SOIL GEOLOGY

Introduction

Soil formation is closely linked to the underlying geology, and this in turn contributes to the characteristics such as drainage and mineral content. The four basic inorganic components are, from largest to smallest, gravel, sand, silt, and clay, which will be present in different proportions.

Mike Norton, USDA, under Creative Commons Attribution-Share Alike 4.0 International license
Texture by Feel to Assess Soil Type

Weathering of the bedrock by physical processes including ice, or chemical processes such as water, oxygen, carbon dioxide, results in different horizons as you move down a soil profile, reflecting the climatic and organic conditions which each layer, or horizon, has been exposed to over time.

BASIC SOIL HORIZONS

A horizon = Topsoil. The layer with most organic matter, with intense biological activity.

B horizon = Subsoil. The layer beneath where soluble minerals and clay particles accumulate. May hold significant water due to iron and clay.

C horizon = bedrock substrate.

These horizons may be subdivided into various distinct layers, whereas some areas may have no soil at all.

So what? Water flow, run-off, drought and flooding

Larger particle sizes allow water to flow through more freely, making them well draining but prone to drought. Whereas small clay particles will hold water well, creating heavier soils that are prone to compaction and low infiltration. Different species will prefer different conditions. Many soils contain a mixture resulting in a loam, which is most suited for agriculture. Poor water infiltration will relate to high levels of water run-off and also increases the risk of topsoil sediment loss if the soil does not have good structure. Note that increasing the biological activity and organic content can significantly ameliorate these issues related to poor underlying mineral particles.

So what? pH, mineral exchange and ecosystems functioning

The mineral content is directly related to the parent rock material composition and can lead to deficits or excess levels of different components which then impacts organisms. However, the availability of minerals and nutrients is a highly complex physical-chemical-biological nexus, which highlights the importance of systems thinking in both ecology and farming.

Soil geology impacts both mineral availability and the soil pH which in turn affects the soil’s physical, chemical, and biological properties and processes, as well as vegetative growth. In particular, the geology of the soil effects plant nutrient availability by determining the chemical forms of different nutrients and altering the chemical reactions. Moisture, temperature, porosity, texture as well as other factors will all impact the mobility of nutrients in the soil.

Regarding pH, the greatest availability for most plant nutrients is between pH 6 and 7. For farmers, the nutrition, growth, and yields of crops can decrease where pH is out of balance. For habitats and natural ecosystems, the species present will be in part related to soil pH, for instance heather thrives in acid peaty soils, whereas cowslips prefer calcareous or alkaline soils.  pH is discussed further on the Soil Ecology page.

Nutrient availability based on soil pH (Soils Handbook, Kentucky Agr. Exp.
Stn. Misc. 383, 1970, p.28)

More on mineral availability: Cation exchange capacity

 “[it is] often the rate of transfer from an unavailable to available nutrient form that is critical in organic systems, rather than the size of the available nutrient pool.” Christine Watson, 2008

Clay particles in the soil have a negative charge which influences the amount and availability of positively charged (cation) nutrients in the soil. The cation exchange capacity (CEC) is a measure of this and can provide another layer of insight into soil functioning and health. It is a very important soil property influencing soil structure stability, nutrient availability, soil pH and the soil’s reaction to fertilisers and other ameliorants (Hazleton and Murphy 2007).

Why are cations important? Many of the key nutrients exist as positively charge ions, known as cations, notably the base (alkali) cations of potassium, sodium, calcium, and magnesium.  The clay mineral and organic matter components of soil have negatively charged sites on their surfaces which bind to these positively charged ions by electrostatic force.  The cation exchange capacity (CEC), or exchange rate, describes the number of cations in the soil solution that are exchangeable, and therefore available for uptake by plant roots, and therefore relates to fertility of the soil.

As soils become more acidic the soil particle surface charges are replaced by H+ reducing the actual sites for cation exchange, and thus have lower CEC.  

Source: The Organic Research Centre

The CEC of soils varies according to the clay %, the type of clay (some have fewer negative charges), soil pH and amount of organic matter. Pure sand has a very low CEC (3-5 meq/100g). Highly weathered or leached soils generally have low CEC. Organic matter has a very high CEC ranging from 150-400 meq/100g, and can therefore have a significant impact on the soil’s CEC.  Because a higher CEC usually indicates more clay and organic matter is present in the soil, high CEC soils generally have greater water holding capacity than low CEC soils. However, some high clay content soils can have very high CEC but may be terrible for growing, so high CEC does not always mean great soil for farming.

Calcium is the dominant cation on the cation exchange capacity in most soils. It can readily desorb and replenish soil solution as needed for plant uptake. However, it is very mobile in soil and can be lost to leaching or precipitated as minerals when not retained by soil particles.

Apart from the total supply of the nutrient inherent in the soil, some believe the ratios are significant:

The Albretch system from the 1930’s has been adopted by some in the USA and a few in Denmark, Germany and the UK, despite some scientists doubting the approach due to lack of evidence. However, there are enthusiasts that believe in its beneficial impact. It is based on Base Cation Saturation Ratio (BCSR) of the soil, which shows saturation as a percentage of the soil’s Cation Exchange Capacity (CEC), and further investigation is probably needed to understand its application in the UK context.  A key understanding from the Albretch system is that a balance of all nutrients is necessary for optimum soil performance, and the most limiting nutrient will limit yield until it is supplied. However, most UK soils are not considered deficient.

A recent review concluded that so long as amounts of exchangeable Ca, Mg and K are high enough to support crop growth and provide adequate crop nutrition, the relative ratios of these cations is relatively irrelevant within the ranges usually observed for most agricultural soils in the UK.

Mark Measures from Agricology reviewed three trials in 2019 and found the BCSR method shows the following trends in one or more of the trials:

  • A positive effect on crop yield in one or more crops (all 3 trials)
  • A positive effect on soil organisms (2)
  • The need for all elements of the BCSR method to be addressed (1)
  • An increase in crop quality (1)
  • An improvement in soil structure (1)
  • A small increase in margin over fertiliser (1)
  • Greater potential in min till systems (1)

Provisional results from these three trials show that the BCSR analysis and management had a positive effect on soil fertility and crop production. However, there is a reasonable amount of evidence across multiple crop trials suggesting that provided the soil contains adequate absolute quantities of Ca, Mg and K the ratios of these cations will not usually effect crop yields in the ratios typically found in most agricultural soils.

So what? Whole ecosystem impact

Soil is the fundamental basis for an ecosystem or habitat. The physical, chemical and geological properties cannot be separated from the biological functioning, whether the microbes in the soil, the trees or crops and therefore the insects, birds and mammals etc that these support.

Although these webpages describe the inorganic and biological elements under separate sections, both need to be considered together as a whole system.

Decisions to make changes in one element should be considered within the context of the impact on the overall system functioning in that geography.

Your soil geology

Cranfield University provides top level data on soils across the UK on the LandIS Soilscapes website, which can be searched by location. For more accurate data for planning purposes etc, the Soils Site Reporter is recommended.

However, the Soilscapes map data is quite coarse, and nothing compares with getting to know your own soil, as it can vary greatly locally even within a small distance.  The Soilscapes site does provide lots of useful information though, including descriptions of different soil types, such as drainage, carbon content and fertility, as well as typical usage for agriculture.

The UK Soil Observatory also has extensive data on soils, including bacterial community structure, invertebrates, soil carbon, chemistry, geophysics, nutrients, pH, textures, type, water, as well as crop maps, and land cover data.

In addition to the tests in the table below that can be obtained for in-the-field use, the Soil Association lists innovative soil testing companies with lab based techniques.

If you want to keep track of your soil test results and data there are various soil tracking apps, as listed in UK Soil Observatory website.

Summary for doing your own simple(ish) inorganic soil assessments

Texture by Feel

Determines particulate composition. Simple but reasonably accurate test. Texture class can be determined fairly well in the field by feeling the sand particles and estimating silt and clay content by flexibility and stickiness.
Find out how:
USDA Guide

Visual Evaluation of Soil Structure (VESS)

Soil structure affects root penetration, water availability to plants and soil aeration. This simple, quick test assesses top 20cm of soil structure based on the appearance and feel of a block of soil dug out with a spade.  
Find out how:
SRUC VESS Method and Scoresheet                          
Soil Mentor VESS Method    

Compaction & Water Infiltration

Water infiltration is related to run-off, flooding and water storage capacity. As well as indicating biological activity it can also be related to direct physical compaction from heavy machinery etc.  
Find out how:
Innovation for Agriculture to test water holding
Soil Mentor guide to infiltration rate test
Soil Mentor monitoring impact of run off  

pH

pH from the soil geology impacts organisms’ growth in the soil, although organisms can also impact pH in the soil. For example, clay soils can have higher, alkaline pH.  Test kits e.g. Bluelab pH pen or Lutron pen, are available to test soil pH. Cheaper and more simple litmus paper tests, or solution based tests can also be purchased easily e.g. from garden centres.  
Find out how:
Soil Mentor method using pen test
NHBS equipment supplies

Nitrogen, Phosphorous, Potassium

Nitrogen, Phosphorous, Potassium (NPK) are essential nutrients for healthy plant growth and the basic components of fertilizers. Nitrate and nitrite in run off can indicate nutrient losses. Accurate enough home nitrogen and phosphate test kits can be obtained from aquarium suppliers (used for monitoring fish tanks). NHBS amongst others, also provides kits for multiple soil minerals.  
Find out how:
Innovation for Agriculture Nutrient Loss from Soils video                          
NHBS soil NPK test

General Links for Soil Testing

AHDB GreatSoils Project summary of different soil assessment methods
AHDB GreatSoils Project Soil Health Scorecard
Soil Mentor website and ap
Innovation for Agriculture Soil Test Decision Support Toolkit: what to test and when
NHBS soil testing equipment online
USDA Natural Resource Conservation Service Soil Test interpretation guide (p51) 

References AND USEFUL RESOURCES

  1. UK Soil Observatory
  2. Soilmentor Soil Tests Guide
  3. Soil Association Soil Advice
  4. Agricology Resource Centre
  5. AHDB GreatSoils
  6. COSMOS-UK soil moisture measuring
  7. Soil Classification on LandIS UK site
  8. LandIS Soilscapes Soil Map
  9. A simple guide for describing soils, June 2020, Government of Western Australia
  10. USDA Natural Resources Conservation Service Soil Quality Site  and test kit guide
  11. USDA Natural Resources Conservation Service Soil Quality for Environmental Health website
  12. Agricology article: Base Cation Saturation Ratio (Albrecht) soil analysis – what is in it for you? Mark Measures, 12 February 2019
  13. Ohio State University, Soil Fertility Lab, Ion Exchange in Soils and Ion Movement and Plant Uptake 2019
  14. Richard Perkins, Regenerative Agriculture 2019 available from https://www.regenerativeagriculturebook.com/
  15. University of Minnesota Extension, Soil cation ratios for crop production
  16. Anja Vieweger, Mark Measures, Dominic Amos. Review of literature and farm experience on Base Cation Saturation Ratio (BCSR) based on the work of W.A. Albrecht. 2017
  17. Watson, C. 2008. Laboratory mineral soil analysis and soil mineral management in organic farming. Review was undertaken by IOTA under the PACA Res project OFO347, funded by Defra