Composting to improve poor soil

Val Bourne / 19 December 2014

Read gardening expert Val Bourne's guide to making compost to improve poor soil.

How to make compost

When you start making compost, the overall aim is to make a multi-layered sandwich using plant material and the most useful system is to construct three square wooden bins, so that one is empty, one is ongoing and one bin (complete with cover) is actively rotting down. This three-bin system is ideal because the empty bin can be used to turn the almost-ready heap - you just move it fork by fork.

This turning process adds air and makes the compost crumbly. You can easily make your own bin and Garden Organic have a simple link on their website to show how it is done. Recycled wood could be used, but as I am organic all the wood used in my garden and allotment is untreated hardwood as chemicals leach out in the soil from tanellised wood.

Buying a compost bin

If you are buying, compost bins come in all shapes and sizes and materials but square wooden bins are more user friendly than plastic ones. If you can shred plant material (with a garden shredder) to form coarse chippings it will rot down much faster.

This is particularly desirable with woody material. The other way to aid decomposition is to turn the heap regularly and, if you can do this, compost could be ready in weeks - at Highgrove they use huge bins and tractors to produce compost in a matter of weeks. Most normal heaps will take months however, but it is a very green way of recycling garden waste - if you have the room.

Selecting what to add to the compost heap

You have to be selective when it comes to compost. Do not add pernicious weeds, seeding weeds or diseased plant material - put these in a dustbin or your green waste bin. The process of producing green waste used by councils involves heat and this should destroy weeds seeds and pathogens.

Keep the heap moist and warm as fungi operate best in warm and wet conditions. Water your heap in dry weather and cover completed heaps with cardboard. Cover all heaps when cooler weather arrives.

Many heaps end up with far too many grass clippings and the compost turns to treacle. There are solutions. If you place the fresh clippings in full sun (on a sheet) and leave them for a day they will partly decompose and go brown.

Turn the grass clippings and leave them for another day or two before adding them to the heap. These partly-rotted brown clippings can also be used as a mulch and mine often go round the raspberry canes.

Read our guide to mulching.

What to put on a compost heap

  • Grass cuttings in moderation
  • Young weeds and nettle tops (but not roots or seed heads)
  • Uncooked fruit and vegetable peelings
  • Vegetable waste - eg potato haulms and outer cabbage leaves, unless diseased
  • Tea bags, leaves, eggshells and coffee grounds
  • Soft green prunings
  • Plain cardboard, cereal packets, toilet roll tubes and egg boxes
  • Plant material - but not seed heads of nuisance plants.

Can be used in moderation on a compost heap

  • Waste paper and junk mail, including shredded confidential waste
  • Tough hedge clippings
  • Woody prunings
  • Leaves (but not too many)

What not to put on a compost heap

  • Meat, fish, dairy products or cooked food
  • Coal and coke ash
  • Cat litter
  • Dog faeces
  • Disposable nappies

Cold heaps and hot heaps

Most gardeners add plant material throughout the growing season and the heap decomposes quite slowly. When you touch the surface it feels pleasantly warm, but not hot. These slowly-rotting heaps are known as 'cold' heaps and the compost is usually dug out after several months.

Learn to touch your heap with the flat of your hand to gauge if the heap is cooling off. When this happens you need to turn the heap so that the less-rotted material ends up on the inside. This will re-ignite the process.

Compost from a traditional 'cold' heap

Compost from 'cold' heaps still takes months rather than weeks and these heaps do not get hot enough to kill seeds off. Consequently if you spread this on the surface, seeds will readily germinate causing you problems. So use this compost below the ground - ideally in double digging. Don’t spread it on the soil’s surface. It will make you work when the seeds in the compost all germinate!

Read more about how compost from a cold heap can be used to improve soil.

Hot compost heaps

Hot heaps are made quickly - often in one day - using masses of green material. The heap reaches a much higher temperature and involves a different, much faster-acting set of microbes. I have used a willow and hazel frame to contain hot heaps.

Cover the heap up as soon as it’s made and it will soon reach a very high temperature. When you sense it’s cooling, turn it and add a natural accelerator such as comfrey leaves or chicken manure.

Compost from a hot heap

Hot heaps produce wonderful weed-free compost in six weeks. This can be spread on the surface and it can be used for seed sowing too because the high temperatures involved kill off unwanted seeds. You will need to sieve this compost before sowing seeds however.

Using accelerators on a compost heap

Certain things speed up the composting process because they provide a nitrogen boost. So if you keep chickens, a hamster, a rabbit or a guinea pig, use their droppings and bedding to form a layer. This boosts decomposition and adds more nitrogen.

Grow comfrey (symphytum), ideally the non-seeding Russian comfrey clone Symphytum x uplandicum 'Bocking 14', close to heaps and chop up the leaves. These will act as an accelerator and Bocking 14 is the best clone of comfrey - although all comfrey leaves work.

Find out how you can also use comfrey as a natural fertiliser by making comfrey tea.

Another system is to use urine, but this tried-and-tested idea is much easier and more attractive to the male than the female!

Once you have good compost, the best method of use for vegetables is to bury it by double digging.

What to do about treacly compost

Many heaps end up with far too many grass clippings and the compost turns to treacle. There are solutions. If you place the fresh grass clippings in full sun (on a sheet) and leave them for a day they will partly decompose and go brown. Turn the clippings and leave them for another day or two before adding them to the heap. These partly rotted brown clippings can also be used as a simple mulch.

How to tell when compost is ready

When compost is ready to use it should look crumbly and dark and it should smell sweet. You should also be able to feel the warmth through your finger tips and perhaps (if it’s been working very well) it will also smell of ammonia.

Using compost in your garden

Every time you plant, add some well-rotted, moisture-retentive organic matter to each hole. Puddle all your plants in fill the hole with water and quickly add the plant (to the puddle) and back fill. The action of the water draining away sucks the soil right down to the roots.

Plant densely, to conserve moisture, and always opt for smaller specimens because they suffer less stress. Keep everything well watered in its first growing season.

You can surface dress with compost. Apply the material in the autumn or early winter, on moist warmish soil, and the the layer of compost will soon disappear into the ground, pulled down by worms.

Mulch round your plants though, not over them, and keep compost way well from any silver plants. They will become green and soft, rather than silvery.

Read our comprehensive plant growing guides to find out how to plant flowers, trees and more.

The opinions expressed are those of the author and are not held by Saga unless specifically stated.

The material is for general information only and does not constitute investment, tax, legal, medical or other form of advice. You should not rely on this information to make (or refrain from making) any decisions. Always obtain independent, professional advice for your own particular situation.