How organic matter breaks down in the soil to make humus:soil carbon pathways

When plants and animals die in the soil the soil organisms such as earthworms, woodlice, slugs, snails as well as the bacteria and fungi use the organic matter as food. The carbon in dead plants and animals is broken down in a number of different ways but two main stages are evident:

1.      the cellular structure and recognised organic substances and minerals in the decomposing organism disintegrate and become unrecognisable as big molecules are broken down to smaller ones.

2.    

           totally new combinations of these broken-down products develop. It is defined as humus when it becomes impossible to distinguish what the original material came from. So humus is an amorphous mix of black or brown gel-like substances of high molecular weight modified from the original tissues by various soil organisms.

There seem to be two main methods by which humus is made in soils

Process 1. The decomposition of organic materials such as cellulose and starch from crop residues and manure. Soil invertebrates eat detritus (dead plants and animals) and make the particles smaller for bacteria to work on, first by breaking them down, then by building them up slowly into humus. An alternative to this is the decomposition of woody organic material such as lignin in crop residues and compost by white rot fungi. Then the products are eventually built up by bacteria into humus.

Organic matter decomposition and formation.  Ankush J

Process 2. Carbon compounds which have been exuded by plant root are used by certain soil fungi which produce mycorrhiza. These form a symbiotic relationship with plant roots. The mycorrhiza use these sugar exudates to make glomalin, a major component of decomposing organic matter and this will eventually be built up into humus.

The process of humus formation through decomposition of organic matter is not very efficient at building humus in soils. Of 100 g of organic matter that is added to the soil maybe 60-80g will be converted back to carbon dioxide by the invertebrates and the bacteria, 3-8 g will be taken up by bacteria to help them to grow, 3-8 g used to make other plant organic compounds and possibly 10-30 g used to make humic compounds.

It is known that around 40% of the sugars made by photosynthesis in plants leaks out of the roots. This can be used by mycorrhizal fungi, some of which invade the plant roots. These fungi supply many nutrients including up to 90% of the plant's nitrogen and phosphorus requirements, plus calcium, potassium, magnesium and iron, as well as essential trace elements such as zinc, boron, copper, cobalt, molybdenum and manganese. They often supply water as well - all in exchange for liquid carbon! (Smith, 2008). So these mycorrhiza effectively increase the coverage of the plant’s root system in the soil and can make the plant much more efficient at absorbing plant nutrients (particularly phosphate) from the soil solution.

Mycorrhizal hyphae colonising the roots of a pine seedling. 

As the mycorrhiza grow they produce a protective surface coat of glomalin, a glycoprotein (protein containing a plant sugar). The glomalin drops off into the soil where it acts as a "super glue," helping sand, silt and clay particles stick to each other as pea sized lumps called aggregates. The small spaces between the aggregates help rainwater to move through the soil more easily. Moisture is absorbed in these aggregates which is protected from evaporation, but the mycorrhiza are able to access this water and supply it direct to the plant roots at times of water shortage. It is substantially due to the gel-like substance of glomalin that it is often stated that 1 kg of humus can hold 4-20 times its own weight of water. And it helps us to understand that soils rich in humus will have a good structure, improved water holding capacity, enhanced infiltration and drainage and enhanced nutrient exchange capacities

Glomalin also stores approximately 25-33% of the total soil carbon and can last in the soil for 7-40 years as part of the soil active fraction.  At some stage the glomalin is either respired to carbon dioxide if the soil is dug, or is converted to humus if the soil is not dug and there are plenty of mycorrhizal fungi in the soil.  

It seems that the process of making humus in soils through the activity of mycorrhizal fungi in process 2 is more efficient than soils which make humus by process 1 because soils that have not been dug generally have higher humus levels.

Another benefit of soils which have abundant mycorrhiza, glomalin and humus is that they have a markedly increased resistance to climate variability.

HCF's has a policy of feeding the soil by adding lots of animal manure and as much compost as is available. However as this article suggests this method is not very effective at converting soil organic matter to humus and may not actually increase soil carbon levels if other farming practices such as digging and leaving the soil bare destroy soil carbon.

Most of the cultivated plots at HCF are likely to be acutely short of mycorrhiza as digging or rotivating breaks up the hyphal strands and disrupts their relatively slow growth. Furthermore our practice of leaving the soil bare and plant free for several months of the year means that during this period the mycorrhiza have no living plants to exchange nutrients with and so decline. One of the challenges for us at HCF is to work out how we can help to develop mycorrhizal growth in the vegetable plots. Maybe we should think more clearly about planting winter cover crop?

References

Organic matter decomposition and formation.  Ankush J

Mycorrhizal hyphae colonising the roots of a pine seedling. Aberdeen mycorrhiza Research Group

Ways of raising soil health

This is our fourth article about how to create and maintain a healthy soil at HCF which will be resistant to and resilient in climate change. Here we will deal with the issue of keeping the soil covered throughout the year.

Maintaining cover can be achieved in at least three different ways, with many different, but beneficial outcomes:

1. Leaving crop residues such as roots in the soil and stems etc on the soil.

In a typical Highbridge Community Farm field the soil is often bare for around six months of the year. Research shows that bare soils lose organic matter as the bacteria use the oxygen and the organic material for respiration and produce carbon dioxide.  Cultivated soils should be planted with another crop as quickly as possible to avoid them losing organic matter. If you can see the soil then it is losing carbon! As soon as a plant crop is harvested and the cover is removed the soil begins to deteriorate.  This is a big issue for us at HCF, as possibly 90% of the organic matter in the manure and compost that we add goes up into the atmosphere as CO2. However, there are several ways of combatting the problem. So basically, when we are clearing beds we shouldn't allow them to lie fallow. Even for a few days. 

When changing over from one crop to the next, the general practice at HCF has been to remove all crop residues including roots. This is disadvantageous for two reasons: first, the soil around the roots always has the best soil structure (tilth or physical condition of the soil) so why break it up?  Around the rhizosphere, the area where the roots grow, the soil forms good aggregates (small soil clumps about 1 cm diameter held together by the soil glue glomalin). The moisture content, degree of aeration, rate of water infiltration and drainage are also good in the root zone as it is where the majority of the microbial activity – therefore nutrient release and biotic glue secretion (aggregation) takes place. So consider leaving the roots and maybe the stems in place after removing the parts of the plant you want to eat!

Second, and very significant, roots of mature crop plants go on producing root exudates even after the stems or stalks, leaves, flowers or seed heads have been removed. Actually the shock of being decapitated causes them to put out a large pulse of exudates giving up much of its stored sugars to the next generation. So when one crop is nearing its end in your plot there are several ways to ensure continuity of plants:

1. Leave root residues in the soil. Cut off the tops- take the part of the plant you want to eat but leave the root residues in the soil. The crop root residues will go on feeding the mycorrhizal fungi, which you want to be in the soil to help the next crop get established.

2. Intercrop your plant so that when one crop is being harvested its roots can be left to feed the other crop as the mycorrhiza take the exuded sugars from the crop that has been harvested and moves them on to the other crop. In this way the mycorrhiza remain alive and functioning effectively and so continue building up soil organic matter and humus.

We know that brassicas such as cabbages, calabrese and cauliflowers do not form mycorrhizal associations with fungi, so if they are planted alone the fungi will starve and die in the soil. Brassicas such as these when sown early may be ready to harvest by July. So it may work well to plant alternate rows of a member of the brassicaceae and a member of the chenopodiaceae such as beetroot, spinach, swiss chard or spinach beet, which will keep the mycorrhiza alive in the soil for the season. Then when the brassica is removed in summer it can be replaced by seeds of French or Runner beans if the date is before mid-July or pot grown French or runner beans if the date is later.

If a vegetable is planted in the spring that needs nearly all the season to grow, such as parsnip, celeriac, courgette, leaf beet, butternut squash or sweetcorn there is little time at the end of the season to grow another vegetable in the same ground after it, but it may be possible to companion plant your desired plant with other plants which will exploit different niches (e.g. plant sweetcorn and squash together). Alternatively as the season is coming to the end for some of the above long season plants then plant another crop in late summer or autumn along-side it and this will stand throughout the winter, and give harvests in the hungry gap the following year (e.g. garlic, spring onions, spring cabbage, broad bean or spinach).  If garlic has been autumn planted then carrots and beetroot can be sowed between the rows of maturing garlic the following spring and will take over as the garlic goes yellow in June.

2. An alternative to intercropping and companion planting is relay cropping.  As one crop is pulled out the next one is ready to go in and take its place. The day you remove the flowers, fruits or leaves of one crop put in some substantial pot grown seedlings of the next crop along with a little organic mulch on the surface to maintain a supply of living roots in the soil.

3. If you have to clear a bed or even just a single row, and have no crop to plant in that space then put on a top dressing of an organic mulch such as compost, crop residues, comfrey leaves, coffee grounds or sweetcorn stalks and then cover with a plastic sheet. This will give the earthworms and microbes something to feed on to tide them over for the period the soil is not growing anything. It will also improve the soil tilth by encouraging worm and microbe activity close to the soil surface and it will slow down, but not stop the loss of organic matter, since no other foods are entering the soil.

2. Sowing cover crops during winter months.  The aim of this is to eliminate long fallow periods where possible. The negative impact of long fallows on soil organisms is particularly important, especially the impact on mycorrhiza fungi. Mycorrhiza need the sugars from living plant roots to survive, and if there is a period where there are no growing plants, the mycorrhiza will reduce in numbers. This can be partially offset by sowing a legume like field beans or the broad bean Aquadulce. There would inevitably be a few weeks delay before the new seeds got going, but the cover crop can be chopped down to ground level in the spring and left on the ground as a mulch and their roots left in the ground to feed the next crop. It is important to be able to remove the cover crop at the right moment and replace quickly with the new spring crop at exactly the right time for the new crop to go in.

Paul Dibden has proposed sowing a cover crop of 80% black oats, 5% brown mustard and 15% oil raddish over the winter of 2019-20.

Cover crops give the following benefits to soils

·         Suppressing weeds

·         Protecting soil from rain/runoff/reducing erosion

·         Increasing soil aggregate stability

·         Adding SOM and increasing SOM

·         Reducing surface crusting

·         Breaking hardpan

·         Fixing nitrogen and scavenging soil nitrogen

·         Suppressing soil pests and diseases

·         Increasing availability of nutrients

3. Use mulches. Our final way of trying to keep the soil covered with plants throughout the year is leave more crop residues on the field as a mulch for the next crop. Team 3 & 4 added bales of hay spread out between their rows of sweetcorn over the summer of 2018 and then left the sweetcorn stalks on the field in the autumn as they planted though their autumn sewn broad beans. As the sweetcorn has slowly rotted over winter and into the spring it has fed the soil with nutrients in similar ratios to compost but also allowed the natural development of soil structure and organisms, as well as the build-up of SOC and water infiltration. Maybe not as good as having roots growing in the soil, but better than sheet plastic and much better than bare ground.

Mulching broad beans with sweetcorn stalks at HCF

A variety of mulches and coverings are available, although not all are currently used at HCF, including; compost, coffee grounds, comfrey leaves, sweetcorn stalks, straw, grass clippings, leaves, newspaper or cardboard, black plastic,  Mypex (woven weed barriers) and woodchip. Some of these mulches will have different effects, so the merits and debits of each should be considered carefully.

In this article hopefully we have learned that fungal mycorrhiza are important for soil health and maintaining soil structure and water holding capacity. They need roots for continued life and growth, but don't survive when the soil is bare for even a few months or when only brassicas are being grown. There are ways of ensuring that roots will be present in the soil throughout the year by leaving crop roots in soils, intercropping, companion planting, relay cropping and sowing winter cover crops. If all else is impossible, use a good mulch or covering, preferably of organic material.

In the next post we will look at a couple more practices that we might focus more on at HCF.