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Potting Mix and Soil Less Media
Historically container plants were grown in mixes based on composted soil and amended with
coarse sand and peat (John Innes mixes).
Today many container plants, whether in nursery, glasshouse or home situations, are grown
in what is called "soil less media" where the soil is replaced by other materials.
The most common materials used for soil less media preparations in New Zealand are:
- Peat (not regarded as renewable and therefore not sustainable organic practice);
- Bark; Sawdust; CompostPumice; and Sand.
Potential materials for soil less media are:
- Ponga; Sphagnum; Rockwool; Cocofibre; and Expanded Clay.
Why soil less media?
Using soil less media can guarantee a growing environment for seedlings which is disease,
pest and weed free. Compost and composted bark will help to suppress diseases.
Why propagating?
In case of high weed pressure in the growing areas transplanting propagated seedlings will
give higher success than direct sowing.
Using propagated plants can mean an earlier start in the season and less time locking up
productive ground in the field.
What makes a good soil less media?
A good mix is free from contamination, can hold water and nutrients well and provides good
drainage at the same time.
It needs to be able to store and supply a good amount of nutrients (high Cation Exchange
Capacity - CEC) and it needs to be especially able to hold on and deliver nitrogen (Nitrogen
drawdown index (NDI) at least 0.7=70% of nitrogen applied remains available after 4 days).
This is the greatest challenge in creating organic soilless media.
Some recipes recommend adding bentonite or zeolite clay to the mix and/or to the ingredient
to be composted for a soil less medium.
Addition of 10% NuFert® to a stabilised, inert composted media is a good way of germinating
seeds and starting seedlings. NuFert is a slow release certified organic fertiliser for growing
plants based on zeolite.
There are various commercial organic certified potting and seed raising mixes on the market.
Leaf mould compost is an excellent substitute for peat and has great water holding capacity
and humus content.
Recipes
A simple seed raising mix:
1 part leaf mould compost plus 1 part sharp river sand plus 1-3 parts mature compost
A simple potting mix:
1 part leaf mould compost plus 1 part sharp river sand plus 3-6 parts mature compost
A deluxe formula:
To make 1 cubic metre of potting mix ~ 400 litres of rich, mature compost (A level wheelbarrow
equals 10 litres) ~ 400 litres of composted bark fines ~ 200 litres of pumice
Add the following: 5 kg of Bentonite or zeolite ~ 1.5 kg of Gypsum ~ 1.5 kg of Dolomite
1 kg of Rockphosphate ~ 1 kg of Ground Kelp
Some recipes recommend the addition of dried Blood or Blood & Bone to media. Not only that
will you need to check with your organic certification rules regarding the option of adding these
materials, but some plant seeds will not germinate well in a high nitrogen mix e.g. brassicas,
tomatoes and lettuces, and high nitrogen mixes tend to attract pest and disease problems
such as Scariad Fly.
References:
HortFACT - Soilless Media in Horticulture, Mike Spiers - HortResearch, Ruakura
