The Tribune

GREEN SCENE

By Gardener Jack

Some gardeners with heavy clay soil swear by adding sand in order to provide aeration. They obviously forgot the formula: clay, plus sand, plus water equals cement, which produces concrete.

The best soil lightener for use in the open garden is peat moss. Fairly cheap when bought in bulk, peat moss provides both aeration and water retention when worked into the existing soil.

When it comes to smaller amounts of soil such as in containers the results provided by perlite and vermiculite are more dramatic. Although very similar in some properties they vary distinctly in others and care should be taken to choose the right one for the job.

Both perlite and vermiculite occur naturally and are mined minerals. Perlite consists mostly of silica (glass) and when heated expands like popcorn. It is sterile and has a neutral pH and its industrial uses include the filtration of beer and house insulation.

Vermiculite is mostly mica (clay) and it also expands greatly under high heat. The name comes from the Latin for 'worm' because of the shape it assumes while expanding. Vermiculite tends to affect the pH of acid soils so care should be taken in its use with acid loving plants. In industry vermiculite is used for fireproofing, acoustic panels, and automobile brake linings.

When used as a soil amendment perlite absorbs some water but not a great amount. Perlite is used mostly to provide aeration to dense soils and can be added in quantities up to 50 percent of the total. Perlite can also be used as a medium for seedlings.

Enough water is retained for the plants' needs and trapped air bubbles provide oxygen. The lightness of perlite allows for rapid and unobstructed root growth. The lightness of perlite also encourages it to rise to the top of the mix when it is watered, where it can easily be blown away by a breeze.

Vermiculite retains much more water than perlite. If over-used in a potting medium it may cause the roots of some plants to rot. It also can be used as a plant starting medium and it is particularly effective at the seed germination stage.

Unlike perlite, vermiculite does not head towards the surface of the soil when you water. Much less vermiculite should be used in potting soil mixes (unless you are growing swamp plants) as it does not provide the degree of aeration that perlite provides.

A couple of years ago I bought a large quantity of commercial cow manure. This is good stuff when added to existing soil and mixed in well, but on its own it tends to dry out and then refuse to get wet again. When this happens to the soil in a container you can immerse it in water until it is forced into absorption.

In an open garden this is impossible, as I found out after adding far too much cow manure to my soil in one particular area. I would water for hours only to find that the soil that looked so wet on top was dry as dust beneath.

The only way I could get the substrate wet was to dig the soil with a trowel while playing a stream of water into it from a hose, thereby creating a slurry. This amused my grandchildren but the effects were temporary as the soil was quick to dry out even with daily watering.

Instead of investing in peat moss I put new seedling in with a generous handful or two of vermiculite. This worked well and by the time I had planted the whole garden plot with seedlings the problem was solved. Vermiculite does not rot away so my investment remains in the soil providing good water retention.

There is one problem with both perlits and vermiculite: they are expensive. There is a finite amount of both in the world and when the supplies run out there will be no more of either.

gardenerjack@coralwave.com