3.4.1. Basic Soil Science

Feed The Soil, Not The Plant.

Healthy soil is the lifeblood of a permanent land-based food system.

If you get the soil right, everything else will work well.  When looking at the most regenerative farming methods of ancient cultures, they all understood this. None of them had microscopes or science degrees but they did have observational skills and thus managed to create soils that have lasted for hundreds of years and retained their fertility. We have the tools of soil science and being able to observe the microbial life of the soil, but you don’t have to be a scientist to have an extremely successful, healthy, and abundant garden that continues to gain more fertility every year. 

Notice that we haven’t started talking about trees and plants yet? That is because it’s more important to understand the structures, or context, that they thrive in. If we can create sufficient water and soil structural support, we can often just throw the plant in there and it will thrive.

We’ve studied microclimates so you have some tools to find or create good microclimates for your plants (and you!). We have different ways of getting water to the plants. We’re now going to learn different strategies and ways to create and maintain vibrant, living soils.

There are a few things that are very helpful to understand about soils. If you understand these handful of things, you will be able to assess a whole lot of relevant things about whatever soil you’re dealing with.

We’re going to cover some theory about soil, and then have you do some simple, low tech assessments on your soil.

What is soil?

First, soil is made up of three aspects. We’ll cover simple ways to work with these three characteristics of soil for maximum benefit to your plants later in this section. We’re giving you a big picture about the components of soil first, and then the details.

• Physical - what is the base, or parent material? The three basic building blocks are clay, silt and sand. These are minerals, and come from rocks, not living things. These minerals hold water and nutrients in different ways, and this influences how plants grow and thus, how we design our food systems.
• Biological - this includes all living things in the soil, whether alive or dead. This means microscopic creatures, roots, worms, moles, etc. Our goal here is to have plenty of rich, diverse lifeforms in our soils.
• Chemical - this includes how acidic the soil is, does it have the ability to hold nutrients through chemical action? Our goal here is to support chemical conditions that are most helpful to plants.

To further break it down, good, healthy soil also contains air and water - lots of it. The living organisms need both of these to stay healthy and abundant.

This pie chart represents what most agricultural soils look like.

There can be very wide differences. In forest systems, there is often much more than 5% organic matter, for instance. Since we are largely focused on creating perennial systems that operate like forests, we tend to go for higher rates of organic matter.

We sometimes create raised garden beds that are almost 100% organic matter. You may need less or more depending on the physical components of your soil which we go into detail about later.

In most healthy soils, air and water make up ½ of the space and perform vital functions for both plants and soil life. So it’s important to keep both of them there. Thus it’s important to avoid compacting your soil. T

his is a great reason not to step on your garden beds or drive heavy equipment on them, and stay in the path! (If you do compact your soil, there are ways to open it up again, like earthworms, or broadforks)

The biological component is only 5%, but within that, there are trillions of beneficial bacteria in one cubic foot of healthy soil!

The biological aspect or organic matter (OM) in healthy, living soil is broken down into approximately these components:

These percentages vary widely depending on the soil type and climate type.

Tropical soil will have more decomposing and fresh residue than temperate forests that have plenty of time to build deep, stable soil. These elements exist in all soils, but in different amounts and forms.

Tilling can destroy stabilized organic matter and living organisms, both of which increase nutrient capacity, water retention, and other major benefits to plants. One important goal is to design ways to grow food that minimize soil disturbance.

Here’s a very short, graphic video on tilled or disturbed vs no-tilled, undisturbed soils. https://www.youtube.com/watch?v=Zb9CZuBIV18

Optional: If you’re interested in seeing more of how this can work, This is another video showing a different aspect of soil infiltration. Natural Resource Conservation Service is a US gov’t agency that is working with commercial agriculture to reduce the negative impact of broadscale agriculture on ecological systems, to rebuild degraded soils and conserve water. https://www.youtube.com/watch?v=ZohI2YR7wQ4

Soil Horizons

The soil horizon indicates the layers of soil beneath the surface.

This is a typical horizon. Keep in mind that every horizon will have different depths of each of these aspects, but they’ll all be there in some form or other.

The surface layer, or O (organic) horizon, includes leaf drop, grass die-off, and manure from animals, all working together to create fertility. You can see bits of twigs, broken down leaves, etc, in this layer. 

This organic material becomes topsoil once fully broken down and mixes with the mineral soil. This topsoil is called the A horizon. 

Subsoil, or B horizon, is below the main fertile area of topsoil. There will be some fertility that has leached into this zone, but less than in the A horizon. This soil is a mix of mineral soil and some leaching from A horizon. 

Below that is the C horizon, which tends to be somewhat broken down rock. The particles will be different sizes. If this is close to the surface, it can be a challenge because plant roots may not be able to penetrate into the rock. 

Bedrock, or R horizon, is sometimes many many feet down, and sometimes, only a few feet, such as on some mountains. This is relatively solid rock but tends to have caves and fissures throughout. Near Miami, Florida, limerock comes up to within a couple of feet of the surface in some areas. It can be necessary to break it up with a jackhammer or ice pick to plant a tree. Quite a commitment!

Alternatively, some arborists and nurseries train trees to grow their roots out rather than down, so when they hit rock they don’t become stunted. This is one reason it’s good to know what your soil horizon is.  

We are mainly concerned with the top 3-20 feet of soil for the purpose of growing plants. 

Below are some examples of different types of soil horizons in different ecosystems.

This illustration shows the different chemical, biological and physical make up of soils in different climates and ecosystems. You can see that some topsoils are a lot more vulnerable and delicate than others. We call these brittle systems.

A brittle ecological system is one that can be easily damaged by change or interaction. This would include a system that is dependent on regular rainfall, or exists in a narrow temperature band, or is easily compacted (ATVs can destroy desert ecologies, for instance), or is not very biodiverse. If one thing goes wrong, the system can rapidly degrade.

A resilient system is one that can recover from changes or abuse. This system might have soils that regenerate rapidly, plants that can handle a wide range of weather and soil conditions, or lots of diversity.

It’s important to know if you’re dealing with a brittle system because to the degree you protect it, it will become less brittle and more resilient. There are other things you can do to help brittle systems become more resilient which we’ll discuss later.

This is a wetlands horizon. Some wetlands are wet most of the year, and some dry out during the dry season. Depending on various factors wetlands could have lots of muck or peat in the o layer, sometimes many feet thick. There can be a lot of organic matter in the B layer too. The lack of oxygen in this system encourages a completely different kind of soil life and microbial interaction that forms a type of “gley” or organic glue beneath the surface (the gleyed mineral soil). 

One reason wetlands are drained and converted to farmland is that they can be super rich in nutrients that have been relatively preserved. Many edible plants can’t handle wet feet (standing water around their roots) and will die if grown in a wetland area, but many plants also thrive at the edge of a wetlands or pond.

Some examples in Florida are bananas, elderberry, and arrowroot. From a permaculture viewpoint, we want to use wetland and flood plain soil in regenerative ways that don’t just deplete it and ruin it, but will allow nature to continue to create that abundance.