Can I reuse old compost?

We’ve all got half a bag of multipurpose compost kicking around the greenhouse and as long as it’s this season’s compost, hasn’t been left out in the rain and smells fresh it should be fine for planting mature plants into pots and containers. It’s a bit risky using it for sowing seeds, taking cuttings or potting up young plants as it may have acquired a few bugs and spores while it’s been lurking in the greenhouse.

If you are not sure, then dig it into the soil somewhere in the garden and use it as a soil conditioner. Compost does have a sell by or use by date, even if it’s not obvious, the best rule of thumb is to buy fresh compost each year from the garden centre when the new stock is delivered and keep this somewhere cool and dry. A shed or garage is better than the greenhouse, which tends to have extremes of temperatures and can get very hot. Compost is finely tuned and contains fertilisers specially balanced for your plants, this formulation can be affected by temperature, so you need to store it correctly for the best results.

Compost that is very old, or that smells or you are really unsure about can be added to the compost heap, mixed in and allowed to rot down, the micro organisms in your compost heap will soon do battle with any bad bugs in the old compost and breathe new life into it. It can then be used as a mulch or soil conditioner after three to six months.

Compost that has been used to support a crop of salad, or beans or some other early veg can be used again to grow salad. Simply remove all the old plants and roots from the last crop, lightly loosen the surface and resow into the compost. Unless the previous crop succumbed to a pest or disease the compost should still support a quick crop. Bear in mind that any nutrients in the used compost will have been depleted so it’s a good idea to mix in some slow release fertiliser or feed your plants as soon as they start to mature.

Last years planters and grow bags can be used to grow crops that don’t need too much fertiliser like carrots or beetroot or a leafy crop like spinach or rocket. Otherwise, empty them onto raised beds and borders and allow the worms to work the compost into the soil.

In the greenhouse border the soil is used again and again, it’s a good idea to dig some of it out and refresh with fresh compost each year. Sometimes the soil can become sick and plants don’t grow well, if this happens remove as much of the old soil and compost as possible and replace with a quality loam based compost. When planting into this area use an additive that contains friendly fungi and bacteria to replenish the lost vitality. You can also consider growing grafted greenhouse plants that have a stronger root system that is resistant to many soil pests and diseases.

Ideally buy just enough compost for your needs each year so that you don’t spend more than you need to. Buying in bulk can save money but only if you use it all. Growing media (compost) is specially formulated for your plants; choose a recognised brand and a product tailored to your plants needs. Why not share a bulk buy with gardening friends early in the season and then if you need some more, just buy a single bag to keep you going.

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