Few people have ever thought about the unfathomable amounts of time that it takes to make soil. Healthy soil takes so long to create that it may as well be measured in geological time scales.
Let me briefly explain how soil is made using a deciduous forest as an example. Deciduous trees drop their leaves every year to create mulch that ultimately breaks down into soil. The leaves fall on top of last year’s leaves and we begin the get a layering effect with newer leaves on top and older leaf material on the bottom. Micro-organisms, macro organisms, and the freeze thaw cycle all act together to break down the leaf material into an unrecognizable mass. This material is what we call humus and it is the basis for healthy soil. Animal waste, wind-blown particles, and fallen limbs of trees all combine to form soil over time.
Over the course of the growing season the plants accumulate minerals, vitamins, and carbon dioxide into their tissues and leaves. When these leaves fall in a natural setting all of that material stays in the local soil so that the plant can uptake the nutrients again in a form of recycling. Considering that the nutrients stay in one place, more or less, the only real inputs to the system are wind-blown particles and carbon dioxide from the atmosphere. Carbon cannot be absorbed through roots and so it accumulates in the soil as humus giving it a light, airy, chunky texture. This also makes soil a fantastic medium for carbon sequestration.
If you can imagine, by analogy, that soil carbon is a sponge cake then the humus is shoe leather. As you can imagine, shoe leather is not very appetizing and so the micro-organisms and the macro organisms do not consume it because there are other forms of food near-by that are easier to chew and easier to digest. The “critters” in the soil eat decaying plant material (sponge cake) and take the carbon into their bodies and when they die or get eaten by a bigger critter the carbon is returned to the soil. This is not to say the “shoe leather” is inedible. When soils in agricultural fields are over fertilized the bacteria begin to consume anything and everything they can find and this leads to soil degradation through the loss of organic matter in the soil. This creates a hard clay-like material that is hydrophobic, meaning that water cannot easily penetrate the surface to the lower levels. This in turn causes surface runoff which in turn causes soil erosion.
So why do we compost? Compost is essentially a human created practice that creates humus in a few months to as little as a few weeks, depending on the method, compared to the natural process which takes several years. The act of piling up organic matter, decomposing it, and redistributing the material does not happen in nature and this makes composting a unique human habit.
Soil micro-organisms in healthy soil consume large quantities of organic matter every year, and if this organic matter is not replenished then the soil will become deficient and the micro-organisms will begin to die. Think of composting as feeding, literally “feeding”, these organisms which in turn provide the plant roots with soluble nutrients in the form of their metabolic waste.
Organic matter must be added to an actively growing field every season in order to sustain the micro-organisms. Imagine one-wheel barrow full of compost thrown onto a field in spring, by the time summer comes to an end only 1-3% of the material in the wheel barrow is left in the soil, the rest has been consumed by the critters and the plant roots. However, that 1-3% is the humus, the indigestible material that the micro-organisms do not want to eat, yet. This is why we must add organic matter every year and as often as possible. In doing so we can replenish soil to keep it healthy, sequester atmospheric carbon, replenish deficient soil, and prevent organic matter from going to a landfill where it will rot and turn to toxic methane gas.
-Posted by Payton