Nature Needs Living Soil

Guest post by Sarah Watkinson

Why farmland nature needs help

Caroline Lucas, speaking at the Oxford Real Farming Conference 2019 this week, criticised the Agriculture Bill at present going through parliament as lacking in firm provisions relating to farmland nature: little on environmental regulation post-Brexit, and nothing on public health. But evidence is accumulating that a robust and accessible natural environment is essential for human mental and physical health. The deterioration of the environment affects the most vulnerable people; children are growing up in ignorance of the joys of wild nature. We forget we are part of creation; humans are just one branch on the tree of life. For us, nature is not a luxury, not just a week-end destination, but a necessity for health and happiness; to be cut off from nature is sensory deprivation.

The need for sustainable farming based on independent science

Michael Gove, on Radio 4 this morning, referred to farming as a manufacturing industry. Not so. Farming is the cultivation and husbandry of land, and the products it brings to market are not manufactured but have to be grown. Farmers are stewards of the land we all inhabit, and as such, deserve not only financial security, but also access to independent advice on best practice.

Through the late twentieth century, the independent government research stations, set up after World War II to offer farmers scientifically sound advice, were disbanded; their staff were dispersed and buildings sold off. Since the advent of 21st century chemical biocides, arable farming in the UK lowlands has been largely shaped by the biotech-related industries, catering to big farmers, and innovation has been mainly market-led. 

Why are we so relaxed about ignoring the highly evolved and sophisticated processes of nature, still barely understood, on which agricultural production depends? In medicine – also an applied science that deals with living things – incredible advances in molecular science have given practitioners the tools for effective targeted interventions with minimal side effects, so that we in rich societies can live long healthy lives. Such scientific depth and detail is scorned by agriculture. 

Irreplaceable and irretrievable beauty is sacrificed for short-term farm profits. Even as the alarm is raised that limited productive capacity is left in many farm soils, the potential effects of biocides on the living soil are barely considered.

Soil microbiota – the underground infrastructure of biodiverse and resilient ecosystems

I live in the country, being lucky enough to have the choice. The land where I walk is mainly under intensive monoculture, but patches of woodland are conserved for pheasant shooting and there is a good network of public footpaths threading through the landscape. The local Oxfordshire limestone soil, if treated as hay meadow, without chemical inputs and cut once a year, becomes naturally colonised by a gorgeous mixture of plants: knapweed, bird-foot trefoil, salad burnet, oxeye daisies, ladies’ mantle, campion, clover, wild marjoram, bellflowers and meadow clary, with associated bees and butterflies. 

Bladon C. of E. Primary School Wildlife Reserve in May. A small former allotment site adjacent to the school, which I have managed since 1998 as a hay meadow, cut annually, without biocides.
Eight and nine-year-olds exploring for interesting plants and animals and reporting finds for discussion.

Landscape-scale biocides are likely to damage soil ecosystems

Disaster struck in June 2013, when about a hundred acres of farmland bordering the village where I live was sprayed with a glyphosate herbicide, and every growing herbaceous plant turned orange and died. A new agricultural contractor renting the land had decided that ‘in difficult trading conditions,’ killing all the plants was the cheapest way of eliminating blackgrass. We who rely on this farmland for our nearby nature, lost something priceless; and the blackgrass thrived. This would not have happened if the farm contractor had not been under severe financial pressure, and if he had had proper scientific advice. 

A Bladon public footpath after herbicide treatment
A Bladon public footpath after herbicide treatment in May 2013 ‘to control blackgrass’. 
ll-advised landscape-scale herbicide treatment of fields in Bladon parish, causing loss of farmland biodiverty

Things looked more hopeful when a few years later the contract was handed to a firm specialising in sustainable methods, with the aim of restoring soil health by better drainage and management to increase its organic matter content – laudable, but with its own problems. Because ploughing releases soil carbon and allows erosion, formerly ploughed fields are now ‘no-till’ or minimal till; the land is not deeply ploughed, and instead crop seeds are inserted directly into the ground with a seed drill. Seeds are often ‘dressed’, for example with a neonicotinoid chemical against flea beetle that accumulates in monoculture oilseed rape. There is strong scientific evidence that neonicotinoids disorient pollinating bees and disrupt bird migration. After the main crop is harvested, the land is sown with a ‘cover crop’ of oat grass, to help conserve soil carbon. Before the next crop is drilled, the oat grass is destroyed, by ‘spraying-off’ the whole field with glyphosate-based broad-spectrum systemic herbicide. The sprayed oat grass turns the fields a jaundiced yellow as it dies.

Surely this sudden mass of dead material must radically affect the entire life of the soil, earthworms, protozoa, micro fauna? Basic soil microbiology suggests that decomposition of dead roots would place a biological oxygen demand on the oxygen-dependent life of the soil? But when I asked the question at the Oxford Real Farming Conference this week, even the sustainable farmers present had no answer. Nobody, it seemed, was researching this obvious question. When whole fields are sprayed off, mycorrhizal fungi, being part of their plant hosts’ living root system, must die with them. A recent study reports that mycorrhiza survive spraying. But such survivors probably consist of small suite of generalist species able to form resistant spores in the soil. These dead soils will surely be difficult to restore.

The effect of broad spectrum systemic herbicide on soil structure is only too evident where spray drifts into the field margins, killing off the last remaining sward of wild plants. Where there is a well-trodden public footpath the sprayed ground quickly becomes a lifeless quagmire. There are no perennials and deep-rooting herbs to collect the soil water and transpire it away.

 Intensive cultivation with additions of nitrogen, phosphate and potassium fertilisers and biocides, enables massive growth of highly bred crop plants, but precludes the highly diverse and complex mycorrhizal infrastructure needed by nature. This is why field and footpath wild margins must be protected and maintained over the years, and kept wide enough to ensure that spray drift cannot undo in one day the gains of the preceding seasons. Under these conditions wild flowers and butterflies return and footpath margins can contribute to connectivity in the landscape.

Mycorrhizal diversity underpins plant diversity and productivity

 I’ve been researching and writing about fungi for my whole long university career. I’ve been lucky enough to witness the exponential increase in understanding of this underground infrastructure made possible by DNA technology.

Aboveground natural communities ultimately depend on a below-ground soil microbiome of minibeasts and microbes. Fungi are particularly important to plants because they grow as microscopic pipes that permeate the soil and transport water and nutrients between soil and root.

Woodland symbiotic ectomycorrhiza
Woodland symbiotic ectomycorrhiza – different mushroom-forming fungi on beech rootlets; each photo shows about 1cm of root. (Bagley Wood, Oxford)

Mycorrhizal fungi enabled the first plants to colonise the land 400 million years ago and still partner the majority of plant species, connecting roots and soil. Lowland farms and woods host two main sorts of mycorrhiza: microscopic ‘arbuscular’ mycorrhiza on the roots of most grassland plant species, and ‘ectomycorrhiza’ in woodland, formed by the larger kinds of fungi that produce mushrooms. 

Ectomycorrhizas grow in partnership with trees to produce the ‘wood-wide web’ of mycelial networks extending up to metres through soil. They may connect plants of different species, and be advantageous to them at different life stages, but they all scavenge soil mineral nutrients in exchange for plant sugars. The ectomycorrhizal population of a woodland soil develops over decades, even centuries. That’s why it’s nonsense to suggest that ‘biodiversity off-setting’ can compensate for the destruction of an ancient woodland.

The invisible wildlife of natural soil is still uncatalogued

Biodiverse grassland soil is structured and stabilised by wefts of roots interconnected by these microscopic ‘arbuscular’ mycorrhizal fungi. They are too small to see in soil, apart from the spores of some species which an expert can spot. The name ‘arbuscular’ comes from the structures they develop inside living root cells, branching like little bushes where the fungal cell membrane and plant cell membrane form one living interface, where mineral nutrients scavenged by the fungus are swapped for sugars made by the plant’s photosynthesis. Effectively, plant and fungus become one flesh.

Both the diversity and productivity of grassland are found to increase with the diversity of arbuscular mycorrhizal fungi added as spores. The partnership doesn’t last for long, but roots in grassland plant communities are continually recolonised as long as there are mycorrhizal roots in the soil, from which newly-arriving plant seedlings can select their fungal partners. Plant and fungus recognise each other by exchanging signal molecules, like passwords. These trigger adjustments in development and metabolism, so that partners combine seamlessly into a single functional unit. Partnership can realise latent potential in the host plant. Mycologist Paola Bonfante’s team have discovered that some plants, including tomatoes, develop some of their appetising features and nutrient quality  in response to forming mycorrhizas

New tools, new understanding

Until the advent of ‘massive parallel sequencing’ and high-throughput analysis of soil fungi, arbuscular mycorrhizas were technically challenging to tell apart; but new programmes of genomic research are now expected to elucidate the taxonomic and functional complexity of arbuscular mycorrhizal fungi far more than has been possible before. I believe, with Paul Nurse, that nature is explicable – even if we will never completely account for all the variability that’s there and which we should always bear in mind.

Acknowledgements: 

Thanks to Jo Cartmell for inviting me to write this guest blog, which was inspired by the recent publication, of ‘Sous la Foret’ by world mycorrhiza expert Prof Francis Martin (HumenSciences, Paris).