Article 4 in Composting Series
Renewing and reusing potting compost is one way we can save some money. It’s also good practice from an environmental viewpoint, being more sustainable.
Crop Rotation
The principle of crop rotation teaches that using the same soil for the same plant family year after year will lead to problems as disease and pests build up. It’s important to consider what the compost has been used for previously and re-use it for plants from a different family.
Nutrient Depletion
Over the season the plants grown in the potting compost will have used up the nutrients. Container grown plants will have been fertilised but commercial fertilisers may not contain micro-nutrients.
Lack of a micro-nutrient (iron, manganese and zinc) will have a disproportional effect in the same way as a lack of a vitamin can make a person very ill.
Practical Method for Reusing Potting Compost
Screening and Removing Debris
Start by removing any plants with their roots to the compost heap. Then empty the pot into a larger container. A wheelbarrow is ideal for this.
Spread the compost out and remove any roots still in there. Look for pests like slugs, snails, their eggs and other insects such as vine weevil. By leaving these spread out on a hard surface the birds will enjoy a treat.
Sterilisation (optional):
Solarisation
This is easily done, effective and basically free. Spread the compost out so it’s around 50 mm (2 inches) deep. Cover with plastic, preferable clear if possible. Left over bubble wrap, wrapping or old compost bags opened up is perfect for this. The sun will heat the compost below. It does depend on some sunny warm weather which can be short supply at the end of the season!
Rotation
Avoid re-using compost previously used for growing the Solanaceae family (potatoes, tomatoes, peppers, aubergines) for growing these plants again in this compost. The next crop will lack the necessary nutrients and there is a danger that blight will be present in the soil and infect a crop in the same family. Best to use the compost for other crops like leeks, garlic, carrots or flowers.
Nutrients
The old potting soil is most likely nutrient depleted. Add slow-release fertiliser pellets which will ensure there are sufficient macro-nutrients available for the new season. Follow the instructions on the fertiliser pack.
Acidity
The growing medium may have acidified over the season. The aim is to have a pH between 5.5 and 6.5. Add a dusting of dolomite lime and mix as you go but do this early and leave to get absorbed before use. Ordinary garden lime is OK but dolomite lime adds magnesium.
Incorporate Garden Soil and Fresh Compost
Mix in fresh compost at a rate of 1 part new to 3 parts old. This will further refresh your old potting compost for re-planting.
Alternatively adding sieved home made compost and or leaf mould will add body and structure to the compost.
Add sieved garden (that hasn’t grown the same plants) at a rate of 1 part to 4 parts old. This will add those small creatures and bacteria needed.
Adding some perlite when mixing will keep the growing medium open and improve water retention. Between 1 part in 10 and 1 part in 5 will be right.
Conclusion
So with your spent compost mix with fresh home made compost and soil to add nutrients. Add some slow release fertiliser, grit or perlite and some leaf mould. This will liven up your spent compost to use again. But don’t use for the same crop like tomatoes or potatoes.
A Note about Slow Release Fertilisers – Environmental concerns. Plastic coated.
Better to use natural fertilisers (see N-P-K article), or add fertiliser a few weeks before you use your re-invigorated potting compost.
Of course manure will supply valuable humus that the fertilisers will not. There’s more information on the N-P-K values of manure in the table below.
Unlike artificial fertilisers, the ‘natural’ fertilisers tend to dissolve slowly and thereby release their nutrients more slowly. More information see article 3 in composting series.
Artificial Fertilisers
These are manufactured chemicals or sometimes mined and processed minerals. They are not usually approved for use in organic systems but they do not have any potential risks like pesticides to humans. More information on artificial fertilisers see article 3 in composting series.
Nutritional Value (N-P-K) of Horse and Cow Manure
| N-P-K | Horse Manure | Cow Manure | |
| Nitrogen Content | 0.7% | 0.6% | |
| Phosphorus Content | 0.3% | 0.4% | |
| Potassium (Potash)Content | 0.6% | 0.5% |