First of all: why should we fertilize our cacti and succulent plants? Let’s say right away that fertilization (or fertilisation, or manuring or mineral nutrition) is necessary for any plant grown in pots, since the amount of soil available is limited and, sooner or later, nutrients will begin to run low. Fertilizer nourishes the plant, enriching the soil of those elements which, through the time, are absorbed by the plant or washed away by the waterings.
Succulent plants should be regularly fertilized to ensure that the right quantity of nutrients is always available during the growing season. For cacti and succulents, however, generic fertilizers should not be used. In other words, a fertilizer commonly used for ornamental, leafy or fruit plants is not at all appropriate for a cactus plant. It is necessary to resort to specific fertilizers, specially balanced for the succulents. Otherwise, the risk is to irreparably compromise the growth and the physical appearance of the plant, making it weak and radically different from the specimens of the same species that grow in the wild.
Beware to Nitrogen
The first big difference is the amount of Nitrogen contained in the product we have choosen to fertilize our plants. The balanced fertilizers contain high dosages, at least equal to those of the other two primary elements, which are Phosphorus and Potassium. The starting point is always the habitat. In nature, cacti and succulents live in sub-desertic zones, where the vegetation is scarce. The leaves that fall to the ground and then decompose and enrich the soil with nitrogen are therefore few. Again, they fall in the dry season and are little attacked by bacteria: this causes little humus to form and that the soils in which the succulents live are scarcely moist, sterile and very little nitrogenous. Phosphorus and Potassium, however, are not lacking. Hence the need to provide these plants with fertilizers with a low Nitrogen content and high levels of Phosphorus and Potassium.
The consequence of highly nitrogenous fertilization on cacti and succulent is as far away from nature as can be. The plants will rapidly grow, will be swollen, pumped, of a bright green. They will also be weaker and more likely to suffer from attacks by parasites or bacteria. The spines, always due to the too fast and pushed growth, will be shorter and thinner and will contribute to confer to the plants an unnatural look, very far from what the cacti have in habitat.
Even with the right fertilizer, however, it is desirable not to exceed if you want plants to grow at the right place, taking on a natural appearance.
Before seeing which fertilizer to use, how much to fertilise and how often to do it, I want to examine in detail the main elements present in soils and fertilizers.
The main elements
The nutrients useful for the life of the plants are a ton. Those really needed are essentially thirteen and are classified into macroelements, mesoelements and microelements. It’s in the absence of one of these thirteen elements that you can have a decline in plant health. Nitrogen (N), Phosphorus (P) and Potassium (K) are the so-called macroelements; Calcium, Magnesium and Sulfur are called mesoelements; Manganese, Boron, Copper, Zinc, Cobalt, Iron and Molybdenum are called microelements. Here below there is a brief description of three macroelements, three mesoelements and the main of the microelements, the iron.
Nitrogen (N) – It is the main element for plant growth and a primary component in chlorophyll photosynthesis. For succulent plants, however, the nitrogen input must be lower if compared to the other plants, because Nitrogen is scarcely present in soils where cacti and succulents usually grow. In nature, in particular, in the sub-desertic zones where the cacti thrive, the vegetation can be very sparse, for this reason, the soil is scarce in leaves which, decomposing, confer humus to the substratum. If for plants flowers or leafy plants the contribution of a vigorous amount of Nitrogen is essential to stimulate the production of flowers and leaves and to make them greener, with cacti an excessive amount of Nitrogen will lead to too-fast growth and an unnatural bulge of the stem. With too nitrogenous fertilisations, the stalks of the cacti and the leaves of the succulents will get a bright green colour. The plants will be pumped and grow rapidly, but at the same time, they will lose their natural shape and will be weaker, therefore less able to face and overcome attacks of parasites or bacteria. Not only that: in cacti, the fastest growth will negatively affect the production of thorns, which will be thinner and weaker, and will influence the appearance of the plant which will be distorted compared to that of the specimens in nature.
Phosphorus (P) – It is one of the most essential elements for the production of flowers, but also for the development of the root system of the plant. Phosphorus contributes to the formation of new cells, essential for the growth of roots, taps, branches, flowers and seeds. This element can be made unavailable to the plant by too hard irrigation water (with too alkaline pH). In case of deficiency of Phosphorus, cacti and succulents will not be able to develop an adequate root system, with the effect that the growth of the plant will slow down considerably or will stop completely. At the same time, blooms will be scarce or completely absent.
Potassium (K) – For succulents, Potassium is essential because it promotes the opening process of the stomata, which in turn regulates the transpiration of the plant. This process results in balanced growth. Not only that: Potassium also regulates the lymphatic flow into the lymphatic ducts of the plant and prevents it from freezing when temperatures drop significantly. Potassium also increases plant resistance to adversity, disease and water scarcity. It is clear that, especially for succulents, this element assumes decisive importance.
Calcium (Ca) – This mesoelement regulates water transport and slows water absorption. Except for epiphytic cacti (which want calcium-poor soils), it is crucial for cacti in general. Calcium strengthens cells, so in cacti, it helps the epidermis to develop with greater consistency and the spines to grow stronger. However, cactus soils should not be rich in calcium. It will be sufficient to add it to the compound (if too acidic) in the form of corn nail (element of animal origin, with horns and nails) or marble dust. Usually, field soil already contains enough calcium for most succulents.
Magnesium (Mg) – Although it is a mesoelement, magnesium is as relevant as macroelements. It is usually scarce in acidic soils (for example, very peaty), present in effusive rocks such as pumice and lapillus. An element of great importance, it is the basis of the chlorophyll process functions (in fact, it is the central nucleus of the chlorophyll molecule). When it is deficient, it leads to a yellowing of the tissues (which in the cacti usually interests the vegetative apices) and to a weak coloration of the flowers.
Sulfur (S) – Pumice and lapillus are rich of this element: perhaps it is also for this reason that these materials are widely used in compounds for cacti and succulents. Sulfur is useful because it promotes the absorption of macroelements by the plant. Having a fungicidal function, plants cultivated in soils rich of sulfur are less prey to fungal attacks. It is not by chance that this element, in combination with copper oxychloride, is used in the main copper fungicides by contact (those, for instance, of light blue colour).
Iron (Fe) – This element is present in soils not excessively rich of lime, which blocks its absorption by plants. It is found in quantities in effusive rocks such as pumice, lapillus and tufa and is very important for any plant. Iron deficiency leads to stunted growth and yellowing of leaves or stems in the case of succulents. Above all, the lack of this element generates ferric chlorosis which drives the plan to deterioration with an evident discolouration of tissues. It is not by chance that iron enters the process of chlorophyll, giving the plants the correct green coloration. Among the symptoms of iron deficiency, there is also growth blockage. In these cases, it is appropriate to intervene with supplements based on this element.
Which fertiliser to use
Cacti and succulent plants need a ternary fertilizer, that is formed by the three macroelements – Nitrogen, Phosphorus and Potassium. Specific fertilizers (both liquid and powdered) can be easily found, with the addition of mesoelements and microelements, all necessary for the proper growth of succulents. However, not all commercially available fertilizers for succulents are equivalent, and it is essential to consider the proportion in which the three macroelements appear.
The Nitrogen content should be significantly lower than that of Phosphorus and Potassium. Indicatively it can be said that a good cactus fertilizer must have a part of Nitrogen, 2 or 3 parts of Phosphorus and 4 or 6 parts of Potassium. The dosages indicated by the various producers are not always the same, but it will be enough to take these proportions into account to choose the right fertiliser. Based on the most commonly used formulations, we can have a ratio of 6-18-36, or 5-15-30, where 5 is for the percentage of nitrogen dosage, 15 is for that of Phosphorus and 30 for that of Potassium.
In short, the Nitrogen content should be at least one-third of that of Phosphorus and one-sixth of that of Potassium.
Absolutely not recommended, for cacti and succulents, are the balanced ternary fertilizers, that have equivalent dosages of the three macroelements (for instance, 10-10-10).
When and how much to fertilize
This is really a complex field since every cacti enthusiast can have his own orientation. There are in fact those who often fertilize and those who do it very few times a year. Let’s start by saying that, of course, the succulents should be fertilized only during the growing season, since most succulents in autumn/winter go into stasis. In this period the plants stop the growth, and their rooting apparatus does not absorb water.
It will be possible to start adding a low amount of fertilizer (usually it is recommended to halve the dosage indicated by the manufacturers on the package) from the first half of April, to stimulate the growth of the plants. Here in North Italy I usually start watering in the middle or end of March, depending on the weather. I give the first two waterings without fertilizer so that the roots begin to wake up and the capillaries to gradually reform. When the plant is active, and the root system is broken down after the dry winter, the plant will be able to properly absorb the nutrients added to the water. The first fertilization, in my case, begins from mid/late April. I am not so convinced that copious fertilization results in an immediate and abundant flowering. I think that plants furnish flowers as an answer to the growth process and the fertilization of the year before. Also, the rest observed in the months before the restart is another possible reason for it.
In my experience, the last fertilization coincides with the beginning of September, when the plants resume vegetating after the slowing down in the warm months, in particular following the aestivation in August.
Every cacti enthusiast will also have his say on the number and frequency of fertilization. There is no practice for everyone: someone fertilizes often, someone else rarely does. Personally, I am for spaced fertilizations, around one every three waterings with only water, for a maximum of three fertilizations per year (from April to September). This is true for traditional fertilization, that is, by adding the right amount of fertilizer to the water (I suggest half of that indicated by the producer). In addition to traditional fertilization, in fact, there is also the fertigation system, that is the low dosage fertilisation provided with each watering. Frankly, I have never experienced this technique because I have realised that with the low fertilization regime I use, plants grow naturally, bloom abundantly and do not show deficiencies of any kind.
I think it also depends on the fact that I use to repot frequently, leaving each plant in the same soil no more than four years. The loam itself is the first fertilizer for the plants: when it is well balanced and fresh, not washed by watering and impoverished by the exploitation that makes it the plant, it already contains the adequate nutrients to grow for at least a couple of years. For almost all the genera I grow, I add to my substrates 10% finely sieved peat or earthworm humus (at this link you can find articles on substrates and elements with which to make them). It is mainly these elements that provide the basic nourishment for compost. The fertilization, therefore, is essential for integrating the impoverished loam with the time, also the washing-away and the absorption of the nutrients by the plant.
In this regard, there is also the slow-release granule fertilizer to be added to the substrate, but I never used it because it is usually very nitrogenous.
The fertilization can be done from above, watering with rain, or from below, pouring the water into a container and soaking the pots. Obviously, this second procedure is impossible to comply with if you have a large number of plants.
Fertilize the seedlings
Usually, I adopt a fertilization regime slightly more pushed for the sowings and for the seedlings of one year. In these cases I use the same fertilizer I use for all my plants, but I fertilize more often, say once yes and once no, to help young plants grow. I have seen that for sowing fertilization is necessary: it stimulates the seedlings born from a few months to develop adequately in time for their first winter. At the same time, the fertilization slightly pushes the growth of the seedlings, which are often still found in the main mineral soil of the sowing, therefore too poor to provide nutrients to the young plants, which have higher growth rates than adult plants.
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