There are many different points of view about fertilizing cacti. This is a rather controversial issue and, as a rule, it is based on someone's experience. Since cacti are very peculiar plants, fertilizers are special for them. Let's figure out what batteries cacti need, and to what extent.
Nitrogen
The first element of nutrition that is mentioned when it comes to feeding ordinary plants - it is necessary for growth. In cacti, the need for nitrogen is extremely limited due to some physiological features and, above all, their slow growth. If there is a lot of nitrogen in the soil, or a lot of nitrogen was in the fertilizer used, then the stem of the cactus begins to grow rapidly, becomes loose and watery. Many children and offspring appear, but they do not develop into full-fledged plants, the skin of the cactus does not have time to grow and bursts. Wounds and scars appear that rot or crust, as a result, the plant will be disfigured or die. Therefore, organic fertilizers are not used for cacti, except in very small doses for epiphytic, rapidly growing cacti. However, it is impossible to completely abandon nitrogen, i.e. if you plant a cactus in clean sand and do not feed, nitrogen starvation may occur.
Potassium
Potassium is a vital nutrient for cacti, it regulates their growth, contributes to the formation of dense leathery covers that protect the body of cacti from temperature changes, winds and bright sun. Potassium also affects the timely ripening of shoots, stimulates good flowering and ripening of berries. The lack of potassium is very difficult to determine on cacti, usually manifested in the absence of flowering, pallor of the skin sometimes in the appearance of a bronze tint. Potassium deficiency can be observed on calcareous soils. This is explained by the fact that during liming, an excessive concentration of calcium ions is formed in the soil solution, which makes it difficult for potassium and magnesium cations to enter the plant. Therefore, for a harmonious balance, one part of potassium must have two parts of lime.
It should be remembered that the same signs as with a lack of potassium can be explained by violations in care - lack of light or high air temperature. Therefore, do not rush to feed - excess potassium is harmful to cacti - growth slows down, roots die off.
Phosphorus
If nitrogen is needed for the tips, then phosphorus is the element that stimulates root growth. It also affects the laying of flower buds and flowering, fruit ripening and seed viability. Therefore, mineral fertilizers with the formula NPK, where equal parts of nitrogen and potassium and more phosphorus, are most suitable for cacti. A lack of phosphorus can be suspected if cacti grow very slowly, even if the temperature and light conditions are observed, they do not bloom.
Calcium
For cacti, calcium is a very important element due to the fact that it is a building material for spines, bristles and hairs. So, for example, back in 1938, when studying the species Cephalocereus senilis, it was found that more than 85% of the dry weight of plants consists of calcium oxalates (Cheavin WHS. The crystals and cystolites found in plant cells. Part 1: crystals. Microscope. 1938; 2 :155–158.). The lack of calcium on cacti will initially manifest itself on acidic soils - this is slow development and growth, a weak root system, thin soft spines, fragility and fragility of the plant .
A healthy strong cactus is practically not injured when falling from the windowsill, the maximum that threatens it is that the children break off, the peduncle breaks. But a healthy cactus that has enough of all the nutrients can be dusted off, put in a pot and put in place. His needles do not break, the trunk will not crack, the roots will not come off. Largely due to proper mineral nutrition.
Composition of the right fertilizer for cacti
We found out that cacti need very little nitrogen than all other plants, so organic fertilizers are almost never used by cactus growers (except in exceptional special cases). Many years of experience in growing cacti show that fertilizers in liquid form are most effective. They are more convenient to use cactus, because it is easier to dose if you have acquired too concentrated fertilizer. By itself, a high concentration of minerals in an aqueous solution does not affect anything, their ratio is important, and we can change the concentration by diluting it with water. The general recommendations are that the concentration of full fertilizer salts should not exceed 1 gram per 1 liter of water.
Such ratios of minerals as, for example, NPK 5-10-5 or 20-30-20 (diluted 4 times the recommended dose) were recognized as successful - to stimulate flowering. At the end of flowering, NPK with the formula of elements in equal parts, for example, 5-5-5 or 20-20-20 (1/4 dose) or 10-10-10 (1/2 dose).
Some experts consider these doses of fertilizer too high. For example, the very popular Schultz fertilizer in Europe has the formula NPK 2-7-7, i.e. nitrogen is even less than in the traditional one. Another fertilizer for cacti "GrowUp" has the formula NPK 2.5-5-4. These fertilizers are specifically designed for cacti, they do not need to be diluted three or four times. In all other cases, the rule works: "it is better to underfeed than overfeed."
It should be especially noted that the requirements of different genera of cacti for minerals are different: epiphytic and semi-epiphytic cacti (primarily Rhipsalis Ripsalis, Schlumbergera Schlumbergers, Epiphyllum Epiphyllum or Hylocereus Hylocereus), do not require a high content of calcium in the soil, tolerate calcareous soils. But they love the sufficient content of biohumus in the soil and they can be fed with ordinary fertilizer for decorative and flowering indoor plants, using a dose, depending on the concentration, equal to or slightly less than that recommended by the manufacturer.
When to feed cacti
Feeding of a very weak concentration can be done every two weeks, from March until September. During the dormant period, which is from October to March, cacti are not fed. Remember that flowering does not always coincide with the growth period! And if flowering occurs in autumn - winter, this is not a reason to feed, usually it is enough only to slightly increase watering.
It is also worth reducing or stopping feeding in the spring-summer season, when rainy cloudy weather lasts for a long time. Metabolism slows down during this period, plants slow down in growth, do not require a large amount of nutrients.
In addition, feeding begins no earlier than three weeks after transplantation.
By the way, sometimes you can read in the literature about feeding cacti with some unconventional methods, for example, sugar or castor oil, etc. However, we strongly do not recommend using such products on cacti, since everything can end quite deplorably. If, nevertheless, you decided to try a new tool, then try it on some one (experienced) not very valuable cactus, observe it for a long time. It may take a year, but the results will please you. But you shouldn't try out various remedies you don't know about throughout the cactus collection.