Soil is not a dirty word

Despite a lifetime working with soil and as a qualified horticulturist, I was astounded to learn that we have misunderstood our soil for a century. World-renowned soil ecologist Dr. Christine Jones professes, “You might as well burn every book written on soil for the past 100 years for all their good – they are wrong!”

This profound insight sent me down a rabbit hole of research – I started digging deeper and simply couldn’t stop! I learned that Dr. Jones was right, but I also unearthed an incredible amount of evidence-based knowledge that all gardeners and landowners need to know. We have managed to lose two-thirds of the world’s level of humus, which has a huge water-holding capacity. We are now seeing the flow-on effect of this extraordinary loss in devastating floods and dust storms, resulting in 7–10 tonnes of topsoil lost per hectare per year. If the researchers are correct, and I believe they are, we only have 65 years’ worth of humus-rich soil remaining.

This is extremely worrying, given we only use 3% of the Earth’s surface to grow our food. Traditional methods of turning and tilling our soil have played a significant role in soil depletion, compounded by the overuse of high-analysis NPK fertilisers. We now use ten times more fertiliser to achieve the same result compared to the early days of application. The routine application of chemical insecticides, fungicides and herbicides has also contributed to the degradation of soil fertility.

The belief that plants take up minerals and of their own accord is fundamentally flawed. Modern technology has enabled a closer look at the symbiotic relationship between living plant roots and the microscopic world beneath our feet. Plants produce food for themselves through photosynthesis; they produce 40% more than they need, using it to attract and feed the microbes in the soil in return for solubilised minerals and nutrients in a plant-available format. Plants can only absorb solubilised minerals, so they rely heavily on the soil’s microbiome to perform the conversion for them. This creates the symbiotic relationship that we previously had no idea existed. In fact, plants have been found to farm microbes in the same way we farm plants – who would have thought?!

Did you know that a gram of soil contains billions of organisms belonging to thousands of species, mostly microbial and largely still unexplored? When they die, they leave microbial necromass (organic carbon) in the soil, which is eventually returned to the atmosphere through the process of respiration carried out by microscopic organisms. However, a substantial part is retained in the soil in the form of soil organic matter.

Tillage (manual turning of the soil) usually increases the rate of soil respiration, leading to the depletion of soil organic matter. Hence working the soil is counterproductive. Since plant roots require oxygen, ventilation is an essential characteristic of soil. This ventilation can be accomplished via networks of interconnected soil pores (humus), which also absorb and hold rainwater, making it readily available to be taken up by plants. Since plants require an almost continuous supply of water, but most regions receive sporadic rainfall, the water-holding capacity of soils is vital for plant survival. And this is just the tip of the iceberg when understanding what soil is and does.

Add to that the fungal network that is akin to an information superhighway stretching large distances, sometimes a kilometre or more, ferrying minerals, nutrients and water to plants. Biochemical signalling also occurs between microbes and plants for communication. Feeding plants fertiliser disconnects this communication. Plants tell the microbes daily what they need, as they need it. The microbes respond by solubilising the minerals in the amounts required, which are then made accessible in a plant-available form in exchange for the excess food the plants produce, known as exudates.

When we feed fertilisers to plants, they can only take up 10–40% of it in the year of application, which means the unused fertiliser gets locked up in soil, washed off or volatilised into the atmosphere. These microbes protect plant root systems from harmful bacteria and fungi, as they have a food source to sustain them. Plant exudates stimulate plant growth, in turn promoting microbes that stimulate healthy root system development and strong plants. Conversely, plants grown with chemicals have little or poorly developed root systems.

As mentioned, microbes have a vested interest in ensuring that plants receive what they need due to their reliance on the plants as a food source. When we feed plants chemical fertilisers (high-analysis NPK), the plants stop feeding the microbes and the plants don’t get all the things they are supposed to get. The “ah ha” moment for me came when I learned that fruits and vegetables grown with excessive fertilisation had over 2,000 less antioxidants than in fruit and vegetables grown in healthy soil – a jaw-dropping realisation. The link between flavour and antioxidants was equally mind-blowing – think tastebud experience between homegrown tomatoes vs. store-bought varieties!

Feeding our soil with organic matter is now critical to keep soil covered and to help microbes feed our plants. We must reduce, if not avoid altogether, the tilling and turning of our soil. The addition of regular compost, as diverse as possible, is the best solution. But don’t dig it in – that will obliterate the soil microbiome! Instead, sift and broadcast well-rotted compost over the mulch layer in your garden. Keep that mulch layer in a constant state of decomposition, just like a forest floor, and you too will create an incredibly biodiverse forest soil, teeming with life and nutrition. We need to return the CO2 we have oxidised through tillage and restore the health of our soil. We gardeners need to be the change as we have no time to waste.