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Organic amendments

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Home composting
Photo: cc flickr (E. v. Muench)

Main amendments

Amendments are substances added to the soil to improve its physical, chemical and biological properties. They fall into two categories: organic amendments and mineral amendments.

Compost

Compost is formed by the fermentation and wetting of animal manure, dead leaves, grass clippings, kitchen scraps (fruit and vegetable trimmings and egg shells), garden waste, etc. Compost makes an excellent amendment and fertilizer for the garden: it adds microorganisms to the soil, provides them with nutrients and shelter, improves the soil structure, balances the pH and supplies plants with essential nutrients. Because these nutrients are released gradually, the plants enjoy a steady food supply.

When to apply it

Early spring is the best time to add compost to sandy soil (so that fewer nutrients will leach out), and late fall for other soil types. Avoid adding compost during a heatwave or the hardening-off period.

The hardening-off period extends from late summer until the leaves fall. During this time, plants get ready for winter: growth is halted, tender twigs on trees and shrubs turn woody (lignified), buds form that will open in spring and the stores of nutrients made by the leaves accumulate in the roots and underground organs. Adding compost at this time may promote the growth of tender shoots that will not have time to harden off before winter.

How to apply compost

  • When preparing a bed, work compost into the top 10 to 15 centimetres. To maintain planted beds, dig the compost in lightly or simply cover it with mulch.
  • To promote proper root growth, compost should be applied to the entire planting area and not just around the plants.
  • To ensure that your plants have access to a complete and varied source of nutrients, it is best to use different types of compost over the years (shrimp compost, sheep manure compost, leaf compost, backyard compost, etc.). All compost you use should be well decomposed.
  • It is best to work compost into the soil a few weeks before seeding.
  • You can mulch with compost.

How much to apply

How much compost you should apply depends on your soil test results and the requirements of your plants. As a general rule for maintaining planted beds, a layer 0.5 to 5 cm thick spread on the soil and worked into the surface is sufficient.

Manure

Manure contains livestock feces, urine and bedding. Its nutrient content varies depending on the animals’ age, diet and bedding, weather conditions, storage methods, etc.

Fresh manure should not be used because it may burn any plants it touches. It is best to compost it or let it decompose for a while before using it. It is ready to use when it smells earthy.

Composted manure is a good all-purpose garden amendment and fertilizer. Apply it in spring to sandy soils (so that fewer nutrients will leach out) and late fall for other soil types. Avoid adding it during a heatwave or the hardening-off period.

When preparing a bed, work composted manure into the top 10 to 15 centimetres. To maintain planted beds, dig it in lightly or simply cover it with mulch. It is best to work composted manure into the soil a few weeks before seeding. How much you should apply depends on your soil test results and the requirements of your plants. As a general rule for maintaining planted beds, a layer 1.5 to 5 cm thick spread on the soil and worked into the surface is sufficient.

Peat moss

Horticultural peat moss consists of largely decomposed plant waste that accumulates in bogs at a rate of 0.5 to 1 mm a year. One of these plants is peat, a type of moss that can hold up to 25 times its volume in water!

Peat moss is used to improve the water retention of sandy soils, lighten compact clays and acidify soils (its pH is between 3.5 and 5). This amendment breaks down very slowly, however, and therefore provides plants and soil organisms with very few nutrients. In addition, when peat moss dries out, it is very difficult to rewet it. Unless it is worked in deeply, it accumulates near the surface, where it forms a crust that prevents water from soaking through.

Lastly, extracting peat tends to destroy peat bogs. These wetlands are home to unique flora and fauna, and take thousands of years to form. For all these reasons, it is best to use peat moss in your garden sparingly.

Black earth

Most of the black earth sold in garden centres in Eastern Canada comes from peat bogs. In fact, it is peat moss in an advanced stage of decomposition (black peat). You can also buy top soil, also known as muck soil. This is soil removed from the surface of some fields or even from woods.

Black earth is usually acidic and poor in nutrients. In addition, it may contain weed seeds.

Organic mulch

Mulches made from plant material also act as amendments. Over time, they decompose and work their way into the soil, adding organic matter.

Mulches that decompose quickly (chopped dead leaves, buckwheat or cocoa bean, etc.) can also be used as fertilizer.

Warning: cocoa beans can poison cats and dogs who eat it.

Ramial chipped wood (RCW)

Some municipalities distribute ramial chipped wood (RCW) free of charge. RCW is made from tree branches and twigs (both hardwood and softwood) less than 7 cm in diameter that have been chipped.

Ramial chipped wood substantially improves the structure of all types of soil and adds many nutrients as it decomposes.

Tips

  • RCW mulch should never include more than 20% softwood, which contains antibiotic substances that may harm the soil.
  • RCW mulch should be neither too fine nor too coarse. Ideally, the wood chips should be 2 to 4 cm.
  • RCW mulch should be applied in fall. If applied in spring, the soil will have a nitrogen deficiency for about 2 months, and this will interfere with plant growth.
  • A 3 to 5 cm thick layer of RCW mulch should be applied around herbaceous plants and a 7 to 10 cm thick layer around trees and shrubs. 
  • You usually won’t need to add any new mulch for about 3 to 5 years. 

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