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Using Perlite in Potted Plants

Original Report by: Bruce Schundler

From an address by George (Doc) and Kay Abraham, to the Perlite Institute, 27 April, 1979

Before discussing various tests that have been conducted, we have to explain why perlite is such an important soil component in today's container grown and potted plants.

Water is a nutrient. A plant is 90% or more water but it also needs oxygen. Roots supply water and oxygen to plants (perhaps 98% of the oxygen a plant uses comes through its roots. Since both water and air (oxygen) move in, through, and out of soil mixes, both water retention and drainage, and aeration are important considerations in any potting mix.

If the soil is poorly drained, or packed due to fine organic particles, oxygen is cut off from the plant. Also, whenever a plant is watered, it forces air out through the bottom of the pot or it bubbles up to the surface. In a loose, porous soil this is fine because it creates a suction effect which draws in fresh air from the top into the air spaces in the soil.

However, if soil is too moist, or lacks proper drainage, oxygen is excluded and roots gasp for air. Also, a lack of oxygen favors high concentrations of carbon dioxide (a waste product), cutting down on root growth, and certain soil organisms, which like high CO2 concentrations, grow. In the stagnant soil these organisms produce toxic chemicals, which have a disastrous effect on the health of plants. Many gardeners discover this first-hand when they set out container-grown plants and often note that the bottom one-inch is often stagnant and smelly.

This coupled with a low pH (that is, high acidity) and a toxic form of ammonia (NH3) helps to kill plants. The role of perlite particles in a soil mix allows air spaces to form where oxygen and water have free movement.

While most growers seem to prefer coarse grades of perlite, an educational effort is needed to share with them how medium and finer grades of perlite are just as effective in preventing oxygen starvation.

General Observations about Perlite

In general, after years of testing and experimentation, several observations can be made about perlite and its use in potted plants:

1-Plots and pots containing fine, medium, and coarse perlite have had exactly the same weight and size as those using traditional peat mixes. Many tests have proved there should be no hesitation in using finer grades, and that 100% perlite could be used and be just as successful as traditional peat mixes.

2-Tests on the effects of various grades of perlite on the rooting of cuttings and in the germination of seeds found that in all cases, regardless of size, rootings were the same in time, size, and quality. In fact, finer grades required less water to maintain healthy roots. In seed germination, however, coarse grades of perlite did not do as well as finer grades.

3-Perlite also was tested for use in drying out flowers. Flowers were placed in pans of fine perlite and covered lightly. After standing for 3-6 days (or long enough for drying to occur), the flowers were removed and dusted off. Flowers also can be dried using fine perlite and a microwave (approximately 3-5 minutes).

4-100% perlite has been used growing orchids. The fine and medium grades have been tried, but coarse perlite did the best.

Conclusions

  1. Perlite is one of nature's best media for growing plants. It does not appear to make any difference which grade is used except with certain plants like orchids.
  2. It is possible to grow most plants in perlite alone, although usually the finer grades and medium grades will work better and require less water.
  3. Seeds can be started in any grade of perlite, but with smaller seeds, finer grades of perlite would be recommended.
  4. Perlite is good for greenhouse benches. And as an added benefit, insects and snails do not like perlite!
  5. Perlite (especially the fine grade) is excellent for drying flowers.
  6. Perlite is ideal for outdoor containers. They can be moved around easily because perlite in the mix lightens it, besides improving drainage.

Hopefully the summation of this presentation has been helpful. If you have any questions, or need more information, please contact the Perlite Institute or your local perlite supplier.

Information given herein is from sources considered reliable, but no guarantee of accuracy can be made or liability assumed.  Your supplier may be able to provide you with more precise data.  Certain compositions or processes involving perlite may be the subject of patents.
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