Soil: an Underappreciated Superhero

There’s some lovely filth over here - Monty Python

Friends often ask me how to get their garden plants to thrive. The answer is very often that the soil is not providing the plant what it needs. Water and sunlight are of paramount importance, of course, but we tend to pay attention to those issues while neglecting the soil. This lack of care towards soil led me to ponder if there is link to our use of soil related words as bad things. For example, farmers are mocked for reveling in quality soil, as in the Monty Python skit referenced in the subtitle of this newsletter. In the US, we use dirt and soil interchangeably while the Brits seem to use dirt only to mean that which makes something unclean. We use the phrase “to soil” something to refer to something getting dirty. Does this blurring of messy and earth muddy the water, as it were, and promote disrespect to soil. Let it not be! Soil is a vastly under appreciated superhero.

And superhero trait number 1? Soil is the way in which rocks, rocks!, manage to support life on land. We know of this terraforming magic from the fossil record, but we can also observe it in the evolution of life over a fresh lava flow. First to arrive on a newly formed or newly exposed rock surface will be algae, lichen, ferns and mosses. These pioneers start to break down the surface of the rock - creating rock grains. Weather accelerates the crumbling of the lava into smaller and smaller grains of rocks and minerals. As the first pioneers die they add organic matter to the grains of eroded rocks. Once enough organic material has accumulated, wildflowers will grow and then shrubs and finally trees arise. In a wet region, a lava flow can become a forest in less than 150 years! Of course, the progression of post-lava-flow forests is rapidly accelerated by the addition of seeds and organic matter (dead plant material primarily) from nearby ecosystems. When starting from scratch, we estimate that the time from when plants first emerged from the oceans onto the land to the time when trees evolved, took 170 million years.

This colonization of bare rock serves to illustrates what soil is. And that is primarily grains of rock. There is also organic matter in soils which supports the life of plants. On an industrial farm, the organic matter makes up a few percent of the soil column, whereas it comprises about 15% in an organic farm. In one study, crop yield increased about 12% for every 1% increase in soil organic matter. In between the rock grains and the organic matter are spaces, or pores. It is in the pores that water and gases move and soil organisms like earth worms live. The pore spaces are essential so plants can respire, and access water and nutrients. The size of the grains of rock, and thus the size and connectivity of the pore space, is thus essential to the fertility of the soil. In fact, soils are classified based on their grain size: sands have grains greater than 0.05 millimeters and less than 2 millimeters. Clays are much smaller and have grains less than 0.002 micrometers. Silts are in between sand and clay in size. The ideal garden soil has a mixture of these three classes of soil grains.

When soil grains are small, as in the case of clay soils, there is less pore space as the grains are well packed. This means less water percolation, less gas exchange and less room for critters. Furthermore, soils with smaller grains have more grain surface area in a given volume than a big grain soil. This is important because water sticks to the negative charges on grain surfaces - more surface means more sticky water. So smaller grain sizes have not just less physical room to percolate water, but also less mobility of water and nutrients. This can be beneficial as clay soils have the potential to hold moisture in dry times and generally have loads of nutrients to offer plants, but clays are susceptible to water logging and to becoming super hard in dry weather. On the other hand, sandy soils, with large grain sizes, lose their water and nutrients to a percolation quite readily making growing most plants a challenge.

Luckily for gardeners, the best way to improve our soil and support our plants is the same for almost any kind of soil you will have in your garden - add organic matter and don’t walk on it. In the case of the small grained clay soils, organic matter literally opens up the pore spaces and allows water and gas exchange. In the case of sands, organic matter provides nutrients and holds water and said nutrients in place for ready access by the plant roots.

How do we access this magic? By incorporating rotted kitchen compost or manure, wood chips, leaves, seaweed, or chicken manure pellets. A liquid feed offers nutrients to our plants but doesn’t do anything to improve the structure, or pore space distribution, of the soil so has limited benefits. In addition, an application of organic matter to the soil moderates the soil temperature and promotes biodiversity in the soil. And perhaps most readily apparent to the gardener, a mulch supresses weeds. What’s the best way to add organic matter to our soil? Some gardeners dig compost in, others double dig. Me? I’m a top dresser - which means I simply put organic matter on the surface of my plant beds. It is far less work then digging in, it doesn’t disrupt the natural ecosystem of the soil, and it doesn’t dredge up weed seeds to the surface. I top dress with a nutrient rich compost (like kitchen compost or well rotted manure) when I make a new bed, when a bed has become unproductive, and also once a year on my veg beds. Ideally, nutrient rich mulching is done in the autumn so the nutrients have time to get worked into the soil column before growth occurs. But in my case, it usually happens when planting time comes around. In addition to the nutrient heavy mulch, I also top dress all of my planting areas with a less nutrient rich cover like wood chips or chopped up leaves as needed. And though that may sound like a lot of work, it vastly reduces weeding and watering time. I’d guess that once a flower bed is established, I spend less than 1/4 of the time I spend on mowing an equivalent area of lawn. There is on exception to the mulch everything practice on my property, my wildflower patch. Wild flowers prefer nutrient poor soil and I want the wildflower seeds to spread there at will.

Lest you think that taking care of your soil is for your benefit only, consider soil’s second superpower - it’s biodiversity bonanza. One single teaspoon of soil is home to 1000 distinct species, millions of individuals and over one hundred meters of fungi networks. One teaspoon full! It is thought that soil is home to over 25% of all living species and also supports all above ground life which is another 50% of the global species. Other global scale superpowers of soils including cleaning and regulating water flow, diluting pollution, and buffering our carbon emissions by absorbing around 1/4 of the carbon we emit every year. Indeed, a study back in 2004 estimated soils ecosystem services at 1.5-13 trillion US dollars a year. But sadly, we are degrading our soils the world over - primarily through erosion at industrial agriculture sites. The UN has estimated that if current trends continue, the loss of soils will results in a dire 10% decrease in our capacity to grow food by 2050. If only there were an alternative food source that took care of soils …

Before I sign off, I want to emphasis the breadth of soil’s superpowers: turning rocks into life, housing 1/4 of the planet’s species and underpinning another 50% of all species, regulating and cleaning water, diluting pollution and slowing global warming by 25%. It’s astonishing that we don’t take soil very seriously. Since it takes up to 1000 years to build just 1 centimeter (0.4 inch) in nature, we can do a great deal by taking care of the soils in our lives. For all of us now, and for the future. And all we have to do to be a soil superhero is spread a little mulch and buy organic foods. It’s a lovely day outside, think I’ll go spread some mulch.