How to Make Compost for Sowing Seeds
Pots, trays, and modules
Sowing seeds into individual pots is convenient if only a few plants are required, but takes up valuable space on a heated bench. For large numbers —of bedding plants, for example — sow the seeds into a single pot then prick out the seedlings into trays as soon as they are large enough to handle.
The alternative is to sow directly into modules (trays divided into individual cells) or into blocks of compost compressed into shape with a special blocking tool. This saves time on pricking out and avoids the damage and shock that can occur to seedlings when they are transplanted. It is the best way to sow seeds that are normally sown in situ — hardy annualand root vegetables such as beetroot, for example.
Germinating seeds and young seedlings are delicate and vulnerable, so getting the sowing mixture right can be critical. It needs to be:
- Free-draining, to minimize risk of fungal disease.
- Free from pests and disease organisms.
- Free from weed seeds.
‘Seed’ composts, used when seedlings are going to be pricked out, need not contain many plant foods as most of the seedlings’ requirements are contained within the seed itself. However, the compost into which they are pricked out, or which is used to fill modules for direct sowing, must contain sufficient food to sustain the young plants for several weeks – ideally until they are planted outside or potted up (using one of the potting compost mixtures).
Because the sowing mixture is so critical, and only used in small quantities, buying a proprietary brand may be preferable to making your own. The organic ‘multi-purpose’ composts on the market should be suitable for most seeds, although you may need a ‘seed’ compost for a few seeds that will only germinate when the nutrient level is very low. If you want your own mixture, use the recipes below as a guide – they should be successful for at least larger seeds such as beans and cucurbits, and for vigorous ones such as brassicas. The special ‘blocking’ composts on the market are designed to hold together well when compressed. They usually rely upon a mixture of different types of peat for their stickiness and are not easy to make yourself.
Home-made sowing composts
You can use these mixtures like multi-purpose composts, in modules and small pots. Check the pH of your mixture. It should be around 6.0. If it is too high, then add dolomite lime or calcified seaweed.
- 1 part sterilized loam, 1 part sieved leaf mould or coir, 1 part sharp sand.
- 2 parts sieved comfrey leaf mould, 3 parts sieved leaf mould.
- For a very low nutrient seed compost, try sieved leaf mould on its own.
Sowing and pricking out
I. Fill a tray or pot loosely with moist sowing compost so that it is level with the top. Firm it down gently, either with a piece of wood made to size or with the bottom of another pot.
2. Sow the seed as evenly as possible, using your fingers, the tip of a knife, or a plant label. It helps to mix fine seed with about twice its volume of silver sand. Do not sow too thickly — this encourages fungal diseases.
3. Cover all but fine seeds with a loose layer of sieved sowing compost to a depth of about twice the diameter of the seed. Leave fine seeds on the surface.
4. Water fine seeds from underneath by standing the tray or pot in a shallow bowl of tepid water. For bigger seeds use a watering-can with a fine rose.
5. Cover the tray or pot with a sheet of clean glass, cling film, or a polythene bag to keep in the moisture until they germinate. For seeds that need darkness for germination, put on an additional thick wad of newspaper, or use black plastic.
6. Prick out the seedlings as soon as they are large enough to handle. Ease them out with a piece of wood or a plant label, holding them only by their seed leaves, never by the stems.
7. Space them out in a tray of moist multi-purpose compost, giving each one enough space to become a sturdy plant. When the tray is full, water with a fine spray from a watering-can or hand sprayer. Alternatively, sow seeds in a tray of individual modules, so no transplanting is necessary.