Ever looked at the magnificent growth of a forest and wonder how it gets by without trucking in fertilizer a few times a year? Compost, that’s how. Well that, and not being planted as a monoculture, but I digress. Composting, or the breaking down of organic matter into usable nutrients has been around as long as trees have been piling up dead leaves underneath them. Human methods used today just take an understanding of what’s happening there and tweak the conditions to suit our needs.
If you go into just about any forest and take a look at the ground, one thing you won’t see much of is bare soil. Instead, the ground is covered in a layer of all sorts of organic matter: leaves, branches, animal droppings, etc. If you dig down a few inches, you’ll notice that the litter gets darker and harder to identify the further down you go, eventually getting to a layer of nice black crumbly stuff. That black crumbly stuff is basically compost: decomposed organic matter from the litter layer. The only difference between that organic layer and the compost you make at home is that it was made without human hands, only the natural interaction of the various organisms that live there. But why does it break down?
Living things, lots and lots of living things we can categorize as decomposers. Decomposing organic matter is worked on by a progression of species, each one suited to different stages of the decomposition process. For example: lions and flies can’t eat grass, but after that grass has been converted by a zebra into meat and manure it is perfectly suited for both. In a home compost pile, things are generally kicked off by bacteria, then fungi, then larger animals like earthworms, termites, etc.
In composting, the two important bulk nutrients are carbon and nitrogen. Carbon chains are used by microorganisms as an energy source, and nitrogen (think protein) is used to build their bodies. Talking to someone about composting, you’ll hear terms like “greens” and “browns” pretty often, which are shorthand terms for nitrogen rich and carbon rich material. Green is used to describe compostable stuff that is rich in nitrogen, because some nitrogen-rich plant material (grass clippings, leaves, etc) are green in color. “Brown” is used to describe more carbon-rich material like wood chips, branches, straw, etc. Personally, I don’t think the whole green/brown thing is a particularly useful rule of thumb, because there are just so many exceptions. For example, manure of any type is a fairly rich nitrogen source, putting it firmly into “green” territory. I don’t know about you, but most manure I’ve seen is brown. Blood and bonemeal are also high in nitrogen, and decidedly not green. I think a better rule of thumb would be “stinks when wet” and “doesn’t stink when wet.” Just about any “green” will stink pretty badly (like manure, roadkill, even grass clippings) if left wet and rotting, while “browns” like wood chips or straw don’t smell much at all. But hey, who am I to argue with convention?
But why do I want compost?
Compost has three main benefits to any soil: physical, chemical and biological.
Physical
Adding organic matter to soils improves their structure. In general it keeps soils loose and friable (crumbly, easily broken apart), which allows for roots to more easily grow through it, and oxygen to more easily penetrate to those roots that need it. Just about any gardener knows that compacted soils are not a good thing, and in general more organic matter means less easily compacted soil. Adding compost to soil can also improve both water retention AND drainage! Sandy soils have a hard time holding on to anything, and adding compost to them increases the water holding capacity (more on that below) of the soil without letting it get waterlogged. In clay soils, which tend to not drain well, adding compost spreads out the tiny little clay particles allowing water to better drain through them.
Chemical
Compost is rich in a number of funky long carbon chain molecules collectively known as humus. These complex organic molecules are filled with binding sites for water and nutrients to temporarily adsorb (like absorption, just on the surface) to, holding them there in the soil until they are needed. A term that is somewhat commonly used to describe the amount of these binding sites in soils is the Cation Exchange Capacity or CEC. A high CEC prevents leaching of nutrients, holding them there until root comes along asking for some. Adding compost increases the CEC of your soil, making it function as a big time-release fertilizer reservoir.
Biological
Increased organic matter in the soil also supports a huge amount of soil life, which help release nutrients for plants and prevent diseases. Just like in our bodies, having a healthy population of good bacteria can help convert nutrients more available and prevent harmful organisms from being able to get a foothold. The same organisms that take your compost pile through the progression from leaves manure and twigs to finished compost are working all the time in healthy soils, breaking down old dead roots, leaf litter, vegetable scraps, dead insects and animals, etc. Worms move through the soil, aerating and micro-tilling it as they eat their way through it.
So, the question is, why wouldn’t you want compost?
How Do I Make Compost?
Compost can be as easy as piling a bunch of plant matter into a pile and ignoring it for a year, to a more intensive 18 Day Compost, vermicomposting (using worms!), or even humanure. Yes, in much of SE Asia human manure is composted, and as you can imagine requires fairly strict adherence to good practices to avoid getting people sick. In fact, a lot of municipal compost is made with shredded plant material combined with sewer sludge, then held at specific temperatures for a set number of days to kill off pathogens.
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