Geoff Bailey’s excellent Ariocarpus presentation sparked a discussion on suitable composts to be used to grow Ariocarpus and other choice, dwarf Mexican cacti. I mentioned I could write something for NN to help our members and Geoff kindly sent me some slides from his presentation to remind me of his recommendations. So what should we use to achieve clean plants, good growth, many flowers and maybe success on the show bench? Also do other cacti and succulents need similar conditions to Ariocarpus?

I shall not discuss here the need for suitable containers, adequate light, air movement and temperature requirements; just focus on the growth medium – the compost. Most succulent plants (including cacti) inhabit weathered mineral soils, where humus (organic matter) is very low or absent. Exceptions include epiphytic (or forest) cacti where humus is plentiful. Many succulents are tolerant of a variety of compost and growing conditions, but all will thrive if their particular requirements are met.

The dictionary gives us clues as to the meanings of the word ‘compost’ – the second is: ‘a composition, a mixture’ – but composed of what and how mixed? Growing media for succulent plants (including cacti) should take account of the plants’ habitats, preferences and growth rates. Remember we generally cultivate our plants in containers for long periods (sometimes rather too long before we repot them) and consequently they deplete their food supplies. Conversely their roots can be damaged either by an excess of fertiliser, or by salts precipitated through the prolonged use of (generally alkaline) tap water.

The principal components of composts are clay, sand/grit/gravel, humus and nutrients. Clay can be sticky but is essential for ‘holding’ water and ‘buffering’ nutrients to make them available slowly. Sand, grit and gravel are different sized rock fragments, composed of various minerals. Humus is decayed organic matter, which encourages micro-organisms. Coir has replaced peat in most commercial potting composts, but both have little ability to ‘buffer’ nutrients and release them slowly to the plant roots. Consequently they are unsuitable for all save a few cacti and succulents.

The space in between the soil particles is termed ‘pore space’ and is vital to the overall compost ‘structure’; that is its ability to provide both water and air to plant roots and not to become solid, claggy or waterlogged. The total soil pore space (dry) can be filled by water or air. One important aspect is the ‘pore space at field capacity’ that is the residual pore space available to air when the soil is thoroughly wet and the surplus water either runs off or drains leaving some pores as airspace. This is important since roots do not work without both available water and oxygen.

The term ‘loam’ means a soil composed of sand, silt, and clay particles in roughly 40:40:20% proportions; considered ideal for both garden and agricultural uses. Loamy soils generally contain more nutrients and humus than sandy soils, have better drainage than silt soils, and are easier to till than clay soils. Loams are gritty, plastic when moist, and retain water well; hence they are ideal for growing crops because they retain nutrients well and hold water while still allowing it to flow freely. They are too nutrient rich and insufficiently free-draining for cacti and succulents, hence garden soil is unsuitable for our purposes.

Compost pH is a measure of acidity/alkalinity, which controls the availability of many nutrients. The pH scale goes from 1 to 14, but the range between 5.0 and 8.0 is of interest to us. ‘Acid’ mixtures are generally in the range 5.0 to 6.5, ‘Neutral’ mixtures in the range 6.5 to 7.5, and ‘Alkaline’ mixtures in the range 7.5 to 8.0. Excessive alkalinity makes iron unavailable and causes precipitation of calcium and magnesium salts, frequently seen as whitish deposits on containers and the soil surface. Plants absorb nutrients not in elemental form, but as compounds or salts which are soluble in water. The only nutrient absorbed in elemental form by roots is oxygen.

Broadly the pH preferences of cacti and succulents are as follows:

Acid: most South American cacti – Copiapoa, Echinopsis, Rebutia, Gymnocalycium, Brazilian species and epiphytes.

Neutral: many cacti and succulents; most preferring the acid side of neutral.

Alkaline: Mexican dwarf cacti – Aztekium, Ariocarpus, Turbinicarpus, Epithelantha etc.

There are many materials commonly used to make up potting composts and some of their physical properties are tabulated below. (The data was collated from various sources.)

When you buy the constituents of your potting compost make sure to get fresh, high-quality, loam-based soil mixes, washed sand/grit and pure fired clay granules, all from reliable sources. Do not store loams through the winter, their nutrient availability and balance will be affected adversely. Also Perlite can cause migration problems since it tends to float and also resembles mealy bug colonies! Ensure you mix the constituents thoroughly, protect your compost from pests and keep it only slightly damp, not wet.

The principal soil-borne plant nutrients are normally split in two groups. The ‘major nutrients’ are nitrogen (N), phosphorous (P), potassium (K), calcium (Ca), magnesium (Mg) and iron (Fe); all are needed either to build the major cell structures or in the chlorophyll molecule. The so-called ‘minor nutrients’ include boron (B), manganese (Mn), copper (Cu), zinc (Zn), molybdenum (Mo), sodium (Na), aluminium (Al), silicon (Si) and chlorine (Cl); all are needed in low concentrations by plant proteins and enzymes essential for plant growth.

Nutrient sources (fertilisers) are generally marked with their available N:P:K ratio; you should select low nitrogen mixtures [NPK ratio 1:1:2 or 1:2:2] such as those suitable for tomatoes. Calcium and magnesium should be in the form of gypsum (for Ca) or crushed dolomitic limestone (for Ca and Mg). Never use chalk or garden lime because they are excessively alkaline. Trace elements are included in many general fertilisers (e.g. Vitax Q4), or in a form for compost mixing as ‘Frit 253A’. A suitable soluble fertiliser is Chempak Cactus & Orchid, which is 8:34:32 NPK plus minor nutrients (Mg, Fe, Mn, Cu, Zn, B, Mo). This can be watered on periodically at half strength during the growing season to maintain nutrient levels. Slow release fertiliser granules can also be incorporated in potting composts, but be wary of the total fertiliser quantity added.

Nutrient deficiency symptoms include chlorosis (yellowing from Mg deficiency), hardening of the growing point, split spines, or excessive corkiness and loss of vigour. The symptoms become evident as the compost becomes excessively alkaline through use of tap water or by salt build-up: repot severely damaged plants or use rainwater to correct the imbalance where the damage is slight.

Suggested compost mixes are shown below; use them as starting points and record your own experiences; also record your approach to watering and feeding. Trust me – you will not remember the details next year! The objective is to find compost recipes that suit your own plants and growing environment.

I hope that you have found this article helpful; you can find general advice in many popular cactus books, but a little research and conversations with other growers can enhance your knowledge. Check whether your plants come from limited habitats or have a wide distribution. Success in our hobby may seem elusive, but a little bit of science can help you a lot! Good growing.