HOW TO MAKE YOUR OWN POTTING MIXES
If you have a compost heap and, better still, if it heats up to produce a dark, friable material, you are on the way to producing some excellent potting mixtures. Dig down into the heart of the heap and extract really well-rotted compost. This is where the highest temperatures will be and hence the compost is less likely to contain weed seeds or disease spores. You can use this material straight from the heap for larger plants in containers. However, if you want to make a more sophisticated mix, consider adding some of the following ingredients.
Leafmould is the dense, dark material that is formed when leaves decompose. If you own a deciduous woodland, you can dig deep through the layers of leaves on the forest floor until you find the dark, rich mould at the base of it all. Most people don’t own woodlands and the most environmentally sound alternative is to pile up autumn leaves and let them rot for a year or two; it takes at least this long to make really fine leafmould. This is a useful addition to home-made potting mixes. It absorbs water and so keeps the mix damp.
A leafmould bin
Compost from a wormery is too rich for most seed mixtures, although it is fine for use with larger plants.
Seaweed powder is great in any potting mix. Seaweed has a good range of trace elements and is very high in potash. Use one teaspoon per bucketful in mixtures for raising seeds. Increase the amount to two tablespoons per bucketful for potting on large plants.
Comfrey leaves should be chopped (wear gloves, as you may get a rash) and put in a black polythene bag. Ideally use alternate layers of comfrey and leafmould. The contents should be just damp rather than wet. Seal the bag and leave for two to five months before using.
Charcoal has a pH of 8–9 and will ‘sweeten’ an acid mix. It will also help to improve moisture retention in the compost. Always use activated (steam-treated) charcoal.
Sand plays a role in most mixes. It helps create air spaces and hence improves drainage. Sharp sand, or silver sand, is usually recommended, but try some building sand first. Rub a bit between your finger and thumb: if it feels silky, it’s no use; if it feels gritty and ‘sharp’, it’s probably OK. Sand from the seaside will have a high salt content, which isn’t ideal for seedlings.
Perlite and vermiculite are natural materials and are acceptable in organic mixtures. They are pH neutral, help improve drainage and will hold three to four times their weight in water. If sand and leafmould are in short supply, perlite or vermiculite will make a good addition to a mix based on dense garden compost or loam.
Manure must be rotted down before use. Cow manure, with organic straw, is one of the best for using in potting mixes. Goat, pig and horse manure are also good, but slightly more concentrated in nutrients. Poultry and pigeon manure can be too strong for potting mixes and certainly shouldn’t be used in the proportions given in the following recipes – a small sprinkling goes a long way. Manure gives a nitrogen boost to any mix. Apply it sparingly; otherwise it may lock up other trace elements. Never use more than one-third manure in a mix, even for the greediest plants.
Garden soil can dry out in a pot to form a concrete-like mass. This doesn’t mean garden soil shouldn’t be used, but do follow a few rules:
• Dig soil from a fertile part of the garden, which has a good humus balance.
• Don’t use soil where disease has been a problem, unless you intend to sterilize it.
• Never use more than two-thirds soil and balance this with leafmould, or a sand and leafmould mix.
• Soil from a molehill can make excellent loam for use in potting mixes – a great way of turning a pest to your advantage.
Lime or wood ashes will raise pH.
Coir fibre is quite acidic and can be used instead of leafmould to lower pH.
Finely shredded newspaper will hold water and can be used if that’s all you have got.
Sawdust will also hold water, but if you use it you run the risk of it robbing nitrogen from the mix as it rots.
Fish, blood and bonemeal will provide nutrients.
Tips and techniques when making potting mixes
• Use a riddle to sieve home-made compost. This will remove any large chunks of material and any wildlife.
• Small quantities of compost can be sterilized in an oven (80˚C/180˚F/Gas mark ½ or S on some cookers) for about one hour, or in the microwave (if sieved and contained in a plastic bag). Large quantities can be spread on a piece of flat metal over an open fire, or you can use a flame weeding device to apply direct heat.
• Mix small quantities in a bucket. For larger amounts use a wheelbarrow.
• Make sure the mix is damp enough before storing it in strong plastic sacks.
• Follow a recipe to make sure minerals aren’t locked up by an imbalance of materials.
1 part compost
2 parts fine leafmould
1 part sharp sand
Or:
1 part good loam (if the loam is very heavy add a little perlite)
1 part leafmould
1 part sharp sand
Recipes for potash-greedy plants like tomatoes
1 part compost
1 part sharp sand
2 tablespoons seaweed meal per bucketful of potting mix
Or:
1 part leafmould
1 part chopped comfrey leaves
NOTE: This mix doesn’t provide nutrients for long and you will need to use a liquid feed after four to six weeks, or top up the containers with more compost.
Comfrey
High-nitrogen and water-retaining recipes for larger plants in containers
1 part leafmould
1 part well-rotted manure
1 part compost
1 part perlite
Or:
3 parts loam
2 parts leafmould
1 part well-rotted manure
1 part sharp sand
Recipe for drought-sensitive plants
8 parts compost
2 parts charcoal
The charcoal controls the availability of water in the mix.