What's in Multi Purpose Compost

Dean Mears over on the Friendly Gardeners Facebook Group posted a brief and not exclusive list about potting compost and various additives. Those more experienced will have their own views, but there may be some information of interest to some and to newbies.

Dean posts a lot of interesting information and is well worth following.

Most potting soils are made up largely of peat moss, bark, and perlite. (Perlite looks like tiny white pellets; it's actually heat-puffed volcanic glass, included to keep the mixture from being too dense.) The best potting compost for your plants is the one you have researched and contains the correct mix and amount of ingredients and drainage materials.

Breakdown by component:

Peat — the basis of most mixes and used in high proportions. Spongy and holds water. It is no longer acceptable to use this so add coconut coir instead.

Perlite — white, light, pebbles formed from superheated volcanic glass. Aids in aeration and water control.

Sand — silicon dioxide. Aids in water drainage. Horticultural sand is the right structure to drain water, contains the correct minerals and should have been washed. If you are on a budget use builders or sharp sand and try to wash it, out of caution.

Vermiculite — helps hold water and provides a slow leak of micronutrients and places for fungi/microbes to aid the plants growth.

Wood chips/Bark — decaying organic matter that provides a slow release of macronutrients and is a “denser sponge” than peat. If cut coarse, can aid in drainage.

Compost — nutrient-rich and microbe rich matter that aids in plant growth. Smells earthy. Well rotted horse manure is a good additive to this.

Glass/Rocks — cheap filler found in not-so-great mixes. Although the rocks could provide a trickle of micronutrients, their weight is enough to make them a lousy ingredient.

Coconut Coir

Retains moisture, creates air pockets for excess moisture to drain away and provides a good environment: Coco coir is one of the most effective growing media for water retention out there. It can absorb up to 10x its weight in water, meaning the roots of your plants will never get dehydrated.

Leaf Mulch - Called 'gardeners gold' it is free, full of nutrients and good fungus and micriorganisms and adds great drainage.

Manures - are strictly speaking derived from animal faeces, urine and bedding, typically straw but sometimes wood chips or hemp fibre. Manure can retain residual weedkiller so the source should be considered.

Mushroom Compost - Is so high in organic matter. It can be used as a soil conditioner and as a mulch. Using mushroom compost will improve your soil's structure and water retention and add nutrients.

Worm castings - The humus in earthworm castings is promoted as helping increase soil's water retention, improve soil aeration and anchor plant nutrients that would otherwise leach away with water. Also, feed beneficial soil microorganisms that produce, store and slowly release plant nutrients into your soil to feed plants.

Virtually no nutritious value and unlikely to be significantly more beneficial, pound for pound, than an organically rich compost or manure mixed with coconut coir.

Garden Soil - Ordinary garden soil is not recommended as a potting mix. It’s usually too heavy and may contain weed seeds, diseases, and insect pests.