Visual Soil Evaluation

Realizing Potential Crop Production with Minimum Environmental Impact

Bruce C. Ball, Lars J. Munkholm, Bruce C. Ball, Lars J. Munkholm

• Publisher: CAB International
• Year: 2015
• Print ISBN: 9781780647456
• eBook ISBN: 9781789244946
• Topic: Technology & Engineering
• Subtopic: Agronomy
• Index: Technology & Engineering

Visual Soil Evaluation provides land users and environmental authorities with the tools to assess soil quality for crop performance. An important tool for ensuring food security, this book appraises the use of visual soil evaluation in determining the potential of different land types for carbon storage, greenhouse gas emissions and nutrient leaching. Providing a guide to diagnosing and rectifying soil problems, it includes: - Full colour illustrations throughout to show variation of soil quality and aid evaluation- A broad range of land types, from abandoned peats to prime arable land- Assessment of soil structure after quality degradation such as compaction, erosion or organic matter lossEssential reading for students, researchers and scientists interested in soil science and crop production, this book is also a valuable tool for policy makers and environmental authorities. A useful handbook assessing yield potential across a range of scales, it places visual soil evaluation in the context of the future sustainable intensification of agriculture.

1 Describing Soil Structures, Rooting and Biological Activity and Recognizing Tillage Effects, Damage and Recovery in Clayey and Sandy Soils

Anne Weill^1* and Lars J. Munkholm²

¹Center of Expertise and Technology Transfer in Organic Agriculture and Local Food Systems (Centre d’expertise et de transfert en agriculture biologique et de proximité – CETAB+), Cégep de Victoriaville, Québec, Canada; ²Department of Agroecology – Soil Physics and Hydropedology, Aarhus University, Tjele, Denmark

*E-mail: [email protected]

Soil compaction and erosion have emerged as major threats to global agriculture as they negatively affect plant production and have detrimental impacts on the environment. Soil compaction is responsible for decreased crop yield and quality, emissions of greenhouse gases and increased water runoff (Hamza and Anderson, 2005; Ball et al., 2008). Unless severe, it is often unrecognized because plant growth can appear normal, especially when mineral fertilizers are used liberally. The major cropping factors affecting soil compaction are the weight of machinery, poor timing of field operations with respect to soil water content and intensification of crop production. Soil erosion is responsible for losses of soil particles, nutrients and agrochemicals resulting in decreased soil fertility as well as eutrophication of rivers and lakes (Rasouli et al., 2014). Site characteristics (rainfall quantity and intensity, slope and soil texture) have strong effects on soil erosion; in addition, important cropping factors related to soil erosion are crop rotation, percentage soil cover and management practices affecting soil structure and compaction (Pimentel et al., 1995; Morgan, 2005). Erosion deposits are mostly silt and fine sand with little structure and porosity and thus resemble soil damaged by compaction. Because compaction plays a central role in soil degradation and yield losses, it has to be properly diagnosed in the field. This can be done by observing soil structure, root development, aeration and evidence of biological activity.

This chapter will therefore focus on describing and illustrating important soil structural features associated with compaction and anaerobic conditions. It will cover the evaluation of soil structure and compaction status for both clayey and sandy soils. Since tillage is often responsible for the creation of a number of anthropic layers, each having a different structure, the identification of the different soil layers will be explained. The use of other indicators of soil compaction such as root development (density, deformation, concentration in cracks or between layers), aeration (soil colour) and biological activity (soil macroporosity of biological origin, rapidity of residue turnover, presence of earthworms) will also be covered.

A quick, preliminary evaluation of soil structure can be done using a spadeful of soil, allowing rapid verification of soil structure over the entire field. Since agricultural practices can often affect soil conditions to a depth of 30–50 cm, and sometimes more, soil condition may have to be investigated to such depths, depending on the situation.

Different tools can be used to assess soil structural quality, either using spade methods (e.g. the visual evaluation of soil structural quality, VESS, Ball et al., 2007; Guimarães et al., 2011), visual soil assessment (VSA, Shepherd et al., 2008; Shepherd, 2009), or profile methods (e.g. Cultural Profile, Manichon,1987; or the SoilPAK method, Mckenzie, 2001). These tools are described by Batey et al., Chapter 2, this volume.

Some helpful information for soil compaction diagnosis should also be collected by checking soil maps and interviewing farmers. The following information should be gathered:

• The origin and characteristic of the soil;

• The field situation; for example, surface and sub-surface drainage situation, crop rotation, yield variation in the field, size of the equipment for manure spreading and timing for spreading, harvesting strategy, tillage and number of passes, depth of tillage, etc.

For the purposes of this chapter a soil is considered to be in good condition if it has good structure, is well aerated and contains a sufficient amount of organic matter in the A horizon to be capable of supporting microbial activity and optimum plant growth.

1.1 Evaluation of Soil Structure

Soil structure is best evaluated considering soil texture because the criteria for assessing structure depend on the clay content. The pressure exerted on the soil by machinery forces aggregates to stick to each other and to form clods. Texture is important because the clods resulting from compacted clayey soil are often hard and difficult to break down, while clods resulting from compacted sandy soils are fairly easy to break. Although the relationship between soil characteristics and clay content lies on a continuous spectrum, the evaluation of soil structure will only be described here for two main, discrete groups labelled as follows: clayey soils (more than 25–30% clay) and sandy soils (less than 25–30% clay). Soil having 20–30% clay content will sometimes behave more like a clayey soil and sometime more like a sandy soil, depending on clay type and the organic matter content.

1.1.1 Evaluation of the structure of clayey soils

The structure of clayey soils can mostly be evaluated by observing the shape of aggregates and clods. When describing structure, soil horizonation needs to be taken into account because organic matter content, root density, aeration and biological activity tend to be much higher in the A horizon and these foster aggregation. This section aims at describing typical good and typical poor structure for clayey soils for both topsoil (A horizon) and subsoil layers (B and C horizon). The structure of naturally recovered clay soil is also described.

1.1.1.1 Soil structure of clayey soils in good condition

TOPSOIL (A HORIZON).

Aggregates of a well-structured clayey topsoil are small, in the 1–10 mm range, and well separated (Fig. 1.1a). They can be observed in some grasslands, some non-cultivated soils and in some areas that are not trafficked (permanent beds, controlled traffic systems). They are also common in intensively tilled top layers of recently cultivated soils.

Fig. 1.1 Aggregates and clods in well-structured clayey soils. (a) Topsoil: small round aggregates, 1–5 mm in size, coming from a healthy A horizon. (b) Topsoil: very rough and porous clod coming from a very biologically active soil. The aggregates should detach from each other when the clod is squeezed. (c) Subsoil: small non-porous, angular, 2–10 mm aggregates. (d) Subsoil: lamellar structure, usually found in soil that contain less clay and more silt.

If the compaction pressure is light enough, the clods that are formed have a rough surface because the aggregates that constitute them keep their individual shapes (Fig. 1.1b). They are porous because of the space between the aggregates (not always visible with the naked eye) and the biological activity which creates pores.

In a non-compacted soil it should be very easy to separate the aggregates in the clod by simply squeezing the clod in the fist. However, to do this the clod must be fairly moist. Clay becomes very hard when it dries, which can give a false impression of being highly compacted.

When examining a spadeful of healthy soil, it is often possible to see an excellent structure with aggregates well separated from each other in the seedbed layer because of the effect of harrowing. Below the seedbed, the clods are rough and easy to break (Fig. 1.2).

Fig. 1.2 Healthy clay soil with mostly aggregates in the top part (seedbed) and rough and porous clods in the bottom part (below seedbed).

SUBSOIL (B AND C HORIZONS).

In a well-structured subsoil the aggregates are small (2–10 mm) and can either be rounded (Fig 1.1a) or more angular in shape (Fig. 1.1c)....

Table of contents

• Cover
• Half Title
• Title
• Copyright
• Contents
• List of Contributors
• Preface
• 1 Describing Soil Structures, Rootingand Biological Activity and Recognizing Tillage Effects, Damage and Recovery in Clayey and Sandy Soils
• 2 Assessing Structural Quality for Crop Performance and for Agronomy (VESS, VSA, SOILpak, Profil Cultural, SubVESS)
• 3 Reduction of Yield Gaps and Improvement of Ecological Function through Local-to-Global Applicationsof Visual Soil Assessment
• 4 Visual Evaluation of Grasslandand Arable Management Impactson Soil Quality
• 5 Choosing and Evaluating Soil Improvements by Subsoiling and Compaction Control
• 6 Valuing the Neglected: Lessons and Methods from an Organic, Anthropic Soil System in the Outer Hebrides
• 7 Evaluating Land Quality for Carbon Storage, Greenhouse Gas Emissions and Nutrient Leaching
• 8 Soil Structure under Adverse Weather/Climate Conditions
• 9 The Expanding Discipline and Role of Visual Soil Evaluation
• Index
• Back