Garden soil can be used as a soil mix ingredient for outdoor containers, but avoid using it indoors, where consistently warm temperatures encourage the growth of soil fungi and may cause an insect colony to hatch. Purchased bags of potting soil, while not truly sterile, generally contain far fewer disease spores and insect larvae, making them a better choice than garden soil for indoor growing.
Bags of potting soil seldom list their specific contents or the ratio of ingredients; the best way to judge what's inside is by lifting the bag. A heavy bag probably contains more clay; the soil inside is dense and won't hold as much oxygen. A light bag contains more peat moss and less clay, which allows more oxygen and moisture around roots, but provides little real soil value. Neither makes a good permanent soil. If a soil-based mixture with clay is called for indoors, it's better to buy the heavy bag and amend it with peat moss and appropriate quantities of other close-to-sterile materials such as perlite, vermiculite and sand. You can use the same amendments outdoors, too, although homemade garden compost is the best amendment. As with garden soil, outdoor materials such as compost and leaf mulch contain spores and insect larvae that proliferate in an indoor climate.
Composted manure is available in bags from garden centres; it's odour-free and looks like heavy black earth. It serves as a soil conditioner and organic fertilizer. Manure purchased in bulk from, say, a nearby farm, may still be fragrant and insufficiently composted-it's what's called “hot” manure, which can burn plant roots. All composted animal manure is valuable to plants, but sheep, poultry and rabbit manures are higher in nutrients than cattle or horse manure.
Horticultural sulfur is a natural element, either mined or recovered as an industrial waste product. It's sold at garden centres as tiny pellets in a box labelled “soil acidifier” or as a powder in large bags labelled “flowers of sulfur.” It's valuable as a soil balancer, adding acid to alkaline soils, and it helps to lower pH. Sulfur is a good substitute for aluminum sulfate, which over time poisons plants.
Shredded bark is available in bags, bales or by the cubic yard. Be sure it's shredded, not chipped (suitable only for mulch). Shredded bark is ideal for woody plants—it aids the exchange of air and water, boosts microbial life and contributes nutrients as it decomposes. Because it uses nitrogen as it degrades in the soil, add some blood meal to prevent potential deficiency.
Sharp builder's sand, a.k.a. coarse sand, creates spaces in a soil mix, allowing oxygen to reach the root zone and promoting efficient drainage. Make sure you buy sharp sand, not fine sand (sold as sandbox or play sand), which holds too much water. Sharp sand, available from building-supply centres, has larger particle sizes; if there are small rock chips in it, so much the better.
Perlite, which looks like bits of white Styrofoam, is super-heated volcanic material. It's a lightweight alternative to sand, providing good aeration and drainage. Perlite contains no nutrients and has a pH rating between 7.0 and 7.5. It's dusty to work with and not good to breathe.
Vermiculite, shiny flakes of super-heated mica, is a natural element. It holds moisture and nutrients in the root zone, releasing them gradually. Mica adds small amounts of potassium and magnesium to the soil, and has a pH between 7.0 and 7.5.
Peat moss, mined from acidic bogs where sphagnum moss plants have decomposed for thousands of years, is a limited natural resource. Fresh or composted shredded leaves are an excellent organic alternative. Peat moss improves the balance of air and water, and also softens soil texture for good root growth. It contains no nutrients, but has a low pH-between 4.0 and 5.0-that benefits alkaline soils. If peat moss dries out, it shrinks and sheds cold water. To re-wet, mix thoroughly with hot tap water before using, or stand plants in pots that contain dried-out peat-based mix in a bucket of warm water until thoroughly wet.
Blood meal is an animal byproduct used as an organic fertilizer. It's an excellent source of nitrogen and won't burn plant roots. Bone meal, also an animal byproduct, is rich in phosphate (11 per cent) and calcium (24 per cent). It stimulates root growth and promotes bud set in flowering plants. Bone meal is a slow-release nutrient that won't burn roots.
Egg shells and oyster shells contain generous amounts of calcium and trace minerals appreciated by some plants. Wash and dry egg shells (90 per cent calcium), then crush with a rolling pin or in a food processor. Pulverized oyster shells (33 per cent calcium) are sometimes available through plant societies.