What is the difference between potting mix, and gardening soil or
topsoil? All soils are not created equal! It may surprise you to know
that potting mixes are manufactured and have very little soil or organic
matter content.
Potting mix is specially blended to hold the correct amount of
moisture in a container, and provide a stable growing medium for pot
plants. Potting mixes can, over time, become dry, worn out and begin to
repel water. It is at this time that we would top up our pots with fresh
organic material, like compost. This will add some much-needed vigour
to the potting mix and help your potted plants thrive.
Garden soils and topsoils are best mixed at least 50/50 with the
native soil to maximise the performance of plants in outdoor planting
beds. When used in a container, garden soils and topsoils may stay too
moist and promote root rot of your plants.
They also have a tendency to shrink and pull away from the sides of
the container when allowed to dry out too much. Our heavy clay soils
also become too heavy in the pot, and can set like concrete. Potting
soil used in an outdoor planting bed may be too well-draining and the
bed could dry out too quickly. It also stops the water from moving
freely through the garden, which will impact plant health.
In summary, potting mix is always best in pots, and garden soil is
best in the garden. To increase the performance of your potting mix,
why not blend 2/3rds potting mix with 1/3 compost?
Extract From The Healthy Urban Habitat