Soil is amazing. It is the medium in which much plant and animal life depends. It is also a dynamic system, changing according to the environmental factors that act upon it, and the influence of humans. It forms part of an intricate ecosystem, interacting with the plants and organisms that live in and on it, the rock beneath it, the topography of the landscape on which it sits and the climate around it. There are several functions that soil performs.
On a basic level, soil provides a medium in which plants can grow. It serves as an anchor for the plant roots, and acts as a holding ‘tank’ of moisture that the roots can access when they need to. A healthy soil, with sufficient moisture, a good balance of gases, and plenty of organic matter is the ideal medium for plants to grow in, and permaculture gardeners have a primary focus on making sure the soil is hospitable to the plants they wish to grow.
The degree of support and anchorage that the soil gives to plants will affect which species will thrive in particular soils. Several variable properties of the soil that affect plant growth include how fine or coarse the texture of the soil is, the degree of aeration, the proportion of organic matter, and the ability to retain moisture.
As an anchor for plant roots and as a water holding tank for needed moisture, soil provides a hospitable place for a plant to take root. Some of the soil properties affecting plant growth include: soil texture (coarse of fine), aggregate size, porosity, aeration (permeability), and water holding capacity.
When moisture falls on the land – either from rain, snow, frost and mist, or from irrigation techniques – the soil absorbs and stores it. This water is then available for both plant to access through their roots, and for the microorganisms that live in the soil, who, like all life on earth, need water to survive. The water also acts as a solvent in which nutrients in the soil can be absorbed by plant roots. This ability of the soil to store moisture is particularly useful in locations where precipitation is intermittent. Increasing the water-holding capacity of your soil also helps preserve water, as less irrigation is needed.
Because gravity is always working on the water, it moves down through the soil. Any moisture that is not used by plants or microorganisms, or does not evaporate from the surface of the land, eventually finds its way down to the water table. The rate at which moisture percolates through the soil depends primarily on the structure. Soil that is predominantly clay particles slows the drainage of water more than soils with more sand particles. Sand particles leave more space between themselves, which gives space for the water to trickle through.
There are several things a permaculture gardener can do to enhance the ability of their soil to retain water. Adding organic matter improves the structure of the soil and promotes more plant and microorganism growth, both of which use water to grow. Planting ground cover crops reduces the rate of moisture evaporation from the surface of the soil, while contouring the land, by creating swales and other water-holding features, is another way to slow the movement of water off the land.
Healthy soil is veritably teeming with life. It is alive with everything from bacteria and fungi to earthworms and beetles. These microorganisms are the primary engines of decomposition in the soil, turning dead and rotting materials into organic matter that they, and plants, need. Like the majority of life, soil microorganisms require some basic things to live, namely air, food, water and a habitat. A healthy soil provides all these and enables the organisms to survive and thrive.
Organic matter provides the organisms with carbon, which they convert into energy, so keeping an abundant supply of organic matter on the soil, through composting and mulching, is a beneficial activity in a permaculture garden. This will also improve the structure of the soil, allowing air to penetrate and so providing the microorganisms with the gases they need for respiration (most organisms are aerobic and require oxygen, but some can survive without oxygen, and are called anaerobes).
You can, in one way, conceive of soil as a vast stomach. It is a digestive marvel. Healthy soil decomposes any organic matter that falls upon it – dead plants and animals – into simpler mineral forms. Living plants and animals then use these resultant forms as energy in the creation of new growth. The primary actors in this decomposition process are bacteria, fungi and insects that live in the soil.
The rate at which these microorganisms break down organic matter is dependent on several variable factors. They require sufficient water and oxygen to be present in the soil, and both the soil temperature and its physical structure will impact upon their activity. Another significant factor is the chemical make-up of the matter to be decomposed. Matter with a high level of nitrogen tends to be decomposed more quickly.
One of the primary elements in organic matter is carbon. The microorganisms use this carbon as an energy source. Because of the soil’s ability to hold a large amount of carbon, keeping soils healthy is an important part of limiting the impact of climate change. When soils are mistreated, by removing the rich surface topsoil, for instance, that carbon is released into the atmosphere, and carbon emissions are a leading cause of global warming. So it is beneficial not only for the plants and animals, but also the planet, for permaculture gardeners to look after their soils.
Plant life is often referred to as the lungs of the world, providing animals (including humans) with the oxygen they need to survive. If that’s the case, you could conceive soil as the diaphragm. The soil interacts with the atmosphere around it to create a beneficial balance of gases within both. Gases including nitrogen, oxygen and carbon dioxide flow into and out of soil so they remain at levels that support the life in the soil. A healthy soil, with lots of organic matter, sufficient moisture and variety of plant life, helps maintain the correct balance of gases.