Peat v. Coir

New thinking about soil amendments

By Ann M. Mason, Fairfax Master Gardener Intern



Northern Virginia gardeners know all too well the problems of planting in our native clay soil. Although rich in nutrients, clay’s tendency to hold water can keep oxygen from reaching plant roots, with can be detrimental to plant health. To improve the situation, gardeners have long turned to peat moss. Mixing it into clay soil allows air to enter and lighten soil structure so that the soil retains moisture without becoming soggy.

But the practice of using peat moss to amend garden soil is no longer the go-to recommendation. While peat is relatively inexpensive and does successfully aerate soil, it is not a sustainable or eco-friendly product.

Peat moss forms when sphagnum moss compresses and decays in water-saturated bogs—a process that can take thousands of years. Left untouched, peat acts as a carbon sink, absorbing carbon dioxide from the atmosphere. Draining and removing peat, however, reverses the process and releases greenhouse gases (carbon dioxide and nitrous oxide) into the air. In addition, while undisturbed peatlands mitigate and store water during heavy rainfall, removing peat changes water-flow direction, which can contribute to flooding and to nutrient leaching. Because peat harvesting causes these negative environmental issues, many scientists and Cooperative Extension Services no longer recommend using peat as a soil additive.

Scientists looking for an alternative to peat that still offers soil-lightening capabilities seem to have found a viable answer in coir. Coir, also called coir pith or coconut dust, is essentially the waste of the coconut. In parts of the world where coconuts are harvested commercially, huge stockpiles of aged coir readily exist. Peat and coir have similar properties and characteristics, making trading one for the other feasible and, in some cases, preferable:

  • Coir has a neutral pH (approximately 6) which differs from the acidic pH of peat. Coir does not need an addition of lime, which would increase the pH above the level for optimum plant growth. At pH of about 4, peat by itself is more acidic than many plants prefer.
  • Coir has extremely high potassium content and low calcium content, which means gardeners do not need to add potassium fertilizers but will need to add calcium, such as calcium sulfate (gypsum), to the soil for plant uptake. Coir is rich in various micronutrients including iron, manganese, zinc and copper. Gardeners will need to add nitrogen to coir to compensate for its limited ability to allow nitrogen uptake by plan