Soil improver

A guide to soil improvement/amendment leading to healthier soil and healthier plants

ploughed field soil

What is a soil improver/conditioner?

A soil improver is an organic material substrate that improves the condition, fertility and structure of the soil. There are many forms of soil improver suitable for various applications. 

How does it work?

It provides ideal conditions for the soil system to flourish leading to healthier soil by improving the symbiotic relationships between root systems, mycorrhizal fungi, nematodes, worms, reducing disease and root rot.

Its benefits

Soil improvement ingredients

Soil improvers come in many forms. We advocate using peat-free products. Peatlands are an important carbon sink and we should stop plundering them. We have listed organic and inorganic soil improvement ingredients, so you can make your own soil conditioner recipe.


Gardeners often create their own compost from green garden and kitchen waste but it is also widely available from shops. You can amend compost with other ingredients to enhance its performance.

This provides a thick layer of mulch and breaks down very slowly. Its mulching action helps water retention and provides humus and aeration to claggy soil. In most conditions tree bark stays where you put it, not blowing or washing away.

Harvesting very small amounts of local woodland soil can help introduce the right kind of fungi and microbes into your soil. But do check with the landowner or your local authority that they are happy for you to take some.

You can buy this from your local farm dry or wet. You can use as a mulch or additive to your compost.

You can add sawdust to improve the texture of the soil. It will make the soil slightly more acidic. Make sure you know where the sawdust has come from. Untreated wood means less heavy metals or pollutants in the sawdust.

You can buy a seaweed feed from your local garden centre or DIY store. Alternatively you can go collect some from the seaside for free*. Remember it contains large amounts of salt. You can either wash it to remove excess salt content or wait for the rain to wash it out. Do avoid contaminating your garden with the salt water washings though.

Seaweed is full of the usual macronutrients and also has calcium. Seaweed has also been known to neutralise fungal infection, disease and pests.

*Check that your local authority allows you to harvest seaweed, as there may be restrictions in your area.

Earthworms, particularly species Aporrectodea and Lumbricus, consume organic material. They then excrete muddy soil. This muddy soil is rich in helpful bacteria, enzymes, soluble nutrients and contains over 50% more humus than topsoil.

Provide the carbohydrate needed by the soil life and in particular the microorganism to grow and get active. They are also a great source of magnesium, calcium, iron and potassium.


Reduces acidity of the soil increasing pH and making nutrients easier to absorb. It also adds calcium and trace magnesium to your soil.

Usually volcanic, but can be glacial rock ground into a fine powder. It contains many minerals needed to promote healthy plant growth. It usually contains the following trace elements Manganese, Titanium, Magnesium, Phosphorus, Iron and others

Contains Calcium and Sulphur.

Contains Iron, Potassium and Magnesium.

Are complex long-chain molecules made up of a chain of sugar molecules. There are studies of algal polysaccarides that show evidence of strong plant growth or biostimulant activity.


These are essentially hydrogels, whose function is to retain and release water and soluble nutrients slowly in soil.

Biochar as a soil improver

For biochar to act as a soil improver, it needs to be inoculated, charged or both.

What is inoculation, charging and activating?

Inoculation refers to the addition of microbes to the biochar, whereas charging refers to the addition of nutrients. Activating biochar is the collective term when you both inoculate and charge the material. Both of these can be done by mixing biochar with a compost. Beneficial microbes can be harvested from an aerobic compost tea and nutrients can be added via animal manure, organic matter and inorganic fertilisers.

There are lots of inoculation and charging recipes on the web. It is likely that specific ‘brews’ will work in different situations and as there are lots of variables, we will be testing our own formulations in due course.

Biochar can be inoculated with any of the organic/inorganic ingredients above to help boost the effect within the soil.

Biochar can be added directly to your home compost, as an activator and given enough time the combined material can be added to the soil for amendment.

Note: Using raw biochar as a soil amendment

If you spread raw biochar into any soil, initially it will act like a sponge, absorbing water and attracting nutrients/minerals from the soil around it and making them less available to the surrounding plants. This can have a detrimental effect on plant health and crop yield, as they will struggle to grow and can eventually die or provide a poor harvest. However, in some cases, you may want to reduce the quality of the soil for plants and crops that prefer poor soil conditions.

How and where can biochar be applied as a soil conditioner?

Biochar can be added to compost to help food and green waste breakdown quicker. It is sometimes known as an activator, accelerator or starter. It helps build the right conditions for microbes to thrive and replicate, which in turn generates a thick sticky humus that is a great store of carbon and water. This process essentially inoculates the biochar and the resulting broken down compost is supercharged and ready to add onto your garden. 

Soil Improver is added to the top layer of the soil in the ‘root zone’ i.e. top 12-36″ or 30-90cm, depending on the plants/vegetables you are planting.

This helps the availability of nutrients and aeration, promoting the health of the plants.

Application rate – 1 to 10-20 mix which equates to 5-10% by volume biochar to compost/organic material.

In many parks and open public spaces, you may see bark mulch in a large circle on the ground underneath a tree. The main root system of a tree is close to the trunk, as such a mulch is applied to inhibit weeds and competing plant growth, allow water to be retained in summer and rain to easily penetrate in the winter. It breaks down very slowly and reduce some macronutrients back into the soil below. For existing trees you can apply a biochar soil improver around the base of the tree or better still inject it into the root ball before mulching. If you are planting a new tree you can add a biochar/manure soil improver into the hole before adding the tree. This sets the tree up for the best chance of survival in its early years.

In the UK we are currently suffering from Ash dieback, where ash tree root systems are infiltrated by a fungus called Hymenoscyphus fraxineus, which attacks the tree by blocking the vascular system reducing water and nutrient intake. It is clear above ground where the tree crown starts to die back and the leaves shrivel and fall off.

In trials undertaken in the past few years, early results show that adding biochar around the roots of some Ash trees has resulted in them not being as susceptible to the fungi. More work is needed, but biochar could be a way to help the native UK Ash to fight back.

To keep a lawn nice and healthy the grass needs to be in healthy soil that has available Nitrogen and has good drainage. The root zone is where all the action takes place, so it’s important to get biochar incorporated at that level for best results.

Existing lawn

It is more difficult to get biochar under the ground to this root zone with an existing lawn, but not impossible. Preferable this can be done in the growing season and done as an add-on to aerating the soil. To aerate the soil you’ll need to create lots of small holes across your lawn, you can use a garden fork for small lawn areas, spikers which you wear like crampons for medium-sized areas or a mechanical aerator to do larger lawns.

Once you have lots of these tiny holes, you have a way to drop biochar down to the root zone where it is needed. We suggest using the finer biochar so that it doesn’t block any of the holes and drops easily to the bottom. You can then top dress with grass cuttings, which will quickly breakdown and sit on top of the biochar, plugging the holes and activating it. Alternatively, you can activate the biochar prior to application.

It is possible to use a biochar injector to aerate the soil and incorporate biochar.

New turf

Laying new turf is an ideal time to incorporate biochar into the soil below. Simply add your biochar soil improver mix onto the topsoil below and then lay the turf directly on top.

Biochar can be added to existing substrates to enhance the growth of small plants that have been potted on, larger plants and saplings.

In regards to potting soil we conducted our own test to see whether there is any significant improvement in germination or early growth. For more details see our inoculation trial page.

Biochar can play a significant role in improving agricultural soils. Many farmers already have the waste products needs to activate it and, in some cases, even produce it.

Animals create a lot of manure, all of which contains the macro nutrients needed for good soil health. Many farmers collect this waste and then heap it in the corner of fields ready for spreading. The problem with this technique is that in spreading the manure, much of it doesn’t get to or stay in the root zone of the crop. As such when it rains the macronutrients run off polluting nearby waterways. It is a waste for the farmers and harmful to the local biosphere.

Biochar is the ideal solution. Farmers can spread a layer of biochar in the animal pens, allowing it to activate in place, with the added benefit of reducing the odours given off. When ready the farmer can take this mixture and spread it onto the fields. The biochar will lock a lot of the nutrients up, increasing crop yields, reducing spend on fertilisers and reducing the pollution run off into the local environment. At the same time it increases the amount of soil carbon, sequestering it for the long term.

Farmer spreading compost

The differences between soil improver, fertiliser and compost

Soil improver vs fertiliser

A soil improver can add macro and micronutrients via organic matter, as well as change it’s density, improving water retention and introducing air. It can be used to change the pH of the soil too. Inorganic fertilisers usually only provide the macronutrients Nitrogen (N), Phosphorus (P), Potassium (K) and sometimes Magnesium (Mg) needed to healthy plant growth.

Soil improver vs compost

Compost is a type of soil improver and will release macro and micronutrients over time. Other soil improver ingredients can be added to the compost to supercharge it.

How do you enrich poor soils (clay, silty, sandy and chalky)?

The addition of biochar improves clay and silty soil, by holding onto nutrients drainage and aerating the soil. For sandy and chalky soil, biochar will hold onto water and nutrients, which will often wash out.

The steps to the right will help improve all these soil types, but be prepared that it will take a few seasons to see the best results.

Note: Don’t walk on newly conditioned soil as this will compact it and undo much of your hard work.

Steps to better soil

1/ Aerate and turn over the soil, just like an existing lawn (see above)

2/ Introduce lots of organic matter with biochar into the turned over soil or the holes created from spiking. It can be beneficial to add a small amount of local woodland soil to introduce local soil microbes.

3/ Plant a cover crop in the first season to help incorporate the organic material you have introduced deeper into the root zone. These can then be chopped back before planting.

4/ Mulch around your new plants to add more structure.

5/ Repeat this process in your second season.