Grouppe Kurosawa: Organic Gardening

Potting Soil

Potting soils or mixes were never intended to replace topsoil as a growth medium for plants. Unfortunately, over the years, potting mixes have been equated to “dirt” and this misconception has led to countless failures in growing plants, vegetables, and especially bulbs. In general, potting mixes are fillers that are designed to retain water and “fill in the holes” when transplanting plants. Most have little or no actual nutritive value for plants. Potting soils can be placed in three general categories.

Basic: This type of potting soil contains sand, perlite, or vermiculite and so-called forest products. You can translate forest products to mean bark and partially decomposed wood. Wood is very difficult to degrade because it contains ligno-cellulose. In the environment, ligno-cellulose is degraded very slowly by fungi. The chemical association between lignin and cellulose adds strength to a plant’s stem or stalk. Wood is a classic example. In order to make paper, which consists only of cellulose fibers, paper companies treat the wood chips with sulfuric acid in order to break the lignin-cellulose bonds. All plants contain lignin to some extent, especially those that require a strong stem. These plants include rice, wheat, barley, and oats. Corn stalks also contain a substantial amount of lignin. In the absence of lignin, these plants would fall over after they reached a certain size. Cellulose alone cannot support the plant’s weight. Ligno-cellulose waste is very plentiful and is a major component of potting mixes.

As an experiment, we purchased two 10-12 inch tomato plants from a nursery, removed the plants and washed the roots to remove the soil and associated microorganisms. These so-called bare root tomato plants died within three weeks when planted into basic potting soil. If the roots are not washed, the viability of the plants is strictly dependent on the amount of good soil that is transplanted with the plants. Bulbs will not grow and flower in basic potting soil. They may germinate and grow a few inches, (many bulbs will sprout in a storage box if they receive the appropriate moisture), but that is about all. Milly learned this the hard way a few years ago when she and her daughter, Carole Dreier, split an order of prized Dahlia bulbs. Carole is an organic gardener and her Dahlias grew magnificently. The flowers were the size of dinner plates. Unfortunately, Milly planted her Dahlias in a basic potting soil. Milly no longer makes this kind of mistake.

Intermediate: Intermediate potting soils include a diversity of ingredients, including humus, fir bark, worm casings, peat moss, chicken manure, bat guano, kelp, kelp meal, and oyster shell. Plants will definitely grow in this artificial soil, but not rapidly or vigorously. The urea in the chicken manure and bat guano adds substantially to the ability of this potting mix to support plant growth. Urea is a natural nitrogen source that is the “miracle” component in the fertilizer Miracle Gro. The urea in Miracle Gro is chemically derived. Miracle-Gro isn’t an organic fertilizer.

Advanced: These soils are spiked with trace amounts of inorganic nitrates, phosphorus and potash in order to support plant growth. The other components are usually trash, such as fir bark, peat moss, sand, and vermiculite or perlite. These soils also are not organic fertilizers.

Don’t forget that these potting mixes are usually pasteurized or heat-treated in order to kill unwanted seeds and microorganisms. As such, and in the absence of soluble minerals or organic nitrogens, such as urea, these so-called soils will not, by themselves, support plant growth. Something in the soil has to breakdown the manure, kelp meal, etc. into simple compounds that the plants can use. We’ve had numerous arguments with people over the quality of potting mixes (“they worked great for me”), because everyone forgets that they mixed the soil from a potted plant with the potting mix when they transplanted. Since the plant’s soil contains microorganisms, gardeners are “innoculating” the potting mix with beneficial microorganisms. In cases where the potting mix contains only bark and other ligno-cellulose waste, there isn’t enough high quality organic matter present to support plant growth. When intermediate potting mixes are used, plant growth will be enhanced to the extent that the potting mix contains ingredients that can be degraded by microorganisms into simpler molecules that plants can use. Plants are wonderful organisms, but they really are stupid. They can’t breakdown organic matter into simpler elements themselves. They are totally dependent on soil microorganisms for processing organic matter. Of course, plants grow wonderfully in hydroponic or all water environments as long as the fertilizer is strictly simple inorganic minerals that the plants can readily assimilate into protein. When the “feedstock” consists of complex organic molecules, as one would expect to find in nature, microorganisms must be present in the feedstock.

We hope this information ends the “transplantation horror” so many hard working gardeners experience. When in doubt, use compost, if available, or find some good topsoil somewhere. For added benefit, make some Bug Juice and use it when transplanting all plants and trees.