Properties and Considerations of Principle Soil Media Ingredients

1. Composts

a) Can be an excellent source of short- and long-term nutrient availability, provide moisture-holding capacity, are a source of bulk density, and provide some degree of drainage and aeration

b) Can be produced on farm, are a way to use animal residues and recycle on-farm nutrients

c) Can be a source of beneficial bacteria and fungi that promote plant health

d) Can also be the source of weed seeds and pathogens if not from a well-managed, high quality source

e) Similarly, if not made from nutrient-rich sources or if too old, they can be a poor source of nutrients

2. Field and garden soils

a) Assuming they are well managed sources, can be a decent source of macro and micro nutrients

b) Can be a source of beneficial bacteria and fungi

c) Can provide valuable bulk density but at the same time contribute considerable weight to propagation mixes

d) If used in too high a proportion, can create a poorly aerated and poorly draining growing environment

e) Can be the source of weed seeds and pathogens if not from a well-managed, high quality site, and can lead to the spread of weed and pathogens as soils are moved from field to greenhouse to new fields

3. Coarse sand

a) Provides excellent drainage and aeration for soil mixes 

b) Provides valuable bulk density but at the same time contributes considerable weight to propagation mixes and is not suitable for use in polystyrene containers because sand readily scars the containers, creating sites that roots will cling to or that can harbor pathogens

c) Although not a renewable resource, it is abundant around the world and thus does not create long- distance transportation impacts 

4. Perlite Of volcanic origin, perlite is a mined mineral, ground, graded, and heated in kilns to 1600ºF, which causes microscopic quantities of water in the ore to turn into a to gas. This in turn causes the raw perlite to expand, popcorn style, to 4–20 times its original size.

a) An excellent source of drainage and aeration in soil mixes, while also being very light weight and easy to handle

b) Can retain 2–3 times its weight in water

c) Is sterile when first removed from its packaging and is therefore not a source of weed seeds or pathogens, and normally has a pH of 7.0

d) Greece, the United States, especially NM, UT and OR, along with China are the biggest producers of perlite

e) Production is very energy intensive, from mining, to expansion of the raw ore, to transport from remote locations to market

f) Alternatives to perlite include sand, pumice, rice hulls, processed corncob waste, and composted grape seed

5. Vermiculite

A micaceous mineral, vermiculite is mined and then processed in kilns heated to 1000ºF. While in the kilns, microscopic water molecules trapped in the ore are vaporized, which in turn causes the ore to exfoliate, accordion style, into a material that has a huge surface to volume ratio.

a) Outstanding ability to hold water, at least four times its own weight

b) High cation exchange capacity (CEC) and especially effective at holding on to K, Mg, Ca, and P

c) By virtue of its size and shape, provides good drainage and aeration, while also being lightweight and easy to handle

d) Is sterile when first removed from its packaging and is therefore not a source of weed seeds or pathogens, and has a pH of 7.0

e) Produced domestically in South Carolina and Virginia, imported from South Africa, Brazil, China, and several other sources

f) Alternatives to vermiculite include partially composted cotton gin waste, ground pine and fir bark, sand, and leaf mold

From Opera

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