Plant Potting Mixes
By: Peter Miles
Tags: Potting, Plant Propagation, Climate Change, Science Education, Horticulture

Plant Potting Mixes

Plant potting media and fertilizers.

Author in plant nursery. Image by author.

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.

Seed Raising with Vermiculite Grade 3. Image — Bunnings, Creative Commons.

With the development and increased use of non-permeable plastic pots a potting media with increased porosity was needed.

In the 1940s the University of California (UC, 2020; UC Mixes, 2011) developed potting mixes consisting of varying percentages of peatmoss and sand and added fertilizer. These UC mixes, as they became known, avoided much of the variability which came from using loams, compost and animal manures.

One of the commonly used mixes consists of 1 part coarse sand : 1 part peat.

To each cubic metre add 1.5kg Hoof and Horn or similar, 1.5kg superphosphate, 140gm potassium nitrate, 140gms potassium sulphate 1.5kgms calcium carbonate and 4.5kgms dolomite.

With the development of Controlled Release Fertilizers CRFs (Buechel, 2020) the Hoof and Horn, superphosphate, potassium nitrate and potassium sulphate in the above mix can be replaced with 2 to 3 kgms of CRFs, eg Osmocote or Nutricote or similar trade name.

Controlled Release Fertilizers consist of nutrient granules coated with a semi permeable membrane to slow the rate the nutrients entering the soil solution, this increases the effective length of time nutrient is supplied, from 3 months to 9 months depending on the product, and reduces over fertilization contamination, leaching and downstream nutrient pollution (Du, Zhou & Shaviv, 2006; Oertli, 1980; Wang, et al., 2020).

Substitute ingredients to the coarse sand and peat in the UC mix can be perlite or polystyrene foam (isolite) instead of sand, these will increase drainage and aeration if needed.

Peat (which is a non-renewable resource) has largely been substituted with pulverised pine bark, the bark from plantation pine trees for construction timber production was previously a waste product. Pulverised pine bark should consist of particles less than 6mm in size with about 40% less than 1mm.

Toxicity from tannins may be a problem in fresh pine bark but this isn’t always the case, composting in moist heaps for 8 weeks should overcome any toxicity.

Nitrogen fixation by decomposing bacteria is less likely when using pine bark as compared to pine sawdust but if necessary, this can be overcome with the addition of nitrogen fertilizer.

The use of non-soil substrates in potting media, sand, peat or pine bark, generally results in less occurrence of plant nursey diseases. This combined with good hygiene practices, such as washing hands and washing tools, plant containers and work bench surfaces, prevents many soil-borne diseases.

Nursery soil-borne diseases are fungi which cause rotting and death of plant tissues and include ground level damping off or collar rot and can affect young seedlings soon after germination.

Root rot in seedling roots and also in stems of cutting is caused by Rhizoctonia fungi and Pythium fungi, good drainage and good ventilation are necessary to prevent these diseases.

Other fungal diseases which can attack the foliage of plants are Botrytis, Schlerotinia and Fusarium and all are discouraged by increased ventilation of the air space above ground level.

Potting mixes may need water added at the mixing stage, enough to reduce dust but not too much making the mix water logged.

It is better not to let pine bark dry out before or after mixing into a potting mix as it is can be difficult to re-wet thoroughly.

Concrete mixers should not be used to mix potting media. Concrete contains heavy aggregate, gravel, which is lifted and then falls as the concrete mixer rotates thereby mixing the contents. Potting mix ingredients are relatively light and just go around and around in a concrete mixer.

Potting media mixers for commercial applications have a horizontally mounted spiral inside which brings the ingredients to the centre before being forced back to the ends.

If you are mixing a few cubic metres, shoveling the ingredients including the fertilizers from one heap to another a few times provides good mixing. This is easier on a concrete floor and keeps the mix cleaner.

Mix ingredients by shoveling from one heap to another then back again. Image by author.

All of these potting mixes come from commercial operations but I haven’t tried them all myself. Caution should be exercised using them the first time and before using them for valuable plants and to ensure you have the ingredients correct.

Measure the pH to ensure it is close to neutral or slightly acidic, see my article on Soil pH.

Toxicity from pine bark or excess fertilizer can be tested by germinating radish seed in the mix. It germinates quickly and is fast growing and will suffer root burn and death if toxins are present.

Alyssum seed can be used in a similar manner to test for excess ammonium.

The lime and dolomite in these mixes may be reduced in areas where the sand contains higher levels of calcium carbonate. Also, the irrigation water used may be ‘hard’ and contain calcium and magnesium.

Test mix for toxicity with radish seed. Image — Pxfuel, Creative Commons.

Potting mixes:

Potting mix for tubes (eg 50x50x150mm container)

2 measures double washed sand (coarse sand with less fine particles)

1 measure peatmoss

0.5 measure isolite (optional) (gives mix more porosity and makes the mix cheaper)

To each cubic metre add:

1.5 kgm Osmocote (3–4 months release)

100 gms Fritted Trace Elements (Fritted — dissolve slowly in soil)

100 gms Iron sulphate

2 kgm Dolomite

1 kgm Calcium carbonate

300 gms Potassium nitrate.

Potting mix for eucalypt species:

2 measures double washed sand

1 measure peatmoss

1 measure podzolic loam (from eucalypt forest)

To each cubic metre add:

2 kgms Mineral Mix

1 kgm Superphosphate

Or alternative fertilizer to above:

3 kgms Sierrablen (controlled release fertilizer)

400 gms Fritted Trace Elements.

Potting mix for Proteaceae species:

1 cubic metre of pulverised pine bark

0.33 cubic metre double washed sand

To each cubic metre add:

400 gms Fritted Trace Elements

2 kgm Calcium carbonate

4 kgm Dolomite

300 gms Potassium nitrate

3 kgms Sierrablen.

A Botanic Garden Potting Mix:

6.5 heap barrows of pulverised pine bark

2.5 heaped barrows of double washed sand

This makes approximately 1 cubic metre. To this add:

400 gms Fritted Trace Elements

2 kgm Superphosphate

2 kgm Garden lime

1.5 kgm osmocote (3–4 months release)

4 kgms Dolomite

300 gms Potassium nitrate

1 kgm Iron sulphate.

Seedling nursery mix.

0.5 cubic metre washed sand

0.5 cubic metre peatmoss

To this add:

2lb (0.9kgms) Potassium sulphate

18oz (500 gms) Iron sulphate

12lb (5.4 kgms) Calcium carbonate

3.5 lb (1.6 kgms) Superphosphate

400gms Fritted Trace Elements.

A Public Housing Authority Plant Nursery mix for container plants.

1 cubic metre washed sand

0.33 cubic metre peatmoss

Add:

400gms Fritted Trace Elements

2 kgm Superphosphate

2 kgm Calcium carbonate

4 kgms Dolomite

300 gms Potassium nitrate

1 kgm Iron sulphate

1.5 kgm osmocote (3–4 months release)

3 kgm osmocote (8–9 months release).

Potting mix with good aeration. Image — Needpix.com, Creative Commons.

References:

Buechel, T. (2020) Basics Of Controlled Release Fertilizers Basics Of Controlled Release Fertilizers | PRO-MIX (pthorticulture.com)

Du, C. W., Zhou, J. M., & Shaviv, A. (2006). Release characteristics of nutrients from polymer-coated compound controlled release fertilizers. Journal of Polymers and the Environment, 14(3), 223–230.

JIC (2020) John Innes Centre https://www.jic.ac.uk/about-us/history-of-plant-microbial-science-at-john-innes-centre/

Oertli, J. J. (1980). Controlled-release fertilizers. Fertilizer research, 1(2), 103–123.

UC Mixes (2011) University California Growing Media Mixes https://afghanag.ucdavis.edu/irrigation-natural-resource/files/soil-growing-mixes.pdf

UC (2020) Growing Media http://cagardenweb.ucanr.edu/Houseplants/Growing_Media/

Sierrablen (2015) https://icl-sf.com/global-en/product-guide/fertilizers/standard-granular-nutrients/sierrablen/

Wang, X., Zhang, X., Han, X., Liu, K., Xu, C., Hu, X., & Jin, Z. (2020). Performance adjustable porous polylactic acid‐based membranes for controlled release fertilizers. Journal of Applied Polymer Science, 138(2), 49649.