A 2015 report from the Food and Agriculture Organization (FAO) estimated that one-third of soils globally are in a moderate to bad state, or are fairly to heavily degraded. Organic matter in UK soils has fallen for the last 30 years or so, according to Martin Wood at soils consultancy Earthcare Technical. Intensive agriculture and a separation of livestock farming from arable farming are partly to blame, with cereal areas in the east of England no longer getting an input of livestock manures.
‘Soil organic matter is really a balance between what is being used up and what is going in. That balance is out of kilter. We need to add more organic matter in to address that,’ says Wood, who recently gave webinars to growers on this topic as part of the Soil Association’s Great Soils programme. The programme aims to inspire UK growers to assess the health of their soils and give them practical solutions to improve them.
Soil organic carbon is a critical ingredient as it energises the network of microbes and animals important in a healthy soil. ‘Organic matter from plants and manner is the fuel that microorganisms need to do their job of releasing nutrients to plants and binding,’ says Richard Bardgett, soil ecologist at the University of Manchester, UK.
Over a 15-year period, the average organic carbon content in arable soils in England and Wales decreased from 3.4% to 2.8% (Nature, 2005, 437, 245). Once levels fall below 3.5%, soils are prone to structural deterioration, while soils stripped of organic matter, or plant cover, are more susceptible to erosion and loss of fertility.
Gains from fertiliser have been pocketed, but soil scientists argue that the nutrient focus needs to be rebooted. As US President Roosvelt wrote in 1937: ‘A nation that destroys its soils, destroys itself’.
Worryingly, the government’s report on UK climate change risk assessment 2017 warned that the proportion of prime farmland is expected to fall from 38% to 9% by 2050 and, in a worse-case scenario, crop growing in eastern England and Scotland could be ended by degraded soil and water shortages. Bardgett, who worked on the report, says proactive measures are needed. He was also part of a team that found around 2bn t or 60% of soil carbon in UK grasslands lies between 30cm and 1m deep (Global Change Biology, 2016, 22, 2929).
Policymakers have taken note. An initiative as part of the COP 21 agreement in Paris in November 2015 advocated a 0.4%/year increase of organic matter in soil, enough to compensate for greenhouse gas emissions. Fortunately, according to Wood, it is not just scientists but growers who are increasingly concerned about the health of UK soils.
So what’s special about organic matter? It has a number of benefits, Wood pointed out during a webinar on ‘soil health and the bottom line’ in July 2016. First, it is a storehouse of crop nutrients, particularly nitrogen, sulfur, phosphorus and trace elements. It contains a high proportion of carbon, and it improves soil structure, which can have a major impact on soil’s water holding capacity. And it also has important effects in terms of species diversity and activity.
‘The emphasis has been primarily on measuring the chemistry of soils, what nutrients are in the soil, the pH, but also phosphorus, potassium and magnesium and adding fertiliser to replace the nutrients taken by crops,’ says Wood. Bag fertiliser comes with a financial and also an environmental cost, with nitrogen lost to the environment due to over-zealous application or bad timing, polluting waterways or being released to the atmosphere as greenhouse gases: nitrous oxide or ammonia. It can depress diversity of plants on pasture farms, with high nitrogen conditions allowing one or two species to dominate. And in China it has led to a significant acidification of croplands (Science, 2010, 237, 1008).
Use of nitrogen fertiliser also leads to nutrient imbalances, as nutrients such as potassium and sulphur, not replenished by fertiliser, became limiting, Bardgett writes in his 2016 book, Earth matters: how soil underlies civilisation. The ledger approach to soil health, measuring elemental inputs and outputs, gave a boost after World War 2, but it is thought to be too simplistic and unsustainable over a longer timeframe, given that soil is a living entity. This has led growers to adopt a number of strategies to improve soil health along yields and perhaps nutrient quality of crops.
‘We’ve added lots of some nutrients, but people are becoming aware that sometimes crops grown under these conditions lack some important micronutrients,’ according to Gerlinde de Deyn, soil scientist at Wageningen University in the Netherlands. ‘There are quite a few soils that don’t have enough micronutrients, so then they don’t end up in the plant. So our food could be more healthy than it is at the moment’
Manures, food waste, crop residues, compost and digestate from anaerobic digesters are all being added now in the UK to assist the biology of soils. In the UK, these are increasingly being tried along with ‘green manures’ – fast growing plants dug back into the ground to replenish nutrients.
‘You can grow a legume like lucerne or clover or buckwheat, which grows rapidly, and then incorporate it into the soil,’ Wood explains. ‘The important thing is to keep the ground with something grown on it, not to leave it bare over winter.’ The plants also have the advantage of different root systems, tapping into new sources of nutrients. ‘Oats have deep, fibrous roots good for soil structure and lucerne has deep roots that can bring nutrients up from deeper down in the soil,’ says Wood. He notes that it is now often not economically viable to leave land fallow for a year, so short duration cover crops are needed.
Organic matter goes hand in hand with good soil structure, which can lead to poor infiltration of water, water runoff, along with agrichemicals, and soil erosion. Soil compaction also happens in intensive systems because of heavy machinery. This can particularly impact crop roots. ‘Water logging leads to a lack of oxygen and poor root growth, and that is associated with reduced organic matter content and intensive cultivation,’ Wood explained in his July 2016 webinar.
A new, more biological, approach to producing crops is now emerging. ‘There is an increasing interest in how to harness the biology of soils,’ Wood says. Standard tests such as for pH remain essential, but novel molecular techniques allow the analysis of soil species diversity. Around 10,000 species of bacteria can live in single gram of soil.
‘Chemical analysis and fertiliser use is like following the recipe. It is straightforward. But biology is much trickier. It is much more complicated,’ Wood says.
One new test also seeks to measure the amount of carbon dioxide produced from soil, an indicator of biological activity, he points out, adding that: ‘there are labs in Britain starting to trial these methods for assessing biological aspects of soil health’. One test developed by the US Department of Agriculture (USDA), in collaboration with a commercial lab in the US, involves first drying soil first and then re-wetting it in a standardised way to encourage a flush of microbial activity, before measuring respiration.
To see if a soil is healthy, you can take a spade to a field and dig out a wedge of soil. ‘Look for earthworm numbers in the soil. A reasonably good population is reassuring,’ says Wood. But labs are now also offering test for soil food webs, where amoebae, worms and nematodes are looked at under a microscope and the soil scored for health and diversity, below and above ground. ‘We have done experiments showing that increasing the number of plant species within grasslands can improve the physical structure of soil, improve the quantities of organic matter and enhance the diversity of organisms,’ says Bardgett. A recent European project also showed that planting multiple grass species in a pasture can return more growth and fodder.
Two other strategies for better soil health are no tillage farming and intercropping. For no till, soil is not ploughed. Seeds are dropped directly into a groove to minimise soil disturbance, which can otherwise boost respiration and release carbon. Residues of previous crops are also retained.
Intercropping puts crops together to complement one other. For instance, nitrogen-fixing legumes, like peas and beans, planted with wheat boosts soil nitrogen levels, but they also smother weeds and protect soil from erosion. It is possible to select crops that release specific enzymes or organic compounds into the soil that mobilise a nutrient. Chickpeas can mobilise phosphorus in soils, spiking its uptake for wheat. Bardgett argues it may be possible in future to select or breed crop varieties that perform better together, or which promote the growth of microbes that mobilise nutrients in soil: ‘It opens the door to the breeding of crops with special characteristics, or traits, that benefit the growth of neighbouring crops, whilst also improving soil.’ A review in 2014 recommended crop programmes consider multiple traits that benefit mixed cropping (New Phytologist, 2014, 206, 107).
Root zone potential
What happens around plant roots is also incredibly important to soils. ‘Roots secrete sugars and polysaccharides that the microorganisms can use,’ says Wood. ‘This together with the gums microbes produce helps stick soil particles together,’ important for increasing its resistance to heavy rains. But future gains in crop yields and more efficient nutrient use could come from a better understanding of the microbial world around roots.
‘Plant biomes could be used to maintain or sustain higher yields without pesticides or huge doses of organic fertilisers,’ says de Deyn. She predicts we will one day encourage a particular suite of organisms in the rhizosphere, the biologically rich area around root systems. ‘We could stimulate microbes to provide what the plant wants in a way that is fine tuned, so that we are not providing an overdose that washes away.’ Already there are some soya bean cultivars inoculated with strains of bacteria that provide them with nitrogen. One approach is to pick plant varieties that release certain compounds that will kick-start the right microbes into action for extracting micronutrients such as zinc. ‘Quite a few plant species also exude organic acids that are helpful in releasing phosphorus,’ de Deyn adds.
In one example, Swiss researchers identified maize cultivars that released odorous terpene from their roots when attacked by root feeding insect larvae (Nature, 2005, 434, 732). The natural compounds attracted nematodes to feed on the larvae. ‘In certain cultivars, the genetic coding for this compound was selected out, so these plants could not cry for help anymore,’ says de Deyn. Better understanding of the interaction between plant chemicals and soil organisms could yield tremendous benefits, she adds: ‘We could make agricultural systems more productive, but in a natural way, making use of soil biota by understanding what is there and how they communicate.’
Microbes that are good for defense or for extracting micronutrients, for example, could be encouraged or inoculated on seeds. ‘This is fundamentally different from a chemical application,’ de Deyn notes.
Technology could also be harnessed for efficient harvesting or ‘intercropping’ of two food crops. And for more targeted fertiliser application. ‘In precision agriculture, you can read the health status of field of crops a reflective spectrum,’ says de Deyn. This is incredibly useful, because soils are rarely homogenous and a large field could have patches with high pH or low nutrient levels or soil compaction. Researchers and some growers are beginning to fly small drones to take images of crop fields for on the spot analysis, for example.
‘We have the technology and advancements to feed all the people, but we for now have forgotten the natural cycles,’ says de Deyn. ‘A healthy soil is one in which organisms can do their job and the cycles of nutrients are in tune with space and time.’
She points to a lack of awareness among the general public and politicians about the importance of soils, despite some positive developments such as the establishment of the Intergovernmental Technical Panel on Soils by the Food and Agriculture Organization (FAO), which had its first meeting in 2013.
‘It is amazing how much food we have produced over the last decades due to intensification, but we have been overloading the system with certain nutrients, especially nitrogen and phosphorus,’ she argues. ‘We can be a lot smarter.’