Course:EOSC311/2020/Soil Mineralogy: the Geology of Agriculture

Focus on how geology affects soil composition and sustainable agricultural practices.

Statement of connection

Different soils are affected by the geology of their areas - geology influences soil formation and determines the nature of a soil's mineral content. Since minerals make up approximately 45% of soil composition, the geologic conditions that produce them play an important role in growing our food. The mineral content of soil can affect its fertility and productivity through the release of and reactions with plant nutrients . The weathering of soil and its minerals is another important topic to touch on, as soil maintenance can only be sustainable if rock particles are removed at the same rate they are added . Other factors such as pore size, water content and drainage in the soil depend on its mineral composition as well. These considerations are important in determining sustainable soil management practices in regards to agricultural land and beyond.

Soil mineralogy and agriculture

What is soil?

The composition of soil. Pore space is accounted for on the right side of this diagram as air and water fill the space equally, or in different ratios according to the condition the soil is in.

Soil constitutes the top layer of the regolith, the unconsolidated rock material that covers the Earth's surface . It is the backbone of the global food system, as it provides support for the growth of plants, crops and livestock. It also regulates water supplies, recycles raw materials, provides habitat for diverse organisms, and serves as an engineering medium.

It is composed of approximately 45% mineral components, 5% organic matter, and 50% air and water (ideally 20-30% each) which fills the pore space. While mineral content varies in soils, it is dominated by clay minerals and quartz, and includes smaller amounts of feldspar and fragments of rock . There are numerous factors that affect the nature of the soil and its rate of formation, including climate, organisms, parent material, the relief of the surface where soil is accumulating, and the length of time which soil formation has been occurring at its location .

How is soil produced?

Soil is produced, in part, by the weathering and erosion of rock material. This weathering can either be mechanical or chemical. Mechanical weathering involves physical processes that occur on the surface of Earth breaking rock into smaller pieces, while chemical weathering involves chemical reactions which change minerals into different forms . These forms are often less affected by other chemical reactions that can occur, but weaken the rock so that it is more susceptible to mechanical weathering . In this way, the two types of weathering encourage one another by weakening rock to ease mechanical weathering and this providing more surface area for chemical weathering to occur . Together, they create the particles and ions that will eventually become sedimentary rock and soil . Climate plays a large part as well, as it is a main determinant for what plant life is present in an area. Plants contribute to soil production by forcing rocks apart with their roots, and contributing organic acids to soils to further chemical weathering .

Parent materials that undergo weathering and form soils can either be bedrock or unconsolidated sediment such as glacial or stream deposits . This parent material plays an important part in determining what kinds of chemical weathering reactions are possible and at which speed they will occur - some are easier to weather than others. For example, minerals such as halite, gypsum, and calcite weather by dissolution and are the easiest to weather, and silicate minerals with lower silica to oxygen ratios are easier to weather than those with higher ratios .

Classifying soils

Sections of land are divided into predominant orders of soil. These are based on the distinctive soil horizons created by environmental conditions that allow certain soil processes to occur.

Soils can be classified according to their horizons, or distinctive layers that represent various soil processes caused by certain environmental conditions . Within Canada, we have orders which describe sets of these horizons (see map to the right).

The soil horizon is made up of 5 layers :

1. O horizon, a layer of organic matter.
2. A horizon, a layer of partly decayed organic matter mixed with mineral material.
3. E horizon, a layer from which some of the clay and iron have been leached out, leaving a pale, sandier layer.
4. B horizon, a layer where clay, iron, and other elements from the overlying soil accumulate.
5. C horizon, a layer which contains broken fragments of rock, reflecting that weathering of the underlying bedrock or sediment has not been completed.

In Canada, podzolic soils dominate. They involve the downward transportation of hydrogen, iron, and aluminum from the upper part of the soil profile, and exhibit an accumulation of clay, iron and aluminum in the B horizon .

Canadian soil classification system

Order	Description	Environment
Forests
Podzolic	Well-developed A and B horizons	Coniferous forests throughout Canada
Luvisolic	Clay-rich B horizon	Northern prairies and central BC, mostly on sedimentary rocks
Brunisolic	Poorly developed or immature soil, that does not have the well-defined horizons of podsol or luvisol	Boreal-forest soils in the discontinuous permafrost areas of central and western Canada, and also in southern BC
Grasslands
Chernozemic	High levels of organic matter and an A horizon at least 10 cm thick	Southern prairies and parts of BC’s southern interior, in areas that experience summer water deficits
Solonetzic	A clay-rich B horizon, commonly with a salt- bearing C horizon	Southern prairies, in areas that experience water deficits during the summer
Glacial and tundra
Cryosolic	Poorly developed soil, mostly C horizon	Permafrost areas of northern Canada
Vertisolic	Clay-rich soils associated with glacial lake deposits	Southern prairies
Other
Organic	Dominated by organic matter; mineral horizons are typically absent	Wetland areas, especially along the western edge of Hudson Bay, and in the area between the prairies and the boreal forest
Regosolic	Does not have a B horizon (i.e., no accumulation of leached minerals)	Unstable sediments including steep slopes prone to landslides, shifting sand dunes, and floodplains where sediments are frequently moved by streams
Gleysolic	Colour patterns related to the absence of oxygen	Water-saturated soils

Sustainability and soil mineralogy

Soil sustainability is achieved when components are removed at the same rate they are replenished; soil forming processes and erosion must be in equilibrium . Yet modern day agricultural practices result in a net outflow of nutrients in agricultural systems that does not occur in natural systems whose nutrient losses due to erosion are replenished at the same rate by weathering of primary minerals or atmospheric deposition . Even under ideal conditions soil takes thousands of years to develop, meaning that human activities which damage soils have long term consequences for ecosystem health and agricultural productivity . Soil sustainability must also be considered across different timescales. This is because mineral matter, organic matter, and water all have different residence times - organic matter can reside in soil for between 1-1000 years, while water can reside for tens of years to minutes . These are both significantly less than that of the mineral content, which relies on geological parent material and does not readily change. Thus, variations in ecosystems, climate, glacial effects, extreme events, or human impact can exacerbate both present-day and long-term damage to soil .

Geological processes and climate change

Soil is fundamentally a product of geological processes and the climate. It forms best under temperate to tropical conditions with moderate precipitation; too much water in an environment can lead to soils that lack nutrients, as they get leached away, leaving behind acidic soils, or producing soil that is low in inorganic nutrients and dominated by organic matter. On the other hand, too little water limits the rate of downward chemical transport meaning that salts and carbonate ions dissolved in upward-moving groundwater can precipitate and build up in sediments, creating a lack of organic activity and matter . The effects of anthropogenic climate change are posing serious threats to soil conditions, affecting these aspects of formation and many more. With global temperatures rising and precipitation patterns changing, there is threat of widespread soil erosion, desertification and leaching by acid rain . Increasing temperatures are also causing permafrost and glaciers to melt, respectively releasing massive amounts of greenhouse gases (GHGs) from permafrost stores and contributing to sea level rise . Rising sea levels will affect coastal areas the most dramatically, bringing contaminants to inland soils and affecting food production .

Yet soil also plays an important role in storing carbon; other than the ocean, soil is the second largest natural carbon sink . It has the capacity to capture carbon dioxide, a prominent GHG, from the air which could eventually help to remove some of the carbon dioxide from the atmosphere . Higher carbon dioxide levels also accelerate chemical weathering, but since this occurs within the geological carbon cycle over very long time scales, this is not an 'easy fix' for climate change .

Agricultural connections and consequences

Processes leading to soil degradation. Note the impact that agriculture, deforestation, climate, nutrient depletion, erosion, and acidification can have on soil degradation.

The fertility and productivity of soil is affected by geological conditions, and nutrient supply varies with soil mineralogy . In general, basaltic parent material tends to generate very fertile soils as it provides lots of phosphorous, iron, magnesium, and calcium . Some unconsolidated materials such as river or flood deposits are also good soils as they tend to be rich in clay minerals and organic matter. One example of this is the Fraser Lowland in British Columbia, one of Canada's prime agricultural areas . They have large surface areas and are negatively charged, meaning that they attract cations, positively charged elements such as calcium, magnesium, iron and potassium - all important nutrients for plant growth . Clays are also important as they affect soil temperature and pH, aggregate sizes and strength, porosity and water-holding capacity . They are the most "reactive and interactive" inorganic compounds in soils . They are often found in conjunction with sand and silt in different ratios, of which loam, roughly equal proportions of all three, is the most fertile .

Agriculture, and by extension our global food system, relies on and will continue to rely on soil to support plant growth. Our reliance on soils for agriculture (and unsustainable management of them) is reflected in the fact that human activities have increased the long-term soil erosion rate by a factor of 30 globally . Industrial, large scale agriculture has also relied on the use of synthetic fertilizers to replenish nutrients that are depleted by intensive farming and monocultures in an attempt to maintain soil health. However, this widespread practice has detrimental effects. Given the time scale of soil water flow, it causes escape of nutrients and eutrophication in other ecosystems .

Strategies for maintaining agricultural soil

Soil degradation commonly occurs by means of wind and water erosion caused by tillage activities and lack of soil cover. Degradation can be monitored by different indicators, or properties that influence a soil's productivity. Physical properties include soil density, infiltration rates, water retention, and aggregation; chemical properties are nutrient availability, pH, and cation exchange capacity (CEC); biological properties include organic matter content, biological activity, roots, and organisms . Proper soil management practices can be adopted in order to maintain the health of all of these properties, and create more ecologically sustainable agricultural systems.

Best practices for soil health in an agricultural setting are as follows :

• Create conditions for the least amount of mechanical disturbance, i.e. minimum and well-timed tillage, keeping off the field when soil is too wet or dry.
• Keep soil covered, i.e. with cover crops.
• Plan for and promote diversity, i.e. crop rotations that prevent pest management issues, reduce weed competition, distribution of nutrient demand, efficient use of nutrient inputs, potential for increased yields.
• Keep a living root in the ground, i.e. perennial grass/sod cover.

On a broader scale, policy changes and the implementation of smaller-scale, sustainable agricultural efforts can be made to ensure the long-term health and productivity of our soils.

Evaluation of connection

Examining how geology affects soil formation and composition can help us understand the complexities of soil, and determine how to manage it sustainably throughout time and changing climatic conditions. Parent material as well as climate, organisms, relief, and time all intertwine to form soil, providing key services that vast ecosystems and societies depend upon. Geology and agriculture are intimately connected with one another through soil, and our modern global food system could not operate without any of the essential processes that define soil formation.

References

This Earth Science resource was created by Course:EOSC311.