Every gardener should strive to build good soil structure in their gardens. It is literally and figuratively the foundation of a healthy garden. Understanding how soil works will make your gardening easier and supercharge your garden’s health.

Us gardeners obsess about good soil structure. We talk about it, read about it, complain about it, and we work on it. We add to our soils, we dig our soils, we test our soils, and we plant in our soils. When asked what they would most like to receive for Christmas, many gardeners say, “a large pile of manure” (much to the frustration of our family and friends who would rather we just asked for socks). But, the thing that we prize above anything else is good soil structure. That beautiful crumbly loam is easy to plant into and easy to weed. That’s the dream.

WHAT IS Good Soil Structure?

Good soil structure to us gardeners is crumbly, soft, and dark in colour. It feels wonderful between our fingers. We can squash it into shapes and then break it apart again with our fingers. It isn’t slimy or dry. It’s almost the exact same consistency you aim for when making a nice crumble topping!

But to plants, good soil structure is something else. It is stable, well-aerated, well-drained and can hold onto plenty of water. Healthy soil is porous, just like a sponge, with plenty of air pockets and aggregates to hold onto water. It also does not have pans or compaction layers. These are common in places that have had machinery drive over them regularly. Compaction layers also occur when we regularly dig our soil to the same depth (usually about 30cm). We inadvertently create compaction layers just beneath this depth. These are problematic because they can trap water above them, creating underwater pools that breed pathogens and suffocate our plants’ roots.

Organisms in the soil build good structure as they move through the layers of the soil. They feed on carbohydrates from live plants, dead organic matter such as leaves, and algae that live within the soil. It is in their interest to preserve the environment around them. So, just like bees build hives and badgers build setts, the soil organisms build soil structure. 

WHY IS GOOD SOIL STRUCTURE IMPORTANT?

Soil structure that is created by organisms in the soil is a very resilient, self-maintaining ecosystem. The organisms in the soil create a soil structure that is the perfect environment for themselves and for the plant roots. You can think of this almost as though the soil organisms were farming the plants above them. The plants feed down little packages of carbohydrates to the life in the soil in exchange for nutrients. They have reciprocity that allows the soil organisms to thrive and the plants to thrive.

In exchange for these carbohydrates that the plants produce through photosynthesis, the soil life maintains the plant’s growing environment. By aggregating the soil particles, they can ensure that the soil can hold onto water and not get waterlogged. This helps plants access oxygen and water at their roots as often as possible. The soil life also releases and sources nutrients for the plants.

Some soil organisms also help fight pests that might seek to predate the plant roots or carry pathogens. Some even go as far as creating a protective barrier around the plant roots. Good drainage in the soil can also protect from pathogens that thrive in anaerobic conditions, such as damping off. Good soil structure is also less prone to compaction or erosion, as soil particles are bound together.

WHAT IS AGGREGATION?

Soil structure that is built naturally is done by a process called aggregation. The tiny particles in the soil are clued together by sticky bacterial sugars called polysaccharides. Then these stuck-together particles are stuck to other particles by another glue called glomalin, produced by fungi. Instead of making the soil compacted and sticky, this process opens air pockets or pores. Coupled with the movements of larger soil-dwelling organisms such as earthworms, this forms a matrix of air pockets and aggregates. 

The aggregates have a slight charge, which helps them to hold on to water. The net result is that our soil is well-aerated, with good moisture retention. In short, a wonderful environment for plant roots that need to drink and respire. 

HOW CAN I CREATE GOOD SOIL STRUCTURE?

We now know that good soil structure is made by particles in the soil being stuck together, forming aggregates. Life in the soil does this for us, so we must encourage soil life in our gardens. There are several ways in which we can do this. The first is to stop using chemicals in our gardens (if you do, of course). Even herbicides can have a huge knock-on effect on the soil ecosystem, where one of the main food sources, besides root exudates, is algae.

Using liquid fertilisers can also have an effect on the life in the soil. The plants, able to get their nutrients easily, cease to “pay” the soil life by providing root exudates. They keep these carbohydrates for themselves, and in turn, they lose the other services that the soil life provides. The soil life begins to die off without its primary food source, and the structure and natural nutrient availability in the soil goes down. This means that our soil health will slowly decrease, and our plants will depend on fertiliser.

Organisms within the soil come up to the surface to feed, as this is where organic matter falls in nature, in the form of dead leaves and plants. In our gardens, we tend to remove this organic matter by harvesting or tidying away. It is good practice to keep tidy beds as this will help to keep pests such as slugs and woodlice at bay. However, we must replace organic matter. One of the best ways to do this is by using compost. This does not provide a habitat for pests and ensures that the soil life is fed.

TRY NO DIG GARDENING

No dig gardening is the practice of laying compost on the tops of your beds once a year and letting the soil life do the digging. It relies on not disturbing the soil but preserving the habitat that the soil life has created for itself.

No dig gardening has become increasingly popular recently, with people finding that their soil health improves, their plants are healthier, and their gardening is less back-breaking!

For more information on No Dig Gardening, I highly recommend the following:

• Stephanie Hafferty. Her blog and social media channels are full of information from her years of experience as a no dig gardener. She is generous with her knowledge and an excellent writer. Her book “No Dig Organic Home and Garden” is one of the best no dig gardening books available.
• Liz Zorab. Liz’s access-all-areas style YouTube channel is a mine of useful information, as is her blog. Liz’s book “Grounded” takes you through her journey as a gardener whilst dispatching useful information and tips about growing food.