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September 2019

What is soil health and why should farmers and growers care about it?

2379 1770 Sectormentor for Soils

This is a guest post by Niels Corfield, soil health, agroforestry and whole farm planning advisor, researcher and advocate. You can find more articles from him on medium.

In this article I’d like to lay out the main tenets of soil health, but before that I want to present what I feel are the main motivations for “going to all that effort”!

Why bother about soil health?

First of all, in this climate change acceptance world, all forms of agriculture, and land use must wipe their face in terms of carbon. Being at least carbon neutral, if not carbon negative (or positive, whichever way you prefer it phrased).

All land, and all production must sequester carbon and it can, it’s been done, one example has raised their soil organic matter (SOM) from 2.4% to 8–11% in 6 years. 0.5–1% per year increases are regularly achieved.

Every single cropping cycle is a growth cycle and therefore should be net cumulative, in terms of carbon — through the process of root exudation. Simply put, all plants have the potential to improve soil (fertility), it’s the simply the management and composition of those plants (in space & in the rotation) that determine this fact. However, this is very hard to achieve when regularly cultivating, for example.

If you’re serious about climate change, business as usual is not an option. The good news is if you rigorously apply the same health principles production can be carbon negative.

What’s in it for us?

Apart from saving the planet, are there any other arguments for adopting novel practices? Is it just about doing good while our livelihoods suffer? Not at all!

Adopting the soil health principles wholeheartedly is one framework that can both address global issues and bring production benefits — showing that cropping (and farming) doesn’t have to be a zero sum game.

How would these benefits manifest? Everyone knows how well their crops look when the land is in good heart. And that that translates to fewer crop failures, less drought-stress, better infiltration and more yield. Moon on a stick? All of these are correlated with better soil health.

Another benefit of better crumb structure will equate to fewer passes to raise a seed bed, where cultivation is practised. That’s less diesel and better windows for field access (a window that’s fast becoming undependable). Either way there’s wins to be had once we harness the latent potential of plants.

Grass & fertility

Perhaps one of the few things that farmers (and growers) can agree on is that the land is “in best heart” the first year out of grass. So, is there something magic about grass, compared to other plants? Does it have unique “healing powers”?

Basically no, but to be clear, what is it we can say about the conditions under grass?

A living root, covered soil, undisturbed, diverse (ideally), some animal impact (potentially) and few if any chemicals.

Cropland (at left) — Grass margin (at right)

Soil fertility (health) can and should be built within the cropping phase of our operations, and can be achieved without going to grass. Not that grassing-down and fallowing aren’t efficacious practices, just that we have more tools available to us that are perhaps more appropriate in a cropping situation. We just need to closely mimic those conditions we find under grass, and that’s where the soil health principles come in — 7 ways to help you select management practices, and to make decisions on your operation that build soil: cropping or fallow, spring or fall.

One key insight — microbes matter

Perhaps the most important single insight that underpins, most, if not all of us or health principles is the understanding that microbes matter. And they matter to us the growers. But, why do they matter? What’s so good about them? And, what do they do for us? There are three parts to the answer:

Firstly they create crumb structure. Their actions bind together disparate soil particles into crumbs or “aggregates”.

Secondly, they feed plants. They digest elements out of the soil mineral matrix (as well as organic matter) and transform them into plant available nutrients. They literally eat rocks. Hitherto known as decomposition but since the process is done enzymatically it’s much more akin to digestion.

This is what’s known as the “microbial bridge”. Simply put, plants (and healthy plants in particular) are nourished by nutrients provided by organisms.

Thirdly microbes protect plants. Through a number of different mechanisms they either out compete or shield plants from disease-causing organisms and often pests as well. Perhaps the poster child of this final point is mycorrhizal fungi that snare root feeding nematodes which when caught are then digested and past up to the host plant.

So the plants are getting a pretty good deal here! They’re getting a nice loose structure to push their roots through easily, room service and their own live-in healthcare service. Are they getting all that for free? Emphatically no, this is where the soil health principles come in, read on..

The Soil Health Principles

Weed roots, rhizosheath forming (at upper left)

1 The Living Root — maintain a living roots in the soil for as long as possible and as often as possible

This is number one on the list because perhaps it is the most important. Because, simply put, roots make soil and they do this by producing root exudates, — carbon secretions that feed (and recruit) soil organisms, which in return deliver plant available compounds, right in the “root zone”.

Gothelney Farm: Rhizosheaths on oat cover crop (Photo: Fred Price)

If you want to see the effect of the living root on soil structure examine the soil around the roots of the healthiest plants on your farm (often a weed!). So the next time you’re out in the field, take a moment to carefully lift one or two plants and examine those roots. You will typically notice finally grained soil that has excellent crumb structure, potentially some of which may be actually stuck to the roots themselves, in what’s called a “rhizosheath”, this is a phenomenon seen where microbes are active and producing the glue like substances that bind together soil particles into aggregates. Where you see rhizosheaths you can infer from that that root exudation is taking place, as, without this carbon (energy) source these organisms would be inactive.

What you are seeing is soil building in action — where you are seeing crumb structure around roots and rhizosheaths. Without roots and root exudates, organisms become inactive, microbial populations become simplified and ultimately aggregates become consumed by those same organisms, since they have nothing else to eat, as these, glue-like substances, are all organic (carbon based) for example polysaccharides, hence are comestible. Literally the structure of aggregates is edible, Think Hansel and Gretel.

2 Covered Soil — protect soil, from wind, rain and sun, with leaves or residues

Maintaining a soil cover is essential. This is a fact that “nature intrinsically understands”, which is why bare soil is such a magnet for weeds, because without a cover, soil degrades quite rapidly, nutrients are lost, soil caps and water runs off. Ultimately that porous structure crumb structure that we all value is sacrificed. We should be taking steps to reduce the frequency of bare soil as well as the length of those periods.

Bare soil is the enemy of soil health. Don’t farm naked as some say.

3 Minimise Disturbance and Compaction — reduce tillage severity and frequency

No dig and no till are perhaps the foster children of the soil health movement but certainly it shouldn’t be reduced to this one single measure. Simply shifting to no-till and making no other changes will not bring the kind of results that we are really looking for. It will not bring health back to the soil on its own, it must be coupled with the other principles and particularly diversity. That said it’s worth expanding on the rationale for not tilling, by laying out the issues with tillage itself.

The first and most important point to make is that tillage destroys soil structure. This seems counter-intuitive because one of the primary reasons for tillage is to improve structure, or to get a tilth. While certainly there is a loosening effect post tillage, this is a short lived phenomenon, however, the structure of the soil is soon lost and it will then settle back into it’s more native, mineral-structured state, more akin to the subsoil. If all conditions are right then the crop can be established in this short phase, but this window is getting more unreliable by the day.

The reason that tillage destroys soil structure is because when we are talking about crumb structure we are talking about aggregates ultimately, and aggregates are made of small soil particles bound together with glue-like substances that have been secreted by organisms. When you till you break open these aggregates, splitting apart these crumbs, exposing them to oxygen at which point “R-strategist” organisms invade the space and rapidly decompose or digest (literally eat) the binding agents that glue together these crumbs. The process of aggregation is directly analogous to that of making biscuits in that the homogeneous flour (and sugar) particles are bound together by the butter or fat into larger non-uniform (heterogeneous) clumps or crumbs. In this analogy the sugar is analogous to sand and the flour is analogous to clay. And the fat is analogous to those binding agents, those polysaccharides that are biologically derived in the same way that a fat is.

Generally speaking if you want retain good crumb structure then reduce tillage — reduce the depth-, the frequency and minimise inversion.

That said, is there a place for tillage as a force for good? That depends much on the starting point. But there does seem to be a small number of clear cases where tillage (or mechanical interventions in general) can help to rectify the situations related to past management.

If your starting point is tired old pasture (But I thought you just said that pasture ticks all the soil health principles? Well yes it does, but management is always superior).See photo below: Old pasture (especially horse) may well be heavily consolidated and there’s only so much that your crop roots and even cover crop roots can do to undo this situation. In horticulture operation the Real Food Garden, they good results with ploughing the existing pasture and then a year of cover cropping before conversion to no dig.

Horse pasture (at right) — Fence line (at left)

4 Diversity — increase diversity in your plantings and rotations

Diversity above ground feeds diversity below ground. If you want your plant to have a balanced diet it needs to associate with a diversity of organisms that mineralize a diversity of nutrients.

It’s been shown that so that the formation of humic substances and stable aggregates is closely correlated with below ground diversity.

Below ground diversity requires a diversity of foods, from a diversity of root exudates etc.

This is one realm where cover crops can really excel. We can bring in so much diversity. And so long as we’re destroying without tilling, we can retain a significant portion of that beneficial structure. This diversity can be so much more, it can include C4 plants, which are rare and general in the UK cropping situations. Plants like maize, millet, sorghum, quinoa, amaranth, and the more common sunflower and buckwheat.

Trill Farm: High diversity (at left) — Low diversity (at right)

All these fancy seeds in a mix sounds expensive don’t they? Well it doesn’t have to be, just source your seeds from the feed merchant, rather than the seed merchant. Bird seed mixes are a great starting point and most of them already have the 8 species or more, recommended: broadleaves and grasses, warm season and cool season. And when the costs are low you can start sowing at proper seed rates which maybe 2 times or 3 times the standard advice. This overcomes the main pitfall people have when experimenting with cover crops: too little seed.

Schumacher College: Amaranth crop

That said, diversity isn’t limited to cover cropping. We can introduce diversity into our cash cropping operations. This is potentially where some of the biggest wins are actually to be had. Given that increasing species diversity from 1 to 2 represents a doubling in diversity that’s a big win. And going from 2 to 3 represents another 50% increase. So, include: catch crops, intercrops and relay crops whether practical.

There should be gains to be had in this case, so long as we doing our due diligence with the other soil health principles — especially around bare soil and the living root. Again we don’t want massive space between the plants being unfilled with roots and uncovered. This is where intercropping and catch cropping can really pay dividends.

5 Feed Soils — hungry organisms need food to stay active and healthy

This is definitely one area where the organic crew got it right. But what do we mean by this? What eats stuff in soil? Well organisms, life, life eats stuff. Not the “soil” per se, as this is simply a matrix of geological degradants.

What actually eats stuff in the soil is soil organisms: the soil food web members. So the question is then, what do these soil organisms like to eat? Really we’re talking about fungi and bacteria in this case, as they make up the vase majority of soil biomass. So, what’s their preferred food? Well in this case it’s root exudates. Simple to utilise (metabolise) foods such as carbohydrates, often as straightforward as glucose. But that is what’s being supplied by the roots, no? So, that’s covered under principal number one — the living root, right?

Well yes. Except we are in a cropping situation which means there may be a break in cropping. Where there’s a break in cropping, the organisms that are present in the soil, those active communities of bacteria for example that have built up, through your good management of cropping (and potentially diverse cropping) will all go dormant, but before they do, that they will consume the most readily available food source present. And what is that in this case? It will be the binding agents that hold the aggregates together. They will literally eat their own homes because there’s nothing else to eat. Evoking that famous phrase “burning the furniture” — when you’re snowed in and it’s freezing outside and you run out of firewood, you’re only alternative to keep warm is to burn the furniture but once you’ve done that where are you going to sit?

So what are we to conclude from this observation? Simply, where a break in cropping exists, provide a subsistence ration. This would typically be over winter. But either way, this is the prime time to apply bulky organic matter. And if all goes well, when you come to plant your next crop you will have a nice surface tilth and an active population of organisms ready to immediately associate with your plants.

The last two soil health principles are perhaps the least relevant to an organic growing situation. However I include them for the purposes of being thorough.

6 Incorporate Animals

Diversify your straight cropping operation by rotating animals or integrating livestock into your system.

So what’s good about animals? What benefits can they bring to a cropping system? Firstly, their manure contains inoculants that are rare, even in good compost and they can carry out the work of distributing (or applying) that manure through their own natural behaviour.

That said the key rationale behind this particular principle is that all ecosystems contain animals as part of the web, and they add a whole new dimension of diversity to the system so perhaps they should come under principal number 4, but in this case the desire is to be explicit.

Ian Boyd’s farm Whittington Lodge

7 Minimise Use of Chemicals

Simply put, the use of chemicals and synthetics undoes all of your good work further up the list.

Firstly, use of synthetic fertiliser or soluble nutrients in general discourage, the production of root exudates. Basically root exudation is a quid-pro-quo for plants. They’re effectively paying for nutrients in a form that they can absorb with energy (or sugars) the only currency plants hold. So, when a plant is artificially nourished through soluble fertiliser applications it’s need to offer exudates is removed, though this is fine in the short term, however, the removal of foods from our friendly minions (the soil microbes) means they will at best go dormant and at worst start chowing down on the aggregates themselves because there’s nothing else to eat and then go dormant.

Fertilisers in salt form, and in very high concentrations, actually force their way into the plant structure. So the plant almost becomes over nourished or certainly has no control over the quantities and types of nutrients that enter the cell structure.

Further to this and perhaps, more obviously is the use of pesticides etc which basically have all sorts of impacts on non-target species. But it goes without saying that applications of fungicides for example have detrimental effects on mycorrhizal fungi, though that said, so does tillage.

Putting it all together

Ultimately the strengths in this approach, this framework, is that the principles can be applied individually or in consort. But ultimately the real wins are to be had where all of the soil health principles are applied both in the cropping phase and fallow periods.

I like to think of the soil health principles as different levers maybe like you might picture in an old signal box, or a graphic equaliser. With each lever representing a different principle. Ultimately, over time want to be able to push all of the levers up to full, bring all of the benefits to bare.

Furthermore, for each principle there’s obviously a range. Similar to gears on a gearbox. Ultimately where limited by equipment or tools, it’s like being unable to change up from 3rd gear for instance. There’s nothing worse than being stuck in 3rd gear on the motorway. If we can gradually increase the upper maximum we can achieve each year we will gradually be able to leverage more of the latent power of nature to grow better crops.

Conclusion

So in conclusion, the soil health principles are another iteration of nature mimicry, offered, in this case to farmers and growers as a tool for decision-making on-farm.

Simply put, where two options are considered for a particular situation, that which scores highest, in terms of the soil health principles, should be preferred, where practical, or when it becomes practical.

I hope that you will use these principles in your day-to-day practice and in your longer-term rotation planning.

And for those that want to find out more, or discuss this in more detail please get in touch or consider joining me on one of my soils courses this autumn. If you have any thoughts or questions, get in touch: info@nielscorfield.com

Soil health courses & info
https://www.facebook.com/pg/nielscorfieldland/events/

Further Reading
Part 2 — Realising the Promise of Soil Health in Organic Horticulture
https://medium.com/@nielscorfield_90202/no-till-for-growers-realising-the-promise-of-soil-health-in-organic-horticulture-646fd553257

Soil Science Latest: How Can We Sequester More Carbon and Build Soil Health?

3024 2257 Sectormentor for Soils

A few weeks ago we presented at Wageningen Soils Conference with Elizabeth Stockdale, Head of Farming, NIAB. We shared about the work we had done together converting the AHDB soils scorecard into an interactive Soil Quality UK Dashboard, with the main focus being “How can we get farmers talking about their soil health, based on soil health tests?”. We had lots of brilliant feedback about the dashboard we built and everyone at the conference was very positive as we talked about the benefits of a very different, more thoughtful, type of user research.

Whilst we were there we also listened to many brilliant talks sharing the latest soil science, some of which were incredibly relevant to the farming community, so we want to share the insights with you.

Building short-term vs long-term Carbon – it’s all down to the microorganisms!

We have all heard of words like humus, humic acid etc in terms of soils and carbon stored in soils. Well it turns out in the words of Johannes Lehman, soil scientist at Cornell University “humus is dead”.

He was very clear that it’s false to think there is such a thing as a long term carbon store that is locked up forever.

The idea that stored organic carbon can be neatly separated into 3 different types (Rapid, labile, stable), which has been the model for many years, is not reflected in the research any longer.

We shouldn’t worry about the idea of building long term carbon vs short term carbon, essentially all carbon that is sucked into the soil via photosynthesis or decaying organisms has the potential to become long-term carbon. What actually matters is which microorganisms are present and what they do with it.

There is no clear silver-bullet pathway to locking up carbon for good, instead we can think of soil organic carbon as an ongoing cycle of carbon gains and losses which we need to constantly manage. To many farmers this may not come as a surprise – that is exactly what we have seen happening in the field. As we practice more regenerative approaches (living roots in the soil, ensuring plenty of plant residues etc) we are quickly seeing the advantages of healthier soils which is so often synonymous with higher soil organic carbon and a more alive soil system (i.e more microorganisms).

Read more in this paper: Microbial models with minimal mineral protection can explain long-term soil organic carbon persistence
Dominic Woolf & Johannes Lehmann  Scientific Reports volume 9, Article number: 6522 (2019)

Johannes showed that soil management techniques on farm to improve food security can have large beneficial effects on soil organic carbon. He drew from an example of changing agricultural practices in Ethiopia where they have been implementing agroforestry, diversified cropping systems, terracing, and many other agroecological practices — first and foremost as a measure to decrease poverty and prevent future drought scenarios. The project was undertaken on 600,000 Ha and had many benefits to the community, food security as well as environmental rehabilitation. However Johannes pointed out that it also had an unintended positive effect of sequestering significant amounts of carbon helping Ethiopia to meet their climate targets.

Read more in this paper: Land restoration in food security programmes: synergies with climate change mitigation
, & Climate Policy Journal Pages 1260-1270 | Received 24 May 2017, Accepted 05 Jan 2018, Published online: 26 Jan 2018

Carbon Current Account and Interest Rates

We discussed these findings with Elizabeth Stockdale, Head of Farming at NIAB, and she gave the analogy that this implies we need to think of our soils like a current account – you are constantly making deposits and withdrawals of carbon, and that will be reflected in your carbon balance. From a farming perspective, what I wanted to know is what is the interest rate on this current account? That essentially would reflect how effective we will be in the long run at sequestering carbon in soils and building up our SOC. The banking analogy also makes a lot of sense, as it reflects the fragility of soil carbon in the face of larger scale disasters and fluxes in the market, as everything can be ‘lost’ at any time.

In summary Johannes Lehmann’s work showed that the focus of building soil organic carbon should be on farm ‘short-term’ carbon cycling interventions (e.g cover cropping, leaving plant residues, agroforestry, composting applications, rotational grazing) and the microorganisms will do the work to build longer term carbon deposits from there! Unfortunately this new understanding of soil organic carbon and the value of microorganisms has not been incorporated into IPCC models yet, so the value of these practices aren’t fully taken into account in global carbon targets…but Johannes said it will all change quickly.

Johannes also posited that the traditional models for understanding soil, based on adsorption and aggregation, although helpful in their way, are not necessary to explain soil organic matter and soil functioning. His work has shown that soil organic carbon and soil function can all be explained by the makeup of microorganisms and the action of microorganisms in the soil. It’s early days for fully understanding this as we still only understand around 5% of all the microorganisms in our soils!

 

The Soil Microbial Carbon Pump – how do microorganisms affect carbon storage in soils?

One model put forward for understanding the effect of microorganisms in sequestering carbon in soils was the Microbial Carbon Pump, explained by Chao Liang from the Chinese Academy of Sciences. The Soil Microbial Carbon Pump is a model for understanding how microbes are an active player in soil carbon storage. Chao showed how it could be applied at many different scales from the rhizosphere (plant root-soil interactions) to the field and landscape scale, which could have implications for understanding the responses of ecosystem carbon processes to global environmental changes.

My interpretation of what he was saying is that there are two types of carbon sequestration pathways through soil microorganisms:

  1. Biomass: The living biomass of the soil i.e living microorganisms taking carbohydrates being offered by plant roots and using it for food, excreting it in the form of slimes, building fungal hyphae etc. This is what Dr Christine Jones refers to as the liquid carbon pathway. (Catabolic pathway)
  2. Necromass: All the dead material, whether plant residues or compost materials etc being broken down and processed by microorganisms. Increasingly scientists are recognising the important role this plays in building soil organic matter. (Anabolic pathway)

Read more in this paper: The Importance of Anabolism in Microbial Control over Carbon Storage
Chao Liang, Joshua P. Schimel, Julie D. Jastrow Nature Microbiology volume2, Article number: 17105 (2017)

Chao showed how this model allowed them to investigate the effects of plant covers on microbial communities and what that meant in terms of the magnitude and composition of the soil carbon pool. He outlined that once the Microbial Carbon Pump processes carbon, it will either be released back to the atmosphere as CO2 – known as the priming effect (i.e decreasing the storage life of the carbon) or further increase the storage life of the carbon – known as the entombing effect. Their results showed that as the fungal proportion in the microorganism community increases the amount of carbon that goes through the entombing effect and becomes longer term carbon significantly increases after a certain time and then remains constantly high, whilst the priming effect although initially peaks, it then decreases and falls significantly below the entombing effect (you can see this in the photos below, where the high fungal content soil sample is the middle graph, highlighted in red)

Essentially what that means is that in soils with a higher proportion of fungi, the carbon is more likely to be turned into longer-life carbon deposits.

What does this mean for farmers?

The scientists are saying that yes it’s all about building microbial communities if you want to increase soil organic carbon.

There is no silver bullet, but it will take new management strategies based on careful monitoring of carbon gains and losses and making sure you are building more carbon on a field than you are taking away.

We can very much see ourselves as key actors when it comes to reducing greenhouse gas emissions and sequestering carbon at a global scale and that employing methodologies that improve soil health and soil microbial communities are the best way to do this. Of course, we already know this is what regenerative agriculture, conservation agriculture, permaculture and so many other farmer-led practices are all about! But it’s good to know the scientists are behind it all, and we hope this will enter the minds of the policy makers sooner rather than later!

We learned a lot from the conference and you will see some changes to Sectormentor for Soils in the coming months as we reflect some of our learnings to bring you some new tools to help you understand what your soil monitoring results mean.