Plants need at least thirteen elements from the soil for proper growth, in addition to carbon, hydrogen and oxygen which are obtained from air and water. Plants generally grow normally until they run out of one nutrient, which then limits growth. Deficiency or excess of nutrients often causes discoloured or deformed growth, which may be characteristic of that particular element, although there is a considerable overlap of symptoms.
Most plants will grow better in the long term if additional nutrients are provided by application of fertilisers. Gross excess of some nutrients can induce deficiency of others. The moral is therefore to use balanced mixtures of nutrients in fertilisers, rather than applying a large amount of one element. Many fertiliser mixtures only specify their Nitrogen (N), Phosphorous (P) and Potassium (K) content on the label of the package, but unless especially pure ingredients were used in their manufacture, it is likely that some trace elements will be present fortuitously. Other fertilisers explicitly list their trace element content. The elements are identical whether inorganic or organic ingredients are used, but the source will affect the speed of their release and their availability.
Boron is the only element required by plants but with no function in animals, except that it may be toxic in excess. Borax is the active ingredient in some products designed to kill ants. In most cases extreme measures have to be taken to induce boron deficiency in plants as such low levels are required. However, it has been suggested that cacti may benefit from higher levels of boron and some people add small amounts of borax when watering.
Silicon is also usually ignored as a micronutrient, and will be generally available in any gritty potting mixture suitable for cacti and succulent plants. However, significant amounts of silicon (as silica) may be present in plant tissues (especially in diatoms, grasses, sedges and horsetails) as a structural element. It must be translocated through the vascular system in solution and precipitated, often in crystalline form, under biochemical control to form precisely defined structures.
The N:P:K balance of some typical formulations are shown below, as quoted by the manufacturers, mainly using examples from the Chempack range which were readily available. This should not be taken as any special endorsement, as other brands will be equally suitable. I like to rotate between several brands to ensure a mix of micronutrients.
For succulent plants, a balanced NPK fertiliser such as Chempack No. 3 is suitable, but cactus growers may prefer a lower nitrogen content such as Chempack No. 8 to ensure that their plants remain compact. Chempak produce a specific cactus and succulent fertiliser with an even lower nitrogen content, in rather small packages. Leafy succulent such as Euphorbias may appreciate a high nitrogen boost (e.g. Chempack No. 2) in the Spring when they start growing, followed by the balanced mixture for the rest of the growing season. For comparison, a typical formulation to encourage tomatoes and other fruit has a very high Potash content as does Cactigrow.
Organic fertilisers may be mixed into the potting mixture when it is being mixed and provide sustained release of nutrients over a long time. The bulky constituents of organic fertilisers help to improve the soil structure. Many organic fertilisers contain essential trace elements and may also encourage beneficial soil micro-organisms. The role, if any, of beneficial mycorrhiza in the growth of succulent plants seems to be largely unknown, despite their demonstrated importance in culture of orchids and many woody plants. However, organic fertilisers will tend to encourage mycorrhyza in contrast to inorganic fertilisers which inhibit them.