What does it mean to fertilise the soil? There is usually an implied understanding that chemical nutrients are to be added, in order to feed plants better. As a result, one ends up focusing on the nutrient needs of particular plants. An alternative approach is to consider soil as a living organism that has the ability, when well fed itself, to provide conditions of fertility. All plants can then thrive, as long as climate and pH are also correct. This approach benefits from knowledge of soil biology and the factors that can promote extra life in the soil. I suggest that soils with an abundance of fungi, bacteria, nematodes, worms, beetles and so forth have the ability to nourish plants with all they need, and to do so in a healthier way than when synthetic nutrients are supplied. In this article I offer a few thoughts on what makes soil fertile, in the biological sense, leading to healthier growth and less need for synthetic chemicals in the garden.

Young seedlings appearing through dark compost soil
Soil is more than a nutrient store for plants. Soil is a living organism respiring and full of life. Photo: Getty images

What is soil?

Instead of seeing soil as a ‘nutrient store’ or ‘bank balance’ of plant food, we might imagine it as a living organism which is respiring and full of life – the skin of our Earth. The next step is to consider how to enhance the lives of all those soil organisms that have the ability both to give a healthy structure to soil, and to make nutrients available to plant roots. Two simple ways of doing this are by keeping a mulch of organic matter on the surface, and by avoiding any unnecessary cultivation. Scientists such as Dr Elaine Ingham have revealed much about soils’ food chain, with invisible bacteria at the bottom and frogs, mice, birds and so on at the top (see below and also www.soilfoodweb.com). At the top of this chain is mankind, which has the ability to either destroy or encourage all the inhabitants underneath.

Maintaining soil health

A first step is to avoid regular use of synthetic chemicals that irritate or even destroy many soil inhabitants. And be extremely careful in their use – for instance, it’s better to use just two or three slug pellets under something like a piece of wood, then retrieve and bin the poisoned slugs. A second step is to avoid cultivating soil as far as possible. Thirdly, most positively, we can increase soil life by adding organic matter to the surface, keeping the most finely decomposed compost for plots where vegetables are grown. Adopting all three of these practices together is self-reinforcing. Not digging soil, for example, will lead to a more healthy soil population and more vibrant plants. Your plants then require less chemical assistance to keep disease at bay, especially when they are well adapted to your type of soil, location and climate.

Buying compost and manure

Home-made compost can be supplemented with bought-in compost or manure. Black and crumbly green waste compost and mushroom compost are often available at reasonable prices, say £20 a tonne, but are not rich in nutrients. Animal manure can often be had for the cost of delivery alone and contains a lot of goodness, but is often lumpy and harder to spread evenly. Horse manure is better for heavy soils and cow manure for lighter soils. Many gardens in the past grew fine plants in soil improved with horse manure.

The soil food chain

Illustrations by Lottie Oldfield

There’s an incredible dynamism and interlinkage to the working parts of our soil. Impairment of any one group of organisms has bad effects on the others. We need them all.

  1. Bacteria are vital to the planet’s health.
    There may be half a million in a teaspoon of healthy soil, mostly helping to decompose organic matter. When bacteria die, the nutrients they recycle become available to plants.
  2. Fungi, unlike bacteria, can travel by increasing in length, helping to aerate soil and move nutrients around. Plant roots use mycorrhizal fungi to fetch and unlock minerals, especially phosphorus.
  3. Protozoa include amoebae, ciliates and flagellates, which work with and, mostly, live off bacteria. Protozoa may supply as much as three quarters of plants’ nitrogen requirements.
  4. Nematodes, or roundworms, are prolific and mostly beneficial, consuming everything below them in the chain, and some above, such as slugs. Above all, nematodes help to mineralise nitrogen.
  5. Arthropods include mites, spiders, beetles, springtails (‘soil fleas’) and millipedes, whose main role is to shred organic matter such as leaves, speeding their decomposition.
  6. Earthworms make casts up to 50 per cent higher in organic matter than surrounding soil. Their digestive enzymes make nutrients more available to plants. They can open up compacted soils and increase soils’ water-holding capacity.
  7. Gastropods are slugs and snails, who play a vital role despite occasionally devastating our plants. Most gastropods live below the surface and convert organic waste to a more decomposed form. Their excretions help bind soil particles together.

Further reading
Teaming with Microbes by Jeff Lowenfels & Wayne Lewis, revised edition, Timber Press, 2010.