- Components of soil structure
- Soil structure at the macro scale
- Poor soil structure
- Good soil structure
- Managing soil structure
- Water flow through soil
- Surface tension
- Soil conditioners for agriculture
- Soil conditioner products
- Experiments with soil conditioners
Components of soil structure
- Soil structure is built from a molecular scale up to a macro scale.
- At the molecular scale, clay platelets join together to form flocculates (see Figure 1 in Cations for soil structure for a diagram of flocculated vs. non-flocculated clays); in a soil there will additionally be particles of silt, microbial debris, metal oxides and so on.
- The exact proportions of each depend on the soil itself: its origins, cultivation and inputs, and environment / weather impacts. These bond together as shown in Figure 1, up to a scale which can be discerned in the field.
- At the molecular scale, structure will be influenced by soil chemistry, including fertiliser inputs, water content and exudates from roots; as larger particles are considered, bacterial and fungal activity become important, for example to move around tiny particles of organic matter and to start ‘gluing’ together pieces of silt and clays.
- Towards the millimetre scale, root hairs and fungal hyphae can aid bonding between particles; and as macro-aggregates are formed, influences such as invertebrate movement, root channels, weathering (freeze / thaw), and water flow become dominant in determining soil structure.
Figure 1. Soil structure components from sub-mm scale upwards (from Brady & Weil, 2008, The nature and properties of soil).
Soil structure at the macro scale
- For practical purposes, the term “soil” encompasses the whole growing medium, including the solid soil alongside pores which may be filled with either air or water.
- Solid aggregates are generally irregularly shaped and thus do not precisely tessellate: the gaps between them are the pores, which are vital to a healthy soil, because:
- they allow the soil to hold water, in which nutrients are dissolved for uptake by plant;
- they hold air, which allows gas exchange at root surfaces, allows aerobic microbes to thrive, and allows aerobic chemistry to occur which influences nutrient availability;
- they provide spaces through which creatures such as worms and insects can travel, and where roots can grow;
- they allow water to move vertically and horizontally through the profile, moving nutrients and preventing build-up of water on the soil surface (i.e. flooding).
Figure 2. Ideal proportions of solid, air and water in a soil.
Poor soil structure
- Poor soil structure can be caused by chemical means resulting in deflocculation of clays (see Cations for soil structure) e.g.
- incorrect fertiliser inputs,
- inundation by saline water.
- Poor soil structure can also be caused by physical compaction e.g.
- too many animals
- too much / too heavy machinery travel.
- Impacts of compaction can include:
- reduced root growth: roots cannot push through highly compacted regions;
- “perched” water, e.g. a layer of water below the soil surface but on top of the compacted zone such as at the bottom of the plough layer;
- slow infiltration causing standing water on soil surface;
- soil zones saturated with water have negative impacts on health of both plants and microbes, as well as upsetting chemical balances within the soil.
Good soil structure
- Ideally, soil will be structured at a macro scale in robust “aggregates”, with vertical fissures between them [link awaited].
- An example is shown in Figure 3, where soil on the right is porous and well aggregated, and the soil on the left is blocky and has few macro-pores.
- Well-structured soil is better able to withstand physical pressures from machinery; furthermore “healthy” soils with diverse, numerous microbes, well-connected pores and good aggregation are better able to withstand other types of perturbation such as chemical or biological changes.
Figure 3. Photographs of “good” (left) and “poor” (right) soil structure in a sandy, slumping soil. Courtesy of Philip Wright
Managing soil structure
- Due to the complex nature of soil structure and the variety of influential factors, many aspects of soil management can impact it, not just the obvious ideas like ploughing.
- Physical cultivation can be managed to optimise the factors described above, for example by:
- growing cover crops to help maintain bacterial and fungal populations. Cover crop mixtures can be chosen to help improve drainage by producing deep, larger root channels, or using plants with more fibrous roots to help bonding between soil aggregates. Having plants growing in the soil helps to provide food for microbes and fungi, therefore avoiding bare ground can help maintain these populations;
- considering using reduced tillage or using a less destructive tillage method where soil and crop types allow [link awaited];
- enhancing overall soil health thus increasing populations of invertebrates and insects which can help by forming their own channels;
- increasing active organic matter content (e.g. application of FYM / composts, using longer-term grassland or leys), which will support microbial and fungal growth as well as providing additional “glue” to bind clay particles together.
Water flow through soil
- It is vital that crops can access sufficient water from the soil in which they grow. Since roots cannot rapidly adapt their location to follow water, application of water must be effective throughout the soil volume explored by the roots.
- The route water takes through soil is governed by factors including:
- Pore size, shape and connectivity (tortuosity)
- Soil moisture content
- Amount of water on top of soil (head)
- Soil chemistry
- water-swelling clay?
- Furthermore the presence on soil surface of plants or a crust (“cap”) of soil will affect what proportion of applied water is able to enter the soil.
- ‘Preferential flow’ is when water moves preferentially through particular channels in the soil, rather than evenly filling the soil volume. This is illustrated in Figure 4: water containing blue dye has been applied to the soil surface and allowed to more down the profile, before an inspection pit was dug. The top portion of the soil has been cultivated and well mixed, thus water is able to travel throughout the volume. In contrast, the lower horizon has not been ploughed and water flow is mainly through macropores / cracks, such as those resulting from root growth or invertebrate activity. For water uptake by plants to occur, roots must be in the same soil volume as the water.
Figure 4. Illustration of preferential flow from:
5. Soil texture can influence how water flows through soil, due to different sizes of soil particles and hence different sizes of pores (silt < clay < sand; see Soil types). Therefore if sudden changes in texture occur either vertically or horizontally, there is likely to be different water flow between the two areas. This is illustrated by Figure 5, where the top layer is silt and the bottom layer is sand; larger pores take longer to fill but drain more rapidly: notice that the water has travelled much further (sideways) through the silt compared to (vertically) through the sand after 400 minutes. This is due to surface tension of the water.
Figure 5. Illustration of the effect of change in soil texture on water flow. From The nature and property of soils by Brady & Weil (See Soil conditioners 2015). The lines and numbers show the time (in minutes) taken by the water to travel that distance.
- Water molecules have the chemical formula H2O, with a slightly negative charge on the oxygen atom and slightly positive charge on the hydrogen atoms. Therefore in liquid water there is an attraction between the negative and positive parts of the molecules and they tend to arrange themselves as shown in Figure 6; although the molecules are always moving around there is an attraction between molecules.
- When water moves through pores, if there is insufficient water to fill the pores this creates tension at the surface of the water because the attraction between molecules is stronger than the pull into empty space. Hence water tension in dry soils is greater than in moist soils; and for a given (non-saturated) soil moisture content, surface tension in soils with larger pores will be higher than in soils with smaller pores (see Figure 7) – and where surface tension is greater, water movement is slower because the attraction between molecules is greater than the pull to move.
Figure 6. The chemical structure of water. The two pairs of dots represent two pairs of electrons with a negative charge; H has slightly positive charge and O has slightly negative charge.
Figure 7. Illustration of water tension between soil particles. a) greater soil water content at lower surface tension; b) lesser soil water content at higher surface tension. Arrows round edges of soil particles show direction in which water is pulled by tension. Diagram from Soil science – methods and application by Rowell (See Soil conditioners 2015).
Soil conditioners for agriculture
- In addition to water tension and preferential flow caused by physical effects, hydrophobicity in soils and capping on the surface can affect movement of water through soils.
- Hydrophobicity is likely to occur on soils which have been allowed to dry out and may particularly be a problem in sandy soils where a layer of organic matter has built up on sand particles, or on soils with very high organic matter content. This effect is the chemical repellence of water from the surface of particles, which effectively stops water flow progress until the depth of water on top of soil forces some flow, usually through a few channels rather than through the whole soil volume.
- Capping is especially a problem on bare (plant-free), silty soils and where irrigation is used, i.e. there is not much ‘pressure’ to force water through the soil surface. At the surface, soil structure is broken down and instead particles stick to one another to form a cap which can then prevent water ingress; this is likely to eventually lead to erosion.
- Soil conditioner products often make use of surfactant and / or flocculant chemistry. Surfactants decrease water tension, while flocculants aid soil structure by binding particles together. Both should be applied in irrigation water and:
- Can only be effective in soil volumes accessed by the product.
- The specific formulation of a soil conditioner product has a large influence on how effective it is; many individual ingredients are listed as soil conditioners but not all will be as effective as each other and a single ingredient may have different efficacy depending on the formulation in which it is contained.
- Products are only as effective as long as their structure is intact: chemical degradation of the product and physical disruption of the treated soil volume will reduce efficacy.
Soil conditioner products
- See Soil conditioners 2015 for specific products which were commercially available at the time the presentation was written. Additionally there are more general materials which can have a positive effect, including:
- gypsum can be an effective flocculant due to its calcium content; see Cations for soil structure for reasons why.
- organic matter (manures, composts, etc) has chemical and biological benefits for soil structure.
- clays may be beneficial depending on soil type and clay type.
- Many other products may be available which claim to have positive effects on soil structure via chemical, biological or physical means. It is important to remember that evidence should be sought for the efficacy of every individual product; simply containing a ‘known’ ingredient is not sufficient and biological products in particular can vary hugely in their composition and efficacy.
Experiments with soil conditioners
The presentation Soil conditioners 2016 includes results from 2015 CMI trials with soil conditioners.
A brief summary of the results is:
- Few significant effects observed on
- bulking rates,
- yield components,
- dry matter and specific gravity.
- Yield not affected by treatment in any case.
- No treatment gave consistently better results for any of the measurements (moisture content, water infiltration rate, soil coverage on tubers).
- Limestone soil gave generally more variable results.
- It had not been possible to measure soil compaction as part of the trial.
Soil structure notes
Soil conditioners 2016 (The potential for commercial soil conditioners to improve soil structure or availability of moisture)
Reducing run-off 2016 (Cultivation techniques for reducing run-off in tramlines)
Soil conditioners 2015 (Introduction to commercial soil conditioners for potential influence on soil moisture and structure)
Soil surfactants 2012 (Potential for soil surfactants to influence availability of water and nutrients)
Notes for 1993 (Chapter 3: Soil structure – measurement of soil characteristics and effects on root growth)
Notes from an external source