Peat moss, also known as sphagnum peat moss or simply peat, has long been the primary ingredient in soil-less seed-starting and transplanting mixes. Environmental concerns, however, have led many growers to consider alternatives. Coir fiber, which is made from ground coconut hulls, is a replacement for peat moss, but gardeners and researchers are still trying to determine which product has superior growing characteristics.
Conflicting Environmental Concerns
Peat moss is partially decomposed organic matter that forms very slowly in acidic bogs and swamps. Interest in coir is driven primarily by concerns that peat is not a sustainable resource because it is harvested more quickly than it accumulates, but this claim is not universally accepted. Opponents of peat also assert that peat harvesting causes permanent damage to wetland ecosystems. Coir fiber, however, is criticized for the financial and environmental cost involved in transporting this material from the coconut-growing tropics to the rest of the world.
Potting Soil and pH
Transplants need a carefully prepared growing medium to compensate for their confined root area, and this medium must also support the rapid growth that growers expect from transplants that are taking up valuable indoor growing space. It is difficult to supply sufficient nutrients in the small quantity of potting soil allotted to each transplant, so nutrients are usually added through soluble fertilizers. But this approach does not work for pH, which is determined primarily by the initial composition of the potting soil. Proper pH is perhaps the most important aspect of a potting mix because it affects the availability of many important nutrients.
Adapting to the pH of Peat
The ideal pH range for garden soil is 6.0 to 6.8, but soil-less potting mixes have the best nutrient availability when the pH is between 5.0 and 5.5. Most typical potting mix ingredients -- such as compost and perlite -- have little effect on the overall pH, so the pH of the final product is often similar to the pH of the peat. With a pH of 3.8 to 4.3, peat moss is very acidic, so agricultural lime must be thoroughly mixed into the peat to raise the pH to acceptable levels.
Chemical Properties of Coir
Coir is equivalent to peat in many ways. It looks like peat and offers similar benefits related to moisture-holding capacity, structural stability and drainage. The pH of coir is significantly higher, though, ranging from 5.8 to 6.5. This means that limestone is not needed to raise the pH, but plant growth may be less than optimal because coir's pH is actually above the ideal range. This higher pH may also be undesirable in regard to plant health -- peat's acidity is sometimes considered beneficial because it suppresses fungal organisms that can injure seedlings.
- Utah State University: A Comparison of Coconut Coir and Peat as Soil-less Media Components for Plant Growth
- Washington State University Extension: The Myth of Permanent Peatlands
- The New Organic Grower; Eliot Coleman
- University of Florida Extension: Coir Dust, a Viable Alternative to Peat Moss
- University of Idaho Extension: What Is Your Substrate Trying to Tell You
- University of Arizona Extension: Growing Media for Container Production in a Greenhouse or Nursery
- Yavapai College: Common Substrate Components
- University of Washington: Plant Growth Media and Fertilizer in the Nursery
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