What is soil organic matter?

Soil organic carbon is organic matter is made up of living organisms, undecomposed plant, animal and micro-organism residues and humus, formed over many years. You cannot have good soil structure without high levels of organic matter. It is the main food reserve for living soil organisms and many of them provide the simple substances that are taken up by the plant roots. The most reliable method of finding out the soil organic matter (SOM) concentration of a soil is by burning (dry combustion) at temperatures of over 900⁰C. The result is normally given as percentage organic matter in the soil. Farmers weekly provide a simple chart for farmers to score the quality of their soil.

Less than 1% very low

Less than 2% low

Less than 4% Medium

Less than 8% High

Over 8% Very high

Typical components of a soil

http://www.cartage.org.lb/en/themes/sciences/botanicalsciences/PlantHormones/PlantHormones/soil.gif

Organic matter is made up of three main components which have been called the living, the dead and the very dead!

Living organisms, plants, animals, soil invertebrates, bacterial and fungi are all considered to be part of soil organic matter, and they play a big role in contributing organic residues to the soil and in formation of more stable types of organic matter. Up to 15% of soil organic matter is living organisms and fresh organic material.

Figure 2 The structure of soil

Active soil organic matter is primarily made up of freshly dead plant and animal residues that break down in a very short time, from a few weeks to a few years. It is sometimes called detritus if it contains partially broken down cells and tissues that are only gradually decomposing. One third to one half of the SOM is active soil organic matter, the detritus of partially and slowly decomposing plant and animal material that may last decades.

Stabilised or passive soil organic matter, is known as humus. It is very dead(!) and not biologically active because it provides very little food for soil organisms. Humus may take hundreds or even thousands of years to fully decompose! Humus is very important as (a) it acts like a sponge and can absorb 4-10 times its weight in water,  (b) it is a way of taking carbon dioxide out of the atmosphere and burying it in the ground (sequestering it) and so helps to mitigate the effects of rising CO₂ levels.

Soil organic carbon from
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Soil organic matter may enter the soil in a number of ways, such as addition of manure, compost, mulch such as sweetcorn stalks or woodchip, leaf mould, coffee grounds, eggshells, chicken pellets or the growing plants in their little pots of compost. But it may also be produced in the soil by the crop and the surrounding weeds and soil animals grow and then die and remaining in or on the soil surface.

Soil organic matter may be removed from the soil when crops are harvested, often with some sticky soil around them, on our boots, when the wind blows it away, or heavy rain washes or leaches it away through the soil.

The global climate is becoming more unstable due to global warming. Last year it was "the Beast from the East" for us at HCF, followed by a warm and fairly dry summer. Who knows what it will be this year? One of our aims at HCF should be to create a soil that is as resilient as possible to climate change. To be this soil it needs high levels of Soil Organic Matter. This soil will be able to hold more moisture, which, among other things, will enable it to cope with longer periods of water shortage or heavy and excessive rains which would otherwise cause leeching or runoff. 

Refs

http://www.cartage.org.lb/en/themes/sciences/botanicalsciences/PlantHormones/PlantHormones/soil.gif
http://soils.usda.gov/sqi/concepts/soil_biology/images/soil_f1_high_res.jpg

Creating a healthy soil at Highbridge Community Farm

Climate change and the ethos of HCF

Most of us are familiar with the broad issues of climate change; an increase in atmospheric carbon dioxide (CO₂) concentration from 278 ppm in the preindustrial period (circa 1750) to 405.5 ppm in 2017; an increase of the greenhouse gas methane from 722 ppb to 1859 ppb in the same period, an increase in nitrous oxide from 270 ppb to 330 ppb in the same period. (Lal, 2019) This has already raised global temperatures by over 1⁰C since the Industrial Revolution with dire consequences as exemplified by the increase in frequency of extreme events throughout the world. Furthermore there is the real likelihood that we will miss the target set at the Paris Climate Conference (COP 21) in 2015 of limiting global warming to 1.5⁰C. (IPCC (Intergovernmental Panel on Climate Change, 2018)

The Highbridge Community Farm ethos (HCF) statement says "We have evolved from the Transition Movement and retain their founding principles - a community-led response to the pressures of fossil fuel depletion and climate change, supporting local economies and moving towards a more viable and sustainable future.  Now a mutual benefit co-operative society in our own right, we work together to produce food for ourselves with minimum use of fossil fuels and chemicals.  We support growing techniques that maintain the natural balance of the soil, preserve wildlife and their habitats, and encourage biodiversity.

Over the nine years of our existence our aim has been to grow good organic food. We have managed the soil to obtain good crops, without ever really addressing the issue of how to  improve the health, fertility and productivity of our soil in an environmentally sustainable way.  Ideally this soil should be resilient to be able to cope with whatever crop is planted in it and cope with whatever combination of weather events that is thrown at it. Probably the best measure of soil health and resilience is one with a high organic carbon content. This is in line with the Climate Accord proposed in Paris in 2015 which initiated the 4 per 1000 programme of raising Soil organic carbon (SOC) in world soils at the annual rate of 0.4% per year to a depth of 40 cm. (Chambers, 2016)  The UK signed up to this initiative and Environment Secretary Michael Gove has undertaken to deliver on this ambitious goal by supporting soil health improvements in the UK. (Eldridge, 2018)

There is an added benefit of raising SOC; the potential lowering of atmospheric CO₂ on a worldwide basis by raising SOC is approximately 84 ppm of CO₂. This burying of SOC in the soil in the form of humus is called sequestration. So raising SOC at HIghbridge Farm will be a win:win. We can play our part at HCF to produce a better, more resilient and productive soil and our efforts will benefit everyone if global CO₂ levels fall!

What is a healthy soil?

Dr F Crotty states "Soil health can be defined as a soil's ability to function and sustain plants, animals and humans as part of the ecosystem." She identifies five main factors that impact the health of the soil and can have a large influence over its capability and resilience to function; they are:

1. Soil structure
2. Soil chemistry
3. Organic matter content
4. Soil biology
5. Water infiltration, retention and movement through the profile (Crotty, 27 July 2017).

Farmers tell us that a good soil

·         drains well and warms up quickly in the spring

·         does not crust after planting

·         soaks up heavy rains with little runoff

·         stores moisture for drought periods

·         has few clods and no hardpan

·         resists erosion and nutrient loss

·         supports high populations of soil organisms

·         has that rich, earthy smell

·         produces healthy, high quality crops and grass

·         are easy to work in a range of conditions.                                                                                                 (LEAF -LInking Environment and farming, 2016)