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Shredded tires in your potting soil


photo by  MARILYN ROTHE´


AS WE ENTER THE NEXT MILLENNIUM, future generations of gardeners may be using potting soils made up of shredded rubber tires, coconut husk fibers and ground almond shells.

Traditional amendments such as peat moss, redwood compost, perlite and vermiculite are becoming more expensive and harder to find, so waste materials are being scrutinized for use, said University of California Davis Extension specialist Richard Evans at a recent workshop on potting soils. The four-hour event offered by the UC Extension last month attracted more than 20 professional growers and homegardeners.

Materials typically used in potting soils, such as peat moss, are no longer as abundant as they used to be, Evans said.

"There is still a lot of debate about the availability of peat moss. There are concerns in a lot of places that the bogs have been destroyed. It's becoming a scarcer commodity, and peat moss is the standard by which other amendments are judged," he explained.

Nearly 50 years ago the UC and Cornell University began tinkering with synthetic mixes in response to the nursery industry's growing demand for container soil. Prior to the invention of synthetic soils, potting mixes used to be composed largely of loam, some peat and sand.

Not only was the supply of loam becoming scarce at that time, native soils had to be pasteurized to destroy weeds' seeds and disease. In addition, some soils had salinity problems and there was always a lack of uniformity from mix to mix. The UC soils were a blessing to the nursery industry and home gardeners alike.

Peat moss, sand and redwood sawdust, in abundance at that time, were prime ingredients for a uniform, sterile soil. Sterilizing was not necessary and the soil could be used immediately after mixing. But like the loams of yesterday these amendments are becoming costly and scarce so the need for new materials is evident.

A substitute for peat moss incorporated in potting soils used by professional growers today is coir dust, fibers from coconut husks. Coir dust has physical properties similar to peat moss.

"It works pretty well. The main problem is it is relatively expensive, but it has nice properties," Evans said. Huge piles of it lie in waste in far away countries like Indonesia, so obtaining it is costly.

Pioneering researchers like Evans have been experimenting with the plethora of used rubber tires.

"It's experimental right now, but they (shredded tires) work pretty well. We grew chrysanthemums in pots and found that the tires had nice physical properties. Problem is, there is a lot of zinc in tires and zinc toxicity became evident," he said. "After a week the chrysanthemums had strange looking margins on the lower leaves." In growing the crop of chrysanthemums, Evans learned that zinc could be leached out of the tires, but then there is the problem of disposing of the zinc waste.

Other amendments on the horizon include ground, composted almond shells, which resemble coffee grounds and rice hulls.

At the workshop students learned the basics of making and evaluating potting soil. While students were introduced to the basic physical and chemical properties of a potting mix, Evans also dispelled myths that persist with gardeners today; for instance, the notion that vitamin B solutions administered to plants will reduce transplant shock. "I've never seen any evidence that it works," Evans said.

Another gimmick on the garden market are hydrogels, moisture holding gels that, when added to potting soil, are supposed to retain water and release it slowly, allowing the gardener to water less often. "I wouldn't use them," Evans said.

"Hydrogels will swell up and hold water, but will also swell up with minerals and nutrients. They cost more than using organic matter and have no real effect whatsoever. Personally I've never found them beneficial," he said.

Some gardeners, in order to improve drainage in large containers, fill the bottom of the container with a layer of rocks or clay pot shards to improve drainage. "Use a porous potting soil; don't put stuff in the bottom the container to increase drainage," Evans advised.

Students learned that the ideal potting soil is one employing the proper portions of organic amendment to form a light, fast draining medium. Such a potting also retains adequate moisture to meet a plant's needs.

Here is an adaptation the the UC soil mix that will make one cubic foot of potting soil. Using a three-pound coffee can for a measuring device, mix:

3 cans peat moss

3 cans sharp sand

3 cans redwood compost

After thoroughly mixing these ingredients add five ounces dolomite lime and five ounces of 5-10-5 fertilizer. Mix thoroughly.


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