The Scoop on Soils, Part 2

DeGroot presentation summarized by Anne Spencer

Nutrient Availability
Soils make nutrients available to plants, and some kinds of particles are better at it than others. Cation (cat-eye-on) Exchange Capacity, or CEC, is a way to measure how many nutrients (fertilizers) are readily held for plants to use. Organic matter (compost) has a high CEC. Inorganic material such as fired clays, sand, akadama, lava, pumice, etc. have very low CEC. Unfired clay and bark have a moderate CEC.

What all this really means to the bonsai grower is that the lower the CEC of your mix, the more often and regularly one must fertilize. A high CEC buffers the soil by evening out the pH and holding fertilizer. This might sound ideal, but for the bonsai grower it means you can't carefully control the amount of fertilizer (which translates to growth) available to your plants. A more inert soil (low CEC) allows you to apply what you need, when you need it. Remember, Dave's bonsai soil mix is 40% bark (organic, moderate CEC), and 60% sand, red lava, and fired clay (non-organic, low CEC). This has some fertilizers, but not too much.

Organic Composts
Dr. Olaf Robero, a famous plant pathologist, was quoted as saying that not all composts are created equally. The best type is composted hardwood bark; next best is aged pine bark; then aged fir bark. Do not use fresh redwood or cedar barks because their tannins make them too toxic for use as soil. Don't use fresh sawdust, either, because it breaks down quickly and steals nitrogen, and you'll have pH problems. One of the worst things you can use is peat moss. It holds too much water and is hard to rewet when dry. Oak bark and rose wood bark would be great.

pH balance
Dave also spent some time talking about organic fertilizers. Organics must be reduced by microbial activity to the same form as chemical fertilizers before the plant can use them. (Sort of like grinding your coffee beans before you make coffee) He feels organic fertilizers like fish emulsion are good, but has had bad luck with Japanese style fertilizer balls. They don't seem to break down fast enough in our Northwest climate and get maggots and mold. Use a timed-release fertilizer, such as Osmocote, or be sure to
fertilize frequently depending on how well your soil holds fertilizer.

Emphasis was placed on using a fairly low nitrogen fertilizer in the spring to avoid a large amount of coarse vegetative growth. The following fertilizers are the ones he uses. Remember that, as curator at Pacific Rim, his goal in fertilizing is to keep his bonsai attractive, with short internodes. It is for maintenance rather than for growing young plants big:

10-30-20 Peters Blossom Booster: bonsai need extra phosphorous from March to the end of May.

0-20-20 or 10-3-10 June to Fall for maintenance (can have more nitrogen)

4-25-35 Peters Conifer Finisher for Fall, or 0-10-10 which helps prepare for winter. Stay away from nitrogen.

0-0-0 (nothing) Nov.- Feb. (no fertilizers)

Dave emphasized that we shouldn't necessarily use the fertilizers or soils he uses. Decide what to use based on your soil, watering habits, etc. If you have the proper pH, you should get good results. He also agreed that a high rate of microbial activity in the soil helps prevent disease. He spoke briefly about mycorrhiza, the types, their value to plants, and the tests currently being conducted to learn more about it. (All good subjects for future articles!)

When you put a bonsai in a pot you automatically place it in a difficult environment. How you manage the tree in this difficult environment is crucial. Before you can make artistically beautiful bonsai, you have to master the horticultural skills to keep your bonsai alive. The soil type, texture, and size, the pots depths, the fertilizing, the watering, the environment, and type of plants you are growing are all interrelated. The challenge is to coordinate them all so it works.