Biochar for trees and woodlands
Following increased enquiries from trees specialists (arboriculturists) and woodland managers regarding biochar for trees, we thought it time to update our blog/article on the subject!
Main areas of interest
- Adding biochar to the root zone of older specimen trees to revitalise growth by reducing soil compaction and poor nutrient availability.
- Adding biochar into the planting hole or pot to help new tree rootstock to thrive and avoid disease
- Using biochar to help establish a 'woodland soil ecosystem' in a prior meadow or cultivated land soil ecosystems.
Biochar works in multiple ways
• The carbon adsorbs nutrients acting as a reservoir for plant nutrient.
• The biochar is porous and holds water.
• The granules do not compress, so it aids irrigation and airflow to the roots.
• It acts as a ‘home’ for beneficial microbes and fungi. These form symbiotic relationships with root hairs and dramatically increase the uptake of nutrients.
• The biochar does not decompose, so it last hundreds of years.
Biochar works best when it is in and around the root zone.
Biochar for mature, specimen trees
A specimen tree could be anything from the glorious cherry tree that blossoms each spring, the Acer full of clour in Autumn to the ancient oak that has stood in the village for hundreds of years.
If you are entrusted with the care of a special and often irreplaceable tree; care is needed. The early evidence is biochar can help revitalise trees, so it is worth considering. The more mature the tree, the more likely the issue will be related to soil compaction which leads to problems with water infiltration, nutrient availability and aeration. This leads to stresses that limit tree health and makes them more vulnerable to disease, drought and waterlogging.
Biochar can be a significant help - but the solution needs to be down in the root zone.
A range of innovative solutions to get biochar into the tree root zone has pooped up in recent years. These include using air-spades to remove all the soil and then putting in back. Using an advanced form of air injection to drill holes and backfill the hole with biochar (eg the Vogt injector). Using drills to create a hole and backfill - from a simple hand drill through to more complex systems such as the Ecosol Drill 'n' fill machine.
In all cases - you are looking to avoid damaging the main roots. Most projects start in the active root zone - a circle around the drip zone of the leaf canopy. If you are going closer to the truck - drill slowly and watch out for any signs of white wood coming out with soil. If this happens, stop immediately and move the drill hole by 30 cm.
Biochar for new trees
Whether you are planting just one tree or many hundreds, the cost can be anywhere from £10-£300 / tree. Many are lost in the first couple of years. Giving them the very best start on planting is justified.
Dig the planting hole and then mix 10-20% by volume of biochar into the backfill soil before using it to backfill the planting hole. You can reduce the cost by adding a thin (1-2cm) layer in the base of the planting hole - but the effect will also be less. DO not get tempted to add more than 20%. Biochar affects soil texture - too much will start to impede rather than help growth.
Biochar for 3-10 year old woodlands
Over the past 10 years, there has and continues to be planting of new woodlands. A lot of these have been sponsored via one of the various Woodland Trust schemes.
There are some cases of top dressing by simply broadcast spreading biochar– but the benefits are going to be less than getting to the root zone.
Often these new woodlands will have 100-1000 trees. All fairly small and tightly packed (pending thining). Not easy to get equipment in and around, and very labour intensive to do in via single drill holes. We think there is some scope to investigate re-using equipment designed for golf lawn care (eg the Ecosol Drill 'n' fill). These machines tend to smaller and drill multiple holes. We will monitor for reviews and feedback.
New Woodlands - soil ecosystems
Quite often new woodlands are planted in grassland (meadows) or previously cultivated arable plots. We often see a grass field drilled and planted with saplings plus stake and the green shield.
This reminds us of the advice from a former colleague who was trained in permaculture Soil ecosystems adapt to support the plants living on them, Hence grass and weeds have different fungi and microbes to a woodland soil. Planting trees in a meadow leaves trees roots in competition to the grassroots.
There are many articles around the use of wood chips to help new woodlands. A good article on the types of wood chip and uses is presented by Linda Chalker-Scott. Whilst a lot of the focus is on water retention and preventing weed growth, we think there might be another benefit.
We are passionate that biochar supports soil microbes and fungi. It may well be the biggest effect they have is to support symbiotic fungi and give them an advantage because the porous structure gives then a defensive wall against predators that would otherwise eat them. If we extend this argument - well-rotted wood chip (at least six months old - do not use the raw new wood chip for mulching - it will hold onto your soil nitrogen) will have wood-based fungi. You are spreading a forest floor ecosystem that supports trees rather than grass.
Don’t forget, raw biochar will adsorb and hold plant nutrients. This is what you want to happen as the soil microbes then inhabit the biochar and form symbiotic relationships with roots to transfer the nutrients to the plants.
However, to prevent short-term leaching of soil nutrients from the soil (and hence temporarily reducing availability to plant roots), you need to activate the biochar. Activation really just means filling up the adsorption sites with nutrients. This can be achieved by soaking the biochar in a dilute liquid NKP solution or by mixing the biochar with compost and leaving it to rest for a few weeks.
(Note SoilFixer has a proprietary method and product for activated biochar - SF60 Our biochar super compost. We mix the biochar into the green waste with trace minerals allow it to compost under controlled conditions. The SF60 has high % of colloidal humus. The humus accumulates in and around the biochar, not only activating it, but radically changing the water retention and CEC properties. We call the activation agent our ‘compost humification agent’ and the resulting product is called SF60 Super Compost /Soil Improver.
Some case studies
Dr Glynn Percival: biochar helps prevent ash die-back. (As featured on BBC Countryfile)
Mattias Gustafsson: use of biochar to revitalise urban trees in Stockholm Biochar Project.