Any growing medium should bind sufficiently to support plants. The organic component should be stable, that is, it won’t readily decompose to cause slumping. It should normally hold plenty of water and provide ample aeration.
Seed compost especially should be well aerated and warm, but contain little available nutrients or salts.
Compost for growing-on and plant containers should hold moisture and contain ample nutrients for the growing plants. Add to all this the individual requirements of different species.
Knowing the attributes of compost ingredients gives you an indication about the effect of including them in a mix or of changing the relative amounts.
Fine particles as in clay give fine pore spaces that hold water tightly and are more often saturated. Course particles, as in grit, provide larger pore spaces that allow water to drain through and the pores which then contain more air.
Garden Soil: Sandy and organic soils may be used in making compost. But soil used alone behaves differently when confined in small volumes. Pot walls alter the water balance and dry the soil which then hardens. The microbial population may not develop normally which affects nutrient supply, while weed seeds and disease organisms present a greater hazard to seedlings and plants.
However garden soil may be used as a constituent when mixed with other components and when pasteurized before use. See below.
Loam: Good loam is made by stacking turves upside down and covering for 6 – 12 months until the grass roots decompose. It is then sieved and pasteurized. This kills weed seeds and potentially harmful microbes. Sterilized loam is used in John Innes formulae.
Loam provides bulk and a fine texture but may become hard and impenetrable when dry. The high nutrient exchange capacity of loam is valuable for potting mixes that also include non humus organic matter like peat and leaf mould. Shortage of loam supplies led to the development of soiless compost. When buying loam, check it comes from a good source.
Garden Compost: Use only fully matured and sieved garden compost for making compost. High humus content holds water and contains plenty of nutrients. It helps to bind the growing medium. Adding grit, vermiculite, or perlite, will improve aeration when the mix is used in pots.
A 'stale' bed of garden compost can be used to remove germinating weed seeds before making compost for potting. Garden compost is easily made at home but it can also be bought locally. You need to check it is free of contaminants.
Re-used compost: Proprietary or rich medium used for growing tomatoes or cucumbers may be re-used in making compost for less hungry plants or seeds. Don’t use if previous plants infected by disease and always pasteurize before re-use.
Pasteurizing: This is done to kill weed seeds and active disease organisms while leaving useful microbes to re-populate. It is done by placing material in an ordinary oven at 800C for 30 minutes. I suggest saving energy by pasteurizing in the oven directly after cooking. Use an earthenware or tin container, certainly not plastic. Pasteurizing may create a smell. I don’t recommend microwaving mineral soils, it could be hazardous.
Leaf Mould: Should be made from fully decomposed leaves (perhaps 2 years old) and sieved. It has a course crumbly texture providing drainage and aeration while being moisture retentive. It does not however, contain abundant nutrients and may be contaminated with weed seeds or disease.
Shredded conifer leaves should only be used to make growing medium for Ericas. Avoid using evergreen leaves like holly, or laurel.
Leaf mould can be used directly for making compost to germinate seeds and is useful for potting mixes and adding to containers.
Worm Compost: Like a jelly when fresh, the high humus content holds water and helps to bind the soil. Adding sand, grit or loam will increase aeration, improve draining and make a more open consistency. Like peat, worm casts do not re-wet easily after drying out. They may not be stable due to continued decomposition which may then cause slumping.
Worm casts are rich in nutrients, especially nitrogen, and its phosphates are more available (ideal for roots). Worm casts are especially good for germinating seeds. When making compost I suggest adding worm casts in a lesser proportion or, add to holes before dropping seeds in.
Coir: A byproduct of coconut production and made from pith that surrounds the fiber. It is highly water absorbent; also, high in lignin and more resistant to decomposition than peat.
It is naturally low in nutrients but subsequent processing can result in higher potassium salt levels. I have read that coir may lock up trace elements like copper and molybdenum which could in turn cause nitrogen deficiency symptoms.
Coir has a fine rather loose consistency. It is a good base for making seed compost, but could ‘swamp’ fine seeds. In nature Coir fiber surrounds the coconut seed and is said to contain natural rooting hormones. When used directly as a seed compost take care not to overwater. Also, transplant when first leaves appear. It is sold as solid bricks and reconstituted by placing these in water.
Peat: Sphagnum peat is used abundantly for making compost. It comes from the dead remains of sphagnum moss accumulated over thousands of years. Real concerns about environmental impact have led producers to look for alternatives. However many gardeners regard peat as irreplaceable in the garden.
Peat is a stable organic material not given to slumping or rapid decomposition. Peat holds plenty of water and care needs to be taken not to over water seedlings. Loam, sand, vermiculite or perlite are useful additives to mix with peat to provide the larger structural pores necessary for sufficient aeration and drainage of excess water. When peat does dry out it difficult to rewet. Peat has no nutrient value until it decomposes to humus when it has high nutrient exchange capacity.
Sphagnum moss: This is made of living moss plants, it is not moss peat. It holds lots of water and has been used in hanging baskets, and for creating a moist atmosphere around plant leaves. It may also release chemicals that inhibit disease organisms. However, I would prefer to see alternatives used.
Sand: Sand is the silicate mineral quartz and has no nutrient value. Fine sand should not be used in making compost as it blocks pores. Course or sharp sand is important for adding larger pore spaces for free drainage and increased aeration. Silver sand has the lime removed.
Vermiculite: It is made from clay minerals. The internal pore space of Vermiculite particles absorb plenty of water while the external space is large to ensure free drainage and aeration. Vermiculite also provides a high nutrient exchange capacity.
Perlite: Fine grades of horticultural perlilte have surface pores enough to absorb plenty of water. Like vermiculite it provides external space between particles to ensure better drainage and aeration. A big advantage to perlite is in being very light.
A big advantage of making compost yourself is that you can deal with smaller amounts, reduce space for storage, put waste material to good use and better adapt the compost to suit plant needs.
You should experiment and try out your own mixes. Make use of the material that you have available in your part of the world.
The garden hand claw is an excellent tool for mixing small quantities together in a bucket or tray.
Some compost making recipes recommended by gardeners (All proportions measured by volume).
The well-known John Innes compost recipes. - compost sold under this name can vary, as the name was not patented.
NOTE: These John Innes compost recipes are not strictly organic where they include Superphosphate or Potassium Sulphate. These formulae detail what should be in the bag and may help you in making compost see below
John Innes seed compost: In the following loam is sterilized and peat and loam are passed through 9mm sieve.
John Innes Compost for Cuttings:
John Innes Composts No's 1 - 3:
plus one of the following fertilizer mixes:-
J.I. Compost No. 1 Fertilizer to add per 1 cubic metre of mix.
J.I. Compost No. 2 Fertilizer to add per 1 cubic metre of mix.
J.I. Compost No. 3 Fertilizer to add per 1 cubic metre of mix.
You'll notice that JI 2 has 2x the fertilizer in no 1, and JI 3 has 3X that in no 1.
The composts are used in increasing order when potting on. So delicate young plants are not harmed by strong fertilizer while robost plants in large containers get enough fertilizer to last.
UPDATES: to this page will consider fully organic alternatives like replacing superphosphate with Bone Meal or Rock Phosphate and making compost mixes for organic tomatoes.
John Innes Ericaceous Compost 2 loam, 1 peat, 1 sand. Fertilizer to add per 1 cubic metre of Ericaceous mix.
More on making compost