Course:EOSC311/2021/The Geology of Soil applied to Agriculture

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Introduction

The dirt beneath ones feet often goes unnoticed but it is key to sustaining human life on Earth. Soil is a priceless resource that provides humans with essential food and resources. Soil is a critical part of successful agriculture and is the original source of the nutrients that are needed grow crops. Throughout human history, our relationship with the soil has affected our ability to cultivate crops and has influenced our success as a civilization. The relationship between humans and food sources affirms soil as the foundation of agriculture. This page focuses on the connection between soil and agriculture and examining how soil health can effect agriculture.

Statement of Connection

Agriculture can be looked at in terms of soil, plants and food production (applied biology). Agricultural soils require the right balance of certain factors and properties and nutrients to be deemed fertile. Soil health determines crop yield, the ability to manage the motion of waterways, the nutritional content of plants grown on it, and the need to use fertilizers and pesticides. Geology influences soil formation (e.g. weathering processes), determines the nature of soil's content (e.g. nutrients, minerals, etc.), and influences other factors/properties (e.g. erosion, water retention) which can affect its fertility and productivity. Therefore, geology can be used to learn more about healthy soils to advance sustainable agricultural production by using geological resources to improve aspects of soil to prevent soil loss and maintain soil fertility.

Soil

The composition of soil. A mixture of minerals (~45%), organic matter (~5%), and empty pore space (~50%, filled with air and water).

What is soil?

Soil is on the outer layer of the earth. It is the loose surface material that covers the Earth’s surface [1]. Soil lies at the interface between earth, air, water, and life. It is a critical ecosystem service provider and supports life on Earth in a number of ways. Soil can function as a median for crop and plant growth, as a water storage or filter, as a modifier of the atmosphere, and as a habitat for organisms [2].

Soil is a product of geological processes. It is composed of a mixture of minerals (~45%), organic matter (~5%), and empty space (~50%, filled with air and water) [3]. The mineralogy of soil can vary but is dominated by clay, silt, and quartz minerals, along with minor amounts of feldspar and small fragments of rock [3].

Soil can be classified according to its distinctive layers (horizons). Soil is made up of 6 horizons [4]:

  1. O horizon, composed of organic matter
  2. A horizon, contains decayed organic matter and minerals
  3. E horizon, clay and iron leach out, sand and silt particles of quartz are left behind
  4. B horizon, clay, iron, and other elements accumulate
  5. C horizon, contains rock fragments
  6. R horizon, consists of large areas of solid rock

How is Soil Formed?

Soil is formed through the process of weathering. Weathering is the breakdown of rocks and minerals on the Earth’s surface [5]. The types of weathering that take place within a region have a major influence on soil composition and texture [6].

Mechanical weathering is the physical processes of disintegrating rocks into smaller particles with no alteration in their molecular structure. Air and water are agents of mechanical weathering and can facilitate a gradual fragmentation of rock particles to sediments which eventually become soil [5]. Chemical weathering involves chemical reactions within rocks that change minerals into different forms. Examples of chemical reactions leading to weathering are hydrolysis, carbonation, oxidation and hydration [5]. Mechanical and chemical weathering reinforce each other, mechanical weathering provides new fresh surfaces for chemical processes, and chemical weathering weakens the rock making it more susceptible to mechanical weathering. These processes create particles that will become sedimentary rock and soil [6].

Factors Affecting How Soil Forms

Soil forms through accumulation and decay of organic matter and through the mechanical and chemical weathering processes. Factors that affect soil formation include climate, parent material, topography, organisms, and time [5].

Climate

Rainfall and temperature influence the type of soils formed in an area. Different climates create diversity in soil properties. Soil forms most readily under temperate to tropical conditions (not cold) and where precipitation amounts are moderate (not dry, but not too wet) [6]. Soil is more exposed to erosion, weathering and leaching of important chemical nutrients in places of heavy rainfall [5]. Acidic soils are more prevalent in such places. Places of low rainfall are less vulnerable to leaching, but have limited downward chemical transportation and the accumulation of salts and carbonate minerals [5][6]. Soils in these regions have high alkalinity. Warmer temperatures encourage rapid weathering as plants and microbial activities flourish while colder climates encourage gradual rock weathering [6].

Parent Material

In soil, organic matter consists of plant and animal material that is in the process of decomposing. When it has fully decomposed it is called humus.

The initial stage of soil formation is the accumulation of the parent materials - the sediments or rocks in which the soils will form [7]. Soil parent material is mainly weathered rock. Parent material can be different types of bedrock and any type of unconsolidated sediments, such as glacial deposits and stream deposits [5]. Soils are residual soils if they develop on bedrock and transported soils if they develop on transported material develop on unconsolidated material [6]. Soil type depends on the parent material from which it was formed. For example, soil formed from weathered granite rocks becomes sand and soil formed from feldspar rock becomes fine clay particles [5].

Topography

Topography is the shape of the earth’s surface area [5]. Soil can develop where surface materials remain in place and are not frequently moved away by mass wasting [6]. The shape of the land surface influences the redistribution of the water received as precipitation. Surfaces that are higher - sloping or convex surfaces lose water by runoff, whereas lower surfaces - concave or depressional receive extra water [7]. Soils cannot develop where the rate of soil formation is less than the rate of erosion, so steep slopes tend to have little or no soil [6]. The steepness of a land increases its vulnerability to water and wind erosion causing the movement of rock sediments and minerals downwards to valleys. Soils in the valleys are darker in color, rich in organic matter and more fertile while soils on hilltops are eroded, less fertile and unfavorable for plant growth [5].

Organisms/Vegetation  

Vegetation helps hold parent material in place, allowing time for soil formation to occur. Plant roots bind soil particles together and increase the entry of water into the soil, reducing runoff and erosion [7]. An area with high vegetation is usually rich in humus. Humus builds up as leaves and dead plant parts are decomposed by soil micro-organisms to become nutrient rich organic matter for the soil [5]. Plants can also produce weathering agents that increase rates of chemical weathering of soil minerals.

Time

Soil can take thousands of years to develop [6]. Soil formation processes are continuous. The additions, removals, and alterations of the surface and minerals are slow or rapid, depending on climate, landscape position, and biological activity [5]. Mineral matter, organic matter, and water all have different timelines - organic matter can reside in soil for between 1-1000 years, while water can reside for tens of years to minutes.

Soil Degradation

Soil degradation is when the quality of soil declines due to natural processes or human-induced processes. Degradation can cause soil to lose certain physical, chemical, or biological qualities [8]. Soil degradation involves acidification, contamination, desertification, erosion and/or salination [9]. When all these processes occur excessively, they can lower crop productivity, limit soil biological activity and increase soil vulnerability to such a degree that the land cannot be used productively [8].

Soil degradation due to soil erosion by rain water washing soil away. The roots of the tiny weeds have been exposed.

Soil degradation commonly occurs by means of wind and water erosion. Soil erosion is a process of moving soil by water or wind - when the soil particles are detached and transported to a different location [10]. It  is the wearing down of the surface of the earth due to the action of wind, water, and gravity [11]. Under natural conditions, the rate of soil formation balances the rate of erosion [6]. However, human practices and activities into natural systems such as agriculture have upset this balance and created erosion that is much higher than the average erosion rate [6]. Soils are held in place by vegetation. With plant cover, the roots bind the soil particles together and lesson erosion. When vegetation is removed, either through cutting trees, or tilling the soil and harvesting crops soil is left bare and exposed. This bare soil is exposed to raindrops and wind. When soil is not protected, wind and water can easily erode it away, taking with them most of the natural fertility and production potential.

Application of Soil and Agriculture

Soil Fertility - Properties for Healthy Soil

Soil fertility refers to the balance of nutrients, minerals, organic matter, soil life, pH and soil structure to sustain agricultural plant growth [12]. Many properties of soil contribute to its fertility and its capability to form functions such as crop production [12]. These properties are interrelated, and no single property can be used as an indicator for soil quality. Five examples of properties that impact the health of soil and have a large influence over its capability to function are nutrients, pH, soil water, salinization, and soil organic matter [13].

Nutrients and Minerals

Complex nutrient cycles incorporate a range of physical, chemical, and biological processes to maintain the essential nutrients (e.g., N, P, C, S) required for plant growth in the soil [14]. Agriculture alters the natural cycling of nutrients in soil. Intensive cultivation and harvesting of crops can extract the soil of plant nutrients. In order to maintain soil fertility for sufficient crop yields these nutrients must be present in the soil. Most nutrients taken up by plants come to plant roots from the soil. There are 17 essential plant nutrients, three come from air and water and 14 come from the soil. The primary macronutrients are needed in the greatest quantities from the soil [14]. Secondary macronutrients are needed in smaller quantities. The micronutrients are needed in very small amounts and, if in excess, can be toxic to plants [14]. Today, farmers add numerous soil amendments to enhance soil fertility, including inorganic chemical fertilizers and organic sources of nutrients, such as manure or compost.

pH

Typically, soil pH values from 5 to 8 (mildly acidic to mildly alkaline) are optimal for plant growth, however there are certain plants species that can tolerate or prefer more acidic or basic conditions [13][14]. Soils in rainy regions tend to become more acidic over time and soils with too low a pH will have trouble growing abundant food. pH controls a wide range of processes and properties that affect soil fertility and plant growth. Soil pH reflects the acidity level in soil [14]. At low pH, essential plant macronutrients (e.g., N, P, K, Ca, Mg, and S) are less bioavailable than at higher pH values, and certain micronutrients (e.g., Fe, Mn, Zn) tend to become more soluble and potentially toxic to plants at low pH values [14]. Maintaining a narrow range in soil pH is beneficial to crop growth.

Soil Water Retention

The soil's ability to retain water is related to its structure and texture - the particle size of the soil. Soil made up of smaller particles has a larger surface area making it easier for it to hold onto water, whereas soil made up of larger particles has a smaller surface area making it less cohesive for water [13]. Additionally, water molecules hold more tightly to finer soil particles than to coarser soil particles. The finer and smaller the particles, the higher the chance that water molecules shall hold on to the soil particles, as opposed to soil that has large and coarse particles that are not cohesive. However, finer and smaller particles do not hold the maximum amount of water. Finer and smaller particles hold water too tightly in very small pores, while larger and coarser particles drain water too rapidly [13]. Therefore, an even mix of finer, coarser,  and medium-sized particles hold the maximum amount of plant available water [13]. A soil that holds more water for crops is more valuable to a farmer compared to a soil that runs out of water quickly.

Salinization

Healthy soil that is able to sustain plant growth.

Salinization occurs when dissolved salts in water rise to the soil surface and accumulate as water evaporates [13]. Salt-affected soils may be too salty to farm, so it is important to reduce excess water to control excess salt production. If irrigation water is too high in salts or applied in insufficient amounts to continually "re-rinse" the soil of salts, then salts can build up in soils preventing crops from growing [13].

Soil Organic Matter

Soil organic matter (SOM) is produced by living organisms and is returned to the soil by going through the decomposition process [15]. SOM is the organic matter component of soil, consisting of plant and animal detritus at decomposition, soil microbes, and substances that soil microbes synthesize [14]. SOM gives soil its deep black colors and nutrient rich benefits. SOM provides numerous benefits to the physical and chemical properties of soil. SOM serves as a reservoir of nutrients for crops, provides soil aggregation, increases nutrient exchange, buffers soil pH, and improves water holding capacity into soil [14].

Preventing Soil Loss from Agriculture and Maintaining Soil Fertility

Soil is a natural resource that is vitally important for growing food. Although soil is a renewable resource, it is lost faster than it is renewed. It can take hundreds or thousands of years for good fertile soil to develop [6]. Humans will continue to rely on agriculture for food and agriculture will continue to rely on soil to support plant/vegetation growth. With human populations continuing to grow, it is extremely important to protect soil resources.

Industrial agricultural practices such as large-scale farming, greater usage of synthetic fertilizers to provide nutrients, planting of monocultures, etc. have harmful effects such as soil depletion, soil infertility, and soil erosion and cause an increase in soil loss [16].

Rates of soil loss from agriculture can be decreased with the adoption of better farming practices. More ecologically sustainable agricultural practices to help to keep soil more fertile and ensure the long-term health and productivity of our soils include [16]:

  • Rotating crops - planting of different types of crops in different parts of the field and at different seasons in a sequential manner to increase soil fertility and soil nutrients
  • Planting nutrient-rich cover crops to cover the soil and reduce erosion and improve soil
  • Planting trees as windbreaks to buffer the effects of wind
  • Low-tillage farming to keep soil in place by disturbing the ground as little as possible
  • Contour farming to conserve rainwater and to reduce soil losses from surface erosion
  • Matting the soil with biodegradable materials - can decompose or decay and turn to soil fertilizer
  • Applying Mulches - acts as a protective covering for plants against extreme weathers, allows water to reach the soil slowly, and thus reduces the impact of rainfall
  • Adding organic material to the soil in the form of plant or animal waste, such as compost or manure, increases the fertility of the soil and improves its ability to hold on to water and nutrients

Evaluation of Connection

Soil can be seen as the foundation of agriculture since it has affected our ability to cultivate crops and has influenced our success as a civilization. Soil has provided humans with the ability to produce food, through agriculture, in order to survive. Geology and agriculture are connected through soil. Processes and factors such as weathering, soil degradation, erosion, parent material, organic matter, etc. all demonstrate the complex and intimate relationship geology has with agricultural soils and the vital role that this relationship plays in our agriculturally dependent society. By looking at how geology affects soil formation, soil composition, soil fertility, and soil loss we can better understand how geology plays an important role in agriculture. This can also help us in understanding the importance of healthy soils and sustainable agriculture. Understanding, evaluating, and balancing detrimental and beneficial agricultural disturbances of soil resources are essential to sustain and improve human well-being.

References

  1. ↑ "What is soil?". Agriculture Victoria. 2021.
  2. ↑ Stack, Lois (2016). "Soil and Plant Nutrition: A Gardener's Perspective". University of Maine.
  3. ↑ 3.0 3.1 Panchuk, Karla. "8.5 Weathering and Soil Formation". University of Saskatchewan.
  4. ↑ "Appendix 1: Soil horizon designations". Food and Agriculture Organization of the United Nations.
  5. ↑ 5.00 5.01 5.02 5.03 5.04 5.05 5.06 5.07 5.08 5.09 5.10 5.11 Efretuei, Arit (2016). "Soil Formation". The Permaculture Research Institute.
  6. ↑ 6.00 6.01 6.02 6.03 6.04 6.05 6.06 6.07 6.08 6.09 6.10 6.11 Earle, Steven. "5.4 Weathering and the Formation of Soil". BCcampus Open Publishing.
  7. ↑ 7.0 7.1 7.2 "Factors". Soils of Canada.
  8. ↑ 8.0 8.1 Begum, Tammana (2021). "Soil degradation: the problems and how to fix them". Natural History Museum.
  9. ↑ Hall, R.; Miller, F. (1989). "Soil degradation: I. Basic processes". Land Degradation & Development. 1: 51–69.
  10. ↑ Ritter, Jim (2018). "Soil Erosion – Causes and Effects". Ministry of Agriculture, Food and Rural Affairs Ontario.
  11. ↑ "Soil management". Agriculture and Agri-Food Canada (AAFC) Government of Canada. 2020.
  12. ↑ 12.0 12.1 National Research Council (1993). Soil and Water Quality. The National Academies Press. pp. Chapter 5.
  13. ↑ 13.0 13.1 13.2 13.3 13.4 13.5 13.6 "Soil Properties and Human Responses to Boost Food Production". Science Education Resource Center. 2018.
  14. ↑ 14.0 14.1 14.2 14.3 14.4 14.5 14.6 14.7 Parikh, Sanjai; James, Bruce (2012). "Soil: The Foundation of Agriculture". Nature Education Knowledge. 3.
  15. ↑ Bot, Alexandra; Benites, José (2005). The importance of soil organic matter. Food and Agriculture Organization of the United Nations. pp. Chapter 1. Introduction.
  16. ↑ 16.0 16.1 "Module 4: Weathering and Soil Formation - Avoiding Soil Loss". Lumen Learning.


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