Improving our soils at HCF; some more thoughts and ideas.

Over the last 150 years many of the world's best agricultural soils have lost between 30-75% of their carbon, and this has contributed billions of tons of CO₂ to the atmosphere.  1/10 of all CO₂ emissions since 1850 have been from farm soils! This has had a major impact on climate change! Many of us will remember learning at school about the creation of the American dust bowls from the prairies in the 1930's. Most of the agricultural soils in southern England, especially those situated on the South Downs have less than 2% soil organic matter and could easily become dust bowls! Around 30% of the world's farm soils have been abandoned in the last 4 decades because of poor soil.

A recent study from the US has clearly shown that organic agricultural practices build healthy soils by storing away appreciably larger amounts of carbon and for longer periods than typical agricultural soils.  Probably the most compelling findings was that on average, organic farms have 44% higher levels of humic acid, (a component of humus) that sequesters carbon over the long term, than soils not managed organically. (Misiewicz, 2017)

So we ask the question; what practices are most important for building soil organic matter while at the same time ensuring good harvests of organic vegetables at HCF? Here are three more suggestions.

1. Minimise soil disturbance

There is an increasing body of evidence that suggests that digging, rotivating or tilling on the larger scale is utterly devastating to the soil if practiced five or six times a year. First it "slices and dices" many of the beneficial soil organisms like worms and destroys their habitat. As we saw in our worm survey on 29th March 2019 there are a group of earthworms, the epigeic worms that live in the surface litter and help to break it down. These actually form quite a large portion of thrushes' diets, and soil ploughing may be partially responsible for their massive population decline of more than 50% between 1970 and 1995.(Lusher, 2019). The anecic worms that are the drainage worms that make deep vertical burrows which help water infiltration and deep plant rooting are also disturbed or destroyed by frequent digging and their numbers decline leading to poorer drained soil. (Chambers, 2019)  

Soils which are tilled mechanically or repeatedly dug and raked to raise a mechanical tilth are often great for planting seeds in. The problem is that it destroys soil structure. It breaks up the soil crumbs or aggregates (particles held together by glomalin) breaking them to a powder. This tilth may only last a few days or weeks before settling back into a packed, consolidated and homogeneous structure with little pore spaces and structure. If heavy rain falls the surface can become a hard pan and be compacted afterwards and resistant to further water infiltration. 

A second problem associated with digging is that it turns the soil, introducing air containing oxygen to pockets in the soil which enables bacteria to thrive. Bacteria increase in numbers if the conditions are right: good levels of oxygen, moisture, nitrogen and carbon. If the soil is rich in nitrogen (has been manured or fertilised), the bacteria will take in as much nitrogen as they can, but they need about 20 times as much carbon as nitrogen. So the bacteria pull as much carbon from soil organic matter as they can, excreting the bulk of it into the atmosphere as carbon dioxide. Once all the easily digestible soil organic matter (SOM) has been used they will then turn to any humus available in the soil and more slowly break that down! This has been giving a yield boost to the farmers on the South Downs, which is what they want, but it is causing the humus levels in the fields on the Downs to fall dangerously low. So as we can see after just a few seasons of tilling the soil becomes carbon depleted.

There are other disadvantages to lots of digging or rotivating: dug or rotivated soils release far more nutrients than the soil plants can use and these leach through the soils or are washed out by heavy rains.  Finally digging stimulates weed seed germination. Weeds love disturbed, bacterially dominated soil because their goal is to grow quickly, set seed, die and get another generation of weed seeds into the soil.

Charles Dowding began practicing a "no-dig" method of soil cultivation in 1983 which seems to obtain good results on an allotment scale (Dowding, 2019).  By not digging a good biological tilth has developed over time. Biological tilth is the kind of crumb structure that good organic soils becomes as the fungal mycorrhiza produce glomalin which causes particles to aggregate together in little clumps roughly 1 cm across. This is often first found in a soil around the roots of healthy plants. It is resistant to weathering and degredation by digging, while at the same time allowing water to enter easily, but holding onto the water so it doesn't drain though too quickly and take the nutrients with it.

To understand better how Dowding's "no-dig" method works watch this 19 minute video Spring in the No- dig garden at https://mail.google.com/mail/u/0/#inbox/KtbxLxGgHQSXRTZNcHgjvZmXGbnJKXDlCL?projector=1 

Initially we may want to consider a "minimal dig" policy and reduce the use of rotivators on the soil.

2. Sheet Mulch in the autumn

We dealt last week with adding a few centimetres of an organic material to a crop as it is being planted out or adding a composted manure or mulch in the autumn before sheet plastic is used to cover the plot. What I am suggesting here is rather different.

As a teenage I remember buying dry tubifix worms to feed to my tanks of tropical fish. But then I discovered I could grow my own worms by placing bread on damp soil in a margarine tub in our shed and a few days later under the bread would be loads of worms! It saved me my pocket money! Sheet mulching is a bit like that on a bigger scale.

In Rwanda 35 years ago we made a deep bed full on any unwanted organic matter: old grass rugs, chicken droppings, sorghum and maize stalks, sheep and goat shed straw and poo, chopped down banana stems, old leather sandals and shoes, paper and wood shavings. Eventually it formed a bed about 20 cm high. We covered it with cardboard and then left it to the microbes to break it down and the worms to mix it for a few months. Then we grow all sorts of wonderful crops on it for several years. Might we not consider making a sheet mulch bed somewhere on the farm by collecting all sorts of unwanted organic matter over the summer: cotton tee shirts, paper, cardboard boxes, coffee grounds, garden weeds, leaf mould, sawdust, straw, urine and biochar? We could then make a deep bed in the autumn, to rot down over winter and grow in next year? By making one sheet mulch bed a year for several years we could slowly move to reduced digging this way!

 3. Focus on building soil resilience.

One of the features of climate change is that the seasons are becoming more unpredictable. Since we began at HCF we have had wet winters, dry winters, very cold late springs, warm early springs, droughts in summer, really damp summers, typical autumns and indian summer autumns! We have had to become flexible with planting dates and careful about hardening off crops grown in the polytunnels. We have lost whole sowings of crops to freak weather and we've lost plots full of potatoes to blight in damp summers. As we are becoming more resilient to climate change so we need our soil to be more resilient: able to cope with water stress, flooding, frost, snow, humidity, heat and sunshine.

Studies show that organic growing systems get around 30 percent higher yields in periods of drought than con­ventional systems due to the increase of SOM and its ability to capture and store water for crops. What causes this higher yield? We know that organic matter holds anything between 4 and 10 times its own weight of water. This is partly because organic matter acts like a sponge and partly because organic matter particles have a charged surface that attracts water so that it adheres to the particle surfaces. So if we can increase our SOM at HCF it is most likely that our soil will be able to hold more water and maybe the plants will be able to keep going without being watered for another 3 or 4 days. This could help a crop survive through a dry spell.  Similarly, after a very heavy period of rain the soil might be able to hold more water and so reduce runoff to flooding rivers and the loss of precious crop nutrients.

So if we are to build soil resilience at HFC we may need to be more proactive in putting organic material such as manure, compost, mulch, crop waste, progro, leaf mulch, wood chip, straw, bio-char and other sources of organic carbon onto our fields and try to avoid letting the majority of it turn to carbon dioxide within a few months by digging or rotivating.

What are your thoughts?

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.