The hows and whys of bonsai soil. Fast, easy, and cheap.
Bonsai trees require a growing medium tailored to keep the trees healthy without causing rampant growth. Remember, we’re not in the timber business. The ideal soil will retain enough moisture for one to two days, provide the proper nutrients, and minimize critters. Everybody you talk to will have the perfect mix, none of which are the same.
My basic mix consists of equal parts, by volume, of drainage material, moisture retention material, and organic material. This meets the criteria listed above. I buy my components in bulk. If you don’t need much just go to Lowe's or HomeDepot and get a bag of cactus mix, add some organic material (we’ll talk about that later) and you’re set to go.
For drainage I use fine poultry grit. This stuff is available at feed stores, farmer’s elevators, or my favorite boutique, Amazon. Twenty five pounds will cost you about $15. You can use any hard, inert, item like small pea gravel, large sand, etc. This material is to keep the soil from getting soggy.
For moisture retention I use small hard fired clay granules. Ideally I use fine Turface if I can get it. This is a product used on golf courses and ball fields available from golf course supply companies. Other products include granular Oil Dry from auto parts stores, or even, cheap fired clay kitty litter. Get the kind that has no additives, no odor control, and is non-clumping. You’ll need to test it by putting a handful in a glass of water for a few days to make sure it doesn’t dissolve or get mushy. Use the stuff right out of the bag so you don’t have to fight your cat for it. The cheapest generic brand from the grocery store usually meets the grade. Depending on which you pick, It’ll run from $3 - 15 per bag. This material absorbs water and releases it as needed by the tree, like a sponge.
The organic material I use is small orchid bark, coarse milled sphagnum moss, or composted bark mulch (last choice). Use something about the size of matchheads. The purpose of this is to provide a buffer when fertilizing. Also, to provide natural nutrients as it decomposes.
If you use commercial cactus soil or rock garden soil you’ll have to mix in organic material since these soils have little or none. You can usually get small bags of orchid bark from the same place you got the soil.
When I mix soil I throw in about a teaspoon of Milorganite (an organic fertilizer, HomeDepot again) per cup of soil. This is a slow release fertilizer that gets trees off to a good start.
Many books recommend sifting soil components to get rid of the very fine particles. I don’t. I tried it and all that happened is that I got dusty and dirty. My trees don’t seem to care.
Okay, we’ve got a basic mix. Now let’s tweak it. You’ll adjust it for climate, watering habits, and tree species. I moved from Minnesota to North Carolina. Now I’m hot and wet instead of cold and dry, so I bump up the clay component to absorb more moisture (for hot) and leave the drainage the same (to take care of the wet). For the species, I look at the typical habitat. Pines usually grow in dry ground, so I bump up the drainage. Larches and bald cypress grow in, or near, water so I almost eliminate the drainage component.
Ideally, you soil should retain moisture for a day or two. Your watering habit has to account for this. Let’s say your tree is due for water in 24 hours, but, the moisture in the pot will start running out in 18 hours. Obviously, it would be better to routinely water in the morning so your tree runs out at 4:00am instead of 4:00pm. I’m retired, I no longer get up at the crack of dawn. I choose to water in the afternoon so I bump up the clay component.
I can’t give you a hard and fast schedule. Here’s how to tell if the tree needs water. Dig down in a corner of the pot. If the soil is dry a half inch down then you’ll need to water. You’ll soon figure out if your soil holds too much or too little water, so adjust accordingly.
I've been doing bonsai since the late 70's. I'm passing on what I've learned in those 35+ years. These are real life techniques and tips that worked for me, but, may not for you. Remember, this is free advice and worth every cent of it.