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The value of compost as a soil amendment

February 11, 2013  By  Mike Jiggens

The entire existence of man is based on the earth’s soils. Crops depend
on soil to grow and are consumed by animal life. Human beings not only
eat the vegetation and fruit it bears which grow from soil, but also
many of the animals which, themselves, depend on plant life for

The cultivation of soils, therefore, is essential for the existence of future life on earth.

January’s Landscape Ontario Congress in Toronto featured a panel discussion about the inner workings of soil and the need to periodically amend it with organic matter to keep it productive. Presented by the Compost Council of Canada, the panel’s three speakers focused on the role of soil in landscaping.

Chelsea Stroud of Waterloo-based Gro-Bark Ltd. spoke of soils, their structure and the means to which they can be amended for the best possible productivity.


“Every year when you’re gardening, you’re taking away from the soil technically by a harvest season or you’re removing clippings or raking leaves,” she said. “You’re removing everything that is normally put back into the soil in a natural environment.”

Engineering that soil by amending it with quality organic matter is what makes it different from a natural soil in areas which have developed over time through leaf litter, sloughed roots, minerals and other things which add to the soil profile.


The top three layers of natural soil include the O horizon (or the organic layer at the top), the A horizon (where seeds germinate and roots grow and which is high in organic material), the E horizon (or eluviation layer which is made of sand and silt and only few minerals) and the B horizon (or subsoil, which contains clay and mineral deposits received from the above layers when mineralized water drips from above).

When referring to topsoil, it is the A horizon being described.

Soil is technically made up of air and water and mineral particles.

“We want it to be about 50 per cent air and water, or mostly pore space that holds the water,” Stroud said.

Soil also contains gravel, sand, silt and clay. In soil tests, it refers to everything below a two-millimetre particle size, unless the customer specifically asks for content greater than two millimetres, which automatically takes out gravel content.

Organic components of soil represent only a small fraction by weight, but plays a vital role in the function of soil. Microbes, fungi and insects are among key organic matter.

“The purpose of soil is to house a plant or to hold a plant up, acting as an anchor.”

Soil both holds and moves nutrients, stores water and oxygen and helps facilitate their movement. It helps to fluctuate temperatures by cooling the root zone area in summer and preventing it from freezing too quickly in winter.

“It’s a very good temperature moderator,” she said.

Soil texture is the proportion of sand, silt and clay combined. A sandy loam soil is typically between 60 and 70 per cent sand, but can be as low as 50. A loam soil, as long as it has sand, silt and clay content, is considered a loam whether it’s a true loam, a sandy loam or a clay loam.

Sand size fraction is an important consideration, Stroud said. Sand, silt and clay determine the texture, but the sand size fraction will determine whether it is a uniform-type sand in the sand part, or if it’s non-uniform sand. Something non-uniform may not drain as quickly while the opposite is apt to be true with uniform sand. The type of plants to be planted will dictate the sand type.

“In turfgrass, you may want it to drain very quickly. In a planting soil, you may want something that’s not quite so uniform.”

Stroud said something to pay attention to when specifying soil is its cation exchange capacity. The CEC is the soil’s ability to retain nutrients and the ability to exchange them. Clay soils have a high CEC, but there may be several reasons one may not desire clay soil because it is slower to drain and is more apt to become compacted. If plans are to plant every year, it may not be the best soil.

Micro and macronutrients in soil are an important consideration. Levels of pH in southern Ontario are often higher than 7.5 while they are frequently below 7 in northern Ontario. The objective, she said, is to go with a soil whose pH is best suited for a particular geographic area. In southern Ontario, however, where soil is often high in pH, many plants prefer something less than 7.5. Although this will throw off certain nutrients, it doesn’t necessarily mean it’s detrimental.

“You might just have to use a different combination of nutrients or more compost—more organic matter in order to bring down the pH or moderate that effect.”

Bulk density deserves consideration, Stroud said, noting it may compact easily and not drain well.
Soil structure is the ability of the soil to stay together. Factors include sand, silt and clay content, but also how they are held together by organic matter. Little clumps can form which is important to note when thinking of drainage, aeration and soil porosity.

Sandy soils have weak structure while clay soils are stronger. Clay particles are usually flat and elongated and, when sitting together, form more of a layer than having pores between them.

Problems which exist when soil structure is poor include reduced root growth for seedlings, drought-associated problems due to either too much drainage or not enough, soil compaction after rain or traffic, poor infiltration, lack of oxygen, and small, stunted plants. In such cases, the plant is more prone to disease.
Organic content accounts for about 5 per cent of a natural soil, but it may not always be the best amount of organic matter, Stroud said.

Types of organic matter include active fraction, stabilized organic matter and fresh organic residue.
Stroud said the best thing to do is to pay particular attention to the plants and they will indicate what is needed. There are many symptoms to suggest soil is poor. Poor water will produce wilting, air problems will yield to disease, light problems will result in chlorotic tissue, and nutrient problems—and if there is poor microbial life—will sometimes lead to disease problems.

Soil sampling and testing will provide a nutritional  analysis and suggest the amount of fertilizer to put down or how much organic matter should be added to the soil. Soil sampling will also determine its physical qualities, including sand, silt and clay content, texture, organic matter content, and saturated hydraulic conductivity.

Compost as a natural soil amendment

Rod Kidnie, national sales manager at All Treat Farms Ltd. with composting facilities in Arthur and Ingersoll, spoke about the importance of compost as a natural soil amendment.

Soils have many diverse cultures, he said, including algie, bacteria, fungi and protozoa.

“These microorganisms are specialists in recycling organic matter,” he said.

It’s this recycling process, that over millions of years, has built up the earth’s productive soils.

In 100 grams of compost, there may be one billion bacteria. Three to 5 per cent of that organic matter is actually microorganisms, he said.

Sources of organic matter include old plant roots, deceased microorganisms, earthworms, insects, lawn and garden residues, shrub and grass clippings as well as added material such as manure, peat moss, peat sedge or compost. Each additive has its advantages and disadvantages. The pH of peat moss is often low, requiring the addition of lime, and doesn’t have a lot of nutrients. Peat sedge has a higher pH but not a lot of nutrients. Compost has high levels of organic matter and lots of nutrients, but may have issues with salt and sodium content.

The benefits of organic matter include improved soil structure, which is important for drainage and root growth, and better water-holding capacity, which is particularly important in sandy soils.

“Just a 5 per cent increase in a soil’s organic matter, you can quadruple the water-holding capacity of that soil,” Kidnie said.

Organic matter enhances the infiltration of air, water and roots, which is especially important in clay soils. He said it is difficult for water to percolate through dense clay unless there is some organic matter present, and it is difficult for roots to work their way down.

Organic matter encourages microorganisms and earthworms which enhance fertility. The organic matter in the top seven or eight inches of soil holds the moisture and nutrients in the root zone where the plants need it.

“Higher levels of organic matter help stimulate the beneficial fungi which, in turn, helps nutrient uptake by those roots.”

Some soils are low in organic matter for various reasons. When new buildings are constructed, the top eight to 10 inches of topsoil are removed, but, once the job has been completed and topsoil is replaced, only about four inches of it is returned. Other potential organic matter such as grass clippings and fallen leaves are raked up and placed at curbside to be treated as waste.

This is where compost comes in, Kidnie said.

“The composting process is the natural biological reduction of organic materials to humus.”

In most cases, compost needs to be mixed with existing soil or peat moss or peat sedge as a soil amendment. It improves fertility structure and should be analyzed for nutrient content prior to its use so that it is utilized in the right application.

The compost quality assurance (CQA) program in compost production has a number of parameters, including pH, carbon-nitrogen ratios, moisture and particle size, soluble salts and percentage of sodium.

Kidnie said landscapers buying large quantities of compost should be asking their suppliers for CQA reports which present detailed accounts of pH, carbon-nitrogen ratios, maturity of the product, heavy metal content and other pieces of information.

“A nice, rich soil high in organic matter is very important. Humans are great at creating waste. The compost producers are recycling that waste and we’re making a valuable resource that’s sustainable.

Improvements in compost quality

Mike Kopansky of Markham-based Miller Waste Systems said the past 10 to 15 years, with help from the Compost Council of Canada, have represented a significant breakthrough in compost quality and one the landscaping industry can trust and reply upon.

The industry has progressed beyond the point of waste simply being diverted from landfills to a time where people are realizing the value of compost.

“People are enjoying reconnecting with nature,” he said.

In addition to improving both soil structure and sustainability as well as benefiting storm water management and nutrient holding, compost is effective in slope stabilization. Rain is absorbed into the top layer of the soil structure so that is can be held and released more slowly without the soil being washed away. Not only does washed away soil create landscaping problems, but it creates issues downstream with storm water management systems clogging rivers and streams with sediments.

Application rates for slope stabilization can range from 11/2 inches to four inches, depending on what is being grown and the steepness of the slope. Two inches is regarded as the norm.

Compost not only improves the organic matter in the soil and its nutrient-holding characteristics, but a lot of biodiversity is being added.

Kopansky said it is important to feed the soil, respect the soil and help it and the existing microorganisms be successful in what they’re doing which is to catch the nutrients, hold them and the moisture and feed them back to the plants.

In landscaping, he said, compost is useful for restoring areas adversely affected by drought.

“It’s not just about growing good plants. It’s about the whole ecosystem and how it’s sustainable when we have a healthier soil.”

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