Biochar encourages the growth of winter salads

As part of The Big Biochar Experiment, five weeks ago I planted 40 pak choi seeds in small plastic pots. 20 went into conventional peat-free seed compost and 20 were planted into a mixture of 10% biochar (by weight) and 90% compost.

Biochar helped greatly. 16 out of the 20 biochar seedlings germinated, compared to 11 without biochar. The biochar seedlings are, on average, healthier, greener and have much better root systems. Some of the biochar seedlings had one or more roots 40 cm long when taken out of the plastic pot. None of the non-biochar plants had roots that had grown sufficiently to leave the pot. This difference was very striking indeed.

Why does biochar have these effects? In particular, why should germination rates be better with biochar? Much more work is needed on this, but potential hypotheses include the impact of black biochar increasing the temperature of the soil by absorbing more of the limited autumn sunlight.

I think the far better root development may possibly have arisen because the biochar made the soil less susceptible to waterlogging. When I took the seedlings out of their pots, the biochar-amended oil was loose and friable, probably encouraging the growth of the root system. By contrast, the unamended soil was dense and overly damp. The improvement from the use of biochar might therefore not have been as marked if I had planted the seedlings in a peat-based compost which would have resisted the effect of heavy rain better.

Whether or not biochar works to improve agricultural and horticultural yields is a vitally important question. Biochar is nearly 100% carbon, and it seems to remain in the soil for many generations. If the carbon in agricultural and wood wastes that would otherwise rot and turn into carbon dioxide were permanently stored in soils around the world, humanity's net CO2 emissions could be significantly reduced. Increasing the carbon content of the world's cropped soils by one tonne per hectare a year would sequester about 5% of global emissions. Since the typical hectare of agricultural land produces several tonnes a year of organic wastes in the form of such things as straw and maize stover, this target is certainly possible. Biochar has important other effects such as reducing nitrogen run-off, thus cutting nitrous oxide emissions and decreasing the need for conventional fertiliser.