Almost every guide to the proper care and feeding of carnivorous plants emphasizes, after using rainwater or distilled water, the proper soil mix. All of them tell enthusiasts that using standard potting mix will kill the plants in a matter of days, and many give recipes for a suitable general carnivore potting mix with a suitable acidity for healthy growth. As a general rule, one part sphagnum moss and one part sand is perfect, and some varieties work best with two parts sphagnum moss, one part sand, and one part shredded orchid bark. But what about the quality of all of these?
With the sphagnum moss and the bark, some authorities warn about making sure that these are of high quality. For instance, many retailers sell sphagnum moss with added fertilizer, which will kill your plants, or the sphagnum is cut with green moss, which has the same effect. Coir, which is shredded coconut hull, can be just as dangerous if not properly prepared, as many suppliers use coir that’s been soaked in or exposed to seawater, and the salt will, again, kill most carnivores. The quality of sphagnum moss can be checked by making sure that a bag or bundle specifically reads “PURE SPHAGNUM MOSS” and smelling the bag or bundle, as contaminated sphagnum moss tends to smell like old manure. Even salt-soaked coir can be used if it’s soaked and rinsed with rainwater, and then drained and dried. But how often does anyone check the quality of the sand they’re using?
Check most references on carnivores, and see what they have to say about sand. You’ll get recommendations not to use sea sand, because it might be contaminated with salt. You might get recommendations to use sharp or builder’s sand, which means all-silica sand. But are you sure that you’re using the right sand?
My question comes from personal experience from two years ago. The first thing I do every spring before my plants come out of dormancy is repot them, and the spring of 2006 was no different. My mistake was not to check the composition of that sand, trusting the label “Builder’s sand” on the bag, and that sand led to most of my plants dying before the end of the summer. True, the 2005-2006 drought and the summer’s heat wave contributed to the carnage, but the main cause was the quality of the sand.
The mistake I made was assuming that the sand being offered was pure silica. Pure silica sand is reasonably water-insoluble, which is why it settles into water instead of dissolving. Some will, but usually not enough to make a difference in horticulture. The problem is that most sand offered for sale can be and is contaminated with anything else that happened to be in the in the mix. If the sand was mined in an area that used to be a marine deposit, for instance, it can be full of shells. If the sand came from a river deposit, it may be full of limestone chunks or other carbonate rocks. All of these contaminants are very alkaline, which is enough of a problem to most carnivores save some purple pitchers (Sarracenia purpurea) or the Portuguese dewy pine (Drosophyllum lusitanicum). Anyone who’s mixed acids and alkalis in high school chemistry class has a pretty good idea of what happens when alkaline components of sands encounter highly acid sphagnum moss: the carbon dioxide emitted won’t hurt the plants at all, but the resultant salts will burn their roots.
Now, if you know for an absolute fact that your sand is pure silica, then feel free to use it as you see fit for any carnivore soil mix you choose. However, considering the problems I mentioned before, testing it beforehand is a good option, especially if the sand would otherwise be unusable. Carbonate-contaminated sand can be treated with acid, such as vinegar, to dissolve the carbonates, but it’s generally not worth the cost unless the sand has special colors that need to be preserved. The project at hand requires:
- a sample of the suspected sand
- a small bottle of common household acid (white vinegar is the easiest to obtain, but ascorbic acid, also known as Vitamin C, works well, too)
- a small container with a sealable cap, such as a single-serving yogurt container or a 35mm film canister
To start, get a handful of the sand and spread it out in your palm. Look for larger particulates within the sand and note the composition. If these pieces appear to be quartz, feldspar, or other components of granite, these bits should be safe. If the larger particulates include bits of limestone, sandstone, or shell, consider how many pieces you find and their size. If the sand contains lots of these chunks, don’t buy it or use it.
Secondly, take a small amount and put it into the container. Soak it down with the acid and watch the reaction. All-silica sand won’t react in the slightest to most household acids, and typical contaminants will spit and hiss a bit. If the sample produces lots of bubbling, particularly a froth, don’t use it, as the sand is far too alkaline to be safe for use with carnivores.
This test won’t work for other potential contaminants: for instance, it won’t test for salts. In that case, the only option is to wash it well with pure water.
If the tested sand is the only available option, and you literally have no other choices as to sources for sand, otherwise unsuitable sand may still be used. The first step is to sift the sand with a wire mesh trainer or riddle to remove large particulates. Afterwards, put the sand into a waterproof container such as a Rubbermaid tray or a plastic bucket and add vinegar or ascorbic acid, stirring repeatedly until the mix stops bubbling. Drain off excess liquid when it stops bubbling and add more acid; keep repeating until you get no further reaction whatsoever. Wash the sand well, and spread it out to dry.
Now, considering the work necessary to make that bad sand usable, see the advantage to using the right sand in the first place? More importantly, are you willing to risk the health of your carnivores on unsuitable sand?