Improving Garden Soil
A Down To Earth Guide

What are the best methods and materials to use for improving garden soil, and which methods are best for your garden and your own soil type?

Before we start, bear in mind that when it comes to improving garden soil, you will undoubtedly get the best results for the least amount of effort by working with what you have and selecting plants that thrive in your particular soil conditions.

Improving Garden Soil: All Soil Types

The following can all be used for improving garden soil, whatever your soil type. The easiest way to use them is by applying them to the soil surface as a mulch.

Leaf mould

This is an excellent all round fibrous soil improver, and it's easy to produce your own by collecting fallen leaves in the autumn, bagging or stacking them, and storing for a year to allow them to decompose into a dark, crumbly mass.

Garden compost

Get into the habit of composting your garden waste to produce a good, totally free soil conditioner. Make sure you don't use it until it is dry and crumbly.

Green waste compost

If you aren't able to produce enough compost from your own garden waste, supplement it by buying compost produced by local councils from recycled green waste. It is cost effective and has the advantage of being produced by the 'hot' composting method which ensures that all weed seeds and other nasties are killed.

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Improving Garden Soil: Sandy Soils

The problem:

These light soils are very fast draining and important nutrients tend to be leached out. They are also prone to erosion and evaporation if left bare of plant cover.

How to improve:

Add plenty of bulky organic matter to improve structure and top up nutrient levels.

What to use and how to use it:

Farmyard manure

Manure needs to stand for a minimum of one year until it is well rotted. The rotting process releases a lot of nitrogen which will actually damage your plants so look for a dark crumbly appearance and an earthy, inoffensive smell. If you are in any doubt leave it in a heap in your own garden until it is ready to use.

Apply as a mulch or lightly fork into the top few centimeters/inches only.

Wool and cotton shoddy

Shoddy is a by-product of the textile industry and garden writer Ursula Buchan reminds us that, in Victorian times, it was a popular nitrogenous fertilizer. To this day, wool shoddy is still used by rhubarb growers in Yorkshire.

Shoddy is ideal for improving garden soil as it is biodegradable, decomposing slowly over many months to gradually release the nutrients that are so beneficial to light, sandy soil.

Pelleted versions are now available giving all the benefits of water retention and gradual nutrient release in an easy to use form. Shoddy also has the great advantage of forming a felt-like mulch that both suppresses weeds and deters slugs.

Apply as a mulch.

Water retaining granules

These soak up water and then release it gradually, and can be helpful when applied on planting to individual specimens, such as ornamental trees, which are sensitive to drought until their root systems become established.

Apply by incorporating into the planting hole according to the manufacturer's instructions.

Green manures

These are crops specially grown to be dug into the soil. Again, they are invaluable for improving soil structure and raising nutrient levels.

Typical green manure plants include rye grass, mustard and legumes such as vetches, clover and lupins.

The legumes have the added benefit that they 'fix' nitrogen in the soil through their root nodules. On light sandy soils green manures also protect the soil surface from erosion and surface cracking.

Apply when you need to leave your soil bare for any length of time or when preparing a new bed. Simply sow the seeds, allow to grow, and then dig the whole lot into the soil.

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Improving Garden Soil: Silty Soils

The problem:

Silty soils have a fine texture and poor structure which can lead to poor drainage and compaction. Although they are more workable than clay they can easily settle into an airless mass.

When and how to cultivate:

Cultivate only when dry enough not to stick to your boots. Autumn is a good time to dig to allow the soil to be broken down into a tilth over winter. Take care not to walk directly on silty soils to avoid compaction. Avoid this by laying down boards to distribute your weight.

How to improve:

The main aim is to improve the structure and alleviate poor drainage and compaction.

What to use and how to use it:

Grit

Grit is by far the best material for improving garden soils that are poorly drained and prone to compaction. Treat selected areas only. Although it is expensive, it remains in the soil providing long term benefits.

To spread the costs and the workload, make the improvements gradually, treating one bed or border at a time. Use grit with a diameter of 6mm (0.25in) and combine it with organic matter.

Apply one to two bucketfuls per square metre/yard and lightly dig into the top 20cm/8in of soil.

Spent mushroom compost

This is very well rotted manure with added lime. It is much lighter than ordinary farmyard manure and doesn't bind the soil in the same way.

Be aware that the added lime will affect the pH of your soil so use with caution if you are growing acid loving plants.

Apply as a mulch around ornamental plants or dig very lightly into the top few centimeters/inches of soil.

Composted bark

This is a good lightweight material for improving garden soil as it promotes better soil structure and drainage.

Buy from a reputable source and make sure it is well rotted as decomposing bark uses nitrogen and your plants will starve!

Apply as a mulch around ornamental plants or dig very lightly into the top few centimeters/inches of soil.

Mineralized straw

This is an excellent lightweight soil improver made up of chopped, partly composted straw waste with added nutrients and minerals.

Apply as a mulch around ornamental plants or dig very lightly into the top few centimeters/inches of soil.

Farmyard manure

Farmyard manure is another good soil improver, provided that it has been allowed to stand for a minimum of one year until it is well rotted.

The rotting process releases a lot of nitrogen which will actually damage your plants so look for a dark crumbly appearance and an earthy, inoffensive smell.

If you are in any doubt leave it in a heap in your own garden until it is ready to use.

Apply as a mulch or lightly fork into the top few centimeters/inches only.

Green manures

These are crops specially grown to be dug into the soil. They are invaluable for improving garden soil as they improve structure and raise nutrient levels. Typical plants include rye grass, mustard and legumes such as vetches, clover and lupins. The legumes have the added benefit that they 'fix' nitrogen in the soil through their root nodules. On silty soils, green manures also protect the soil surface from erosion and surface cracking.

Apply when you need to leave your soil bare for any length of time or when preparing a new bed. Simply sow the seeds, allow to grow, and then dig the whole lot into the soil.

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Improving Garden Soil: Clay Soils

The problem:

Heavy clay based soils have poor structure which leads to poor drainage and waterlogging in wet weather, and compaction during dry spells.

When and how to cultivate:

Do not attempt to dig when very wet or dry and hard. If you need to dig, do it in the autumn when the soil is just moist and then leave it over winter for the clods to break down.

Take great care not to walk on wet clay as you will destroy all your good work; lay down boards to distribute your weight.

How to improve:

The main aim is to improve the structure to aid drainage and allow better root penetration. A word of caution - do not add farmyard manure as this effectively binds clay soils into an unworkable mass.

What to use and how to use it:

Grit

This is by far the best material for improving clay soils that are heavy and poorly drained. Although it is expensive, it remains in the soil providing long term benefits.

To avoid a large initial financial outlay make the improvements gradually, treating one bed or border at a time. In areas that are too large to treat, select plants that tolerate heavy, clay soil.

Apply a 10cm/4in thick layer of 6mm/0.25in diameter grit and lightly dig into the top 20cm/8in of soil.

Spent mushroom compost

Spent mushroom compost is very well rotted manure but with added lime. It is lighter than ordinary farmyard manure and doesn't bind the soil in the same way.

The added lime will affect the pH of your soil so use with caution if you are growing acid loving plants.

Apply as a mulch around ornamental plants or dig very lightly into the top few centimeters/inches of soil.

Composted bark

This is another lightweight material that promotes better soil structure and drainage.

Buy from a reputable source and make sure it is well rotted as decomposing bark uses nitrogen and your plants will starve!

Apply as a mulch around ornamental plants or dig very lightly into the top few centimeters/inches of soil.

Mineralized straw

A good lightweight soil improver made up of chopped, partly composted straw waste with added nutrients and minerals.

Apply as a mulch around ornamental plants or dig very lightly into the top few centimeters/inches of soil.

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Improving Garden Soil: Chalk Soils

The problem:

They are thin and very fast draining so nutrients tend to be leached out. The problems are made worse if left bare of plant cover. They are very alkaline so you will not be able to grow acid loving plants.

When and how to cultivate:

They can be worked at any time of the year but if you need to dig, spring is the best time, a few weeks before sowing. As the topsoil is not usually deep, keep your digging shallow.

How to improve:

Add plenty of bulky organic matter to improve structure and retain moisture.

What to use and how to use it:

Farmyard manure

Manure that has been allowed to stand for a minimum of one year until it is well rotted.

The rotting process releases a lot of nitrogen which will actually damage your plants so look for a dark crumbly appearance and an earthy, inoffensive smell.

If you are in any doubt leave it in a heap in your own garden until it is ready to use. Manure is also acidic so will help to counteract the alkalinity of chalk soil.

Apply as a mulch.

Green manures

These crops are specially grown to be dug into the soil. They are invaluable for improving garden soil as they improve structure and raise nutrient levels.

Typical plants include rye grass, mustard and legumes such as vetches, clover and lupins.

The legumes have the added benefit that they 'fix' nitrogen in the soil through their root nodules. On thin, chalky soils, green manures also protect the soil surface from erosion.

Apply when you need to leave your soil bare for any length of time or when preparing a new bed. Simply sow the seeds, allow to grow, and then dig the whole lot into the soil.

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Improving Garden Soil: Peaty Soils

The problem:

These soils are usually very acidic so growing some types of fruit and vegetables can be difficult.

The acidity also makes them inhospitable to earthworms and other beneficial organisms.

If they dry out it can be difficult to rehydrate them, and if they are waterlogged they may need artificial drainage.

When and how to cultivate:

They can be worked at any time of the year.

How to improve:

Water in dry weather if necessary and add lime to fruit and vegetable plots. They already have a high content of organic matter so this does not need to be applied.

What to use and how to use it:

Ground limestone

Ground limestone (calcium carbonate) can be applied to selected planting areas to raise the pH and encourage earthworms and other beneficial soil organisms.

Although more expensive, this form of lime is longer lasting and safer to use than slaked lime or hydrated lime.

Apply in autumn or winter, preferably on a windless day to prevent it from being blown about. Wear a dust mask to prevent inhalation.

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You may also be interested in these related pages...

The techniques of raised bed gardening and gardening in containers are a good way to get around some of the problems caused by a less than ideal soil.

You can find out more about garden soil at healthy garden soil.

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