It is repotting season and for the past few years, you may have acquired your bonsai soil from vendors, the kindness of others, or from the club stash. But now you may have more trees than ever and handouts of those bags of premixed soil are getting more expensive and you’re feeling guilty about hitting up your bud for 10 gallons of soil. Is now the time to attempt your own mix? Sure! It’s actually quite easy, but some planning is necessary.

First of all, you need to determine what your soil components will be. There is a wide variety of components and each provides different characteristics to suit your local environmental conditions. For the Bay Area, we require a free draining mix for our wet (usually) winters but enough water retention to handle those dry windy days of late spring and autumn. A good basic mix is equal parts pumice, lava rock, and akadama (a hardened clay medium exported from Japan and available from many vendors).

Next, you will need to round up your soil components. There are a couple suppliers in the area. For small amounts, I use American Soil Products in Berkeley. They sell large bags (2 cu ft) of cleaned pumice and lave rock for around 5 dollars per bag. The bags easily fit in the trunk of your car! Akadama can be purchased at any of the local shows or fundraisers where bonsai vendors are present (see the Mammoth Fundraiser for the Bonsai Collection at Lake Merritt). It is available in three sizes (small, medium, and large). A bag of the medium size is ideal because it contains a great mix of small to medium pieces due to breakdown. Akadama is the most expensive component at >$25/ bag. Cheaper alternatives are available such as pine/redwood bark and turfice. The akadama is the water retention component in your mix, so its use is your own personal choice (see numerous discussions on web blogs/chat rooms/internet forums for pro/con arguments).

Next, you will need a few supplies and tools for preparing your soil components.
1. A roll of plastic sheeting (plastic drop cloth etc)
2. Screens/sifters of various sizes
3. Soil scoops
4. Buckets for storing your prepared components and soil.

The pumice, lava rock, and akadama you purchase will be full of dust and fines. These must be removed. But first, the lava rock and pumice will most likely be wet. So, you need to dry them before sifting out the dust. I use sheets of plastic spread out on my patio. Both the pumice and live rock take days to dry completely. So, spread each bag over a sheet of plastic in a dry sunny area. Every other day you may want to redistribute the rocks so they are completely dry. Sometimes I will wash the gravel a few times in a bucket to remove most of the dust, then spread it out on the plastic.

After your material has dried, you must separate according to size. For small amounts of soil, you can use a simple hoop sifter available from most bonsai suppliers ($15). These sifters usually include three different interchangeable screens (2/16”, 3/16”, 5/16” sizes). Most people start with the smallest screen to remove the dust and fines. I begin with the largest screen to remove he largest pieces first and then move on to the smaller sizes. Each time, I move the sifted material into a bucket for storage. You will require many containers for storing soil components. At the the end of the process you will have three sizes of lava rock, pumice, and akadama. Plus, you will need individual buckets for mixing and holding your different soil mixes. You may wish to buy Rubbermaid containers, but a better alternative exists. If you have a cat or know a cat owner, make sure to buy your kitty litter in plastic tubs. They have re-sealable lids and are stackable. They are the perfect size for keeping your mixes.

So, now that you have all your components, it’s time to mix. 1 part of each component is ideal. I make two mixes, a 1/4” mix for, shohin trees and top dressing, and a 1/2” mix for my larger trees. The largest pieces of pumice and lava rock can be used as the drainage layers during repotting or for filling space in large growing containers. I even make a 3/4” mix for my largest trees that require a lot of a drainage.

Even though you have sifted your soil throughout the process, you should always sift your soil mix prior to repotting. Dust always accumulates and can impede drainage in your bonsai pots.

So, Yes, I’ll admit that preparing your own soil is a lot of work, but it is very satisfying once it is all mixed and your trees are all soaking their feet in it. Don’t be scared and give it a try.

Lawrence LeClaire

Tags: Beginner, Repotting, soil