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A Sense of Humus: The Environmental Benefits of Good Soil
by Gord Miller, Environmental Commissioner of Ontario
August 29, 2010

Environmental Commissioner of Ontario; January 11, 2010 in ECO Commentary

Most people have some sense of "humus" — the rich, dark, earthy-smelling material found in most top soils and in all well-aged compost piles. Few of us, however, have a real appreciation of its full range of environmental benefits. Even the people whose job it is to find solutions to climate change and other environmental problems often overlook the potentially huge role of soil in general, and soil organic matter (SOM) in particular, in addressing these issues. Soil scientists have had to fight hard to get decision-makers to even consider creating a major role for soil solutions in on-going climate negotiations.

In fact, soils are enormous carbon "sinks"; that is, they are capable of holding large amounts of carbon indefinitely (most of it as humus), reducing atmospheric carbon and mitigating climate change. There is more carbon held in soils worldwide than there is in either the vegetation above ground or in the atmosphere. Furthermore, most agricultural and pasture soils have lost about half of their SOM over the past few decades, so there is ample scope for improvement. Another benefit conferred by organic matter is an increased capacity for holding water, reducing the need for irrigation and preventing flooding and erosion. In fact, the more carbon sequestered in soil, the more water it can hold – a win-win-win scenario for agriculture, the environment, and the economy.

To get an idea of the potential benefits of raising the levels of soil organic matter, let’s consider two close-to-home examples: a football field; and an average urban lawn.

A typical CFL football field has an area of about 4645 m2, which is just a bit less than half of one hectare. Assuming a topsoil depth of 30 cm, there are about 1400 cubic meters of topsoil, which weigh about 1700 tonnes. If the folks who manage the field were to add just 17 tonnes of well-made compost (about one large truck load) as a top-dressing to the surface of the football field each year (preferably right after aerating), they would increase the carbon level of the soil by 1% (total dry weight of soil) over a period of 10 years. What does this mean for the environment?

First of all, it means that each year the field would sequester about 6 tonnes of CO2 (enough to offset the annual CO2 emissions produced by four average passenger cars). Secondly, each year the field’s soil will have increased its water-holding capacity by about 7 cubic meters, or the amount held by 42 barrels (the big ones they use to ship oil). This is water that does not have to be added to the field via irrigation to keep the turf healthy and green in dry periods.

After ten years, when the extra 1% of organic matter has been sequestered, the amount of water-holding capacity will have increased by about 70 cubic meters (imagine a very large tractor-trailer full of water sitting in the middle of the field!). In addition, about 60 tonnes of CO2 will have been permanently sequestered. These benefits could be realized for at least 50 years for most agricultural fields, parks, recreational fields, and gardens, before maximum organic-matter content would be attained (after which, of course, the benefits can be retained, but not increased).

To accomplish the equivalent soil-carbon increase on your 90 m2 (1000 ft2) lawn, you would need to add only 1/50 of 17 tonnes, or 340 kgs (a pick-up truck load). Your lawn would then be able to hold about 4/5 of a barrel more water in its upper 30 cm than it did before. You would also be off-setting about 1/12 of the CO2 produced by your vehicle, if it is a standard one and you drive as much as the average North American. Since this is so easy to do, you might want to increase your soil’s C-content by twice that rate, so that the percentage goes up 1 point over five years and two points over 10 years.

With this simple act, you would be offsetting 1/6 of your car’s emissions (or somebody else, if you don’t drive) and increasing the water-holding capacity by 1 and 3/5 barrels each year.

The benefits don’t stop there. If the turf (football field or lawn) is managed properly, the addition of the compost will preclude the need for commercial fertilizer and pesticides, further reducing the turf’s carbon footprint and eliminating the potential risks associated with pesticide use. Moreover, by supporting the composting industry, you will be helping to create local jobs, boost the economy, and provide stable, long-term markets for the compost produced from your own organic residuals.
All of these benefits, simply by developing a better sense of humus.

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