A SOIL GROWERâS FOUNDATION
Iâm looking out over my backyard, iced tea in hand. It is a late summer day, and the vegetable garden is a riot of color and texture. Hollyhocks and cosmos have grown to giant proportions and reach skyward between the large leaves of winter squashes. Tomatoes and beans twine upward on trellises. Carrots, beets, and cucumbers are hidden treasures between the broad green leaves of collards and kale. Clovers cover pathways between the beds. The herb garden is a jumble of lavender, rosemary, thyme, and calendula, buzzing noisily with the bees that scavenge the plantsâ nectar. Small fruits swell on apple trees that are spread between clumps of currants and raspberries. The only thing that I canât see when I look out over my garden is the soil.
Nonetheless, beneath this verdant garden jungle is a world equally, if not more, alive. The living soil is a beating, breathing, eating, growing, digesting, dying, and throbbing organism. More accurately, it is a collection of organisms so abundant that they rival all of earthâs aboveground creatures in number and type. One teaspoon of this microscopic world contains vastly more individuals than the human populations of New York, London, Hong Kong, Tokyo, Mexico City, Chicago, and Moscow put together. This living system functions at once as the lungs, filter, and food source for the planet. Almost every molecule of earthâs water, air, and nutrients has passed through the soil at some point in its cycle. In doing so, these materials are consumed and transformed by the billions and billions of organisms that inhabit the top 12 inches of gardens, orchards, and lawns. Although invisible to us, the teeming life below ground creates the conditions necessary for thriving life above ground. Know this and you will know the secret to successful gardening.
For me, the complexity of a soil can be boiled down to a basic premise: grow healthy soil to grow healthy plants. An afficionado of garden science, Iâve researched soil in minute detail, from its chemical composition to the way clay minerals interact. In my garden, however, I only use two basic soil concepts to grow healthy soils: What does the living soil need? and What does the nonliving soil provide? This chapter spells out the needs of the living soil, while giving you the tools to know a healthy soil when you see it. In my own experience, once I gained the eyes to see soil health, taking care of the soil became second nature.
A garden thrives when built from the ground up. Diverse, vertical layers of living plants cover the living soil, which is the foundation and source for whole-garden bounty.
ITâS ALIVE!
So what is soil anyway? We walk on it every day. We avoid it when it turns to mud. We move it around to plant seeds or weed garden beds.
In any soil-science class, the first things a student learns are the four fundamental components of a soil. If you looked at your soil in profile, you would see that almost half, by volume, is composed of minerals. These are the little bits of broken-up rock that took centuries of wind, flowing water, ice, and rain to be worn down to particles that range from small to really, really small. You can think of this as the nonliving backbone of the soil.
Looking at a soil profile, we easily see the solid minerals that make up about half of the soil. The other halfâempty pores filled with water and airâis harder to see. Though barely visible (the living and organic portion of the soil is less than 5 percent by volume), it drives almost all of the soilâs, and thus our gardenâs, processes and functions.
Almost the entire remaining half of the soil is composed of the area between the minerals, collectively termed pore space. The size of this pore space also ranges in size, from large channels we can see to small microscopic spaces, and it is filled with a mixture of water and air. The amount of water or air that fills the pore space fluctuates depending on rainfall or irrigation but is roughly equal parts of the two. Water clings to the sides of the solid soil, while air fills the spaces in between.
These three componentsâminerals, water, and airâmake up nearly all of what we see when we look at our soil. With only these three ingredients, however, we would be living on a barren planet. It is the remaining 5 to 10 percent of the soil that actually makes plant life possible. This fraction, the fourth component of soil, is soil organic matter.
Soil organic matter is all the material in the soil that currently is or once was living. This includes the leaves that fall in the autumn and the manure tilled under in the spring. It includes active plant roots and the unrecognizable, decayed roots from five years ago. It includes the living and dead bodies of microorganisms and the material that they excrete. It includes the worms, spiders, and nematodes that we can and canât see. It even includes the cardboard and paper that we use to mulch the soil surface. Yes, that cardboard was once a tree and that tree was once living.
Most of a soil is made up of minerals, and the water and air-containing pore space between them. A small but mighty portion (less than 5 percent) of the soil is organic matter, made up of living, dead, and very dead plants and organisms. Organic matter gives soil its dark black color and is the basis for building soil health.
Despite its small percentage, the organic part of the soil exerts control over almost all of a soilâs properties. Organic matter changes what you can do with a difficult clay or sandy soil. It increases how much water the soil can hold and when it can be released to plants. It even stores and recycles plant nutrients in the bodies of living and dead bacteria and fungus. Without organic matter, a soil is merely broken-down rock.
Within the relatively small fraction of soil that is organic matter, only 5 percent is actually alive. In terms of whole-soil compositions, the means that less than 0.5 percent of the entire soil is living! The remainder of organic matter in the soil is in some stage of decomposition. Together, the living and decomposing organic portions of your soil are a tangle of live, newly dead, and long-dead organisms and plants that hold the soil system together. Nonetheless, if such a tiny fraction of the soil is actually alive, why do we call it a living soil?
This mighty 0.5 percent of a soil that lives and breathes at any given time is a powerhouse of activity whose effect vastly outweighs its proportional size or weight. Itâs the living engine of the garden that feeds plants by constantly consuming and excreting nutrients. It creates a rich ecosystem that protects the garden from pest and disease. A living soil is even res...