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The Rhizosphere Forest Layer

Within the rhizosphere, or root zone of a forest garden, we can find a range of roots, corms, rhizomes, and tubers that are edible, or which have a range of other uses. These plants of the root layer remind us that root forms are very important to forest garden design, and also that we need to delve deeper to see what is really going on in a food forest ecosystem.

In many ways, the layer of the food forest or forest garden that lies below the soil surface is the most important layer of all. But this layer is not predominantly important for the food and other resources it can produce. Rather, it is the life within the soil that makes the root layer, or rhizosphere, so important.

The Root Layer In a Food Forest

It is important to recognise that the layers of a food forest or forest garden do not end at the soil surface. The ecological function of the food forest, just like a natural forest or woodland, is dependent on many important processes that occur below the soil, and on the varied web of life that is found there.

Roots are important, whether we are talking about our own roots and a sense of belonging in our worlds, or about the physical structures of plants below the ground. Of course, in a way, the two senses of the word are connected. A healthy rhizosphere is crucial for that sense of 'rightness' or feeling in place and at home, that comes from a truly healthy, functioning ecosystem.

In permaculture, we must always delve deeper, looking to the roots of things, both literally and metaphorically speaking. In a literal sense, looking to the roots of the plants that we include in each of the other layers of the system is important. As is looking at other features of the rhizosphere.

In order to create an effective food forest or forest garden, we must understand what lies below, and foster at least a fundamental understanding of the soil ecosystem upon which we depend. When we learn more about the soil, this makes us better able to protect and enhance it, and to understand how our plants interact with this part of the system as a whole and individually.

The Importance of the Rhizosphere

What lies below the soil is not something that we should forget about, even though it is not something that we normally see. The root-zone, or rhizosphere is the area of the topsoil where there is a lot going on in any garden, farm or other planting scheme.

In a food forest or forest garden, we are trying to mimic a natural woodland or forest, but introducing species that can better meet our needs than an entirely native and natural environment.

This means not only reproducing the same types of plant community, with plants to fill each of the different roles that plants would fill in the wild. It also means trying to reproduce the forest below the ground – doing our best to recreate the rich, fertile forest floor and what lies beneath the understories and leaf litter.

Looking more closely at the rhizosphere in food forest design is important for two main reasons. The first reason is that without understanding the soil at least to a degree, we cannot hope to ensure the fertile conditions that our food forest plants will require, nor can we hope to adequately maintain that fertility over time.

The second reason is that understanding what is going on with plant roots allows us to better combine those plants to ensure that there is not excessive competition, that plants can co-operate, and that the polycultures we create work well as a whole.

Elements of the Rhizosphere

In order to understand the root layer of a food forest, we need to take a step back and look at what actually constitutes this part of the system – really, an ecosystem in its own right.

It is important to remember that soil is far more than just 'dirt', and when we treat it like dirt we cannot for long expect it to sustain the life upon which we depend.

Soil is a living ecosystem, which is made up of mineral particles, organic matter, air, water, and soil-life, from earthworms to micro-organisms like bacteria and fungi. Thinking about each of these different components of soil can be important in choosing the right plants for the right places and creating a food forest or forest garden.

Soil Minerals

One determinant of soil type is the size of the particles it contains. Clay soil has particles less than 0.002mm in size. Silty soil has particles of 0.002 – 0.05mm in size. And sandy soil has particles of 0.05 – 2mm in size.

The composition of the soil depends on the environment and on the substrate below the soil layers. The location will determine any of the different characteristics of the soil, along with the size of the particles within it.

The smaller the particles (the more clay the soil), the more it will retain water and nutrients. The larger the particles the more free-draining the soil will be.

Soil Organic Matter

Another very important part of the composition of the soil is organic matter. This is any living material from plants or animals that is in a state of decomposition. The amount of organic matter in the soil is a key determinant of healthy soil structure and good soil fertility.

As we create and tend a food forest or forest garden, increasing soil organic matter is a key priority. As the area transitions into a tree-based habitat, it should start cycles that perpetuate the carbon cycle and increase soil organic matter naturally over time as plants die back at the end of the season and deciduous trees drop their leaves.

We also add organic matter by chopping and dropping, to speed up and shortcut natural processes to speed ecological flourishing in the food forest or forest garden.

Air & Water

The amount and balance of air and water within a soil ecosystem are also important considerations. Aeration and water saturation will of course have a bearing on which plants will do best in a particular area and on how fertile the area is.

By choosing the right plants, making the right design decisions, and taking the right approaches, we can potentially limit problems in these areas and avoid compaction, flooding, waterlogging, drought etc...

Fungi, Bacteria and Other Soil Life

The above elements within the root layer zone are all important. But even more important than each of the above is the life we find below the soil surface – not just the life we can see with the naked eye, but microscopic life like fungi and bacteria too. Feeding on and in organic matter, and of course adding to it, microbes in the soil are the glue that holds our our soil and our ecosystems together.

Without fungi, we simply would not be able to garden as we do. So many of the processes on which we depend as organic gardeners cannot function without a healthy population of a range of different fungi. The strands of fungal hyphae spread out through the soil, working between soil particles and solubilising nutrients to make them available for uptake by plant roots.

The unbroken chains of fungal growth spread through the rhizosphere, binding the soil together and transporting water and nutrients to where they are required. And other fungi help us in our gardens in other ways – breaking down organic matter, and performing other beneficial functions.

Specialist fungi, called mycorrhizae, work by forming symbiotic relationships with plants – effectively increasing exponentially the surface area of their root mass. They gain what they need by co-operating with organisms very different to themselves. These mycorrhizae networks make up what has been called the 'wood wide web'. They can be crucial in creating a fully ecologically functioning food forest or forest garden.

Bacteria and other micro-organisms also play important roles below the soil. In a tree-based ecosystem, however, we are generally aiming to achieve a fungal-dominant rather than a bacteria-dominant soil system. Through our design decisions and maintenance methods in a food forest or a forest garden, we can help this fungal-dominant system progress and mature to help the plants that we grow.

Roots, Rhizomes, Bulbs, Corms, Tubers Etc..

Within the soil, of course, we will find the roots and other underground structures of the different plants that grow naturally or that we have planted on the site. It is through these structures of their anatomies that plants obtain what they need from their environment, and sometimes, how they spread.

As you will discover, there are numerous plant within this category that can fill your root zone of your forest garden with yields of food, medicine and more.

Creating a Healthy Rhizosphere

Creating a healthy root layer in a food forest or forest garden involves both taking care of the soil, and learning more about roots and underground bodies of plants in order to make the right decisions about which plants we choose, where we place them and how we combine them within a planting scheme.

Caring for the Soil

From the first steps we take to prepare an area to plant a forest garden or food forest, to the ongoing maintenance of the system, we always need to keep soil health front and centre in our minds. Remember that one of the things our systems aim to replicate about a natural forest or woodland is the healthy, humus-rich, life-filled and fertile soil of the treed environment.

Caring for the soil means thinking about organic matter content, air and water, but it is helpful to remember that primarily, all our efforts should be geared towards creating an ideal home and habitat for all the beneficial life that is in the soil – especially, when trying to create a functioning forest or woodland ecosystem, fungal life.

To create a more fungal dominant environment, we can lay mulch of ramial wood and ensure that dead wood can remain to break down on the forest floor. We can create fungal composts to enrich the soil where required. And we can ensure that we keep promoting healthy natural cycles within the systems that we create.

Learning About Roots To Make the Right Decisions

Whenever we choose plants for our gardens, siting them in specific places and combining them in specific ways, we are relying upon the complex processes that take place below the ground. Roots are, of course, the conduits through which plants take up water and nutrients, the way in which they interact with the complex soil web.

The form, growth and size of a plants root system, therefore, is one important thing to consider. Yet when choosing and combining plants, the hidden world below the soil is all too often overlooked. A plant's roots are as distinctive, and characterise the plant just as much, as what we can see above the ground.

So understanding the roots of plants can help us to understand their needs. It can help us determine where to place them. Understanding roots in a garden is important because it helps us to understand where plants will grow well. It helps us to see which plants will be overly competitive with one another, and which might make for good companions.

For example, we might place plants with deeper tap roots around a fruit tree, as part of its guild. Because a deeper rooted plant may not compete as much with a shallower rooted species. And can be used to draw up nutrients from further down in the soil.

Beyond this, looking at roots and other elements of plant physiology below the ground can also help us to understand how we can sequester more carbon, prevent soil erosion and run off, and boost fertility and soil health.

The patterns of root systems in nature can also inspire us and teach us as we create and maintain our gardens. In permaculture, we often mimic nature's patterns in our designs, and the ways in which roots branch out can be inspiring for the design of many a site's layout.

Edible and Useful Yields in the Rhizosphere

Of course, as well as looking at roots, rhizomes, bulbs, corms, tubers etc. as elements within the rhizosphere, and thinking about how these interact with their environment below the soil and with one another, we should also be sure to think about the yields that can be obtained from the parts of plants that persist below the soil surface.

Just as we can look at the edible, medicinal, crafting and constructional uses of the aboveground portions of plants, so too we can consider all the uses of roots and other parts of plants that lie below the soil surface. Many, of course, are food sources, provide medicinal benefits, or are useful in a wide range of other ways.

Choosing Plants With Edible or Useful Roots for a Food Forest

Choosing plants with edible or useful roots for a food forest, we should be sure to think about:

  • Whether particular plants are well suited to the specific site and the environmental conditions to be found there.
  • How the plants will interact with the soil and other plants nearby.
  • The persistence and/or methods of propagation for the plants in question. (Are they perennial, biennial or annual? Will they remain in place or need to be sown or planted each year?)
  • What yields the plants in question can provide, and when and how these are harvested.

We must be sure to think about roots and the soil of the rhizosphere if we stand any chance of creating a food forest or forest garden that really can stand the test of time.