Gardening: The science of soil

Keep surfaces covered by plants and mulch as often as possible so you get the best from your soil. Photo / Meg Liptrot

2015 is International Year of Soils. Whether you call it dirt, mud or spend hours scrubbing to get it out of your kid's sports gear, we are completely reliant on the health of our soil - our wellbeing depends on it. A year dedicated to soil might sound a bit dull, but a universe is teeming under our feet.

If you want to become a better gardener, you need to understand that soil is paramount to growing healthy plants that are naturally resistant to pests and disease.

When I studied horticulture, one of the core subjects was soil science. I learned about soil chemistry, "cation exchange capacity" and other curious concepts. Some of it was daunting, and dare I say it, a tad dry.

Some great resources are now available online, making the science of soil more digestible and fascinating, too. Did you know it can take up to 1000 years to form 1cm of topsoil?

A must-see documentary, Symphony of the Soil, celebrates the life of soil beautifully and has interviews from the world's top soil scientists and organic farmers. It deepened my knowledge and rekindled my enthusiasm for the subject.

It's alive

Soil is how terrestrial life began. Over thousands of years, soil formation begins with the weathering of the "parent" rock beneath. In Aotearoa, the first stages of soil creation can be found now on Rangitoto Island.

This volcano last erupted about 550 years ago, and the early building blocks of soil are appearing in its rocky crevices, as seabirds and pioneer plant species provide organic matter for biological soil life to take hold.

Soil is not as inert as you think. It is the earth's "living skin". Half of it is solid, the rest is filled with air, water vapour and a small amount of liquid/water. Within these spaces, living organisms thrive.

I loved this comment by Dr Elaine Ingham in the film: "It's Times Square on New Year's Eve all the time in the soil. When you take soil and you put it under a microscope it's a place full of life."

The relationship between soil and plants is a mysterious thing. Plants purposely feed beneficial bacteria and fungi by providing simple sugars, proteins and carbohydrates. They create a "castle wall" with these biological allies, helping keep out the bad guys: soil-borne pests and disease.

The key to healthy soil is providing organic matter via green manure crops, compost and mulch.

This becomes food for the bacteria and fungi, which in turn are digested by protozoa that are microscopic feeding machines.

The digestive waste from protozoa provides all the nutrients a plant needs. In natural systems we're not feeding plants, we're feeding the soil. Once the cycle is balanced, the soil doesn't require fertiliser inputs.

Why should we care?

Soil takes a heck of a long time to form and we hardly give it a second thought.

The UN has chosen to highlight the importance of soil to change the way we manage our land for food security and resilience. Our very existence depends on the health of our soils, but it is being degraded wherever human habitation occurs.

The "Earth as an Apple" demonstration is a clever way to show how little arable land we have. An apple is sliced until a thin 1/32 sliver is left. The skin on that slice is all the useful soil we have on Earth, and we're losing it at a great rate. Even the best foodgrowing soils are being excavated, built on or sealed over.

Poor land-use practices, such as the use of chemical fertilisers and herbicides, kill soil life. Lack of natural cover on the soil surface causes erosion. Our topsoil is fast disappearing, making its way into stormwater systems and tributaries and out to sea, or is simply blowing away as dust.

Thankfully it has been proven that sustainable, organic agriculture is the best way to manage our soils and can meet the demands of the globe's increasing population. The Soil and Health Association's mantra says it all: Healthy soil, healthy food, healthy people.

What Can We Do?

• Compost is key. Honour the "Law of Return" by putting in what you take out. If you're removing crops, return carbon and nitrogen to the soil by growing cover-crops such as lupin, or by adding compost.

• Keep soil surfaces covered with plants and mulch.

• Avoid the use of herbicides or chemical fertilisers. They can kill soil life.

• Support locally grown, organic food, grown by small, well-managed farms.

Resources

Must see: symphonyofthesoil.com - view on Vimeo.

Get involved: Head to Wynyard Quarter (by the Viaduct bridge) and get tips on organic foodgrowing and composting from October 10-18. compostcollective.org.nz

International Year of Soils: fao.org/soils-2015/en/