Soil structure and its benefits - An evidence synthesis - Royal Society
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Soil structure and its benefits An evidence synthesis
CONTENTS
Contents
The Royal Society is the independent scientific
academy of the UK, dedicated to promoting
excellence in science. The Society’s evidence
synthesis reports draw together evidence on
topics where the evidence is new, uncertain, Executive summary 4
complex or contested, and which are relevant to
current policy debate. They follow the ‘principles Introduction 7
for good evidence synthesis for policy’ outlined in
the joint Royal Society and Academy of Medical Chapter one: Soil structure and associated benefits 12
Sciences publication ‘Evidence synthesis for policy’ Biodiversity 13
and aim to be inclusive, rigorous, transparent Agricultural productivity 17
and accessible. Topics are selected following Clean water and flood prevention 18
consultation with a wide range of stakeholders
Climate change mitigation 21
including scientists, policymakers, and industry
and NGO professionals. Chapter two: Measurements 25
This report is part of a series of evidence Chapter three: Interventions 32
syntheses on agriculture and environment topics Interventions to minimise soil erosion and degradation 32
as part of the Royal Society’s Living Landscapes
Interventions to mitigate soil compaction 35
policy programme. For further information see
royalsociety.org/living-landscapes Chapter four: Discussion 39
Illustrative examples 43
Annex 1: Acknowledgements 51
Annex 2: Methodology 53
References 55
Soil structure and its benefits
Issued: April 2020
ISBN: 978-1-78252-458-8
© The Royal Society
The text of this work is licensed under the terms
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This report can be viewed online at:
royalsociety.org/soil-structure-benefits
Cover image © narvikk.
Soil structure and its benefits 3
EXECUTIVE SUMMARY EXECUTIVE SUMMARY
Executive summary
This report synthesises the evidence on Summary of findings Clean water and flood prevention rates, due to high soil moisture and decreased
the relationship between soil structure and Our findings specify the benefits that arise Soil can act as ‘natural flood management rates of microbial decomposition8,9. Changes in
Soil provides a All of the benefits
benefits, focusing mainly on agricultural, from maintaining a well-structured soil. infrastructure’4 by increasing water infiltration land use can have large impacts on soil carbon
wide range of mineral soil. Soil provides a wide range into the ground and also by providing natural levels. Meta-analysis studies have shown that described here
benefits to human of benefits to human society, including Biodiversity water storage, for example through uptake into land use conversion from forest to agriculture can be delivered
agricultural productivity, clean water and flood Biodiversity and soil structure are closely linked; root systems. However, both these benefits results in loss of soil organic carbon10,11. In
society, including in parallel, with
prevention, and climate change mitigation. soil structure influences the nature and activity are negatively affected by compacted soil contrast, the restoration of former crop fields to
agricultural In addition, soil contains high levels of of soil organisms, while soil organisms affect the structure. Compaction of the pores within the grassland or forests can restore soil carbon12. good soil structure
productivity, clean biodiversity and directly supports ecosystem physical structure of the soil. Good soil structure soil reduces the ability of rainfall to infiltrate leading to increased
water and flood services and other terrestrial biodiversity. benefits a number of species and habitats. In the soil5 and acts as an obstacle to root Win-wins, trade-offs and caveats yields, enhanced
addition, soil biodiversity, and its associated penetration6. The degree to which soil can All of the benefits described here can be
prevention, and biodiversity,
There is growing awareness of the importance influence on soil structure, contributes to contribute to flood prevention is strongly delivered in parallel, with good soil structure
climate change of soil structure, particularly its porosity and a range of ecosystem functions such as reliant on it being well-structured. When water leading to increased yields, enhanced improved carbon
mitigation. In permeability to water and gases, for the delivery decomposition of dead matter and nutrient flows over the surface of the land it can also biodiversity, improved carbon sequestration sequestration and
of these benefits. Good soil management is cycling. Soil also contributes to ecosystem have negative impacts on water quality. For and improved water storage. However, there
addition, soil improved water
therefore of paramount importance. Despite services such as support of above-ground example, rather than steadily infiltrating the may be some trade-offs in terms of prioritising
contains high levels this, there is currently no single policy biodiversity, control of plant, animal and human soil, surface runoff can increase the erosion of or enhancing one of these benefits above storage. However,
of biodiversity and dedicated to maintaining high quality soil at pests and diseases, and climate regulation. topsoil and wash chemicals out of the soil and others. For example, interventions to reduce there may be some
directly supports the United Nations (UN), European Union (EU) into aquatic ecosystems, potentially leading to erosion and improve water quality may lead to trade-offs in terms
or UK national level. The UK’s departure from Agricultural productivity the pollution of waterways and eutrophication7. short-term reductions in crop yield.
ecosystem services of prioritising or
the EU presents an important opportunity to Soil is required for 95% of global food
and other terrestrial ensure that UK policies relating to soil health production1. There is a correlation between Climate change mitigation What is less clear from the published evidence enhancing one
biodiversity. incentivise best practice in land management. improvements in soil structure and increasing Soil is the largest terrestrial store of organic is the relationship between an action to of these benefits
grain yield of cereals2. A well-structured carbon and its potential as a carbon sink improve soil structure (for example adding
above others.
This synthesis presents the evidence on four soil can improve crop productivity through means it could have an important role in more organic residues back to the soil) and the
benefits provided by well-structured soil: providing a habitat for earthworms and other climate change mitigation. There is growing magnitude of change in the associated benefit
biodiversity, agricultural productivity, clean soil organisms. Compacted soil is often interest in soil management practices that help (for example the increase in soil organic
water and flood prevention, and climate change associated with a decrease in yield through increase levels of soil carbon stocks. Many carbon). Furthermore, quantifying the scale of
mitigation. It summarises the measurements detrimental effects on the crop’s root system. interventions that improve soil carbon levels an intervention’s benefits to farmers and land
that can be used to monitor soil structure and Improved soil structure can help to prevent also improve soil structure and contribute managers is difficult due to the variability in
the interventions that land managers can make soil erosion, where the upper layer of soil is to the maintenance of healthy soil. There is measures of soil structure. The UK has over
to improve the structure of their soil. The report displaced. Soil erosion significantly affects the debate over the extent to which practices that 700 soil types, determined by variations in
concludes with a series of illustrative examples productivity of soil, with Defra estimating that increase soil organic carbon can play a role geology, climate, plant and animal ecology,
to demonstrate the trade-offs and co-benefits the total cost of erosion in England and Wales in climate change mitigation. The capacity for and land use13. It can therefore be difficult to
that can arise from the different interventions is in the region of £150 million a year3. soil carbon sequestration depends on soil type monitor when and why ‘meaningful’ changes
to improve soil structure. and land use. For example, the soil of wetlands to soil structure (for the better or for the worse)
and peatlands accumulates carbon at faster have occurred14.
4 Soil structure and its benefits Soil structure and its benefits 5
EXECUTIVE SUMMARY INTRODUCTION
Introduction
Soil structure is just one element of well- The appropriateness of semi-quantitative and Soil provides a wide range of benefits to Soil contains high levels of biodiversity
functioning soil. Thus, a measure of soil fully-quantitative measurements depends on human society (Figure 1), including producing – ten grams of soil may contain 10 billion
Semi-quantitative
structure may be of little relevance if the the desired outcome. If the aim is to improve food, providing clean water, reducing the risk bacterial cells, representing more than 1
approaches that soil of concern is providing a platform for soil quality through a participative, low cost, of flooding and mitigating climate change million species17 – and directly supports
farmers and land human activities, or storing geological and decentralised system that incentivises land through carbon sequestration. other terrestrial biodiversity.
archaeological heritage15. managers to engage and self-evaluate the
managers can
impacts of their land management techniques, FIGURE 1
use themselves, Trade-offs also exist for potential metrics then the semi-quantitative approach may
and that are also of soil structure that could be used to be the most appropriate. Alternatively, a soil The range of functions and services that soil provides18.
inexpensive incentivise good soil management within a monitoring approach that uses the expertise
future agricultural policy. Semi-quantitative of agricultural scientists, with techniques
and quick to
approaches that farmers and land managers that are more expensive but more detailed
apply, have the can use themselves, and that are also and objective, may be more appropriate Provision
of support
advantage that inexpensive and quick to apply, have the for mapping the current state of soil across
for human Flood
advantage that they can be used repeatedly the UK and demonstrating more detailed
they can be used infrastructures mitigation
over time, and by the main user of the land. trends over time. Either of these approaches Provision of
repeatedly over They can provide an overall indicator of could feasibly be used as part of a new land raw materials
Filtering of
time, and by the whether different visual aspects of the soil management policy. The two approaches are Hydrology
nutrients and
main user of are ‘good’ or ‘poor’, which may be enough not mutually exclusive, and it may be that both Storing, filtering contaminents
to inform land managers, farmers and could be used in a tiered approach. Any future Provision of Agronomy and transforming
the land. food, wood nutrients, Biodiversity
government on whether the soil is generally approach should be designed with industry and fibre
Biomass substances pool, such as
production, habitats,
improving or degrading over time. However, support and participation from farmers, including agriculture
and water
species and Climatology Carbon storage
these approaches would be unsuitable for retailers, water treatment companies and other and forestry genes and greenhouse
the development of a rigorous and reliable stakeholders. It will be important to ensure that gases regulation
national soil monitoring programme, which land managers in the future have access to Physical and
Ecosystem Soil Soil cultural environment
would require measures to collect objective a practicable set of indicators to monitor and for humans and
services functions science
data that could be analysed statistically, and clear standards to meet. human activities
Detoxification
ideally could provide information on a regional and the recycling
or even global scale16. Archive of of wastes
geological and Source of
archaeological raw material
Cultural heritage Acting as
identity carbon pool
Regulation of
pests and disease
Ecology populations
Heritage
values
Recreation
Aesthetics
6 Soil structure and its benefits Soil structure and its benefits 7
INTRODUCTION INTRODUCTION
There is a growing awareness of these of UK policy in relation to soil include a pledge As the UK leaves the EU, a new agricultural evidence pathway between soil management,
benefits and the role of good soil management by the UK Government that all of England’s policy will replace the current Common soil structure, and the benefits that good soil
There are a
in delivering them. For example, concerns soil will be managed sustainably by 203028. Agricultural Policy (CAP). It is important to structure provides therefore has current policy
have recently been raised regarding the More recently this has been reaffirmed in ensure that UK and devolved administration relevance to all four UK nations. range of different
continued ability of soil to support food the Government’s 25 Year Environment Plan policies and incentives relating to soil soil types and
production for a growing human population19. which states that “by 2030 we want all of promote best practice in land management There are a range of different soil types
therefore the
England’s soil to be managed sustainably, and to deliver multiple beneficial outcomes. For and therefore the management and range
Soil is included across a wide range of we will use natural capital thinking to develop England, the new Agriculture Bill will include of benefits provided are likely to be very management and
different United Nations (UN) goals and appropriate soil metrics and management a payment scheme based on ‘public money context specific. The majority of the scientific range of benefits
agreements20, European Union (EU) policies21 approaches”29. The Welsh Government for public goods’ and this explicitly mentions literature focuses on the structure of mineral provided are
and directives22,23 and national policies and includes the loss of soil carbon as one of the rewarding good soil management37. In soil (where the parent material is rock), which
likely to be very
legislation24,25,26 amongst others. However, national indicators tracking progress towards Wales a new Sustainable Farm Scheme is makes up the majority of UK agricultural soil.
while soil is currently subject to a variety of achieving its seven well-being goals30. under development around the principle of It is for this reason that the evidence synthesis context specific.
international and national targets, there is sustainability38 while in Scotland many current will focus primarily on the soil structure and
currently no single policy dedicated to soil Alongside policy mechanisms, good soil CAP schemes will continue as under the EU, benefits of mineral soil of managed grassland
at the UN, EU or UK level, unlike for example management is promoted by incentives at least in the short term39. In Northern Ireland, or arable land. For inclusion criteria please
the EU Water Framework Directive for water. offered to farmers and land managers by a move away from area-based payments is see Annex 2: Methodology.
This issue was highlighted in the 2016 supermarkets, food production companies and under consideration. In all instances, evidence
Parliamentary Inquiry into Soil Health by the water treatment companies. Some examples connecting the action or intervention taken There are various benefits that good soil
Environmental Audit Committee27. Examples of these are given in Box 1. by the farmer or land manager to a feature structure can help deliver. For this report, the
(such as soil structure) and a beneficial focus will be on four benefits where there is
BOX 1
outcome will be vital. sufficient evidence to draw upon: biodiversity,
agricultural productivity, clean water and flood
Examples of soil management incentives offered by industry Focus of this synthesis prevention and climate change mitigation.
This evidence synthesis summarises the
• M
arks and Spencer have launched ‘Plan • W
essex Water Ltd, a water and sewerage published evidence about the relationship Soil structure
A 2025’, an eight-year transformation plan business serving the south west of between soil structure and the benefits it A description of soil and its structure is
focussed on social and environmental England has also used various means provides. It also examines the measurements provided in Box 2. For soil used in agriculture,
issues, including soil health31 – including advice, negotiation and of soil structure that land managers and a ‘well-structured soil’ will have a continuous
financial contributions – to change the scientists can use, and the interventions network of pore spaces to allow drainage of
• N
estlé have partnered with First Milk
practices of farmers and landowners to available to improve soil structure and water, free movement of air and unrestricted
to incentivise farmers to improve
reduce soil erosion and runoff to improve prevent degradation. development of roots40. These features
environmental sustainability (including
water quality34 enable functions, such as nutrient cycling and
soil). The scheme sees farmers being
Soil structure was selected as the topic of water and oxygen transport, which promotes
paid directly through their contracts for • T
ried & Tested is a voluntary initiative
focus following two stakeholder workshops ecosystem services such as increasing soil
delivering quality agri-environment work delivered by a series of industry partners
in March and July 2019. Soil structure was fertility and water purification. In this synthesis,
through a points-based system32 and aims to help farmers to improve
chosen due to its relationship with water and we will refer to soil with these features, as
nutrient management planning through
• A
SDA have collaborated with LEAF gas permeability and the beneficial outcomes ‘well-structured’. In addition to supporting
practical nutrient, manure and feed
(Linking Environment And Farming), to that this permeability supports – such as those food production, many of these soil structural
planning guidance35
produce ‘six simple steps’ for farmers described in Chapter one. Soil structure is also features also provide a range of other benefits.
to improve the performance, health and • C
FE is a partnership initiative which a property of the soil which can be measured These are explored in detail in Chapter one.
long-term sustainability of their soil33 promotes good environmental management and potentially rewarded as part of any new For other soil types, outside the scope of this
through productive farming practices36 payment scheme emerging from a new synthesis, we note that the qualities of a ‘well-
agricultural policy framework. Providing the structured’ soil will be different.
8 Soil structure and its benefits Soil structure and its benefits 9
INTRODUCTION INTRODUCTION
BOX 2 FIGURE 2
Mineral soil and its structure A mineral soil profile.
What is soil? The three main types of soil particle are clay, sand and
Soil forms the uppermost layer of the Earth’s crust, silt. Note that silt in this instance refers to soil which
Organic matter
and mineral soil consists of a mixture of organic matter, originates from the erosion of rock and is not associated
minerals, gases and water. with river deposits. These three types vary in the size Topsoil
of their constituent particles, which leads to different
Soil develops gradually over time, as weathering of the
properties (Table 1). The combination of these three Subsoil
bedrock on the Earth’s surface combines with decaying
particles determines the soil type (Figure 3). Soil type and
organic matter. Soil typically develops in layers (also
structure have important ramifications for how soil behaves
known as horizons) which are distinct from one another
under different weather conditions and land management
in colour and texture (Figure 2). The bottom layer, the Parent rock
regimes. It is important to consider the type of soil
bedrock, is a solid mass of rock and provides the ‘parent’
present in fields for a number of reasons: to assess the
material for the soil and influences its type. For example,
risk of drought or flooding; to determine the vulnerability
clay particles are derived from fine-grained rocks such Bedrock
of the soil to compaction; and when considering the
as shale while sandy particles tend to come from the
measurement of the characteristics of soil degradation,
weathering of sandstone. Soils formed over chalk and
as all of these differ between the different soil types.
limestone are naturally thin because these rocks do not
give rise to clay or sand particles. Partially weathered What do we mean by soil structure? FIGURE 3
rocks form the basis of the parent rock layer. We refer to soil structure as the arrangement of solids
and pore spaces within soil. Soil solids are soil minerals Soil texture triangle, showing the different soil types and combinations of clay, sand
Unless regularly ploughed, the topmost layer is made up
and organic particles that (with metal ions, organic and silt particles41.
of organic matter, including leaf litter, at various stages of
matter, root hairs, bacterial secretions and fungi) ‘clump’
decomposition. Below is the surface soil (often referred 100
together to form aggregates. Aggregates (also known
to as ‘topsoil’) which is typically 10 – 25 cm deep. Topsoil 95 0
as peds) vary in composition, shape and size, and in their 5
is a combination of organic and mineral components; 90
stability towards the erosive forces of water. The size 85 10
it usually has the highest biodiversity and the most 15
and continuity of soil pores surrounding the aggregates 80
nutrients. The layer below, the subsoil, has a similar 20
is important for air, water and nutrient transport. 75
composition although contains more minerals which 25
70
have been leached (moved down) by rainwater. Soil structure influences water retention and movement, 65 30
root penetration, carbon storage, susceptibility to erosion, 60 35
Clay
40
)
y (%
and fertility – meaning it underpins many benefits.
Sil
55
45
t (%
50
Cla
50
)
45
TABLE 1 40 Silty 55
35 Sandy clay 60
clay 65
30
Properties of soil particle types42. Silty 70
25 Sandy clay Clay loam
20 clay loam 75
loam 80
Soil particle type Particle size (mm) Water retention Characteristics 15
85
10 Silt
Clay < 0.002 Drains slowly, high water retention Heavy, slow to warm up, prone to Sandy loam Sandy silt loam 90
5 Loamy loam
compaction and drying out in summer 95
0 Sand sand
100
Silt 0.002 – 0.05 Retains a moderate amount of water Easily compacted and prone to erosion 100 95 90 85 80 75 70 65 60 55 50 45 40 35 30 25 20 15 10 5 0
Sand 0.05 – 2.00 Fast draining, therefore often dry Warms up quickly, often acidic Sand (%)
10 Soil structure and its benefits Soil structure and its benefits 11
CHAPTER ONE CHAPTER ONE
Soil structure and associated benefits
The previous chapter described the societal benefits. See Box 3 for more detail on The remainder of this chapter presents the correlate to ‘useful’ biodiversity; an increase
characteristics associated with well-structured the distinction between natural capital assets, evidence linking soil structure in agricultural in biodiversity could mean an increase in
Soil communities
soil. As a natural capital asset, soil can be ecosystem services and ecosystem benefits. soil with the four following benefits: pathogens or pest populations. The chapter
managed to generate goods and services. In This chapter examines the role of soil structure will identify the types of organisms that are are extremely
• Biodiversity
conjunction with other inputs such as human in delivering some of these benefits. beneficial to soil structure and functioning. diverse, with millions
labour, these goods and services generate • Agricultural productivity
of species and
How soil structure supports biodiversity
• Clean water and flood prevention billions of individual
BOX 3 Soil structure supports biodiversity by
• Climate change mitigation providing a habitat for the many organisms that organisms, ranging
Natural capital assets, ecosystem services and ecosystem benefits 43 live within it. Soil communities are extremely from microscopic
Biodiversity diverse, with millions of species and billions of
bacteria, archaea
Natural capital assets Benefits Wild species diversity and abundance can be individual organisms, ranging from microscopic
The elements of nature that directly Changes in human welfare (or wellbeing) that viewed as an ecosystem benefit in its own bacteria, archaea and fungi, through to and fungi, through
or indirectly produce value to people. result from the use or consumption of goods, right, in terms of cultural or aesthetic value. Soil larger organisms, such as earthworms, ants to larger organisms,
Individual assets include ecological or from the knowledge that something exists organisms also underpin several ecosystem and moles (Figure 5). It is estimated that soil
such as earthworms,
communities, species, soil, land, (for example, from knowing that a rare or services, such as pollination, biological pest and leaf litter is home to about one quarter
freshwaters, minerals, sub-soil resources, charismatic species exists even though an control and soil fertility, which deliver additional of vertebrate and invertebrate species on ants and moles.
oceans, the atmosphere, and the natural individual may never see it). Benefits can benefits including food production. the planet46. This level of biodiversity is
processes that underpin their functioning. be both positive and negative (disbenefits). supported by the diverse microhabitats that
Examples of benefits are the aesthetic Soil organisms and soil structure are closely well-structured soil provides and that are
Ecosystem services and recreational benefits of wild species linked and have a reciprocal relationship: created through variations in soil structural
Functions carried out by the natural diversity, food and agricultural productivity, soil structure influences the nature and features such as soil texture (the relative
environment (eg pollination, carbon clean water and prevention of flooding, and activity of soil organisms and other terrestrial content of soil particles of different sizes eg
sequestration) from nature that can be turned climate change mitigation. Benefits are the organisms, while soil organisms affect the clays, silts and sands), water availability and
into benefits (eg food, hazard protection) goods provided by ecosystem services. physical structure of the soil and support well- nutrient availability.
when combined with human input (eg labour, functioning soil and wider ecosystems.
machinery). See Figure 4 for further examples. Soil structure affects the composition of soil
This section identifies where good soil communities in a number of ways. For example,
structure is linked to biodiversity and also bacterial diversity is affected by soil particle
FIGURE 4 how soil organisms can improve soil structure size, with a higher percentage of larger sand
and functioning. We define biodiversity particles (ie coarser soil) causing a significant
Flow of natural capital assets, ecosystem services and the benefits that can be obtained44. as “the variability among living organisms increase in bacterial species richness47. The
from all sources including, [among other ability of soil structure to hold moisture is linked
Natural capital assets, Ecosystem services provided Benefits obtained, things] terrestrial, marine and other aquatic to a high microbial diversity and more robust
for example: by these assets, for example: for example: ecosystems and the ecological complexes populations of soil mesofauna (animals between
of which they are part; this includes diversity 0.1 – 2 mm in size, such as tardigrades, Figure
Species Pollination Food
within species, between species and of 5e) and macrofauna (animals more than 2 mm in
Communities Biomass Energy ecosystems”45. It is important to note that an size, such as earthworms, Figure 5k) compared
Landscapes Carbon sequestration Clean water increase in biodiversity does not necessarily to dry soil48. Likewise, in one study the diversity
Ecosystems Erosion protection Clean air
Soil Water purification Recreation
Water Hazard protection
Air Wildlife conservation
Equitable climates
12 Soil structure and its benefits Soil structure and its benefits 13
CHAPTER ONE CHAPTER ONE
and composition of fungal communities was was demonstrated in a UK-wide study where How soil biology supports soil structure The release of organic compounds by root
strongly influenced by soil type and land animal richness was predominantly determined Soil organisms contribute to a range of systems, known as root exudates, into the
Soil biodiversity Soil microbial
use intensity49. Critically, soil biodiversity by land use intensity and unaffected by soil ecosystem functions (see Figure 1 and surrounding soil systems has been shown to
should not be should not be treated as a single entity but properties, whereas microbial richness was Figure 4) such as nutrient cycling and soil act like a glue and bind soil particles together. communities can
treated as a single as a complex array of communities, which are driven by broader environmental properties, formation, and ecosystem services, such as Root exudates increase soil stability and in directly affect
differently affected by various factors. This including soil50. the control of pests and diseases, as well the longer term have been shown to reduce
entity but as a soil structure
as supporting above ground biodiversity52. the ability of water to flow through the soil58,59.
complex array However, the relationships between species Additionally, root exudates strongly influence and functionality
FIGURE 5
of communities. diversity and ecosystem functions and the composition of soil microorganisms60. through their
A selection of organisms in soil communities51. services are complex; the direction and Mycorrhizal fungi, beneficial fungi that form a roles in cycling
strength of the effect of species diversity symbiosis (close interaction) with plant roots,
soil nutrients and
varies highly and other factors may also play can also change the soil structure by physically
a role in driving ecosystem functioning53. enmeshing soil aggregates in their hyphae (the storing carbon.
branching structures of fungi). Recent evidence
Certain organisms, such as plants and their suggests that a particular glycoprotein released
associated root systems or animals such as by certain mycorrhizal fungi is involved in the
earthworms, act as ‘soil engineers’ and can aggregation of soil particles61.
modify soil structure, pore size, porosity, bulk
density, organic matter and water content54. Soil microbial communities can directly affect
soil structure and functionality through their
Plants and their root systems influence the roles in cycling soil nutrients and storing
physical and biological properties of the carbon62. Microbes known as cyanobacteria
soil55. Denser, finer root systems bind soil produce extracellular substances that alter
more effectively than thicker, sparser root the soil pore structure and form biological soil
systems, and thereby increase soil stability56. crusts. These soil crusts help to stabilise the
The growth of roots physically displaces soil soil. Once the cyanobacteria have colonised
particles; hence, larger roots increase soil and created a soil crust, other organisms
density adjacent to the root, whereas finer such as fungi, lichens, bryophytes, and
roots can decrease density by increasing algae also colonise the crust63. This helps to
soil porosity57. prevent soil erosion in arid or wet regions and
(a) root fungi (b) decomposer fungi (c) bacteria (d) nematode (e) tardigrade aerates the soil. This aeration by microbes
(f) collembolan (g) mite (h) worm (i) millipede ( j) centipede also helps to cycle nutrients by decomposing
(k) earthworm (l) ants (m) woodlice (n) flatworm (o) mole organic matter, making vital nutrients such as
phosphorus, potassium and nitrogen available
to be taken up by plants64.
14 Soil structure and its benefits Soil structure and its benefits 15
CHAPTER ONE CHAPTER ONE
Earthworms have an important role in Certain managed farm systems can change Agricultural productivity A high density of earthworms is linked to
maintaining and enhancing soil structure. the nature and complexity of the communities Soil is required for 95% of global food improved agricultural productivity. Arable soil
Earthworms have Compacted soil
They act as ‘ecosystem engineers’ by found in soil75. A meta-analysis revealed that production79. Over the next 30 years, our food typically contains 150 – 350 earthworms per m2
an important role physically burrowing in, and aerating, the more intensively managed soil is associated system will experience an unprecedented and high populations (>400 earthworms per m2) occurs under
in maintaining and soil and strongly influence the physical and with higher levels of microbial richness demand as global population increases to 9.7 of earthworms are linked to significant benefits the wheels of
chemical characteristics of soil layers65. (number of species), but leads to declines billion people by 205080. Meeting the nutritional in crop productivity86. A 2014 study found that
enhancing soil tractors and heavy
They play a vital role in mixing organic matter in the number of larger soil animals such demands of 2 billion more people may on average earthworm presence in agricultural
structure. They in the soil66, cycling nutrients67 and creating as earthworms76. It is important to consider require either radical societal adaptation (eg soil leads to a 25% increase in crop yield and machinery, and
act as ‘ecosystem new microhabitats for soil organisms68. ‘functional biodiversity’ in these cases; there replacing most meat and dairy with plant-based a 23% increase in aboveground biodiversity87. this is associated
engineers’ Earthworm burrows also increase water may be instances where the overall number alternatives81), or a considerable increase in Well-structured soil can affect crop productivity with decreases
filtration and reduce runoff on the soil of species may be higher in one system, for the efficiency of global agricultural production, by providing a habitat for earthworms.
by physically in crop yield due
surface, thus reducing soil erosion. instance due to a higher number of microbes, distribution and waste management82, and most Earthworms can be negatively impacted by
burrowing in, but it may not have the same beneficial effects likely a combination of all of these measures. certain farm management systems. A global to detrimental
and aerating, Human agricultural activity can have a as a soil ecosystem that contains organisms Here we describe the ways in which soil meta-analysis revealed that conventional effects on the
strong impact on soil biological activity and of a range of sizes that carry out a range structure can enhance agricultural yields. till regimes decrease the abundance of
the soil and crop’s root system.
diversity69. Tillage, which involves digging, of functions. earthworms88. One study indicated significantly
strongly influence stirring and turning over soil, strongly reduces It is well known that soil structure can affect reduced earthworm numbers with increased Compaction also
the physical the numbers of most organisms within Changes to the balance of communities of soil crop yield83. One study in which soil structure fertiliser and pesticide inputs89. reduces water
and chemical the soil70,71. Soil compaction by agricultural organisms has implications for the resilience was visually scored (with a high score (9 – 10) infiltration and
machinery has been shown to reduce of food production in the face of extreme indicating a good soil structure and a low Compacted soil occurs under the wheels
characteristics water uptake.
soil microbial biomass72, and reduces soil events. Bacterial dominated communities are score (1 – 2) indicating a poor soil structure that of tractors and heavy machinery, and this is
of soil layers. pore size which affects the movements of slower to recover from drought events77. As ‘consists entirely of big clods, smooth dense associated with decreases in crop yield due to
worms and larger soil animals73. Earthworm more extreme weather events occur in the crack faces, roots only in cracks’84), found there detrimental effects on the crop’s root system.
populations have been reported to decrease future, resilient food production is likely to was a correlation between good soil structure Compaction also reduces water infiltration and
in response to soil compaction74. become increasingly important; functional soil and higher grain yield of cereals. It found yield water uptake90. Compaction from machinery
biodiversity may have a role in supporting this78. increases of 300 – 350 kg ha-1 for each unit can be reduced through the use of fixed tracks
increase in the soil structure score85. for wheels (to achieve non-trafficked crop
The role of soil structure in promoting growing zones). For example, implementing
agricultural productivity is explored in the non-trafficked zones (as measured by soil
next section. porosity) significantly improved the structure
of topsoil when compared with conventional
random traffic farming – this correlated with
an average yield increase of 6 – 10% in green
peas, spinach and planted onions91.
16 Soil structure and its benefits Soil structure and its benefits 17
CHAPTER ONE CHAPTER ONE
The physical structure of the soil also Clean water and flood prevention FIGURE 6
determines the likelihood of soil erosion, Well-structured soil filters water between the
The most recent Well-structured
which can negatively affect agricultural atmosphere, groundwater, lakes and rivers, Soil compaction reduces the available space for soil, air and water, limiting pathways
estimate of the productivity. Soil erosion is the removal of the improving water quality and availability. for root growth102. soil filters water
impact of erosion top layer of soil by water or wind. Generally, Soil water represents only 0.05% of global between the
soil with higher porosity, faster infiltration rates freshwater and 0.001% of global water96,
on productivity atmosphere,
and higher levels of organic matter is more yet is crucial for supporting all terrestrial life. Well-structured soil Compacted soil
losses was around resistant to erosion92. Erosion is estimated groundwater,
£40 million a year to move around 2.2 million tonnes of topsoil Soil is made up of solid particles, air pockets (or lakes and rivers,
in England and per year in the UK alone, with the soil often pores) and water (Figure 6). The effectiveness improving water
ending up in watercourses93. The topsoil of soil water storage depends on the soil
Wales, as a result quality and
layer contains the highest concentration of texture and on the pore space between soil
of reduced yield organic matter and microorganisms and thus particles, which is determined by factors availability.
and increased its loss significantly affects the productivity, such as soil organic matter97. The pore size
structure and functionality of the soil. The distribution affects aeration, water holding
costs, with the total
most recent estimate of the impact of erosion capacity, and drainage capacity of soil98. When
cost of erosion in on productivity losses was around £40 million soil structure is degraded due to compaction,
England and Wales a year in England and Wales, as a result of the pores are pressed together, reducing the
in the region of reduced yield and increased costs, with the space where air and water are normally stored
50% soil particles Less than 50%
total cost of erosion in England and Wales in (Figure 6). This significantly reduces the ability
£150 million a year. 25% water water and air
the region of £150 million a year94. A review of water to vertically infiltrate the soil and
25% air
of 24 studies in the UK found that yields thus increases surface runoff and the risk of
decreased on average by 4% per 10 cm depth flooding99. It also limits the pathways available
of soil loss through erosion95. for crop roots, affecting agricultural yields100,
and leads to greater soil erosion and the
Agriculture relies on well-structured soil for pollution of waterways101. Flooding and surface runoff Compaction damage changes the soil pore
its ability to store and provide water to plants. Soil can act as ‘natural flood management structure and reduces the ability of soil to absorb
The role of good soil structure in providing infrastructure’103 by lowering the risk of heavy rainfall, leading to the rapid flow of water
clean water and flood prevention is explored flooding through: 1) increased water infiltration into lakes and rivers105. Subsoil compaction can
in the next section. into the soil and 2) providing natural storage, lead to the formation of a plough pan (Figure 7),
for example via uptake into root systems. a layer in the subsurface of the soil that has a
Well-structured soil structure reduces high density and a lower porosity than the soil
surface runoff. directly above or below it. This is the result of
pressure applied by machinery during tillage.
The amount of water retained in soil (available With tillage, the formation of a plough pan in
water capacity) is positively related to levels the subsoil changes the direction of water flow
of soil organic matter. Soil organic matter through the soil by impeding vertical infiltration
enhances soil water retention because of and enhancing the horizontal flow106. This
its hydrophilic (water-attracting) nature and results in two major issues depending on the
ability to increase soil aggregate formation time of year: 1) it can increase the risk of flooding
and stability, thereby enhancing porosity in winter and 2) reduce the soil’s capacity to deal
and infiltration104. with heat shocks in summer107,108.
18 Soil structure and its benefits Soil structure and its benefits 19CHAPTER ONE CHAPTER ONE
FIGURE 7 Livestock grazing can also cause soil Climate change mitigation
compaction. Root systems are affected by Soil structure and carbon sequestration
Soil is the largest
Plants growing in (a) well-structured soil and (b) compacted soil109. both topsoil and subsoil compaction, with Soil is the largest terrestrial store of organic
compacted soil acting as an obstacle to root carbon, and contains twice as much carbon as terrestrial store of
penetration (Figure 7)110. In one study, soil the atmosphere118. Soil management and the organic carbon,
Surface crust permeability on a highly grazed pasture was resultant soil structure can affect the carbon
and contains twice
increased and rainwater runoff was reduced content of soil119. Soil carbon sequestration
Tightly packed by reducing the number of livestock and refers to the long-term accumulation of as much carbon as
crumbs planting trees111. In another study, water was carbon in soil. Sequestration occurs when the atmosphere.
found to infiltrate into forest hillslope soil, carbon input (for example, from leaf litter,
Porous Large blocks but run off the surface of sloped, compacted residues, roots, or manure) exceeds carbon
(loose-fitting) with few cracks grassland soil. This was due to the larger root losses (mostly through the respiration of soil
crumbs and water uptake by trees, and lower soil moisture organisms, increased by soil disturbance)120.
blocks Subsoil
compaction, in the forest soil compared to the compacted Even small changes in the soil carbon pool
including a grassland soil112. Thus, there is a need to have the potential to significantly influence
plough pan appropriately value and preserve the ‘green the concentration of carbon dioxide in the
storage’ of water by trees113,114. atmosphere. There is increasing interest in
a) Well-structured soil b) Compacted soil enhancing the carbon content of soil as a
Soil compaction reduces the depth of crop means of reducing the amount of carbon
rooting and the supply of water to root dioxide in the atmosphere. The Royal
systems, reducing crop growth. This increases Society’s report on greenhouse gas removal
the likelihood of surface runoff and soil recommended that if the UK were to achieve
erosion. The water carries with it fine sediment, its target to be net zero by 2050, a key
organic material, crop nutrients, pesticides action would be to ‘ramp-up’ soil carbon
and microbes115. It also results in an increased sequestration across large UK land-areas
need for fertiliser input as chemicals are through changes in agricultural practices
washed away rather than retained in the soil116. (see Box 4)121 . There are a number of co-
These chemicals can become contaminants benefits of improving soil carbon sequestration
in aquatic ecosystems and a threat to (including improved soil structure) which
human health117. makes such strategies to increase soil carbon
‘win-win’ or ‘no-regrets’ strategies122.
Soil moisture also has an important role in the
regulation of another benefit derived from
well-structured soil, the mitigation of climate
change, as discussed in the next section.
20 Soil structure and its benefits Soil structure and its benefits 21CHAPTER ONE CHAPTER ONE
Carbon forms a significant part of the total Input of carbon from the atmosphere to soil Changes in land use can have large impacts as root and microbial biomass, than annual
soil organic matter, which consists of plant is indirect, enabled by plant photosynthesis, on the capacity for soil carbon sequestration. wheat agriculture134. The use of permanent
Many interventions
residues, living microbes, fresh and partially which converts atmospheric carbon dioxide Meta-analyses have concluded that conversion vegetation in agricultural land, for instance
decomposed detritus (dead organic matter) into simple sugars. These sugars are from forest to arable cropping systems reduces with agroforestry (the intentional combination which increase
and humus (stable organic layer). Soil organic incorporated into organic matter for plant soil organic carbon123,124. Since 1750, between of perennial shrubs and trees with annual levels of soil
carbon levels are therefore directly related to growth or used as a source of energy. 40 and 90 billion metric tonnes of carbon has crops such as cereals) and conservation
organic carbon,
levels of soil organic matter and result from Decomposition of plants transfers the been lost from soil globally, through cultivation buffer strips (strips of land with permanent
the interaction of several processes. Carbon organic carbon captured from the air, into and disturbance125 – 127. However, carbon vegetation), can aid carbon sequestration, also improve
enters long-term storage in soil as organic the soil. The constant flux of carbon in the sequestration due to the restoration of former in addition to providing other benefits, such soil structure
carbon from plant material and is incorporated environment is known as the carbon cycle, cropland to grassland or forests can reduce, as improving soil quality and structure, and contribute to
into the soil through decomposition. and is depicted in Figure 8. or in rare cases even exceed, carbon deficits reducing erosion and supporting wildlife.
other soil functions.
resulting from previous land use128. Therefore, The associated improvement in soil structure
BOX 4 FIGURE 8 croplands have high potential for future will contribute to the increased regulation In particular, the
carbon sequestration and, with appropriate of water flows and other soil functions135,136. addition of organic
Agricultural interventions that The carbon cycle. land management, can play an important role Interventions to stabilise carbon within the soil
matter to soil
enhance soil structure and may in climate change mitigation. This increase in or even increase how much carbon can be
soil organic carbon would also improve soil sequestered are being investigated to help increases nutrients
increase soil carbon storage
structure and the range of other soil functions achieve net-zero emission targets and mitigate and water retention,
ATMOSPHERE
Plants remove carbon
• Reducing tillage intensity and frequency dioxide from the air and benefits which are associated with this. climate change137,138. which reduces
via photosynthesis
• C
rop rotations – including use of grass soil erosion.
Land management practices also affect soil When considering land management
and livestock structure and carbon sequestration (Box 4). interventions to improve soil structure and
• S
witching to perennial crops instead Practices such as reduced till129 and growth carbon sequestration, for example through
Carbon dioxide
BIOSPHERE
of annual crops released of soil cover crops have been shown to promoting an increase in soil organic
into the air increase levels of soil organic carbon130, matter, there are potential effects on other
• U
sing cover crops and eliminating via respiration though the extent to which this has a large role greenhouse gas emissions.
fallow periods (Figure 11)
in climate change mitigation is debated131,132.
• A
pplying of manure and sludge, and Many of these interventions which improve Soil organic carbon can be released back
PEDOSPHERE
other waste materials levels of soil organic carbon also improve into the atmosphere as carbon dioxide
Plants break down soil structure and contribute to other soil via respiration of plants, soil animals and
• Adjusting irrigation methods into organic carbon
functions. In particular, the addition of organic microbes, including decomposers. Carbon
• Changing grazing regimes matter to soil increases nutrients (reducing the in the form of methane can also be released
• Implementing conservation buffer strips need for synthetic fertiliser), increases water into the atmosphere from soil organisms when
retention and reduces soil erosion. Measures decomposition takes place in the absence of
• Agroforestry
to protect soil structure from compaction and oxygen (eg under waterlogged conditions).
degradation, such as optimisation of grazing Methane is produced in anoxic (low-oxygen)
intensity, also enhance organic matter content environments, including submerged soil, by
and hence carbon sequestration133. microorganisms (methanogenic bacteria) that
excrete it as a by-product139. Methane is a more
The use of perennial vegetation (crops potent greenhouse gas than carbon dioxide,
which do not need to be replanted after with 34 times the global warming potential
each harvest) rather than annual crops also over 100 years140. However, increasing soil
increases soil carbon. For example, perennial organic matter for carbon sequestration is
grassland has been found to have higher soil expected to have only a negligible impact
carbon throughout the soil profile, particularly on soil methane emissions141.
22 Soil structure and its benefits Soil structure and its benefits 23CHAPTER ONE CHAPTER TWO
Measurements
Nitrous oxide is also a potent greenhouse in the autumn, limiting the time available for This chapter summarises the currently Visual field assessments and scorecards
gas and can be emitted from agricultural soil soil management and tillage150. available methods for measuring soil structure. Visual assessments of soil conducted
The impacts of
particularly where nitrogen fertiliser is used. There is substantial evidence that land in the field offer the potential to gather
these climatic Increasing soil organic matter or carbon In addition to the effects on soil structure, management practices affect soil structure semi-quantitative information for use in
changes on soil sequestration would also increase organic climate change may also alter rates of and therefore impact the benefits that soil can monitoring soil condition and avoid possible
nitrogen levels in the soil which could increase soil carbon sequestration. Increased offer157 – 159. Part of the commitment in the 25 errors caused by transporting samples to a
structure include
nitrous oxide emissions, though the likely effect atmospheric carbon may increase plant Year Environment Plan to manage England’s laboratory. Some assessments have been
greater erosion is hard to quantify142. Emission rates of nitrous growth and therefore increase carbon dioxide soil sustainably by 2030, involves developing deliberately designed to be readily understood
and loss of soil oxide are increased in wet and compacted capture from the air into the soil through “appropriate soil metrics and management and easy to teach to non-soil scientists (for an
organic matter. soil143,144. The release of nitrous oxide from photosynthesis. However, due to erosion, approaches”160. Detecting any deterioration in example, see Table 2)165.
manure applications may offset any increase loss of organic matter and changes to the soil structure early is also important due to the
in soil organic carbon as a carbon sink145. frequency and intensity of droughts and flood long timescales (up to 190 years161) required for A variety of visual soil description
events151, climate change could also lead to a severely deteriorated soil to fully recover162. assessments exist including the SOILpak
The effect of climate change on soil structure loss of carbon to the atmosphere. As a result, score166 and Le profil cultural167 and variations
Soil structure and the ecosystems it supports predicting the composite effects of climate There is a vast array of methods to measure of one of the most well-known methods,
are intimately linked to the climate. The change on soil is highly challenging152,153. soil structure163 with advantages and designed by Peerlkamp168.
production of carbon dioxide in soil comes A recent meta-analysis predicted that a disadvantages to each. Measurements
almost entirely from root respiration and business-as-usual climate warming scenario for soil structure suitable for use in the UK Visual assessments differ in several important
microbial decomposition of organic matter. (ie a global average soil surface temperature were recently reviewed164. The types of ways including the depth of the soil under
Carbon cycle processes are temperature- increase of 2 °C over the next 35 years) would measurements differ by who conducts the consideration, how the soil is handled prior
dependent146. Moreover, environmental drive the loss of between 5 – 115 billion metric measurement (typically either a land manager, to assessment, and the emphasis placed on
conditions beyond temperature contribute tonnes of carbon from the soil by 2050154. academic scientist or soil consultant); where particular features of soil structure169. Most
to changes in decomposition rate and It should be noted that the sites with the the measurement is performed (at the site methods attempt to minimise subjective errors
carbon sequestration147. For example, dry greatest losses were predicted to be wet, itself or samples analysed in a laboratory); with clearly defined rules and scoring criteria170.
or waterlogged (lacking oxygen) soil has organic carbon-rich sites common in upland accuracy (for example whether they are based They are relatively low cost and straightforward
decreased decomposition rates, sometimes areas of the UK. on discrete samples from a field or whether to perform, with experts suggesting that
leading to accumulation of soil organic carbon the entire field is measured); scale (from assessment of a soil sample by this method
in areas such as peatland bogs. Furthermore, There are also indirect effects of climate field to whole catchment); and finally, cost. would typically take less than an hour when
high levels of precipitation on fast-draining soil change on soil structure and the role of soil The main groups of measurements that are performed by an experienced user171.
can lead to loss of carbon from soil through in the carbon cycle, including changes in performed on soil to measure its structure are
washing away dissolved organic carbon and soil biodiversity and soil composition155. The summarised below.
soil erosion. effects of these changes on the decomposer
community requires further research due to
Climate change in the UK is expected to the complex interactions of temperature and
result in hotter, drier summers and warmer, moisture compounded by regional variations
wetter winters, and additionally an increased and differing soil types156.
occurrence of extreme weather events such
as drought, storms and floods148. The impacts The next chapter of this evidence synthesis
of these climatic changes on soil structure looks in more detail at some of the
include greater erosion and loss of soil organic measurements used by land managers and
matter149. Furthermore, wetter winters and scientists to assess soil structure and the
therefore wetter soil, increases the risk of benefits it can provide.
soil compaction from grazing livestock and
livestock may have to be housed indoors for
longer. Likewise, for arable farming, there will
be a longer wait until the soil dries out in the
spring and potential challenges with harvesting
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