Arable Matters: The Good, the Bad and the Ugly in our soils

In my naivety, I asked him what was wrong with it – and he told me that when the repair man took the back off the set, he discovered that the problem had been caused by all the dead cowboys and Indians down the back.

Of course, this was also back in the days when Westerns were easier to understand, when the good guys were always 'good' and the bad guys were always 'bad' – and sometimes to make this even easier for the young fan base to understand, the heroes wore white hats and the villains wore black hats.

For some reason, I was reminded of this when I came upon a recent research paper investigating the micro-organisms which play such an important role in the health of our soils – and which aimed to identify the goodies and the baddies of the piece, and what influenced their balance.

While the simplistic portrayal in the early Westerns of an often brutal period of American history has been subjected to much revision and re-working over the years, the way in which our soils are viewed has also been considerably revised – and never more so than in these current eco-conscious times.

It’s not all that long ago that soils were viewed a bit like a simple chemistry set, with the levels of a set number of macro-nutrients like nitrogen, phosphorous and potash along with the pH level being viewed as the major markers of a soil’s health.

A few years down the line, though, the important role played by micro-nutrients and trace elements were also brought into the equation – as scientists realised that they too performed an important role in feeding and supporting the crops, and grassland we wanted to grow.

In more recent years, new and improved techniques, including genome analytics, have also advanced our understanding of the crucial role played by the millions of bacteria, fungi and other microbes as we been able to measure and appreciate the wide range and genetic diversity which lives in our soils – and which can help or hinder our efforts to grow crops.

On top of this, since world-wide recognition of the climate emergency we are currently facing, there has been a huge increase in the interest in the important role which our soils can play through their ability to store and sequester the carbon which, in various gaseous forms, is held to be the major contributor to global warming.

But the research paper I’m speaking about looked at the impact of arable farming on soil ecosystems – and claimed it had created an underworld where undesirable bugs and fungi could muscle their way in to prosper at the expense of their helpful and beneficial counterparts.

I’m aware I’m swapping metaphors here, but researchers at the venerable Rothamsted Research Institute claimed that ploughing land and using fertilisers and sprays created the sort of chaos in the microbial world which encouraged what they termed a 'gangsters paradise', where the bad guys come out on top.

Mycorrhizal fungi, including those that form mutual beneficial associations with plants and play important roles in plant nutrition, were reduced under such a regime in favour of pathogenic fungi that survive by attacking insects, plants and lichen, according to the study’s lead researcher, Professor Andy Neal.

The research also found the more ‘nutritionally monotonous’ arable soil environment led many bacteria to ‘reduce their running costs’ by jettisoning more than 600 of the genes usually needed when faced with the diverse range of food sources found in grasslands or pastures.

“Farming practices cause physical disruption and alter the nutritional inputs, which means less diverse plant materials and more readily available nitrogen to soil. As a result, some species lose out allowing new ones – often with very different ways of making a living - to thrive. Even those that survive have had to change the way they live their lives,” he said.

Commenting on the research which looked at the effect of converting grassland to either arable or bare soil he said that while the richness of species didn’t change very much, new species moved in which often filled different ecological roles.

According to Prof Neal, this change in the natural balance meant that typical measures of soil biodiversity weren’t adequately explaining what was going on in soil – as farming not only changed the number and relatedness of the species present, but also the genetic complement of the community as a whole.

Using soil samples from the long running Highfield-Ley experiment, the team from Rothamsted, working with the US Department of Energy’s Pacific Northwest National Laboratory, compared arable soils with their original grassland state, as well as bare soils which had been left fallow for over 50 years.

A wheat field from the Highfield ley experiment which is being used to benchmark the good, the bad and the ugly factors in today's soil structures

The researchers also saw a greater variety of bacteria in arable soil, whilst the total number of species of archaea – a group of single cell organisms members of which generate the greenhouse gas nitrous oxide as a by-product of ammonia oxidation – also increased in response to fertilisation.

Prof Neal added that the results also show that the responses of these three different types of organisms varied markedly depending on the physical and chemical practices adopted by farmers managing the land:

“As elsewhere, biodiversity loss in soil was of great concern,” he said. “We rely on soil to grow almost all of our food, but perhaps surprisingly we know little about how the way we manage soils affects the microbial communities which support soil fertility, provide clean water and regulate greenhouse gas emissions.”

But against this slightly depressing view of what goes on in our soil, the James Hutton Institute provided a small troop of cavalry riding over the hill in the shape of some good news from the world of soil research.

For recent work which the institute was involved in showed that those wonderful suppliers of free nitrogen – legumes – have the ability to be quite discerning about the root nodule bugs they team up with to allow them to fix nitrogen.

Scientists found that legume plants, such as peas and beans, can make ‘smart’ management decisions when it comes to interacting with their symbiotic bacterial partners to harness nitrogen from the atmosphere.

As we know, these plants host nitrogen-fixing bacteria in specialised root growths called nodules, but these bacteria demand sugar from the plant in exchange for the nitrogen that they supply. The study found that legumes can weigh up the different bacterial partners available and then only provide sugar to the best strains, cutting off those that are 'less good'.

“Crucially, we have shown that the plant has an even finer control over its nitrogen-fixing bacterial ‘guests’ in that it can recognise if a strain is relatively better than another,” said the institute’s Euan James.

This means that if the symbiotic bacteria native to an area are underperforming, then inoculating the crop with an ‘elite’ performing strain would see it preferentially selected.

This news can only increase our confidence in the use of bacterial inoculants in seed dressings – and can reassure us that sometimes we can help make sure that the good guys win in the end.