Soil is a combination of minerals, organic matter, air and water. On a farm, plants are often grown directly in soil fields. Container gardens on the other hand, often use a potting mix for a growing media that contain little actual soil (which is not necessarily a bad thing).
The minerals in soil are usually in the forms of sand, silt or clay. The reason that clays pack together so tightly (which is good for pottery, but not as good in large amounts for plants) is particle size.
A pile of boulders will have lots of airspace between them (perhaps large enough to sit in). A pile of pebbles, however, will fit together much closer, so will have less airspace between them. Sand, silt and clays are just finer grades of rocks. Ideally they will have a surface that will allow for air and water to nestle between the particles.
Too much clay in soil allows it to pack so closely together that it can limit the amount of air available and cause difficulties with growth. This is why soils high in clay are often amended with a lighter media. At least a little clay can be very beneficial because as the comparatively large surface area allows for easier mineral access for the plants.
Also, since clay tends to have a negative charge, it attracts positively charged nutrient ions, allowing them to be held to await the root system. Granite dust can be used as a substitute or to diversify the mineral content of a media.
There are drawbacks to using soil in containers. It is very heavy, which makes sense since it is mostly a pile of very small rocks and moisture. It also drains best in containers that are larger and deeper than are usually convenient. And since drainage is critical, soil as a container media is prone to overwatering. While these limitations have little impact on a field of flowers, they do have an impact when used in flower pots.
To overcome these problems, potting soils are often made out of lighter materials known as soilless mediums. Some media holds water better than others, which means that they can go longer between waterings—but the trade-off is they are more apt to have overwatering issues.
Rockwool for example, holds water very well, but cannot be allowed to sit in water for long or else the plants will drown. Clay pellets, on the other end of the spectrum, do not hold water as well and must be watered often; however, they are very difficult to overwater. Perlite has a nice balance of water retention and aeration, but floats when dry.
Commercial mixes are available, which can be very convenient. One benefit to purchasing premixed potting mixes is that they often include a variety of components and save the consumer from having too many half-filled bags of ingredients in the garage. Also, mixing large amounts of potting mix by hand can be strenuous work, and miscalculations with the amendments used in the recipe used can be damaging to the plants.
Still, potting mixes can be made at home, and almost every gardener I’ve run into that mixes their own has their own recipe. For a general purpose potting soil, mix the following:
1 to 2 parts plant compost (homemade or from a trusted source).
Plant compost can be made at home with relative ease in a backyard compost bin. Make sure the compost is mature before use; it should be broken down into unidentifiable bits and have a pleasant “earthy” aroma. I use compost made from garden/lawn waste and veggie scraps. If purchasing, make sure to obtain clean, high-quality plant compost that does not contain biosolids. Composted animal manure can be substituted for up to half of the total compost.
1 part peat, coir or wood chips
Although peat has been a staple of potting mixes for many years, there is concern about overharvesting and environmental impact. Fortunately coir (which is a by-product of the coconut industry) is both a renewable resource and an acceptable alternative. I have switched to coir entirely in my own garden. The quality of coir on the market today is much improved from some of the early versions available to the consumer. Since the peat and coir are used interchangeably, a combination of the two can be used. Wood chips vary greatly in size and quality, so inspect carefully before purchasing.
1 part perlite, vermiculite or expanded recycled glass
The compost and coir hold water well, and adding perlite or vermiculite helps lighten the mix. Although my personal preference is for the larger perlite in potting mixes, most of my potting mix perlite comes from my hydroponic system and is of the smaller variety (which also works). Expanded recycled glass is similar in function to large perlite.
1 part quality earthworm castings (optional)
Earthworm castings contain an array of beneficial micro-organisms and bacteria picked up as it passes through the worms.
Any quantity of a high-quality commercial potting mix (optional)
Several of the beneficial additives in these mixes can be helpful in even small amounts, so including some with your homemade mix can improve the overall quality of the product.
Mix the three (or five) ingredients well, and use as you would normal potting mix. This is a nice starting place, but there are other amendments that you might want to consider as well. Silica sand or rock powder, for example, can be included to add weight to the mixture and for the slow release of trace minerals over the course of the season (just use these in in moderation).
Nitrogen additives—such as alfalfa meal, blood meal, cottonseed meal, seabird guano, feather meal or fish meal—can give the plants a starting source of nitrogen. Phosphorus can be added in forms like rock phosphate (which has a very slow release time, making it a good choice to add to the starting mix), bone meal or high-phosphorus guanos and manures.
Potassium is found in potash and langbeinite, which can be added to the base mix. By adding nitrogen, phosphorus and potassium sources to the starting mix, reliance on early additional fertilization is reduced. As these initial sources are used up, additional nutrients can be top-dressed and watered into the mix, or replaced by normal plant feedings.
Three other amendments to consider adding to the mix are kelp, lime and mycorrhizal fungi. Kelp adds many helpful micronutrients, beneficial plant hormones and small amounts of other plant growth aids, such as enzymes.
Limes—such as shell meal, limestone or dolomite—can be used to raise pH in acidic conditions and are a source of calcium (and, in the case of dolomite, magnesium).
Limes are commonly used to offset the lower pH of peat (if used), so should not be used if a large amount of earthworm castings have been added, as they also raise pH. Adding mycorrhizal fungi powders can introduce a colony if one is not already present. A light dusting of the roots during transplanting can make sure the powder comes in contact with the host roots.
There are a variety of other recipes available to try, and other additives and amendments that can be used. Making perfect potting soil is much like making the perfect meatloaf; there is a general consensus about approximately what should be in the end-product, but the exact details on ingredients and amounts vary from person to person.
In my own garden, I use a combination of homemade and commercial potting mixes. I mix my own more often than not, but if I need more potting mix than I have compost ready, I use a couple bags of store-bought, or I’ll throw in some premium mix in along with the other ingredients if I’m making a large batch.
Since I send any healthy potting mix back through the compost pile after harvest (after rinsing if I suspect a lot of salt residue), along with the new material being composted, the potting mix is continually being refreshed and re-amended, and waste is kept to a minimum.
Even if a gardener chooses the convenience of purchasing a commercial potting mix over making their own, they should have some understanding of the ingredients so they can make informed selections.
Although gardening non-hydroponically is commonly referred to as “soil” by lay people, soil is actually an uncommon container growing media. Soilless mixes are much more common.
Starting with a good potting mix can make a world of difference in how well a container garden performs.