The other day I was chatting to a neighbour about, of all things, Casuarina sp. (’She-Oak’) and the mulch they create from all the ‘needles’ they shed. (Casuarinas seem to be a very much under-appreciated and much-maligned tree, but they really are well worth getting to know!).

I found it fascinating that our very different backgrounds made us see the same thing in very different ways. He was a horticulturalist and saw things from the point of view of plants. For him, the mulch is a weed-suppressant, but he wasn’t sure if it was because of some compound(s) in the ‘needles’, or simply because of the dense covering they make.

Me, I saw things from the point of view of soil microbes and saw the ‘needles’ as not only a great cover that cooled the soil for microbial life, but which fed that life as well, via the abundance of carbon they input. But his comment certainly had me thinking, and I did go and confirm that the mulch itself doesn’t contain plant-killing compounds, though the roots release allelopathic compounds. I was also fascinated to discover these roots harbour nitrogen-fixing microbes!

So what do casuarinas have to do with jujubes? Not much really! But our conversation really had me thinking about those ‘needles’ and what they did or did not do to the ground beneath them. And as we often don’t see what we are unaware of, that led in a convoluted way to this week’s topic: that true soil fertility requires three components, all interacting in a complex and holistic way also not always apparent.

Soil Fertility

There is more to a fertile soil than simply adding NPK and other nutrients as appropriate. This doesn’t really address, much less acknowledge, all three components of a truly fertile soil: its physical fertility, its chemical fertility, and its biological fertility. Let’s explore each of these now.

Physical Fertility

A soil’s physical fertility is determined by its physical properties: its structure, its texture, its ability to retain water, and its friability.

A sandy soil may be easy for roots to push through, but a sandy soil is also quick to drain and does not hold much organic matter. A high-clay soil doesn’t drain quickly and does hold a lot of organic matter, but roots may struggle to push through thick clay, or simply rot if waterlogged.

Both of these soil types can still support plant life, especially those adapted for those conditions, but neither soil is of ‘ideal’ physical fertility.

A soil’s unique combination of physical properties determines its physical fertility, and this physical fertility in turn affects its chemical properties, and chemical fertility.

Chemical Fertility

Chemical fertility is a measure of a soil’s acidity or alkalinity (its pH), and the chemicals (read: nutrients) it can or cannot retain due to this pH. A soil’s salinity (amount of dissolved sodium chloride in the soil water) and sodicity (amount of sodium ions attached to clay particles) also influence chemical fertility.

Chemical fertility is, in part, determined by the underlying physical fertility as it arises from the underlying physical properties of a soil — the bedrock it derived from determined which minerals became available in that soil, and those minerals in turn determined which nutrients become available to plants.

A soil from limestone bedrock will be very high in calcium for example, but these soils are alkaline, which can cause  deficiencies in some nutrients and toxicities in others. Basalt bedrock generates soils high in iron, magnesium, calcium and phosphorus, and are typically regarded as ‘fertile’ soils. Granite and sandstone bedrocks result in not-so-chemically-fertile sandy soils.

The sodicity mentioned above, though an extreme and undesirable soil property, is another example of how physical and chemical fertility are intertwined.

Sodicity influences physical fertility, in that sodium ions bound to clay particles causes structural problems by disrupting the stability of soil crumbs when wet. And sodicity also influences chemical fertility in that those same sodium ions prevent other more desirable ions (nutrients) from binding to clay particles.

There are other examples of physical fertility shaping chemical fertility. A soil with a good amount of clay and humus colloidal structure has a higher physical fertility than a fast-draining sand low in organic matter. And these colloids also increase a soil’s chemical fertility by providing bonding sites for nutrients, making those more plant-available and less susceptible to leaching.

Physical and chemical fertility are very much interconnected, but it’s the third part of soil fertility, biological fertility, which completes the trinity and makes a truly fertile soil.

Biological Fertility

The biological fertility of a soil is shaped by both macroscopic  and microscopic organisms.

Macroscopic Organisms

Macroscopic soil organisms include earthworms and arthropods.

The mucus that earthworms secrete as they tunnel through soil improves that soil’s structure by binding soil mineral particles (clays and silts) together into irregularly-shaped soil crumbs (or aggregates). These aggregates stabilise the soil by making them less resistant to erosion, and their irregular sizes lead to a more porous and loose soil through which water, air and roots can travel. Soil crumbs also contain organic matter, which, together with clay particles, binds plant nutrients and holds them in the soil where they may otherwise leach beyond the reach of roots.

Earthworm and arthropod burrows additionally help with aeration and water infiltration, and the organisms themselves have important roles in breaking organic matter down into smaller pieces which can then be further degraded by microbes and made plant-available.

Microscopic Organisms

Soil microscopic organisms include fungi, bacteria, archaea, and even algae and viruses! Protozoa also feature — we’d regard some of them as disease-causing, but protozoa have an important role in maintaining balance in the soil by feeding on bacteria.

Of the fungi, the saprophytic fungi are decomposers which break down dead plant material, while the mycorrhizal fungi make available to plants phosphorus, micronutrients and water in exchange for carbohydrates.

The bacteria and archaea are without a doubt the most diverse in their contributions to biological fertility. Some are crucial in the nitrogen cycle while others, with their unusual energy sources are important in soil creation (pedogenesis) and making mineral nutrients available to plants. Some are photosynthetic, and others produce polysaccharides which stabilise soil by binding sand particles together and absorbing water.

Algae can produce large amounts of oxygen in the soil through photosynthesis as well as mop up excess nitrates that would otherwise leach through the soil profile. Viruses have roles in gene transfers between species and bacterial population control.

Putting it All Together

Each of the three components of physical, chemical and biological fertility discussed above can ‘carry’ a soil individually and support plant life to some degree. There are plenty of naturally-occurring soils deficit in any or even all three of these components, and plenty of others that have sadly been destroyed by our interference in any or even all three of these components, and yet some kind of plant life can usually be found growing on these.

Plants will grow better in a physically fertile soil of excellent texture and drainage than in soils that aren’t. But their potential is limited if that physically fertile soil is deficient in chemical fertility — which is why, of course, chemical fertilisers are continuously added to maintain good plant health. This is especially so in agricultural soils where vast amounts of nutrients ‘artificially’ leave the land by way of produce. In a more natural environment these nutrients would stay in place, being recycled through decomposition of animal and plant matter and the breakdown of manures.

Which brings us to the third component of soil fertility: its biology. A soil can be of good physical and chemical fertility, but both will degrade over time without the biological input. It is biological fertility that makes soil a unified whole by continually improving physical and chemical fertilities in a positive feedback loop.

Organisms improve physical fertility by amalgamating mineral particles and organic matter into soil crumbs, which increases soil stability, loosens a soil, and improves aeration and water infiltration. The longer a soil is left undisturbed, the more this physical fertility can develop, and is a major reason for the major shift in agriculture from traditional heavy ploughing to less disruptive reduced- and no-till farming over the decades.

Organisms also improve the chemical fertility. Macroorganisms hasten the breakdown of organic matter by turning large pieces into smaller pieces with larger surface areas for microbial decomposition. This decomposition releases nutrients back into the soil in simpler plant-available forms. Other microbes add nutrients via nitrogen-fixation or pedogenesis.

Thus a truly healthy soil is one rich in physical, chemical and biological fertility, with all three components interacting holistically. The more biologically fertile a soil, the more physically and chemically fertile it becomes. The more physically and chemically fertile it becomes, the more biologically fertile it becomes.