Compost and Global Warming

Compost

I’m not too sure about talking about compost to allotment holders – they will know all about the subject already. So I want to set the business in the context of global warming, the existential crisis facing humanity. Perhaps such an approach might give rise to some fresh thinking.

Let’s look at why we make compost. The two obvious things are

1.       To improve the soil

2.       To get rid of waste vegetable matter.

But there are a couple of other aspects I’d like to include:

3.       Biodiversity

4.       Global Warming

As you probably all know, the addition of organic matter to the soil improves fertility. Humus binds to clay particles and allows them to hang on to plant nutrient elements that would otherwise be leached out of the soil as rainwater drains through. The organic matter also provides food directly for mycorrhizal fungi. These are essential for many plants, helping in the breakdown of minerals to provide nutrients and in transporting nutrients through the soil, effectively acting as an extended root system.

The food we take from a veg plot is only a small part of the biomass that grows each year. We have to do something with the rest and we don’t want to lose the mineral matter it contains since this will be needed by the next year’s crop. Composting is an essential step in the cycle.

Let’s look closer at this cycle. Actually it’s more complicated than a simple circle so let’s take a system approach, looking at inputs and output to our veg plot. The output we actually want is the food we carry off and eat. This depletes our plot of the various elements it contains and have to be replaced by equivalent inputs.

After water, carbon is the main one, freely gained from the air by photosynthesis. Nitrogen comes next, also from the air but fixed with much greater difficulty. All the other elements are derived from the weathering of soil minerals, ultimately from the rocks below.  Some growers do a bit of cheating, they bring in nitrogen, potassium and phosphorous in the form of a bag of NPK fertiliser. This is just taking phosphate and potash from rocks elsewhere in the world and burning natural gas, methane, in the process required to fix nitrogen. This is ultimately unsustainable and organic permaculture seeks to avoid such inputs. If we get the soil right these inputs from afar are not needed and composting plays the key role here.

Let me say a bit about how I do the composting. Everybody’s situation is different and what suits me may not suit everybody. I’ve a large garden – six and a half acres in all – but much of it is growing trees or a meadow and doesn’t get involved in the composting business. I have a veg plot much the size of a standard allotment and some big areas of flower beds and lawns.

What goes into the compost? As far as I am concerned everything that was once alive is potential compost and that includes wood, eggshells, all food waste, lawn-mowings and anything else that once lived. I put kitchen waste into a plastic composting bin, along with a little garden waste and some lawn-mowings. Everything goes in, eggshells, meat and fish, orange peel and anything else you may have heard shouldn’t be composted! Using a bin keeps things that might be unsightly, tidy and stops rats and pheasants and foxes scattering the stuff about. Actually I have two such bins. When one is full I start the other and by the time that is full the first is done.

All the garden waste, except woody material, just goes onto a big pile. When the pile is big enough, maybe half a year’s worth, I start a new pile. Some folk are very keen to ensure that their compost reaches a high temperature and adding lawn-mowings usually heats things up quickly. The advantages are that composting goes faster and weed seeds are killed.

My experience is that however hot things get there are always parts that stay cool and weed enough weed seeds survive to make that objective never realised. It might in an industrial scale system but for domestic purposes expecting one’s compost heat to kill all the weeds seeds is to invite inevitable disappointment.

As for speed of composting, yes, that’s useful but I don’t get too hung up about that. It will rot eventually whatever you do and maybe we should extend the slow-food movement to slow-composting. Anyway, I’ve got plenty of space so there’s room for two heaps and a third if the first isn’t ready soon enough. If you are limited for space then speed may be more important and getting the optimum mixture of high-nitrogen waste such as lawn-mowings with coarser material, keeping the compost damp and neither too wet nor dry, and periodically turning it over, will all help to speed things along.

What of the woody material? This takes much longer to rot and while you don’t get much, if any, from a veg plot (fruit bush cuttings are about it), our garden certainly produces a great deal as we have a lot of trees and shrubs. Anything getting on for an inch thick goes to the woodpile and, when dry, will heat our house, but we still have masses of smaller woody stuff. We never have bonfires. Instead we just pile it up and if it takes several years to rot then so be it. Because I have plenty of space I don’t much mind if heaps of dead plant material are left lying about for a long time. I actually welcome this.

A few years ago I joined the fashion and made a bug hotel – a box with lots of hollow sticks for the wild solitary bees to nest in. We certainly have plenty of these bees but the hotel never got used, and the reason is pretty obvious. We have great heaps of slowly decaying sticks that produce veritable cities for the bugs and bees; why would they want an extra facility?

A large heap of woody material, slowly decomposing, is a biological factory producing biodiversity. Starting with the bacteria, algae and fungi, through the myriad of small creatures, the pile anchors the food chain all the way up to the badgers and buzzards. Yes it provides a hiding place for slugs and snails but it does the same for hedgehogs. That’s what the organic permaculture approach to food-growing is all about.

And so we come to the elephant in all our rooms, global warming. This, being the existential crisis facing humanity, ought to be the thing that dominates everything we do. The biggest factor is our habit of burning fossil carbon and so adding the greenhouse gas carbon dioxide to the air. The obvious solutions are to stop burning carbon and also to use photosynthesis to remove the CO2 already emitted and store the carbon. This is where the allotment holders can play their part. What we should try to do is maximise the amount of carbon stored on our plots.

Those piles of dead sticks are our contribution to global warming mitigation. The bonfire destroys all the good work. If we can get an extra tonne of carbon stored on our plot, whether in organic matter incorporated in the soil or woody stuff in heaps, that’s a tonne knocked off our carbon footprint. Every little helps.