Why Fresh Compost is Always Best

This month, we explore what can happen if compost is stored incorrectly – and what to do with leftovers…

Anyone who has read the small print on the back of our SylvaGrow bags will see that we recommend using the product within the season of purchase. This may seem a rather vague statement – how long is a season after all? But it’s there to convey that fresh compost is always best.

A bag of compost sold in a garden centre is usually a combination of two or more bulk ingredients. For example, SylvaGrow contains our own composted woodfibre/bark, together with a small percentage of coir. Fertilisers and lime are also added in order to provide all the nutrients that the plant will require for the first few weeks, and to give the correct pH level.



Creating a stable base?

As growing media manufacturers, we have to consider two things when designing our products. That the product may be used within days of being made. But equally, it may sit in a garden centre for many weeks before being sold.

The fertilisers are added as dry powders. But as soon as they are blended and come into contact with the bulk ingredients they become slightly damp. So, we have to choose fertilisers that remain stable even under moist conditions.

However, plants only take up nutrients when they are dissolved in water. So in order to be usable by the plants it is necessary for the fertilisers to go into solution. Therefore, as well as staying stable in an unused bag of compost, the fertilisers also have to gradually dissolve once the compost is in use.


Getting the right balance

Fertiliser technology has improved greatly over the years. In normal conditions it is not too difficult to achieve this delicate balance between stability in the bag and availability to the plant.

However, if the bag of compost is left outside, on a pallet in a garden centre for example things can be very different. If rain is allowed to penetrate, the wetness can be enough to mimic a situation similar to a pot being watered. The fertilisers can prematurely go into solution. Then, when you first water it, far more nutrient is leached away than would be the case in a well-stored bag.

Even in a well-kept bag of compost, with an optimum moisture content, there can be a gradual change in the availability of some of the nutrients. This is caused by both biological and chemical mechanisms. Although usually slow, it is another reason why using fresh compost achieves optimum performance.


What to look for in the shops

When purchasing compost, always try to make sure the stock is new. The packaging should look clean and fresh. If the bag feels really heavy this could be an indicator that it has taken in rainfall so avoid bags like this.

On SylvaGrow bags, the date of manufacture is ink-jetted onto the side of the pack. Look for it near the base or the top so you can check how old the stock is.


SylvaGrow John Innes 2 potting up peat free compost

Use up half-used bags of peat free compost

What to do with leftovers?

A half-used bag of compost can attract all sorts of garden life. Slugs, snails and various insects will all happily find their way in. Although none are likely to cause serious problems – this is not an ideal way to start plants off.

If you have part bags of compost left over from one season to another, these are best used for applications where the compost quality is less important. Spread onto borders as a soil improver, use in the bottom half of large containers, or for containers of bulbs.

Remember – fresh compost is always best!