Have you ever read the labels of potting soil? What is all that stuff, anyway, and why is it in the mix? Some ingredients make you wonder how dirt got so complicated.
Think about the job that soil needs to do. It holds plants upright; the mix needs to have substance that will not wash away and expose roots. Soil also needs nutrients to feed the plants and a means for water to percolate through and supply the plants with enough moisture.
When you think of it in those simple terms, the ingredients make sense.
Here are a few basic components of soil mixes:
• Fir bark – Tree bark ground to add substance to finer ingredients and allow smaller particles to cling together. Because it’s a natural material, it will break down and release nutrients into the mix as well.
• Compost – Organic substances (not fresh or raw) broken down that add high amounts of nutrients and microbial activity.
Typically, compost is a blend of cow manure, but it also can be chicken manure, bio-solids, fish by-products, bat guano and more.
• Worm castings – The remnants of what earthworms leave behind after they munch their way through soil and organic materials. Worm castings are a good source of nutrients, is pH-balanced and adds micro-organisms to help soil stay alive.
• Sand – Small particles of rock. Sand is the best way to allow space in-between the heavier ingredients of the mix. The air pockets add good water drainage and healthy air exchange so soils don’t become boggy or sour.
• Perlite – Small white pieces of light volcanic rock product. Perlite is used to loosen soil and prevent heaviness and compaction. It helps maintain drainage and soil structure.
• Vermiculite – The little shiny flecks in potting soil mixes, a form of mica rock used for aeration and water retention.
• Peat moss – Harvested from sphagnum peat beds, it provides aeration and helps the soil retain moisture. It also is used to lower pH.
• Coco peat – An alternative to peat moss made from the coir or husk fibers of coconut. It lasts three times longer than peat moss and is used for water retention, nutrient storage and to balance pH.
Most all-purpose soilless mixes are blended with bark, peat moss, perlite and other nutrients.
The latest trend in commercially bagged soils is a concoction of these typical ingredients, plus moisture-retaining gels and time-released fertilizers. Read the label on specialty potting mixes to avoid using them for the wrong plantings.
Time-release fertilizer mixes work well for heavy-blooming annuals that need a steady stream of nutrients through the summer.
Specialty mixes for orchids have ingredients that take longer to break down to provide good drainage and root stabilization.
Succulent and cactus mixes have more sand for good drainage.
Seed-starting mixes are larger ratios of perlite or vermiculite for lightweight blends that have good moisture retention.
Now that you know, here are a few recipes to make your own mixes:
• Homemade seed starting mix: one part perlite to two parts well-matured compost. Add to two parts water-soaked peat or coir.
• Basic soil-based potting mix: one part well-matured compost, one part garden topsoil (use only the first few inches of clean, weed-free garden soil), one part clean sand and one-half part perlite.In the Garden columnist Sue Goetz can be reached at email@example.com.