All good potting media should meet the needsWorking from tried-and-true recipes is a good idea, especially at the beginning, and we will give you a few recipes for organic mixes. Dont be frightened to experiment with different blends for different plants. When experimenting, begin by making small batches and give them a thorough evaluation. The next step entails assembling the components and arranging the equipment, space, and labor for mixing and storage. Storage can present its own challenges, and weed seed contamination must be prevented. Contrary to what some critics say, organic growers are permitted to use a wide array of materials in growing media. The challenge is more a matter of assuring consistent quality of ingredients than of finding enough of them. The section that follows features a brief description of some of the most common materials used in creating organic growing media and discussions of some issues that surround them.
The trend in conventional gardening for many years has been the increased use of soil-less media. A major reason for this has been concern about soil-borne plant diseases and the excessive density of mixes where soil is a dominant ingredient. However, soil is still used in some organic blends.
Clean commercial topsoil is acceptable for use as a natural ingredient. Be certain to check the label or inquire, however, to be certain that it has not been treated with prohibited ingredients to kill microbes and weed seeds. If using soil from the farm or garden, use only the best. Consider solarising, steam pasteurization, or oven heating if there is any history of soil-borne diseases. Microwave technology is effective as a sterilisation process but will likely be prohibited for organic crops. It goes without saying that soil contaminated with pesticides, prohibited fertilizer materials, or environmental pollutants should not be used. Certifiers may require that any soil used must come from land in certified organic production.
Choice of sand in a growing mix can make a difference. Coarse sand called builder's sand adds air space to the potting mix. Fine sand settles into the spaces between other ingredients and makes a dense mix that tends to exclude 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 pot plants that will be moved ocasionally. Sand is the least expensive and most readily available larger-particle material. Compost is perhaps the most common media ingredient among organic growers. Cheaper than more traditional components like peat moss, compost holds water well, provides nutrients, and can be made right in the garden. The quality of the compost depends in part on how it is made, but especially on what it is made from. The variability in commercial composts is one of the main reasons why it is less common in commercial organic media. Experienced compost makers know that compost quality is directly affected by the raw ingredients. If the ingredients are low in nutrients, the resulting compost will also be nutrient-poor. To produce a high-quality compost, it is advisable to make it according to a recipe using a specific blend of balanced ingredients rather than simply using whatever ingredients come to hand. For more information about making compost, jump to our compost page!
Click here for our compost making page! In many circumstances, compost has been observed to suppress plant disease. Researchers have found that vegetable and herb seedlings raised in a mix of 40% vermiculite, 30% peat moss, and 30% composted cow manure had faster growth and less incidence of disease than those raised in a 40% vermiculite/60% peat moss mix. Sphagnum peat moss is the most commonly used soil-less medium because of its wide availability and relatively low cost. Peat moss is a very stable organic material that holds a great deal of water and air and does not decompose quickly. Peat moss is quite acidic (pH 3.5 to 4.0); limestone is commonly added to the mix to balance the pH. Younger, lighter-colored peat moss does a better job of providing air space than do older, darker peats that have few large pores. Organic growers are urged to be cautious when purchasing peat moss. Many commercial sources are treated with wetting agents. Since all but a very few of the commercial wetting agents are prohibited in organic production, the buyer should assume that any product with an unspecified wetting agent is probably prohibited and avoid making a purchase.
Spent mushroom compost is wonderful stuff, and sometimes you will get a mushroom or two with it! It's great in the garden, and good in potting mixes as long as you dont use too much. It's too coarse for seedling mixes. Other forms of peat can be used in growing media, though not all may be readily found in the marketplace. Sphagnum peat moss is the most common peat and represents its least-decomposed form. Light, dark, and black peats typically describe the same substance in various stages of decomposition; dark peats are more advanced in decomposition than lighter ones. There are also some differences in the base or original vegetation that decomposes to make peat. Besides the peat formed by decomposing sphagnum moss, other peat materials are derived from reeds, sedges, and grasses. Reed sedge peat is typically very dark or black and does not have visible peat fibers. It is very difficult to rewet when dried and readily "fixes" phosphate. While the darker grades are more commonly used for amending horticultural soils, some media blends still use them. Any type of peat will work in mixes, but different results can be expected with each. Blending of different types of peat is apparently done quite often. Peat materials are not considered hazardous to handle, but growers should be aware of a rare but serious disease called sporotrichosis. Also known as "sporo," this fungal disease can be contracted through cuts and scratches in the skin. Symptoms begin with small postules on the skin, which swell and turn red. The disease can spread to bones and internal organs. It is a treatable condition that becomes deadly only in extreme cases. Chances of contracting this condition are low, but it is advisable to wear gloves and long sleeves when working with peat, and to wash thoroughly afterwards.
Ground-up newspapers can be an ingredient in growing media as a substitute for peat moss. Newsprint should not be more than 25% by volume of the mix. Avoid the inclusion of glossy paper or paper with colored inks, as these are prohibited.
Alfalfa (lucerne) may be a good locally-available alternative to peat moss. Alfalfa provides nutrients, especially nitrogen that are released slowly. Raw alfalfa must be processed before use in growing media. Dried alfalfa is ground and passed through a 2-cm screen. Water is added and the alfalfa is allowed to decompose for 20 days. It is then air-dried for another 20 days before use. Kenaf (Hibiscus cannabinus) is a fibrous plant grown in warmer regions. At the end of the growing season, kenaf is harvested and portions of the plant are used to make paper. The waste products can be used as growing media. Kenaf stalks contain two different fiber types, bast and core. It is the core material that is most suitable as a potting mix ingredient. Growers who have used kenaf have seen excellent results. Two greenhouse studies conducted in 1993 and 1995 showed that coarse-grade kenaf core in a 1:1 ratio with peat moss may be a suitable replacement for bark. The quality of sawdust used as media depends on tree type. Because they contain allelopathic chemicals, some cedars, walnut, redwood, acacias, eucalypyts can be toxic to plants. Oak, hickory and maple are reputed to tie up soil nitrogen more readily than sawdust from evergreens. Sawdust from treated or painted lumber is not allowed in organic production.
The mix above is nearly ready, it just needs the mushroom compost mixed in.
Perlite is a volcanic rock that has been heated and expanded to become a lightweight white material. It 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 three to four times its weight in water, yet will not become soggy. It is much lighter than, and can be used instead of sand. Vermiculite is a micaceous mineral that is expanded in a furnace, forming a lightweight aggregate. Handled gently, vermiculite provides plenty of air space 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. Seedling mix. 6 parts compost 3 parts soil 1-2 parts sand 1-2 parts aged manure 1 part peat moss, pre-wet and sifted 1-2 parts leaf mold, if available 1 6" pot bone meal Potting mix 1 wheelbarrow-load sifted soil 1 wheelbarrow-load aged manure 1 wheelbarrow-load sifted old flat mix 5 shovelfuls sifted peat 2 4" pots bone meal 2 4" pots trace mineral powder 2 4" pots blood meal Potting mix One part each by weight: compost sharp sand loam Potting mix 1/3 mature compost or leaf mold, sieved 1/3 fine garden loam 1/3 coarse sand (builder's sand)
Potting mix 1 part sphagnum peat 1 part peat humus 1 part compost 1 part sharp sand (builder's) All potting mix recipes should not rely on just compost to provide nutrients. Chicken manure pellets or a similar fertiliser should be added. A slow release fertiliser is useful too. Seedling mixes should be finely sieved or crumbled.
The mix above is pretty good one. Top left is well-rotted compost, top right is loam, bottom left is coarse river sand, and bottom right is perlite. I'd add some chicken fertiliser pellets too, not too many, two or three good double handfuls to a wheelbarrow. Good luck with your gardening, may the rain fall and your weeds fail, Patrick.
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