Soil Organic Matter and Biology

Earthworms can increase productivity and soil health (Photo: D. Long)

Earthworms can increase productivity and soil health (Photo: D. Long)

The following piece is a non-technical synopsis of a Soil Wealth fact sheet: Carbon Storage in Vegetable Soils and an extensive review of soil organic matter in Tasmanian soils by Bill Cotching, one of Tasmania's very own soil gurus. All italicised words are links that will take you to relevant fact sheets and other levy funded resources. 

Main messages:

1.     Soil organic matter has multiple positive effects on soil health

2.     Plant roots and soil biology are important components and processes within SOM

3.     Soil organic matter is influenced by soil type and climatic conditions.

4.     Cropping soils require constant inputs of organic matter to avoid declining SOM levels.

Why does SOM matter? Well, for soil health, few things matter more. Even though SOM is a small fraction (1-5%) of your soil volume, and the living organisms within it a fraction of that fraction, they are a powerful force for good. Sometimes called 'the underground herd' there are trillions of living organisms in every teaspoon of a healthy soil. They rely on and cycle the SOM. SOM both supports and is generated by the soil biology, and requires constant input in a cropping system to avoid decline. Other bonuses you can get from good SOM levels are: increased carbon sequestration in your soils, and so reduced greenhouse gas emissions, reduced erosion, improvements in bulk density, cation exchange capacity and nutrient holding ability and pH buffering.

The amount of soil organic matter (SOM) that your soil can comfortably maintain - for example under perennial pasture - is highly dependent on the soil type and texture, as well as mean annual rainfall and temperature. Ferrosols and other soils with a high clay fraction, have a higher potential SOM content, as the SOM is protected by the clay. Soils in high rainfall areas also have higher potential SOM, due to more vegetative growth and slower decomposition in wet conditions. The rate you can build SOM will also vary on different soil types. If you don't know what soil types you have on your place, you have a date with a spade in your paddocks. One thing that all of Tassie's soils have in common, though, is that under cropping agriculture, the SOM declines: rapidly at first, then more slowly, but steadily. It is very likely that almost all of our agricultural soils have a much lower SOM content than they could have. All soil types function more effectively with a SOM content of 2% or more. 

The soil organisms that make up part of the SOM are doing crucial work including: nutrient cycling, nitrogen fixation, breakdown of complex compounds, producing sticky glues and polymers that help soil aggregation, converting chemical elements to plant available nutrients, suppressing disease through competition and predation, teaming up with plant roots in many mutually beneficial ways, creating pores for better infiltration, improving water infiltration and holding capacity and more! Don't even get me started on fungal hyphae and plant-to-plant communication!

 What does this teeming mass of thanklessly hard-working underground life need to thrive? SOM content is important but there's a new school of thought that it is living roots that are the real engine for this ecosystem. Plants make sugars from the sun for growth and metabolism, but they are releasing a significant proportion of this energy into the soil around their roots to boost their co-evolved support team in the soil. 

Another important factor is good soil structure. Some larger organisms, famously the earthworms, can move soil, but most soil life needs existing air spaces and water films to move about and live their lives, so avoiding compaction is important. Plant roots also benefit from good structure and all three, roots, biology and structure, have important complex, feedback loops underground.

We need to talk about tillage. There's a number of reasons why reducing tillage benefits your SOM and soil biology. Cultivation is particularly hard on earthworms - incredible drivers of aeration, structure and fertility - as well as soil fungi - important for disease suppression and structural stability. Tillage operations also produce a burst of very rapid decomposition of SOM due to exposure and mixing. Any reduction in tillage will have a positive effect on SOM, including controlled traffic farming, smaller tractors, less power passes, strip-till and no-till.

The key is to reduce loss, and increase inputs of organic matter. Other management approaches for doing this include avoiding over-fertilisation, using soil and plant tissue testing, use integrated pest management (IPM) to reduce pesticide harm to soil insects, using cover crops to protect soil and keep living roots in the ground, using mulches and composts to increase SOM, healthy crop growth via careful irrigation management and drainage design.

Lots more information can be found in the carbon storage fact sheet, and heaps of great resources are housed at

Exciting times in the rhizosphere  (Photo: D. Roberts-Thomson)

Exciting times in the rhizosphere
(Photo: D. Roberts-Thomson)

N-fixing nodulation in a tic bean root system (Photo D. Roberts-Thomson)

N-fixing nodulation in a tic bean root system (Photo D. Roberts-Thomson)