Plant Potting Mixes
Plant potting media and fertilizers.
Over the years I have collected lists of ingredients and the added fertilizers for different potting mixes used in plant production nurseries. These are listed at the end of the article for those plants people who would like to make their own potting media mixes.
Many home gardeners and hobby plant propagators prepare their own potting mixes from what ingredients they have on hand and by trial and error. Judging the suitability of the mix by appearance, feel and smell and if this is providing good quality plants, it is gardeners experience that should be followed.
But much researched has been conducted over the years to ensure potting mixes are designed so that they perform well, are reliable and don’t have variable performance caused by too much fertilizer, incorrect pH or lack of aeration.
In the 1930s the John Innes Institute in England (JIC, 2020) researched and developed several potting mixes using loam, coarse sand and peat.
John Innes seed raising mix:
2 parts loam : 1 part coarse sand : 1 part peat
To each cubic metre add 1.2 kilograms superphosphate and 600grams calcium carbonate (garden lime).
An alternative, more modern, seed raising mix is Vermiculite, Grade number 3, straight out of the bag and not mixed with other materials. It is therefore very clean; it holds water well and I have successfully used vermiculite for many germination seasons.
John Innes mix for plants in containers:
7 parts loam : 2 parts coarse sand : 3 part peat
To each cubic metre add 1.2 kilograms blood & bone (13%N) or similar, 600 grams superphosphate, 600gms potassium sulphate and 600gms calcium carbonate (garden lime).
The loam used was mostly from alluvial deposits by rivers and streams.
These older John Innes mixes were more suited to terracotta pots which are porous and both release moisture and let air penetrate through the sides of the pot.