The highest and most humble in the soil


When we talk about soil health, we are of course referring to the living component of the soil – the bacteria, fungi, protozoa, nematodes, mites, collembola, beetles and worms. You may note that I have listed these in order of size which is the way soil microbiologists classify the different orders in the soil.
The bacteria and archaea are the smallest free-living organisms. Yes, viruses are smaller again, but they need hosts to survive. Interestingly, up until maybe 20 years ago, our only knowledge of archaea concerned the group of organisms that were termed ‘extremophiles’ because they were only found in extreme environments such as salt lakes, sulphurous vents beneath the oceans, or hot springs. Archaea are so named because they were understood to be ancient, possibly pre-dating bacteria. So, 20 or so years ago, genomic techniques were introduced to soils. These tools allowed an examination of all DNA in the soil, and to our surprise, it was revealed that about 40% of what we believed were bacteria, were in fact archaea. Archaea are now understood to be a critical part of the soil’s communities and are involved in the nutrient exchange and community stability.

At the other end of the size, a spectrum is the elephants of the soil – the earthworms. We all know that earthworms are beneficial and in general terms, more is better. I am engaged by Horticulture Innovation Australia with colleagues from Blue Environment to carry out a three-year study into worms and their contribution to soil health in commercial vegetable production. We have studied worm types and numbers on 18 sites across the country – 3 in each state. The extensive nature of our study means that we can look at worms under a wide range of soil types, climatic zones, and management practices. Interestingly worm counts vary very widely ranging from zero worms on some sites to over 700 on others. Interestingly our highest scores were found on a Tasmanian farm – as was our lowest!

Not only do worms move soil around, degrade organic matter and prepare it for further decomposition by smaller organisms, and deposit mineral-rich casting behind, they also have a crucial role in aerating the soil. The attached images are taken from our top-scoring site in Tassie. Look closely, and you will see a soil riddled with holes which act as conduits for air and water movement – critical functions for healthy soils
. It is very interesting to note that this photo was taken under a carrot seed crop and the soil is a clay loam. Normally carrots are grown in sandy soils, but the very high degree of aeration in this soil contributed to a very healthy crop.

So, big and small, all soil organisms have equally important roles to play in maintaining soil condition, cycling nutrients and containing pest or disease outbreaks. Part of the work we do at SESL when specifying soils is about ensuring
that conditions are optimised for biological function and the contributions they make to vigorous, healthy plant growth.