COMPOST, POTTING & STARTING SOILS

"To buy or not to buy starting and growing mixes, that is the question..."

I don't know whether it is nobler or not to mix your own; but for most backyard gardeners, including me,  the decision is probably mostly a matter of convenience.  My backyard is big enough for me to have  a compost pile that handles our yard waste and kitchen scraps, but I don't have enough space  to warrant creating my own specialized soil mixes because of the quantities of ingredients that I would have buy to make it economically advantageous. I also know that my compost pile never heats up enough to pasteurize itself, and I am quite sure that I would encounter resistance if I were to propose sterilizing the compost in the oven.

Although creating your own specialized soil mixes may not be your cup of tea, composting is something that you should do. If you are interested in composting and have not yet started, this Cornell website provides information about how to start:

 Small Scale or Backyard Composting.

However, for those of us lured by the siren's call of convenience, there are  numerous organic and conventional commercial soil mixes piling up now in stores near you.  Both types contain some combination of organic and inorganic components.  Most combine  peat or sphagnum moss with perlite or vermiculite and add either synthetic or natural  ingredients (e.g. bonemeal, fishmeal, feather material or vegetable compost) for fertilizer.

Source: CWMI

Starting mixes need to be relatively fine and uniform in texture, disease and weed free, allow for soil aeration and retain moisture to minimize the risk of drying out. Growing or potting mixes have  similar characteristics, but  soil particle size is  larger; and they typically include more nutrients, i.e. fertilizers, to promote and sustain plant growth after germination.

You can find many formulas for do-it-yourself soil mixes on the Internet. They seem to follow one of  two variations. The first formula is very simple and  is widely utilized   in  commercial greenhouses: 

                    1/2  vermiculite or perlite
                    1/2  peat

This mix was developed at Cornell University in the 1960's by researchers,  James W. Boodley and Raymond Sheldrake, Jr. You can find out more about it and how to tweak it for various plants from Cornell's Information Bulletin 46: Cornell Peat-Lite Mixes for Commercial Growing.  If you use this mixture in small plug cells, plan on transplanting the seedlings into larger pots with a  growing mix. If you are using larger cells or pots  with the Peat-Lite mix,  then it will be necessary to  fertilize the seedlings to sustain growth until they can be set out.

The second formula    approximates retail commercial mixes. It includes both perlite and vermiculite with generous compost and fiber components to satisfy fertilizing  and aeration needs. Using this recipe you could start seeds in larger plugs or cells and not have to transplant seedlings.

                   4 parts pasteurized compost 
                   1 part perlite
                   1 part vermiculite
                   2 parts peat or coco fiber

Natural Ingredients

And now,  in case you have started wondering what the major ingredients actually are in starting or potting mixes...  Here are brief descriptions:

Peat moss   Source:  U Texas

Peat Moss - This is a natural organic product and not to be confused with  products labeled "sphagnum moss." Peat is the submerged and long dead remains of sphagnum moss along with anything else that might have died and sunk with the moss into the bog.  Peat moss frequently serves as the organic component of economical  planting mixtures and is also  a practical way to add organic material quickly to garden soils.  Being organic, it will continue to decompose in your garden over several years and will eventually need to be replaced. Because it can absorb and hold a lot of water,  peat can also help to keep sandy soils from drying out. However,  it is acidic; and if you use it extensively in your gardens, you should monitor soil pH and occasionally amend the soil  with  a little lime.

Coarse perlite   Source: groworganic.org

Perlite - This is also natural product. As a material, it is an amorphous volcanic glass, i.e. a non-crystalline material and, therefore, not considered to be a mineral. Perlite derives from obsidian,  a product of rapidly cooling felsic lava that    contains high amounts of both silica  and  entrapped water.  The perlite found  in soil mixes has been crushed and then heated driving off the water.   The  fragments expand like popcorn kernels during the heating process.  In soil mixes these expanded fragments can  reabsorb  water and, because of their irregular shape,  also allow for air circulation. Its industrial applications  include use in plasters, insulation, filtration and as a stabilizer for explosives.

Sphagnum Moss     Source: U of Wisconsin 

Sphagnum Moss - This is another natural organic product, but it is derived from the harvesting of  living sphagnum mosses. Unlike its dead remains, i.e. peat, it is chemically neutral despite growing in an acidic environment that it has helped create. Being chemically neutral means that you do not have to worry about it acidifying your garden soil; however, it is a lot more expensive than peat.  Like peat, sphagnum moss also retains water well and is used especially in seed starting mixes and in container gardening.  Orchid growers and growers of carnivorous plants love sphagnum moss too because it provides the natural soil environment that their pets need.

N.B. Sometimes the word "sphagnum" appears on packaged soil products, but if the labeling does not explicitly state "sphagnum moss," it is probably something else, frequently peat.

Vermiculite   Source:  Vermiculite.org

Vermiculite - This is also a natural product.  As a material, it is a  hydrous phyllosilicate mineral,  i.e. it formed into layered sheets with a definite crystalline structure (think mica) resulting from the interaction of  felsic and mafic  rocks.   Unlike perlite, vermiculite contains relatively high amounts of magnesium, potassium, phosphorus, some calcium and relatively low amounts of  silica.    Vermiculite is also  heated to drive off its entrapped water and "exfoliate" (i.e. separate) its layers. Like peat and perlite, as a soil amendment, vermiculite can retain water, but the presence of the other  elements both  serve as buffers against  changes in soil pH and as  sources for some plant nutrients.  In addition to its horticultural uses, vermiculite has many industrial and construction applications including those of insulation, fire retardant and waste water filtration.