The keys to creating the perfect soil environment for your organic crops are first understanding the needs of your plants and second, knowing the ingredients to good soil and what they do.
Sphagnum peat moss. Also called peat moss or simply peat. Peat moss is a very stable source of organic material that holds a great deal of water and air and does not decompose quickly. Peat moss is quite acidic (pH 3.5-4.0); limestone is added to the mix to balance the pH. Younger, lighter colored peat moss does a better job of providing air space than does older, darker peats that have few large pores for air space. A wide range of quality exists. Peat is the most widely used soilless medium, because of its wide availability and relatively low cost.
Composted pine bark. Its high lignin content makes it slow to degrade, so it nourishes beneficial organisms for months. Can be substituted in part for peat moss. Lightens the mix.
Sawdust. Similar to peat moss in most ways. Quality depends on type of tree: cedar, walnut, and redwood can be toxic to plants, and oak, hickory and maple deplete soil nitrogen more so than sawdust from evergreen needle trees.
Vermiculite: Handled gently, vermiculite provides plenty of air spaces in a mix. Handled roughly, vermiculite compacts and loses its ability to hold air. Vermiculite holds water and fertilizer in the potting mix. It also contains calcium and magnesium, and has a near-neutral pH. Vermiculite comes in different grades. Medium grade is usually used for starting seeds. A coarse grade can be used in soil mix for older plants.
Perlite: Perlite is a volcanic rock that has been heated and expanded to become a lightweight, white material. Perlite is sterile and pH-neutral. When added to a soil mix, perlite can improve air space and water drainage. It is a hard material that does not break apart easily. Perlite pieces create tiny air tunnels, that allow water and air to flow freely to the roots. Perlite will hold from 3 to 4 times its weight in water, yet will not become soggy. Perlite can be used instead of sand to reduce the soil mix weight. It holds very little water and costs are relatively high. It is much lighter than sand and can be used instead of it.
Limestone: Calcium carbonate (CaC03) or calcium magnesium carbonate (called dolomitic limestone) is used to adjust pH. The range can be adjusted for specific crops, but a pH range of 5.5 to 6 is ideal for most crops. Lime should be well-ground.
Sand: Coarse sand (sometimes called builderís sand) will add air space to the potting mix. Fine sand settles into the spaces between other ingredients and makes a dense mix that excludes air. Clean, washed sand has a near-neutral pH and little if any food value for plants. Sand is much heavier than any other ingredient used in potting mixes. The added weight is good for tall, top-heavy plants that might blow or tip over, but it is not the best choice for plants that will be shipped or moved a lot. Sand is the least expensive and most readily available source of larger-particle material.
Commercial Starter Fertilizer: Most commercial mixes contain starter fertilizer blends. Whether the blends are made of micro- or macro-nutrients, they are usually formed from synthetic fertilizer. These fertilizers are prohibited from use in potting media used for certified organic production.
Wetting Agents: Wetting agents are a common ingredient in commercial potting mixes. They are included to help regulate moisture, improve aeration, and increase nutrient availability. Most wetting agents do not qualify for approval by organic certifying agencies. Check with a local certifying agent to see if there are any approved materials with wetting properties. Safers Soap® products have surfactant qualities and might be suitable. Some certifying agencies also accept Shakleeís Basic-H® and similar materials considered to be biodegradable. Wetting agents should be used conservatively. Some commercial materials have been found to be deleterious at too high a concentration (1). A University of California Peat-Lite mix employs 3 oz. of wetting agent for a 17 cubic foot media mix consisting of equal portions of peat moss and vermiculite(2). The wetting qualities of compost-based mixes may be superior to peat-based mixtures and may not need wetting agents.
