Like well-nourished children who outgrow their clothes and shoes, a potted plant will also eventually outgrow its container home when it is lovingly cared for. Though it can be nerve-wracking to upend a favorite houseplant to move it into a larger pot, it is more harmful to the plant in the long run to let it suffocate in inadequate accommodations. Knowing how much potting mix to use when repotting a plant and how firmly to pack soil in around its roots are two key components of a successful repot.
Method for Repotting Plants
After removing a potted plant from its old container, brush away most of the old soil and inspect the root ball for problems. A plant with dense, circling roots should always be loosened thoroughly before repotting.
When repotting a plant, don’t treat the pot like a sand castle mold at the beach -- the goal isn’t to get as much potting mix in the pot as possible. Add a handful of soil at a time, gently but firmly packing each scoop in and around the roots so it still feels springy and spongy when pressed. Add more soil until the roots are completely covered and the crown of the plant is just above the soil line. Continue to mash the soil down with gentle pressure until the surface of the soil is even, and then water it well. A plant in a properly packed pot should not sag, lean or shift, and should not come out of the pot when pulled on gently. An over-packed pot will feel dense and heavy, and water will drain slowly.
About Soil Compaction
Although you may think plants only need soil, water and sunshine to survive, plant roots also need access to plenty of air. In outdoor soils, earthworms and other burrowing creatures provide valuable aeration services by breaking up tightly bound soil particles as they push through. Soil that has been compressed, or compacted, lacks the tiny air pockets critical for plants to survive.
Plants in pots need a lightweight soil mixture for good aeration, as potted plants typically lack earthworms to aerate the soil around their roots. Soil dug up from the yard should never be used for growing container plants, as it will compact tightly with each successive watering and drying cycle. Eventually, the plant roots will have little access to air and will be more susceptible to drowning and rotting because a tightly compacted soil drains water slowly.
Components of Potting Soil
Potting soils are specially formulated to provide adequate air, moisture and nutrients while also serving as a sturdy anchor for the plants. These fluffy blends usually include varying ratios of several materials. Peat moss and sphagnum moss are two main components, both of which help retain water to prolong the time between watering. Some form of tree bark is also usually present, and aids with drainage. Compost, composted manure and leaf mold provide a source of nutrients, along with other materials that provide trace nutrients, like bone, blood and feather meal, or cottonseed hulls. Other materials, like perlite, vermiculite and sand, in varying quantities, increase or decrease the porosity of the mix.