Which soil to use is a very large topic, and in this post I will mainly discuss soil in terms of sowing. I will, however, do a post fairly soon going into more detail about various soil components and soil types for growing cacti after the seedling stage.
Cacti seeds in general do not need any particular soil or compost to germinate, but it is wise to plan ahead a bit. After all, most cacti seedlings grow fairly slowly, and depending on how fast you grow them (and also the species) they may stay in the soil you sow them in for several months to several years. Thus it is helpful for the seedlings if the soil is of a good quality.
Soils for sowing and for general growing too, can be either organic based or completely inorganic. When it comes to sowing it is important that the seedlings never dry out completely and it’s perhaps easiest to achieve this with an organic based soil. How much organic matter to use is a matter of preference, though personally I never use more than 50 % organic matter in any soil (unless it’s required by some particular species).
The remaining parts by volume of such an organic based soil should be made up of material that will help drainage and aeration of the soil. These are both very important factors for the successful growing of cacti and succulents. To the organic matter should therefore be added sand/grit/perlite or similar substances. It is not particularly important to choose one thing or another – the main objective is to make the soil well-drained and well-aerated. Personally I like to add material like perlite and/or pumice which are light-weight, helps aeration and drainage, and also retains a lot of water in its pores which are then slowly released into the soil as it dries out helping to keep an even level of moisture for a longer time.
Grain size should be between 1-5 mm, and a mix of this range will do fine. If you add say, 1/3 (medium/coarse) sand, 1/3 (fine) gravel, and 1/3 perlite to the organic matter you should get a good well-drained soil that is light, loose, and not too coarse. It is not critical to use the specific materials above in the specific amounts. You could easily use only perlite or fine gravel, or coarse sand to add to the organic matter. If you have access to good quality sandy loam where you live, then that is a perfectly good material to add to the organic matter. The soil is ready to use when you can grab a handful of the (dry) soil-mix, squeeze it in your hand and have it crumble easily apart as you open your hand. If it does not crumble easily, simply add more drainage material.
|Organic based soil with coir. The white particles are perlite (ca. 3-5 mm wide).|
If you don’t want to make a soil-mix of your own, it is certainly possible to use the kind of ready made cactus sowing/growing mix you find in flower stores. Depending on where you live these may be of a good quality or a very poor one. In Norway, at least, they mainly contain peat (sphagnum moss) and some sand, as well as sometimes a little bit of perlite. Seeds will germinate just fine in this, and many species will grow fine in it too. However, personally I moved away from using peat a few years ago. I’ve never been a fan of it but in Norway it is difficult to find suitable alternatives. Leaf mold or composts are not usually sold in flower stores (on account of peat being cheaper to produce and hence more profitable). Some nurseries and garden centres do still produce and sell composts, so it’s not impossible to get hold of, but one has to work a bit to find them.
There are many reasons why I dislike peat. It is a magnet for the little black sciara fly which lays eggs in it that develop into larvae that then eat into the roots and flesh of the plants causing rot and subsequent plant death. It takes a long time to dry out completely, and is almost impossible to re-wet once it has done so – the first of which is a problem for a great number of species, while the second is obviously a problem when you want to water the plant again (particularly after the long winter rest). It also breaks down fairly quickly and in doing so it becomes compacted and squeezes air out of the soil, which is a problem for cacti. Peat has a tendency to cling tightly to the roots, so when it comes to repotting it is difficult to remove all the peat without damaging the (often) tender roots. The companies making the peat-based products often add nitrogen-rich fertiliser as well, which is a problem not just because you don’t want your cacti to be given too much fertiliser, but especially because too much nitrogen will quickly lead to abnormal growth and/or root burn in cacti.
Having said that, most cacti will grow acceptably well in a peat based soil so long as you make sure to add plenty of grit, sand, perlite or something similar that will keep the soil from becoming too compacted and allow good drainage and aeration. Be prepared to lose plants unexpectedly though!
For my part I have gone over to using coir instead of peat as the organic component of my potting mix. Coir is the hairs from the husks of coconuts, and a by-product of the coconut industry. It’s inert and contains next to no nutrients, so fertilising is required. It fairly quickly dries out in a pot and is easily re-wetted. This also means it is not a magnet for sciara flies like peat. It does often come already fertilised though, so that is something to be aware of. It does not compact the way peat does or cling to the roots. As an important aside, coir is an environmentally friendly product bringing income to many poor people in developing countries, whereas peat is not. The excavation of peat leads to the destruction of bogs which are an important habitat for a great number of plants, animals and insects, is non-renewable (short-term), and also leads to increased CO2-emissions. I will not advise anyone to use peat if it’s at all possible to use other growing material.
Though I like coir, if I can find a good supplier of high quality compost or leaf mold in the future I will probably switch to that. Being inert, coir does have certain advantages, but for species who enjoy a good bit of organic content in their growing medium I think a good compost or leaf mold is preferable. The fact that using coir requires the use of fertiliser may also be a negative for some. In fact, for a sizeable collection of plants I would perhaps not recommend coir as warmly. The necessary increased use of fertiliser leads to quicker build-up of salts in the soil which in turn means that more frequent repottings are required. In a large collection this is time consuming and depending on the soil of choice, may be expensive too.
I have also sown in 100 % inorganic soil which has many benefits compared with an organic based soil, but also some drawbacks. Perhaps the biggest benefit in the short term is that an inorganic soil is not a very friendly environment for fungi which can quickly and easily decimate a pot or ten of nice and plump little seedlings. Nor does algae thrive (as well) in this environment. The biggest drawback in the short term is that it is more difficult to keep the soil from drying out completely (after you remove the pots from their moist atmospheres under plastic), which is a very important thing for the development of healthy seedlings.
|100 % inorganic soil consisting of many different components (see text further down).
The white particles (perlite) is 3-5 mm wide, for scale.
In the longer term another drawback is that the seedlings grow more slowly in an inorganic soil. This may or may not be an issue, but because the soil much more easily dries out the seedlings require more frequent waterings and one has to remember to do this. However, the seedlings are much less at risk from rot in an inorganic soil, so frequent waterings are less of an issue than with organic soils. And because they are less at risk of rot this kind of soil is not a bad option at all for the more difficult species. After the seedlings I sow in inorganic soil come out of their humid enclosed atmosphere I very rarely suffer deaths from rot. Of probably more than 200 seedlings grown this way I can probably count on one hand the number of plants that have died over the previous year (including genera such as Geohintonia, Pelecyphora, Epithelantha, some rot prone Mammilarias and other difficult species).
The only exception is Sclerocactus, Pediocactus and the other particularly difficult species from the USA. But then again, they seem to die at the drop of a hat no matter what you do. Still, I believe those genera are certainly prime candidates for sowing in 100 % inorganic soil.
The inorganic soil I use is a mixture of sand, gravel, perlite, vermiculite, crushed brick, clay, pumice, zeolite, volcanic gravel, expanded clay, expanded shale and diatomaceous earth. I don’t necessarily use all those things together, but I like the soil to be a mix of many different substances and for it to offer the plant all the macro and micro nutrients it needs. As with the organic soil mixture, I am doing a bit of experimenting with all this, so if you want to try an inorganic soil it is not necessary to use all the above components. Indeed, you could perfectly well use only crushed rocks of various kinds as the sowing material (obviously crushed to a fine grade, though).
That’s it for the soil part. As I mentioned at the beginning of this post, I will make another entry a little later going into more detail about soil components.
Now for discussing germination!