Cactus Soil Drainage Tests 

It has a been a long journey to find a well performing cactus soil. Spike & Bloom was born out of a love for these amazing spiky plants, but also out of major frustration with the cacti soil options at stores… or lack there of to be honest.

Typical Cactus Soil

It is really difficult to find a decent cactus soil. You can completely forget about finding a proper cactus soil at your big box store. Stores such as Lowes and Home Depot, etc. only contain the biggest brands which ironically make the worst possible soils for your spiky friends.

Did you know that practically all of the big brands of soil are mostly peat moss. Yep that moisture retaining soil ingredient that constitutes your average potting soil. Putting aside the questionable ecological ethics of peat moss, we still wouldn’t recommend much peat in a proper cactus soil mix.

Peat moss is used exclusively by large soil manufacturers to lock in the moisture. But peat moss is not so easily renewable.

Why would we want to add something to our cactus soil that both underperforms and is not the most ecologically responsible? Surely we could do better than that we thought?

So if peat moss is not the best choice for a cactus plant’s health what is?

What Make a Good Cactus Soil?

It’s important to note that cacti are indigenous to many parts of the earth. This means that there is not a single perfect cactus soil for 100% of all species of cacti. There will aways be a few outlier plants that require special treatment.

But, there are many overlapping needs by plants and most cactus that you are likely to have in your home will have some basic needs.

For starters, a good cactus soil should be well draining. This means that much of the water should drain through the soil and out the bottom of the pot. This automatically disqualifies those horrible “potting soil” mixtures that the big brands manufacture.

Even if they state that it it “cactus mix”, don’t believe them. Sure you can stick a cactus in that soil, but it won’t be happy.

Cacti in Nature

The typical cactus that you would have growing in your home is indigenous to warmer, drier regions of the world. In these regions the soil is not anything like a typical potting soil.

Most cactus soil in nature is a bit rocky and sandy. It doesn’t contain a large amount of organic matter and nothing even close to resembling peat moss. Contrast this with the dark brown garden soil that might be in your backyard. This is not what most cacti expect to dip their roots into!

Just remember that we are generalizing a bit. Of course there are some outlier cases for cacti!

Cactus Soil Drainage Tests

When we first discovered that we needed to test our soils for moisture retention we had a few options and many questions.

At first we conceived that we could use a moisture meter and test the cactus soil that way.
For starters we didn’t have a reliable moisture meter, but more importantly we realized that the moisture could and would vary wildly depending on many factors. Factors such as how long the soil remained in contact with the water could alter the results dramatically. The depth at which you test the soil also matters.

We eventually settled on a simple yet reliable method of measuring the drainage rate of each soil we tested. If we new how much water a soil drained we could easily gauge how moist a soil was.

So with a reliable plan we got to work!

We started off by taking well known soils and measuring their drainage rates. Most of the off-the-shelf cactus soils were really high in peat moss content and didn’t even absorb water with their first contact with water. Because peat moss is hydrophobic the water typically runs right through the soil with the first watering. In subsequent waterings the peat holds on to almost all of the water.

For this reason we introduced a bit of a “hold time” with our water retention testing.

Not that we wanted to be using peat moss in any way but we did want a way to reliably compare various soils as a starting point. At that time, we had no idea what our soil would consist of.

Peat most was very unreliable. With a single watering it drained nearly 100% of the water and once it was already wet it drained nearly 0%. This surely makes working with peat moss unreliable and confusing for the typical cactus plant owner.

Our overall focus was to eventually design a cactus soil mixture that was easy to use, allow a predictable drainage amount and be close the the native soil of these plants. This immediately disqualified peat moss as a main ingredient for a good cactus soil.

We measured sample soils by volume to keep things consistent. We also measured the amount of water to be added by volume as well.

Please dear reader, appreciate the fact that we worked on this for months!

We settled on using 200ml of soil, aggregates, or solids and 200ml of water.

We put a tiny screen in the bottom of the funnel so no soil could fall out but water could easily pass through the soil and out of the bottom of the funnel.

We stopped up the bottom of the funnel with a finger and added the water. Next we let the water sit in the soil for a full 2 minutes. Finally we let the water drain out and measured how much water drained.

There is of course a nuance to most things but that is the gist of what we did. We also did additional tests to see how the various soils drained after they were pre-wet from the first experiment.

These experiments were an eye opening experience for sure.

Not only did we test all the major soil mixtures, but we tested just about every raw ingredient individually used in horticulture… sands, rocks, organic materials, peat, coco coir, lava rocks, the list seemed endless.

Some of the results were surprising.

I never could have anticipated just how much water vermiculate absorbs. Prior to the experiments I would have thought that vermiculite would have drained nearly 100% of the water immediately.

Nope.

Our soil tests revealed that vermiculite held onto 23% of the water! That’s right, vermiculite, that light, airy soil additive only drained 77% of the water added to it.

Things were getting interesting.

Not only did some of the soil ingredients shock us in their ability to drain water but the combinations of materials shocked us too. It turns out that 2 high draining materials could actually drain less water when used together as opposed to separately! This had much to do with the particle sizes of the materials being tested.

After many, many months of testing soil ingredients we arrived at what we believe is a superior cactus soil. It reliably drains well and has just the right amount of organic matter and acidy to keep the typical cactus plant extremely healthy and happy!