stage.

The four 5 cm square pots in the foreground and the one on the left at the back all contain the same Astrophytum
myriostigma var. nudum
 as in the two photos above. They are now 14 months old and ready for a new repotting only six months after their previous one. If I had given them bigger pots and more breathing room in the first repotting they would all likely be larger still than they are at this point. Notice that the variety nudum refers to plants without the characteristic white flecks all Astrophytum species have to a larger or lesser degree. It is not a 100 % stable variety though, as you can see some of them with dense flocking. The other three pots in the background are filled with regular Astrophytum myriostigma and most are densely flocked, yet still a couple have almost no flocks at all.

It’s a bit intuitive as well. When you see the soil clearly drying out or some seedlings beginning to shrink a bit, it’s time to water. And if you see that some seedlings are rotting and algae and mosses are growing freely it highly likely means too moist conditions. With a little bit of practice it isn’t too difficult to reach a happy balance. Most seedlings a also forgiving of mistakes.

Whether you water from above or below, or only spray, is a matter of choice, though small seedlings may quickly be disturbed by overhead watering. Personally I usually only spray to add a bit of moisture if I think the pots are slightly on the dry side, yet not so dry as to need a full watering, while I only water from below when it’s time to water them properly. Any excess water in the tray should be removed within an hour or two, though it’s probably best to do it right away so as not to forget all about it.

pH of the water

The pH of the soil is very important, and any major imbalances one way or another will likely stunt the growth of your plants or stop them growing altogether. Most cacti and succulents receive water by way of rain which is acidic. Usually rainwater has a pH of around 5 but, coupled with thunder clouds (which often happens in summer) the pH of rainwater can reach as low as 2-3. Tap water on the other hand may have a much higher pH value depending on where you live in the world. In Oslo, the water from my tap has a pH of around 8,5 and is bordering on being classified as “hard” water. 
What this means for the plants is that if your water is hard, it will lead to the soil eventually having a higher pH value than most cacti are happy with. Cacti (most of them, at least) seem to do best with a pH of around 5,5, which incidentally is around the level of rain water. All the growers who swear by rainwater therefore have a point. I remember the curator of the local museum where I grew up and who, together with his wife, is an avid cactus and succulent collector, only used rainwater. I didn’t quite understand why at the time, but now I do.

A bottle with pH test liquid to check the pH of the water mix I use.

Still, it’s not necessarily easy to only use rainwater, at least not for me living in an apartment. To make my water more acidic I therefore add a bit of vinegar now. The amount of vinegar will vary depending on the pH of your tap water and the amount of water you’ll use. For me I add about 3 ml of vinegar to approximately 10 litres of water. According to my (admittedly cheap) pH-testing kit this seems to yield a pH-value of around 5-6. I plan to test this on my larger collection in Kristiansand next year and see whether there is any noticeable effect.
Many cacti, particularly in Mexico, grow in or directly on limestone which is alkaline. Some growers have therefore believed that the soils of these plants should be alkaline. However, the plants only absorb water and nutrients when it rains, and the acidic rain reacts with the limestone to release nutrients from it that the plants then absorb. Once the rain has stopped and the water drained off (which happens quickly) the plants do not interact further with the soil other than to merely use it for anchorage. These species will therefore benefit just as greatly from slightly acidic water as other cacti and succulents growing naturally in less alkaline/more acidic soils. 
As an aside, anecdotal evidence from many growers suggest that seeds of several cactus species germinate better in slightly acidic conditions. E.g. Steven Brack of Mesa Garden in New Mexico has suggested that seeds of many species sown outdoors germinate best just after thunder storms when exposed to fairly acidic rain. It may certainly be the case that seeds of some species have chemical inhibitors present to prevent untimely germination and are only deactivated when more acidic water is present.


The topic of which kind of fertiliser to use (and, indeed, whether to fertilise at all) is apparently a much debated one. It seems every book and every article on cacti differs slightly in opinions. Some of it may have to do with the very different soil cacti inhabit, with some living in completely inorganic soil and others in soils consisting of mainly organic matter. Some of it may have to do with the difference in growth rates between species, where some will hardly grow at all in a year while others will race ahead. Some of it may also be down to the vary big differences in the multitude of fertiliser available on the market, and the knowledge of how they effect the plants.
It may immediately seem counter-intuitive, but it’s perfectly possible for cacti and (I assume) almost every other plant in the world to grow in e.g. completely inorganic soil or, indeed, without any soil at all. The latter method, known as hydroponic growing, can be an extremely successful way of growing plants. To grow them this way they require fertiliser to provide all the essential nutrients they need. 
Most cacti, however, are able to grow in a completely inorganic soil without the use of fertiliser. They manage this by extracting all the nutrients they need from the soil itself. In order to achieve this the soil must be made up of components containing all the stuff they need. For anyone interested in trying this method out, I advise reading an excellent guide on this called “The Stone Eaters” by Dag Panco which can be downloaded free of charge from the website of the Romanian cactus and succulent journal “Xerophilia” which is a top journal in itself:
Still, most hobbyists growing cacti and succulents won’t grow their plants in inorganic soils in which the plants must work hard on their own to get food. Many hobbyists will grow their plants in nutrient-rich composts and require less fertiliser, but even so most of us will fertilise their plants whether grown in organic based or inorganic soils. Some prefer to fertilise once in the spring and maybe once in summer, some prefer slow-release granules that will release nutrients slowly over a whole year, and other prefer to fertilise regularly throughout the growing season.

One of the three fertilisers I use (they belong to the same series).
This one is without nitrogen, but high in phosphorous and potassium.

One might think that it sounds best to just give them a lot of fertiliser and have them grow faster, but it doesn’t necessarily work that way. First of all, too much fertiliser will lead to a fairly rapid build-up of salts in the soil which, in addition to being damaging to the roots of the plant, also leads to pH-imbalances in the soil – in fact this is one of the main reasons why one should repot from time to time even if it seems like the plant doesn’t need a bigger pot. Secondly, too much fertiliser can lead to an imbalance in the level and amount of nutrients available to the plant, most common of which is too high levels of nitrogen. 
Nitrogen is a vital element for all plants and is the first letter of the N-P-K symbol on all fertilisers. Too little of it will stunt or halt growth, yet too much of it may cause abnormal growth and also root-burn. If you fertilise slow-growing cacti and succulents with too much nitrogen, they may grow abnormally and elongate, and also be more at risk of disease and pests 
Phosphorus is the second letter in the N-P-K symbol, and is perhaps most important in regulating flowering. Fertilisers with relatively more of this element are usually employed when growers wish to increase the flower yield on their plants, though I don’t know of any research on how this affects cacti specifically.
Potassium is the third letter in the N-P-K symbol and in cacti, at least, is very important for the development of spines and for strengthening the epidermis (the outer cell layer of the plant skin) which helps the plant against disease and pests. Fertilisers marketed at cacti are usually enriched in this element.
The second of the three fertilisers I use. This one has a relatively higher
amount of potassium. On the bottle you can notice how it crystallizes when
it dries. Such a build-up of salts can also occur in the soil and cause problems
for the plants if not repotted.

Apart from these, there are many other elements cacti (and most other plants) need in various quantities such as calcium, molybdenum, manganese, zinc and iron. Fertilisers containing all essential elements will usually cover the needs of cacti, although it is advisable to find a fertiliser with relatively more calcium as this is a more important element for cacti than most plants. Like potassium it helps in developing spines and strengthening the epidermis. It is also advisable to find a fertiliser containing not just all essential elements, but also all beneficial elements as classified by Arnon & Stout in 1939.

Personally I use three different liquid fertilisers at the moment. One is enriched in potassium, one in phosphorous and potassium, and one in nitrogen. The one enriched in nitrogen also contains all essential and many beneficial elements, as well as having a fairly high amount of calcium. I use all three fertilisers with almost every watering at probably a quarter of the recommended strength. I try to mix them evenly, though without knowing the exact amounts of the various elements by percentage it is difficult to say what the exact amount of nitrogen, phosphorous and potassium are in relation to each other in the final water mix. Fertilisers always give the N-P-K amount in numbers (e.g. 20-8-12) which is a measure of the amount of the different elements in the fertiliser in relation to each other. I believe my final mixture is something like 10-8-15 which I’m quite happy with, though there might ideally have been slightly more phosphorous and potassium.

The third of the three fertilisers. This one is rich in nitrogen and
also contains all essential as well as some beneficial elements.

The reason I only add fertiliser to about a quarter of the recommended amount is because most cacti and succulents are slow-growing and if fed too much will quickly grow abnormally. I have not really experimented with adding or reducing the amount of fertiliser in the mix, but it seems to work fine for me so far. Once in a while I water without any fertiliser, and then usually from above. I believe it’s a good idea to vary between watering from below and above, and watering from above once in a while also helps wash away some of the built-up salts.

Finally, with regards to fertiliser, it is important to remember that fertilising the plants can be compared to feeding in humans. A varied diet containing all the essential nutrients humans need is essential for good health, and it’s the same in plants. It need not be tremendously complicated, but one should keep in mind that plants – just as humans – can react both positively and negatively to their diet.


In addition to all the elements a plant needs, it is also possible to add e.g. enzymes, hormones, bacteria, fungi and more to regulate the plant’s metabolic system or life cycle, or the soil environment. This is a bit more complicated than fertilisers and I will readily admit that I’m certainly no expert.

Three additives containing various substances meant to stimulate plant and root growth,
promote flowering and help prevent diseases and pests. Notice the one on the left has an
Opuntia in the background – this must mean it’s good! 🙂

Together with the fertiliser I add a small amount of additives in liquid form containing enzymes, vitamins, humic acids, amino acids and essential oils that are supposed to stimulate root and plant growth, flowering and the plant’s natural defences. I don’t know their efficacy, though it seems to me there is a noticeable effect. Particularly the root stimulants I believe have led to a dramatically increased growth rate in most of my plants. I also believe the flowering stimulants have led to much earlier flowering in some species than would otherwise be normal. I have had very little trouble with disease and pests so it may be that the stimulant I add to increase the plant’s natural defences is working, though I am less certain of the efficacy of this particular stimulant.

Another root stimulant. This one contains slightly
different ingredients than the one in the picture
above. It also contains humic acids.
An additive containing fulvic acids. These are
supposed to help growth and enabling the roots to
more easily absorb nutrients.
I had never used any such additives before, but after realising how widely many of these substances are used in other parts of the horticultural industry I thought they may well be very beneficial for cacti too. Though I haven’t run any scientific tests trying to make out the respective effects of the various additives, it definitely seems as if they are helping stimulate growth. Regular watering, high temperatures and long days are all very conducive to healthy and strong growth, but it seems to me that many of the plants are growing almost too well for these additives not to have a very real effect. The images below show examples of the very well developed root systems of the seedlings.

Frailea asterioides at 13 months old. These have been grown in an inorganic soil,
so they are probably not as big as they could have been if grown in an organic-based soil.
The root systems are very well developed, though. The label is 8 cm long from tip to tip for scale.

Astrophytum capricorne var. crassispinoides cv. ‘Taiho’ at 14 months old.
The root systems are large and very well developed. These have also been grown
in an inorganic soil. The label is 8 cm long from tip to tip for scale.

Lophophora koehresii also at 14 months old. Like L. diffusa this member of the genus
does not contain hallucinogens. The tap roots are very large and very well developed.
These have also been grown in an inorganic soil and the first time they were repotted, they
were placed in a 7 cm deep pot (like the one that can be seen in the previous post) to allow
the tap root to grow larger and longer. The label is 8 cm long from tip to tip for scale.

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