The world below the surface is a complex, organized, and fragile ecosystem, and a basic understanding of its makeup is essential if you want your garden, lawn, and landscape to be prosperous.
Walking across your yard, perhaps while mowing the lawn, have you ever wondered: ‘what’s really under my feet?’
For most of us, our knowledge of what’s down there is limited to:
a) what see on the surface (usually grass)
b) what we generally refer to as “dirt” (with a few pesky stones thrown in whenever we bore a hole for a fence post!).
What follows is a simplified examination of the way an undisturbed (healthy) soil profile will look when excavated. Unfortunately, this is not the way most soil profiles look in urban or suburban landscapes. Our modern way of life has exacted a heavy toll on the Natural World – this is apparent in myriad ways – but perhaps most evident, when we look below grade in our own yards.
The layers below the surface:
Like an onion (or an ogre for all you Shrek fans), soil forms in layers (horizons), and each layer has unique characteristics.
Because we are focusing on the basics here, we will concentrate on the three major layers. They are:
Topsoil (horizon A), Subsoil (horizon B), and Parent Material (horizon C).
An understanding of how these layers function will help you become a better grower.
Topsoil (horizon A)
The Topsoil horizon is the most fertile layer for plant growth. Usually only 5 to 10 inches in depth, it is here that the highest concentrations of plant root systems can be found.
It is in the Topsoil layer that minerals – delivered up through the Subsoil, from the Parent Material – combine with organic matter to create a subterranean ecosystem populated by insects, microorganisms, and fungi (including mycorrhizal fungi); all of which, further assist and support healthy plant growth above.
This image shows the incredibly rich topsoil under an undisturbed Prairie Ecosystem. The wonderful rich brown colour signals the extraordinary humus levels in this highly productive soil.
Subsoil (horizon B)
The Subsoil horizon exists below the Topsoil and is usually composed of small particles of sand, silt, and clay.
Though its mineral content can be high, Subsoil is low in organic matter and lacks the biodiversity of the Topsoil above, making it far less suitable for plant growth. As well, its tightly compacted nature makes it difficult for root systems to penetrate.
Though not good for growing, it is essential for providing a stable base for what lies above and for storing and delivering minerals to the topsoil. Like a sponge, the Subsoil horizon also absorbs and drains excess moisture and provides an anchor for the roots of larger plants and trees.
Parent Material (horizon C)
The Parent Material horizon is the underlying geological material (either solid rock or compacted rock deposits) from which the overlying soil layers/horizons are formed.
The Parent Material is not a layer at which plants grow or roots penetrate, but it is the source of the mineral content of the looser soil compounds that form above.
The concentrations of certain minerals can be evident at the surface and can have an impact on the soil’s characteristics, to which anyone who has seen the iron-rich, red soil of Prince Edward Island can attest.
There are technically 3 additional soil layers/horizons; two of which, horizons “O” and “E” (a top-layer of undisturbed organic matter, and a thin layer of leached sand and silt) are rarely present in human-affected areas (urban/suburban developments).
The third is the Bedrock horizon, called horizon R. No matter where you are on earth, if you dig deep enough, you will eventually hit bedrock. It’s the parent material to the Parent Material.
It serves as a general reminder that, regardless of what we grow on its surface, we are all still living on a giant orb hurtling through space.