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
http://soils.usda.gov/sqi/concepts/soil_biology/images/soil_f1_high_res.jpgaption

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)

How is our soil doing for earthworms?

Earthworm survey of soil quality 30.3.19

In early February and March 2018 Dr Jackie Stroud, a Natural Environment Research Council Soil Security Fellow at Rothamsted Research, led and co-ordinated a project to study the worms in farm soils. A total of 126 farmers took part. Participating farmers volunteered to dig 10 pits, each 20 cm x 20 cm x 20 cm, in one field. They counted the number of adult worms in the sample. Adults are identified as those having a saddle on their bodies. The total number of worms were counted, then the juveniles returned to the soil. An identification guide allowed them to allocate any sightings to one of the three main types of earthworm. The adults were then split into small surface red ones (epigeic), small or medium pale worms which were grey, pink or a darker green (endogeic) or larger pencil sized worms which were heavily pigmented red or black (anecic).

Each of these worm groups has a different function. The epigeic surface worms breakdown surface litter and are a good source of food for native birds such as thrushes and blackbirds. The endogeic topsoil worms mix soil and mobilise nutrients for plant uptake and so support plant productivity. The anecic, deep burrowing large worms are the drainage worms which can form 2 metre vertical burrows which help with water infiltration and deep plant burrowing.

On Saturday 30 March 2019 we conducted the same experiment over our ten plots, with a few teams adding a second count. Here are all our results

Earthworm Sampling at HCF 30.3.19

Plot	Total worms	Epigeic worms	Endogeic worms	Anecic worms	Grown last year	Manure added in autumn?	Compost added in autumn	Roto- Last year	Roto - this year	Plastic over winter
2	14	0	3	0	Brassicas	No	No	No	No	No
4	15	5	3	1	Fennel	Yes	No	No	No	No
6	7	0	6	1	Parsnips	Yes	No	No	No	No
8	30	2	5	0	Broad beans	Yes	No	No	No	No
10	7	0	6	1	Potatoes	No	No	Yes	No	Yes
12	0	0	0	0	Onions	Yes	No	No	No	Yes
14	31	0	4	5	Potatoes	Yes	No	No	Yes	Yes
16	4	0	2	0	Onions	No	No	No	No	Yes
18	16	1	4	0	Potatoes	Yes	No	Yes	Yes	Yes
20	13	1	1	0	Carrots	Yes	No	Yes	No	Cardboard mulch
Total	141	9	37	9
No of plots with worm	9	4	9	5
Fruit	3	0	3	1	Rhubarb	Yes	No	No	No	No
1	15	0	4	0	Squash	No	No	No	No	No
11	2	0	0	0	Potatoes	Yes	No	No	No	Yes
3	13	1	8	0	Sweetcorn	Yes	No	No	No	Sweetcorn mulch

How do our results compare with Dr Stroud's research for 126 farms? Dr Stroud analysed the results she obtained from farmers on 5 counts:

(a) Total number of soil pits with ≥1 earthworm (juveniles or any adults below),

She found that the average field had 9 earthworms per spadeful,

We found an average of 14.1 worms per count

(b) Total number of soil pits with ≥1 adult epigeic surface earthworm.

She found that 21% had no sightings of these worms and 42% of fields had very few. These worms were significantly less likely to be found in fields that had been tilled (ploughed or dug). Low numbers in a field suggest a lack of surface litter which earthworms can pull into the soil.

We found that 6/10 of our plots had no epigeic worms and a further 3/10 had very few.

(c) Total number of soil pits with ≥1 adult endogeic topsoil earthworms.

She found that 67% of fields had good presence of these worms.

We found that 9/10 of our plots had >1 adult endogeic worm. Endogeic topsoil worms mix soil and mobilise nutrients for plant uptake and so help to raise crop productivity.

(d) Total number of soil pits with ≥1 adult anecic deep burrower worm.

She found 16% of these fields had none of these worms and a further 23% had 1 or 2 out of the 10 samples that were examined that had these deep burrowing worms.

We found that 6/10 plots had no anecic worms and another 3/10 plots had just one anecic worm.

The lack of anecic worms was of concern to Dr Srroud because they are 'drainage worms' with vertical burrows that aid water infiltration and help stop fields getting waterlogged. The deep-burrowing worms have slow reproduction rates so recovery in their populations could take a decade under changed management practices. Deep burrowers may not always be in the topsoil, so it is important to look for pencil sized vertical burrows at the bottom of the hole of a pile of straw or stones on the surface (a midden) which overlies a vertical burrow. If these are observed then we recorded that an anecic worm has been observed.

(e) Total number of soil pits with high numbers (≥ 16 earthworms per pit, ≥400 earthworms per m²) of earthworms (total number including all juveniles and adults). She found that one in 10 fields had more than 16 worms per spade. Top fields had around 27-30 worms per sample.

We had an average of 14.1 worms per pit which gives us an estimated 353 worms per square metre. Only two of our plots reached “top field” numbers of worms.

Dr Stroud concluded that 42% of fields had sub-optimal numbers of worms (defined as <10% presence for at least one ecological group) and may be "overworked" leading to absence or scarcity of surface dwelling and deep-burrowing worms. Only 15% of fields had a good presence of all three groups. Her results indicate that tillage is likely to reduce the numbers of surface and deep burrowers first. This may help explain the alarming decline of the song thrush which often feeds in fields on these worms. Topsoil worms are generally least affected by over-cultivation.

What do you think we should conclude about our results? Clearly they are just a snapshot, giving us a baseline for further monitoring. Our results may have been affected by dry weather the previous week, but we can expect the worm numbers in the top 20cms to fall further as the weather warms up. Should we do more repeats on the plots surveyed? Should we do studies on the other plots? Should we be thinking of changing our soil management practices in the light of these results?

I should be really grateful if you would write your comments, observations and thoughts below, so that we can have a dialogue about how we are caring for the soil at HCF.

Refs
J L Stroud Soil health pilot study in England.: Outcomes from an on-farm earthworm survey. 2019  https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0203909    Viewed online March 2019