Recarbonize the world
Exploring Biochar for carbon sequestration

Testing the hypothesis

When something new comes to our attention we often accept it without question. Even with all the evidence presented on the Internet about Biochar, it makes sense to do some tests to prove for yourself whether or not the claims can be replicated.

I have an allotment where I grow vegetables for my family, and at times of glut, friends and neighbours. The soil is extremely heavy clay that swings between so wet you can’t dig it in the winter to rock hard so you can’t dig it in the summer. 2019 proved a lucky year in that February was so dry I managed to dig a lot of the plot. We are trying to move to a ‘no dig’ system but that requires soil improvement on a large scale.

So the hypothesis is that: Biochar can provide both a short term and a long term improvement to soil.

I have already spread one area with two year old composted leaf-mould. Another has composted horse manure and wood shavings and a third has composted garden and kitchen waste available from the local recycling company who compost local green bin collections. All of these work to a certain extent but they require constant replenishment as they break down. I have contemplated using lime to ‘sweeten’ the clay soil and make it flocculate into a more crumb like structure but there are three things against this. The application of lime causes humus to break down faster into CO2. The pH of the soil is only temporarily raised and the lime has to be mined from somewhere else.
To make our first batch of biochar, I’ve taken a sack of locally produced lump-wood charcoal and crush it into little bits. I did this by using a watering can to wet the charcoal and then fed the wet charcoal into our garden shredder. This prevented charcoal dust being ejected from the shredder into the air.

One test will be to see if it can improve growth in the greenhouse. By taking some sieved leaf-mould and adding fertiliser such as blood and bone. Divide this into two and add crushed charcoal at about 10% by volume to one lot. I’ll then use both lots to start tomato seedlings. As they are potted on those with biochar will get potted on into the biochar mix. If the reports on the Internet are correct, we should see round 50% more tomatoes in the first pick.

A second test will be to add biochar to seed drills when sowing on the allotment. Of particular interest in this case will be whether the plants grow a bigger root system. I have been amazed at how small the root ball of our runner bean plants have been. Scarcely more than six inches across they hardly seem adequate for the size of plant.

A third test will be to dig a spade depth trench and mix biochar into the full depth before planting. In the winter, digging down one spade depth often brings up saturated clay. In heavy rain, water runs across the surface from the field above so I have dug an interceptor ditch and filled it with gravel. This catches the surface run off and redirects it underground but it is then below the reach of the roots of the crops. If amending the soil with biochar means the water soaks into the soil instead of running down the hill on the surface, the water holding capacity and drainage should be improved.