Q&A: soil improvers

By Val Bourne

What is the best way to improve my soil?

Improving the soil involves two aspects - firstly adding more nutrients and secondly adding humus (organic material like garden compost or manure) to improve the structure. Adding organic material makes the soil airier and this aids root development and tends to improve drainage on heavy soil. Once the organic material breaks down, after a year or so, the process may need repeating.

The main consideration is your soil. Those on heavy clay may wish to follow the Great Dixter method of adding coarse grit to heavy clay to improve drainage. Adding grit has the advantage of being permanent and clay is already a fertile medium. Once the digging is done it would then be possible to top dress with a sprinkling of a slow-release fertiliser like blood, fish and bone every March and September.

Many plants, including most silvers and tap-rooted plants like eryngiums and verbascums, prefer a poorer diet. So soil improving needs to be targeted in certain areas - where vegetables are to be grown, or where moisture-loving plants like phlox are to be planted. Making your own compost heap is the first step, within six months you will have your own free, friable soil improver.

Making your own compost heap

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 (www.homecomposting.org.uk). 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.

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.

Be selective

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 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.

The ideal compost components

  • Grass cuttings in moderation
  • Young weeds and nettle tops (but not roots)
  • 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

Can be used in moderation

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

Complete no-nos

  • 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!

Hot 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 (see below).

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

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 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.

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.

Double digging (for manure and compost)

Double digging involves removing one spit (or a spade's depth) of soil. The bottom of the trench is then roughly dug over with a fork to break it up to improve drainage.

At this stage garden compost or manure can be incorporated into the trench and covered with the remaining soil to make a raised fertile mound. This is perfect for legumes (like peas and beans) and you can even fill one of these trenches with vegetable waste and leave it over winter.

Buying in manure

Animal manure - can be a problem

I am old enough to remember that when a horse went down a London street everyone would rush out with a bucket and shovel to collect the precious muck. One horse apparently produces between 1600 and 1800 pounds per year and in Susan Campbell’s The History of Kitchen Gardening she describes the perfect diet for a horse kept for its manure. It’s quite a thought. But these days most horses are bedded out on woodchippings rather than straw and this makes their manure less user friendly.

A modern problem - aminopyralid

Animal manure has recently caused great problems for gardeners because some batches have contained a chemical that kills crops and many gardeners have had problems in the last three or four years. The problem has been linked to a new herbicide called aminopyralid (a hormone weed killer) being used on grassland by the farmers. The animals either feed on the grass, or ingest silage during the winter months. The residues come out in the manure. The most sensitive crops affected are tomatoes, peas, beans, carrots, potatoes and lettuce. Many farmers now buy in fodder and they may not know whether it’s been treated or not. But manure from cows and horses, once so valued by the vegetable gardener, is now to be viewed cautiously. For this reason I have stopped buying in manure, but it has made my compost heaps even more necessary.

Apparently a good way to tell if the manure is fine is to buy a tomato plant and repot it using some of the manure.

Green waste

Councils all across the country now have green waste collection schemes and they sell their compost. Schemes vary because many councils sell it straight to a commercial compost company. Others seem to sell their own. Ring you council offices to find out their system. If you can buy it cheaply you will be able to dig this straight in without fear of unwanted seedlings.


Mulching is double-edged sword when it comes to improving the soil because the decomposition process uses up nitrogen on the soil surface -robbing the ground of nutrients.

The way round this is to use a nitrogen feed (like chicken pellets or powdered 6X chicken manure) before you apply your mulch of bark.

The other golden rule is to always mulch wet, warm soil to trap moisture. Mulching on dry soil prevents rain penetrating the ground.

Just feeding

Slow-release fertilisers are much better at releasing nutrients in the best way. Blood, fish and bone is generally the best and the powder is easy to spread.

There are water-based fertilisers that deliver a fast rush of nitrogen and they promote a lot of soft leaf. This soft growth is unattractive to look at and more prone to pets attack - it’s almost a gourmet meal for aphids. It could be used on brassicas, but isn’t suitable for strawberries in flower.

If you want to promote flower and fruit opt for a tomato feed or make your own comfrey tea.

Comfrey tea

  • Bocking 14 Comfrey (you can buy plants from The Organic Gardening Catalogue 01932 253 666) makes the best comfrey tea.
  • The process is very smelly.
  • Leaves are cut and placed into a bucket with a small amount of water and within days an evil-smelling slurry is created.
  • Once the decomposition process is complete (after 2 weeks) the liquid is diluted - one part tea to twenty parts of water.
  • This liquid has the same nutrient values as tomato feed and it’s free - as long as you can put up with the stench as it decomposes.
  • Garden Organic have a drainpipe system that keeps the smell down.
  • It can be used an all crops and in hanging baskets etc.
  • Once diluted, it is fairly pleasant to sniff, but you do need to acquire Bocking 14 - this has the correct nutrients for the job. The Natural Gardener
  • You can also chop up any comfrey leaves and add them to your compost heap as an accelerator, or you can put them at the bottom of potato trenches as a food for your crop.

The Natural Gardener by Val Bourne is published by Frances Lincoln at £14.99.  Order from the Saga Bookshop and you'll receive a 20% discount AND free delivery when you spend over £15. Simply call FREEPHONE 0800 904 7216 (lines are open Mon-Fri 9am-5pm).


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  • Barry Wenborn

    Posted: Wednesday 03 September 2014

    I have a problem with my soil, the drainage is too quick, eg. if it rains one day, the next day the soil is dry.
    Also the soil itself seems very light an crumbly.
    How can I make the soil more heavy so as to perhaps retain more water because plants that are in the soil do not show much growth.
    Thankyou Barry


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