Seed Compost


There are numerous types of 'compost' available from suppliers and garden centres these days, the best known being; John Innes seed and potting composts,these are classified as 'soil based' composts.

The original seed compost; formulated by the John Innes Institute, consisted of;

  • 2 parts sterilised loam.

  • 1 part peat and 1 part sand (parts by volume)

  • 400grams(14oz) superphosphate.

  • 115grams (4 oz) ground chalk* or limestone* per bushel (5 cubic feet [0.15 cubic metres])
Ji Seed Compost

* The chalk element should be omitted when mixing compost for lime-hating plants.

The formula was never patented. and seed composts, sold under the name of John Innes, are not always made to the Institute's specifications.

Sadly, John Innes composts are now becoming less common due to manufacturer's having difficulty in obtaining good loam.

A question often asked is; how do loam-based composts compare against soil-less compost ?

Loam based compost:

This added weight however can be useful in counter-balancing large potted plants, and in the event of the compost drying out it is easily re-moistened.

Soil-less compost:

To overcome this issue; peat based composts have a little lime* added to them to lift the pH value to around 6.5.

* Less in ericaceous composts.

As a general rule, the fertiliser in any of the general composts is only available to the plants for about 5/6 weeks.

After this period, supplementary feeding is generally required.

DIY composts;

The keen amateur gardener may attempt to make his / her own compost then find that they have difficulty with sterilisation!

There are a number of small electric sterilising units available on the market, but the amount of compost being made may not justify the cost of such a unit.

As an alternative; small amounts of soil may be sterilised in a domestic steamer.

34 litre Sterilising Unit


The Basic Seed Compost recipe:

Most compost mixes use the same bulking material, which up until recent times was peat, but this is soon to change, due to recent European legislation,which dictates that the use of peat in compost mixes is to be gradually phased out.

The proposal is; that peat is to be phased out by 2020 for the amateur gardening market, and by 2030 for professional growers.

As a consequence; commercial compost makers are currently experimenting with various recycled material such as; organic green garden waste, sawdust, wood shavings, and Coir, (a waste material derived from the husks of coconuts) to replace peat in potting composts.

At the time of writing, compost makers have been experimenting with peat reduced, and peat free recipes, resulting in mixed results and some rather bad press on the 'quality' of these products.

Hopefully this situation will resolve itsself by 2020.

Meanwhile; and for the purpose of these recipes; 'peat' has been deemed to be the bulking agent.

In commercial mixes the basic ingredients are; Peat, Sand, pH adjustors and fertilisers.

Recipe Ingredients;

Silver Sand;

Is a washed horticultural grade lime free sand, where the particle size is usually under 1mm in size.

The purpose of the sand is to aerate the peat and improve the passage of moisture (water) through the compost.

Silver Sand

River or Sharp sand:

Is sometimes used as a more economical alternative.

This type of sand has a much lager particle size, and may contain lime which is not good if an ericaceous compost is required.

This is not to say it should be avoided as its particle size can benefit the mix if the peat being used is not sieved.


Is a natural mineral that has been heated to produce a porous material to aerate the peat, and improve the passage of moisture (water) through the compost.



Is natural mineral with a 'plate like' structure that is a pre-heated to give it good moisture holding attributes.

It is usually available in three grades;

  • Seed Grade: Is a fine textured structure, ideal for covering fine seeds after sowing, particularly if the seed requires light'to assist germination.

  • Medium Grade: A multi-purpose grade suited to seed and cuttings composts, and sometimes it is used for protecting bulbs and tubers in storage.

  • Coarse Grade: The larger particle size makes it a good material for aerating potting composts and retaining moisture.

*Subject to your quantity needs; sifting the 'coarse grade' through medium / fine sieves will avoid the need to purchase all three grades.


Seed Grade
Medium Grade
Coarse Grade

Seed Compost Recipe:

A wide range of media can be used to germinate seeds, with experience, you will learn to determine which one works best for you.

  • The germinating medium should be fine in texture, be of uniform consistency, yet well aerated and loose.

  • It should be free of insects, disease organisms, weeds, and weed seeds.

  • It should also be of low fertility and capable of holding moisture, yet have good drainage properties.

Commercial mixes generally have low fertility, so seedlings must be watered with a dilute fertiliser solution soon after germination and emergence.

Garden soil should never be used as a seed compost for a number of reasons e.g;

It is not sterile, it is too heavy, plus it does not drain well!


Using a 10 litre (2 gallon) bucket measure out;

6 buckets of peat and 3 buckets of sand, perlite or vermiculite.

*There is no need to add fertiliser, as there should be sufficient feeding in the Endosperm within the seed to sustain the seedling until pricking out time.

Mix the ingredients thoroughly to produce 70-75 litres of compost.

Sieved Peat
Sieved Peat and Silver Sand
Sieved Peat and Perlite
Sieved Peat and Vermiculite


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