Gardening Tips: Compost, Compost, Compost

Editor’s Note: We’re dipping into Michelle Smith’s archives to provide timely tips for gardeners.


What to do this week

It may seem sometimes like the gardener’s answer to everything is compost. Got sandy, light soil? Add compost to help absorb water and slow the leaching of nutrients. Your soil is heavy, wet, clay? Compost will help the fine, sticky particles clump together and improve the drainage and the soil structure. And that’s just a start. There are other good reasons that compost and organic matter in general are so important to good growing.

Healthy soil is alive with all kind of microorganisms — bacteria and fungi that work together to decompose organic material and release the nutrients for use by the next set of green, growing things. There can be billions of these microbes in every gram of soil. For a lot of the last century, agronomists believed that the necessary nutrients could be supplied by using chemical additives in the form of nitrates, phosphates and potassium. It was a narrow view of how plants take up nutrients. It’s like trying to cure an iron deficiency by chewing on a cast iron frying pan. If you think of the total nutrients in the soil as a whole circle, without organic matter plants can ordinarily only take up a small piece of the pie. Add organic matter and good soil structure and you increase the bio-available nutrients significantly. Plants, like everyone else, need their nutrients in a form they can digest.

By M Tullottes (Own work) [Public domain], via Wikimedia Commons

Compost. (Photo by M Tullottes, own work, Public Domain, via Wikimedia Commons)

Phosphorous, for example, is an important nutrient but since it is largely insoluble it is difficult to amend soils deficient in it. Fortunately, in healthy soil there is a thing called vascular arbuscular mycorrhizae or VAM, which sounds like a cleaning product, but isn’t. This symbiotic fungus digests soil phosphorous, nitrogen and other micronutrients and makes them available to plants.

Plants, especially the graminae, or grass family, encourage the VAM by secreting sugars around their roots that the mycorrhizae gobble up. It has been shown that applying chemical nitrogen fertilizer actually kills off the VAM, meaning that even more fertilizer has to be applied. Any time the soil is stressed, the mycorrhizae suffers, whether it is from over-tillage, excessive fertilization, poor crop rotations or compaction. Plus, decaying plant matter normally releases carbon into the atmosphere – think global warming – but healthy mycorrhizal networks actually capture and store that carbon.

All this to say you should concentrate on building your soil to get the healthiest plants. Choose slow-releasing organic fertilizers if you must fertilize at all. Don’t overwork your soil – the top six inches is the most active and important anyway. And add as much well-rotted compost as you can manage.

It is important to use only completely finished compost – half-finished stuff actually secretes hormones that suppress germination and may harbor all kinds of unfriendly organisms that would normally be rendered harmless after decomposition. The basis of good compost is carbon – leaves, straw – and nitrogen – green grass clippings, fresh manure, worm castings – with the ideal ratio of carbon to nitrogen about 25-30 to 1. Too much carbon and decomposition will slow and stall. Too much nitrogenous material tends to be wet and heavy and will generate bad smells. This is excess nitrogen off-gassing into the atmosphere, and we don’t want that. Add some high carbon-absorbing materials, like newspapers or dry leaves, and turn it over to get it back on track.

You can get the most composting efficiency by turning it every couple of weeks, but if, like me, you are too busy with other things, you can just leave it in a pile for a year or so. Make sure you place some branches on the bottom to ensure good air circulation if you do it this way. Try not to add weeds that have flowered and set seed to your pile. Many weed seeds will survive the lower temperatures of a backyard compost pile and you don’t want to add these to your garden. Likewise, don’t add the roots of invasive weeds like yarrow or couch grass for the same reason.

Finished compost is very forgiving. It rarely gives off a burst of excessive fertilization so you can spread it around as you like, if you’ve got plenty. If you don’t – and gardeners never really have enough – concentrate your efforts on things like onions and squashes that are heavy feeders or your perennial fruiting shrubs and trees. Be sure your compost is properly finished before you use it on leafy greens to avoid bacterial contamination when eating them fresh. It is rare but not unheard of, especially if you use fresh manures in the pile.

Featured image: A compost bin in the garden of Feeringbury Manor by Acabashi (own work), CC BY-SA 4.0, via Wikimedia Commons.



Market gardener, farmer, workshop leader, seed-saver, political candidate and mother, Michelle Smith has spent over 30 years coping with the challenges of our bioregion and in the process has built a store of practical and technical knowledge. The Inverness resident has served on the board of Seeds of Diversity Canada and represented Alternative Producers with the Federation of Agriculture but can do nothing about her hair. She is pictured with a head of Club Wheat, a seed that shares her approach to hairdressing.




This column was first published on 10 April 2019.