First, a little ultra-simple plant biology. Plants maintain themselves and grow by taking in and processing nutrients, just as animals do. But the obvious difference is that (with few exceptions) plants can’t go get them, it has to come to them. One major portion of their needs is met when they absorb sunlight and take oxygen from the air. The rest must come from the soil.
Ready made soil mixtures are available from gardening sections at nurseries, in hardware stores, Wal-Mart and elsewhere. But just like convenience foods they’re more expensive and it can be difficult to tell exactly how much of each component they contain. For absolute beginners they’re a good temporary choice, though.
Sooner or later, the avid Bonsai enthusiast is going to want to mix his or her own soil. And, since bonsai are confined to a small pot most of the year, year in and year out, that soil will need to be supplemented and occasionally replaced. Make sure yours has the following attributes.
As with so many things about bonsai, apparently contradictory elements must be carefully balanced to maximize the health of the tree and create the desired appearance.
Though a normal, not a dwarf, species, the tree must be pruned to be kept small. Though growing toward the light, as most plants do, it must be wired and shaped to create the desired appearance. But nowhere do these competing elements need to be so precisely balanced as in the preparation of the soil.
Bonsai soil must be able to retain water well, since excessive drying is the easiest, and most common, way to kill a bonsai tree. Many so-called ‘mallsai’, bonsai bought at a store in the local mall, are nearly dead by the time they’re purchased since they don’t receive the proper amount of water and care.
Water is essential to life in itself, but it also acts as a vehicle to deliver nutrients through the roots. Humus, the organic components of soil that remain after decomposition of organisms, along with clay are the two major factors that help retain water and nutrients.
But the soil can’t be allowed to retain water too well. It has to provide good drainage. When too much moisture remains in the pot, whether through excess watering or compacted soil, root rot is almost inevitable.
Proper drainage is achieved, in part, by infusing the soil with small pieces of gravel. That helps create small spaces in the earth through which water can readily travel. Water then drains through the dirt, into the base and out the hole found in all bonsai pots.
Examine a commercially packaged quantity of bonsai soil and you’ll often see small white chunks. Those ‘aggregates’ as they’re called, help provide the soil with the correct amount of drainage.
In addition to allowing water to pass through, and not pool around the base to rot the roots, good draining soil allows for the easy passage of vital gases both in and out of the mixture.
Carbon dioxide, oxygen and nitrogen need to pass in and out of the plant and soil in order for photosynthesis and other essential biochemical processes to proceed properly.
Good soil will have all those health supporting physical characteristics.