Conservation Comment: Carbon in soil - Phil Thomsen

Phil Thomsen writes about the benefits of composting.

There is a way that we can all do to reduce carbon dioxide build-up – we can remove it from the atmosphere ourselves, through our gardens.

Plants remove CO2 from the atmosphere, converting it into complex foods, which can be used for their own vital functions, or else for food for humans (eg the feijoas and lettuces that we grow in our garden). However, this CO2 is then released back to the atmosphere through decomposition, unless it is retained in a long-term form, such as wood for construction, which is seldom the case with garden plants.

There is a way that we can help to remove CO2 long-term – by storing it in our soil. Macro-organisms (eg worms) and micro-organisms (eg fungi) break down organic matter, releasing CO2 back to the air. However a portion of it is retained in a durable form, as humus. Humus doesn't last forever, as it is still biologically active, but it is long-lasting, and can be continually replenished.

Humus is what makes topsoil dark – usually subsoil is a brown or chestnut colour. Humus is the "secret sauce" that improves water and nutrient retention, drainage and aeration, in any soil – all in one handy package! There are various ways that we can build our humus levels, including composting our scraps, biochar (charcoal), and applying mulch to the surface.

I compost my kitchen scraps in a rodent-proof bin. I have a second, more open bin with slatted wooden sides where I compost bulky material. However, the main way that I return organic matter is simply by leaving it on the surface. When I harvest vegetables, I leave waste material such as carrot tops behind on the ground.

Likewise, most weeds I simply pull out and lie on the soil. I do this during dry, warm and windy conditions, when the weeds dessicate and die quickly. In winter I put the weeds into my bulk bin, otherwise they would simply grow back into the ground. I do have some exceptions – I put really problematic weeds such as oxalis and tradescantia (wandering willie) into my wheely bin, as they are extremely difficult to kill. However sending organic matter off in the wheely bin is counter-productive and should be minimised. The anaerobic decomposition at rubbish dumps releases methane, a potent greenhouse gas.

I spread fallen leaves over the soil, and likewise with lawn clippings. I spread the latter out over the soil, as it can get smelly and attract flies if dumped in a clump. I spread most prunings onto the soil – I trim larger woody stems to about 30cm lengths, which encourages decomposition. Earthworms gradually drag this material into the soil, where micro-organisms energetically convert it to humus.

There are further advantages to having organic matter on the soil – it protects the soil from damage from wind and sun, forming an insulating layer. It reduces drying out of the soil, benefitting plant growth. And finally, a layer of organic matter on the surface is a barrier to the germination of weed seeds.

Mulch can also be provided through purchased material such as bark. Bark has evolved to protect trees from attack by insects or microbes, hence it is much more long-lasting than other materials, though it does break down over time. Artificial weedmat prevents this process, and has a range of other negative impacts. If you feel the need to put a layer between the soil and the mulch, newspaper can be spread first – three to four sheets is about the right level. Cardboard can be used the same way, but be sure to remove any tape first – it is immortal in soil.

Soil humus is one of the ways in which carbon is sequestered globally. Many soils are losing their humus owing to tillage, crop removal and the use of herbicides. However, we can all do our bit by increasing the humus levels in our gardens.

Phil Thomsen is a life-long gardener and conservationist.