After a prolonged drought over the summer of 2019 in the Cambridgeshire area, the fields of clay soil were cracked and hard as concrete as one might expect. Then the autumn rains came, much more extreme than usual. This made lifting root crops hard work and heavy machinery puddled and compacted the soil making it hard to prepare a seedbed to sow autumn sown crops. In a lot of places it is simply not sensible to try and cultivate the soil, which means switching to spring sown crops – assuming the weather improves enough to get on the fields to sow them. We can’t rely on a lucky break like last February when I found the soil was cultivatable for a couple of weeks.
Assuming these extremes are becoming the new norm, we need an effective solution. Saturated and compacted soils become anaerobic, which is bad for plant growth. One possible solution is to add biochar to the topsoil. This has a number of benefits;
- Biochar raises the pH of the soil, colloquially known as sweetening the soil. This causes clay to adopt a more crumbly structure allowing more air and drainage.
- Biochar has a large internal surface area that has cationic properties. The surface charge allows carbon to adsorb fertiliser and water. By bonding closely with the chemicals and water, the biochar can hold a lot more nutrients and water than its volume suggests.
- The structure of biochar provides shelter for fungi and bacteria which plants rely on to exchange sugar for minerals through their roots.
- Applying biochar to a clay soil is a long term amendment because, unlike compost, animal waste or peat it does not break down into carbon dioxide and methane.
- Improved drainage means the land can be cultivated more easily with less smearing and compaction.
One area of concern is that the biochar might soak up the existing fertiliser ahead of the fungi getting established enough to access it. This would lead to a drop in yields unless the biochar is properly charged with nutrients before application. Mixing the charcoal with available sources of nutrients will help in this regard.