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 compos 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])
* 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 type of compost normally contains a larger reserve of plant nutrients, and trace elements, so the plants need less additional fertiliser during the growing season, thus making feeding at a particular time less critical.
- Some plants grow better in this type of compost.
- On the down side it is heavy to carry and less pleasant to handle when wet
- Due to a lack of good quality loam, the compost quality can sometimes be quite variable, and it can deteriorate in storage.
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:
- Is generally lighter to handle, more pleasant to use, and major brands are available all over the country, plus many plants prefer them.
- On the downside, they can soon run out of plant food, and can be quite difficult to re-wet.
- Natural Peat; has a pH level of 2.5 / 4.5, and this is considered to be far too low for most plants. To overcome this issue Peat based composts have a little lime* added to them to lift the pH value to around 6.5.
- The fertiliser content is in accordance with the type of compost being mixed, for example;
* Less in ericaceous composts.
Ji 1 has one measure of fertiliser added to the base mix- Ji 2 has two measures of fertiliser added and Ji 3 has three measures of fertiliser added.
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.
Method:
- Fill the domestic steamer no more than 6 in deep.
- Raise the temperature around 80°C (180°F) and sterilise the soil for ten minutes.
- Once sterilised; rapidly cool the steamer by dipping it in ice cold water.
- Once cooled the compost is ready for use!
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 legislation,which dictates that the use of Peat in compost mixes is to be gradually phased out.
The proposal is; Peat is to be phased out by 2020 for the amateur gardening market, and by 2030 for professional growers.
At the time of writing; Commercial Compost makers are experimenting with various recycled material e.g. Organic Green Garden Waste, Sawdust, Wood Shavings, and Coir (a waste material derived from the husks of coconuts) to replace Peat in potting composts.
The basic ingredients to date in Commercial mixes is Peat, Sand, pH adjustors and Fertilisers, with each supplier having their own recipes.
Note: As these experiments are still 'on-going' 'Peat' has been deemed to be the bulking agent in the following data. As an alternative you could substitute M/P compost in lieu of Peat
DIY 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.
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.
Perlite:
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.
Vermiculite:
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.
DIY 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!
Ingredients;
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.