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Probably the least understood gardening essential.

Climate Considerations
Climate appears to have an effect on how different soil mixes have performed. Conservatories in warmer climates have problems with topsoil or organic based soil and better results using a mix heavy in aggregate or sand. This base is then periodically amended with compost or other organic material. This helps compensate for the rapid breakdown of the organic content of topsoil that can leave a difficult clay base. Conservatories in cooler climates appear to favor mixes based on topsoil, or organic based planting mixes, many using local field soil or "sandy loam". The breakdown of the organics is less of a problem in cooler temperatures.

Information about Propagating Plants from Purdue University

The Ingredients
A general average of those conservatories using a soil mix would be 1 to 2 parts aggregate such as coarse sand or gravel to 3 parts organic material such as peat, compost, or bark, adjusted according to the preferences of the species being grown.

The definition of aggregate, topsoil, sandy loam, and compost, varies from region to region and even person to person. "Compost" may refer to the product of a variety of materials and at various stages of decomposition. The interpretation can make a tremendous difference in the end product. To help moderate this problem, a brief discussion of materials follows.

Topsoil is difficult to define on a non-local term, but unofficially refers to an organic and clay based media with at least a small amount of sand or other aggregate. Topsoil should be pasteurized (or certified free of weeds and pathogens) and tested for nutrient content, buffer potential and pH. Although the best natural topsoil may initially be a perfect growing media, high tropical temperatures and humidity, irrigation, and plant growth may quickly reduce it to a poor growing media without the natural amendment of decaying plant material. Some conservatories have experimented with leaving fallen leaves and branches in plantings, but abandoned the idea for lack of public acceptance. It may be better to remove the material and compost behind the scene and reapply.

Indoor Gardening Books

Compost varies in definition even more than topsoil. Any compost should be tested for nutrient content and amended as necessary to compensate for nitrogen lost in the breakdown process. It is recommended that it be aged before incorporation in any mix, especially bark composts.

Manure composts vary greatly and sometimes contain weeds and undesirable pathogens if not properly composted. Specify minimum composting times and temperatures. Pasteurization takes place at 180 F. Look for sources that offer a guaranteed product. Most gardens surveyed have an in-house composting program for discarded plant material. These programs may not be able to provide enough for large projects, but are useful for spot amendment and top dressing. Cattle lots, local farmers, and horse stables are other sources for compostable materials.

Bark compost seems to be a renewable resource. Composted material is available from timber processing companies and local landfills. Bark composts should not be used green in soil mixes. The quantity of nitrogen it uses in decomposition can deplete the mix. Some conservatories are using a product called 'Natures Helper', a ground pine bark that has been well composted combined with washed white sand.

Secrets to Great Soil: A Grower's Guide to Composting, Mulching, and Creating Healthy, Fertile Soil for Your Garden and Lawn (Storey's Gardening Skills Illustrated)
by Elizabeth P. Stell 

Peats and bark fiber Peat moss is a common base for soilless mixes and a widely used amendment to soil mixes. It is valuable for growing plants requiring lower pH media. However, it has a limited life in high temperature and humidity applications and may eventually become a limited resource in some areas due to environmental concerns. This may make it an inappropriate choice for large permanent applications. Other mosses may also become hard to acquire.

Tree fern fiber, though a perfect media for many tropical species, has already become scarce, its source a protected species. Osmunda and coconut fiber are possible alternatives.

Fir bark and other shredded barks work well to loosen soils. As in bark composts, care should be taken to insure nutrient compensation for nitrogen lost in decomposition.

Commercial soilless mixes are created mostly for container growing, where no plant grows long in the same container. Some conservatories use these as an amendment to their soil mixes. As permanent media in large plantings, these suffer the same problems as topsoil and organic based media; breakdown, compaction and the loss of air capacity and porosity. Perlite in these mixes can cause aesthetic and cultural problems (see discussion under aggregates). Vermiculite may hold too much moisture, causing a mass planting to become spongy or soggy. Many gardens use soilless mixes use it to start the plants, graduating to a soil-soilless combination, then to the soil based planting beds.

Aggregate used in mixes include sand, perlite, gravel, hadite, granite chips, pumice, lava stone, etc. They provide space in the soil needed for air, water percolation, nutrient holding capacity and body.

Coarse river sand is probably the most widely used aggregate. It provides an inert base for almost any soil mix. It can sometimes cause problems by washing into drainage systems. This can be remedied by using a membrane under plantings to separate mixes from drainage material. Sand can harbor pathogens, so precautions should be taken.

Gravel used in soil mixes is usually limestone or chert. Limestone in this form is relatively inert and shouldn't be a pH consideration. Limestone adds a great deal of weight to mixes. This is a consideration in transporting and installing the mix. Granite chips and decomposed granite are becoming a more popular choice. They are good medias or amendments for plants that require well drained conditions and do not tolerate a high pH.

Perlite is widely used in soilless mixes, but is used in only about a third of those conservatories surveyed. It is a potential problem in areas with fluoride in the water, causing toxicity in some species with thin leaves and variegations. Perlite also tends to float out of large plantings and may be unsightly in naturalistic settings. Perlite does come in a variety of grades and qualities. Styrofoam is used as a perlite substitute, but is even more notorious for floating out and blowing around.

Vermiculite is an expanded mica product and is used as an ingredient in soilless mixes for container growing. It is not recommended for use in large plantings as it compresses easily and can cause soil to stay too wet. Vermiculite was used by one conservatory surveyed, but only in orchid mix.

Charcoal used as an aggregate keeps soils 'sweet', and has value somewhere between bark and lava rock. It is used as a base layer in planters and as a filter media. Charcoal is also used to contain chemical spills and thus may capture nutrients and pesticides used in plant cultures. The ability of plants to recapture these elements and in what form is unknown to this author. However, charcoal has proven to be an excellent aggregate in epiphytic culture and a good substrate in planting beds.

One garden surveyed used hadite as an aggregate. It is totally inert, a good filler, but expensive. "Turface" (or kitty litter) is a form of hadite.

Lava rock is used in many tropical mixes and is available in many grades. This is much lighter than chert or limestone gravel, and more attractive than many other aggregates. It is also used in epiphytic cultures.

Best of Luck with your gardening season ....
and let us know how you make out.

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