Home-made Seed and Potting Compost

Homemade garden compost is usually used to enrich and add organic matter to the soil but it can also be used to make seed and potting mixes.  I would recommend that all home composters give homemade seed and potting composts a try. Indeed, I have included  a “compost comparison" project as part of our 2016 Compost Awareness activities and hope that you will join in once full details are available. Compost Week 2015/16

The requirements of a homemade seed of potting compost are the same as for those provided commercially. It should be:

  •  free from pests and pathogens (plant and human)
  •  moisture retentive while allowing air circulation
  •   of a uniform consistency (sieving may be necessary to achiev this)
  • of a texture that will allow root development  and support for the seedling
  • and contain sufficient nutrients.

 

 When making homemade compost mixes only mature i.e. fully decomposed compost should be used and this should be sieved (screened) so that it consists of small particles of a relatively consistent size. 

The cheapest shop option for small quantities is a  Seedling Potting Riddle this has a  4mm galvanised mesh sieveto provide a  compost for planting seed. 

A hand operated  Rotary Soil Sievis a more expensive option which  will create a compost, suitable for  for Seed and Potting compost. Dry compost from the bin on the seive  and the handle is rotated pushing the material  through the 2.5cm/1" x 1.5cm/0.5" mesh. Both of the above can be purchased from http://www.harrodhorticultural.com and other suppliers.

 While sieves, such as the above, can be purchased it is relatively easy to make a wooden a box frame of a size to fit onto your wheelbarrow. A strong weld mesh base with 6mm holes should be fitted securely to the frame. Remember it will be used  quite roughly so make sure that the frame and base are well secured.

Homemade Seed and Potting Compost - Risks

Making homemade seed or potting compost using garden compost direct from the bin or wormery for germinating seeds,  potting on  young seedlings and rooting  plants from stem cuttings risks the introduction of  weed seeds,  pathogens or organisms that  may damage tender plants . The same risks exist if untreated soil is used as part of the compost mix.  

It is not necessary to heat treat the garden compost or soil mixtures used for potting on those plants that are almost ready to be transplanted into the garden. 

Oven sterilisation of Compost

Sterilising the compost or soil in Pyrex dish or metal roasting tray (covered by foil) in a preheated oven at 160C  for an hour is an effective means of sterilising the compost.  The dish or tray should be filled to a depth of not more than  four inches so that the compost in the centre  reaches the required temperature.  The tray of sterilised compost should  be remove from oven and allowed  to cool before use.

Gardening Knowledge at www.gardeningknowhow.com  suggest a similar method but using lower temperatures involving baking at  82-93 C. (180-200F)  for at least 30 minutes. It is suggested that heating to higher temperatures can result in the production of toxins

Sterilisation is an effective way of eliminating the risk but it means that everything is killed  is including beneficial organisms. 

Heating the garden compost, or soil, at a lower temperature with remove pathogens  without damaging nutrients .  Motherearth  (www.motherearthnews.com/homesteading-and-livestock)  suggest a method for the pasteurization of compost so as to  destroy harmful organisms but not the  beneficial ones.  Compost is placed in an aluminium baking pan (I prefer a Pyrex baking tray) again to a depth of  4 inches.  A  meat thermometer is placed in the compost at centre of the tray, so that it can be read while the tray is in the oven.  Pre-heat the  oven  to 93C ( 200F)  and insert  the tray. Bake for 30 minutes once the thermometer records a temperature of  71C (160F).  Cool the compost before use. 

Sterilizing Compost with a Microwave

Gardening Know How (www.gardeningknowhow.com)  suggest a method of sterilising compost or soil using a  microwave.  About a kilogram  of  compost is  heated in  a microwavable container covered by Clingfilm with added ventilation holes.

The compost is heated for about 90 seconds per every kilogram (approx 2  pounds) on full power. 

Alternatively a kilogram of moist soil can be microwaved in an open  polypropylene bag for 2 to 2 .5 minutes on full power (650 watt oven). The bag is closed and  allowed  to cool before removing.

A   problem common to all methods involving invading the kitchen is that the sterilisation or pasteurisation  will give rise to an earthy smell that may not be popular with everyone in the household.  An alternative could be used a propane gas Outdoor Camp Oven.  

Seed Compost

 Seeds contain stored  nutrients and will  need few additional nutrients to germinate and start to grow , so compost straight from the bin or wormery will be too rich to be used as a seed compost. It will also be of the wrong texture and may not provide adequate drainage.

While  Leafmould has a low nutrient level and could  be used as initial seed compost the seedlings would need to be pricked out and transplanted almost immediately after they have germinated to provide sufficient nutrients for future growth.

It is therefore better to make a seed compost  containing just sufficient  nutrients  to provide for germination and initial growth. Then, as the seedlings grow, and  need more nutrients transplant them into larger pot containing  a potting compost with a  richer mix  of nutrients. Potting onto larger pots, may be required several times, increasing the size of the pot gradually, when the roots nearly fill the existing pot.

  In addition to providing nutrients the compost should provide drainage, so that the soil does not become waterlogged and cause the seedling to rot, while retaining sufficient moisture for the plant to grow. It will need to contain air spaces so that soil microbes and the roots will have oxygen while being of an even consistency free from lumps.  This favours a light or fine textured  mix but it must be sufficiently firm to retain and support the seedling as it grows to a size suitable for potting on. It should also retain its volume in the pot and be free from pest and disease.

 

Not much to ask.

Homemade Seed Compost Mixes

Leaf mould

  • Leaf mould, two years old.

Once germinated seedlings need to be transplanted quickly

Leaf mould and loam

  • One part Leaf mould
  • One part Loam -

Needs careful watering, not suitable for very small seed

Leaf mould, compost, molehill soil

 

  •  One part kitchen compost,
  •  One leaf mould and
  •  One part molehill soil

 

Leaf mould/compost, Molehill soil and sand

  •  Two parts molehill soil
  • One part sand
  • One part sieved leaf mould or garden compost.

 

Leaf mould, compost and soil

  • One part Loam
  • One part leaf mould
  • One part garden compost -

a good, general mix

Leaf mould and worm compost

  • Three parts  leaf mould
  • One part  wormcast

a rich mix

Leaf mould, loam, sand

  • One part loam; sieved and ‘pasteurised’
  • One part leaf mould, composted bark, or coir.
  • One part sharp sand or vermiculite.

 

Compost, loam, sand

  • Two parts  loam
  • One part compost
  • One part sharp sand

 

Loam, compost, leaf mould and vermiculite mix

  • One part loam,
  • Two parts garden compost,
  • Five parts  leaf mould,
  • One part  vermiculite,

 

 

Loam, compost, leaf mould and sand or grit

  • One part loam,
  • Two parts garden compost,
  • One part  leaf mould,
  • One part sand or grit.

 

A variant of this mix can be used as a potting mix when the young seedlings when ready to transplant  (see below).

The above mixes are taken from a variety of sources many appearing several times in various publications and on different web pages

Home Potting Compost Mixes

When the seedling has rooted it will need to the transplanted to a mix containing more nutrients  to support growth until it is of a size suitable for planting out. Additiona ingredients, such as sharp sand or vermiculite can be added  be added to the potting mix to produce a  freer draining compost.

These free draining composts can also be used for cuttings.


Loam, leaf  mould

  • One part Loam
  • One part  leaf mould or coir

Will not sustain plants very long

Leaf mould, worm compost

  • Three parts leaf mould
  • 1 part worm compost

The worm compost makes this a mix richer in nutrients

Leaf mould, loam compost

  • One part  loam
  • One part leaf mould
  • One part garden compost.

 

Leaf mould, loam, compost, sand or grit

  • One part leaf mould
  • One part loam,
  • One part garden compost,
  • One part sand or grit

This nutrient rich mix should have a good structure

Leaf mould, manure , loam

  • Three parts  Loam
  • One part  rotted manure
  • One part leaf mould

Very rich mix

Leaf mould, loam, compost

  • One part  leaf mould or coir
  • One part Loam 
  • Two parts garden compost

Good, general  mix.

Coir, compost, vermiculite, worm-castings

  • One part pre-soaked Coir
  • Two parts garden  compost
  • One part Vermiculite
  • 100-200g worm castings

 

Leaf mould worm compost

  • One part worm compost or rotted manure
  • Three parts leaf mould, composted bark or coir

 

Compost, soil, sand or vermiculite

  • One  part garden compost;
  • One  part topsoil
  • ·        One  part sand, vermiculite or  perlite

This mix  has good drainage

 

Compost, soil, sand

  • One part  mature compost or leaf mould,
  • One part garden topsoil
  • One part  sharp sand (or vermiculite or perlite)

Heavier than modern peat mixes, but still has good drainage.

Compost, loam, sand

  • One part mature garden compost or leaf mould, sieved
  • One part fine garden loam
  • One  coarse sand

 

Sources:

 Among the sources, in addition to Garden Organic, are:

http://www.askorganic.co.uk/organicgardening

 George Kuepper, "Potting Mixes for Certified Organic Production," National Sustainable Agriculture Information Service 

http://www.motherearthnews.com/

http://themicrogardener.com/

 

 

 

 

Cutting mix

A suitable compost for cuttings will must provide  good drainage so that the cuttings do not rot  as well as sufficient nutrients to support growth.   

 

Loam, garden compost grit or vermiculite

  • One part loam (sieved but not pasteurised).
  • One part garden compost, leafmould or composted bark.
  • One part horticultural grit, sharp sand, or vermiculite

Soil, compost, sand

  • Seven parts molehill soil,
  • Three parts garden compost/leafmould
  • Two parts sand.

Leaf mould, compost , grit

  • Four part  leaf mould,
  • Five parts grit,
  • One  part garden compost.

Compost for container Plants

When permanently growing plants  in pots or containers a more nutrient rich mix will be required. It is necessary to provide sufficient nutrients to feed the plant during both its growth and flowering and/or fruiting when growing plants such as tomatoes and peppers. I addition to the compost I would suggest using a liquid feed when they a fruiting. worm compost can also be used to provid an additional nutrient boost.

 

Compost, loam or leaf mould

  • One part loam, coir or leaf mould
  • Three parts garden compost

Supplement with liquid feed and/or worm castings when fruiting

Leaf mould, compost

  • Three parts leaf mould
  • One part worm compost or castings

 

 

 

  • Peas sown

    The following photos showing a comparison of leafmould and compost are taken at Whittington Primary School in Shropshire.

  • Peas week 1

  • Peas A week 2

  • Peas A Week 2

  • Peas A Week 3

  • Peas A Week 4

  • Peas A Week 5

  • Peas B Week 2

  • Peas B Week 4

  • Sowing Sunflowers

  • Sunflowers Week 1

  • Sunflowers Week 2

  • Sunflowers A Week 3

  • Sunflowers B Week 3

  • Sunflowers Week 4

Sunflowers
Potting Compost v Seeslings left in Leafmould Seed Compost