Reuse or Recycle Potting Soil

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Reusing potting soil or bedding plant soil from a previous year is generally a bad idea and may ultimately turn out to be false economy. If access to fresh soil is limited, few choices other than the reuse of potting soil may present themselves. Whenever possible however, the best choice is to choose fresh, rich soil for potting and starting bedding plants.

Minerals and Nutrients

Used soil has diminished nutrient value, less organic matter, and in the extreme may be little better than barren soil. The nutrients have been used up and and the physical characteristics of the soil are degraded by the plants previously grown in it.

Other Problems

Used soil may inherit problems from the previous season and from careless storage over winter. Unwelcome gifts may have been left in the soil by pets, overwintering insects, and weed seeds. It may contain nematodes, insect eggs, mites, mold, fungus, bacteria, and other undesirable elements such as viruses which can attack, destroy, and devour tender new seedlings or the roots of newly potted plants.

Used Soil Remedies are not Always Successful

It is often suggested to bake soil at 350 degrees F. to kill off insects and egg casings. Heat does kill weed seeds, insects, and weed roots, but it also damages the soil.

Although baking the soil at 350 degrees will sterilize the soil, it will also virtually burn and destroy all organic matter in the soil and reduce it to the equivalent of ash and mineral soil with no organic elements remaining.

Soil treated in this manner will lose tilth, the friable, spongy ability to compress, expand, and hold moisture. It will tend to dry out quickly and pack hard. In the extreme, soil without organic content will dry out to the extent of becoming hydrophobic, and will not absorb water. Sterile potting soil is unproductive and unfriendly to new plant roots unless completely rejuvenated with a new addition of garden soil and organic matter. Remediation with peat moss, sawdust, compost, and other organic material such as natural animal fertilizers are necessary to replace the organic content that was destroyed by the severe heating process.

A much better procedure utilizes steam to kill insects and destroy any insect egg casings it may contain. Place the soil in a container large enough to hold a second, smaller open container of water. Cover tightly and place it in the oven. Remember, water boils at 212 degrees F, and much hotter steam is formed.

Bake the soil at 220 degrees F for two hours. The formation of steam in the humid, closed atmosphere will effectively sterilize and destroy all living bugs, insect eggs, fungus and molds without burning up the essential organic matter.

When the soil is cooled, add an extra 10% peat moss to it, enrich it with compost, and add 2% by weight of healthy garden soil. These additions restore the natural flora and fauna to the potting soil necessary in nature for good plant health.

Even sterilized and rejuvenated, it is a questionable practice to use the soil for the same type of plants, since they will demand the same essential nutrients, and any species-adapted plant viruses are very difficult to destroy. It is logical and preferable—and probably a reasonable idea to reuse potting soil to pot or grow different species of plants only – if at all. Use some new potting soil instead. Toss the old potting soil into the compost instead, and rebuild it naturally for the best outcome with your potted and bedding plants.

Potting Soil

Potting Soil (Photo:

Understanding Soil and Soil Components

Soil is an ecosystem with biotic and abiotic elements co-existing in harmony. It is important to understand soil and its components in order to tap into the richness of the plant community. Take a look at the important soil components that we often take for granted, with a glimpse at the structure of the soil layers.
Soil is a living entity. It is an ecosystem in itself with biotic and abiotic elements working and living together in harmony. Soil may be termed ‘rich’ when it is rich in the essential soil elements, which in turn proves favorable to plant growth.

Soil may be divided into two sections. They are soil layers and soil components.

Soil Layers

The soil has three distinct layers, viz. the topsoil, subsoil and bedrock.

Top soil is the living part of the soil, very fertile and rich in organic matter, minerals, air and water. The bacteria in this layer convert minerals and organic matter into plant food. In chalky soil, this layer is approximately 5 cm deep, while in rich loamy soil it reaches to a depth of even 1 meter. The shallower the top soil, the less fertile it is. In the process of soil preparation, you should never bury the top soil under the subsoil.

The subsoil is the layer below the topsoil. It is lighter in color and lacks humus, water and air. During soil preparation, do not raise the subsoil above the topsoil.

Bedrock is the parent material of the soil present in a particular patch of land, is below the subsoil, and is made up of larger soil material like stones and rocks. It is a rich mineral base.

Soil components

There are four basic soil components, viz., organic matter, minerals, air and water.

Organic Matter

Organic matter are living organisms, dead organic matter and humus. The living organisms may be microscopic like eelworms, bacteria and fungi, while the macro-organisms may be insects, earthworms, etc. Moles are a good example of macro-organisms.

Dead organic matter are materials or remains from once-upon-a-time living things in the soil. Examples are dead roots, leaves, insects, twigs, etc. It serves as a base for humus formation and bacterial activity. Totally decomposed dead organic matter results in humus.

Plant and animal remains decomposed in the soil with a horde of living and dead bacteria form the humus. Bacteria and other micro-organisms are responsible for this decay. Humus may be described as a dark, jelly-like substance that binds mineral particles together into what is known as crumbs. Crumb soil in a loamy texture is considered to be the most fertile soil for plants.


Minerals form the non-living skeleton of the soil which is derived from the weathering of rocks. The parent rock (present in the bedrock) determines the fertility and size of the mineral particles. Based on particle size, particle names may be sand, silt, clay, stones and gravel.

  • Coarse Sand: Gritty feel (0.6 – 2 mm in diameter)
  • Medium Sand: less gritty (0.2 – 0.6 mm in diameter
  • Fine Sand: grittiness not easily felt (0.02 – 0.2 mm in diameter)
  • Silt: Soapy or silky feel (0.002 – 0.02 mm in diameter)
  • Clay: Sticky (less than 0.002 mm)
  • Stones: over 2 mm size, and is a sizeable piece of rock
  • Gravel: over 2mm weathered fragments smaller than stones but with very little distinction from small stones.


Air is part of the main life support and is required for the breakdown of organic matter. The process results in the release of nutrients into the soil. Air movement within the soil is a means of ventilation and release of toxic gas buildup in the soil. Movement of air is through the air pores.

Water / Soil water

Soil water is basically a water based solution which is rich in dissolved organic and inorganic materials. Nitrates, potash and phosphates are apt examples of plant nutrients found in soil water.

It would be misleading to refer to soil as merely plant anchorage. In order to grow a thriving garden, bear these above points in mind. They will get you way ahead in preparing your garden for planting season.

Did You Know …

A fully functioning soil reduces the risk of floods and protects underground water supplies by neutralising or filtering out potential pollutants and storing as much as 3750 tonnes of water per hectare.

© Raymond Alexander Kukkee

Raymond is a freelance author and writer who practices traditional and experimental gardening using natural, sustainable methods in the challenging Zone 3 environs of Northwestern Ontario. Read more articles by Raymond.

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