Human society has developed through utilization of our planet's resources in amazingly unique, creative, and productive ways that have furthered human evolution and sustained global societies. Of these resources, soil and water have provided humans with the ability to produce food, through agriculture, for our sustenance. In exploring the link between soil and agriculture, this article will highlight 1) our transition from hunter-gatherer to agrarian societies; 2) the major soil properties that contribute to fertile soils; 3) the impacts of intensive agriculture on soil degradation; and 4) the basic concepts of sustainable agriculture and soil management. These topics will be discussed to demonstrate the vital role that soils play in our agriculturally-dependent society.

Agriculture and Human Society

Human use and management of soil and water resources have shaped the development, persistence, decline, and regeneration of human civilizations that are sustained by agriculture (Harlan 1992, Hillel 1992). Soil and water are essential natural resources for our domesticated animal- and plant-based food production systems. Although of fundamental importance today, agriculture is a relatively recent human innovation that spread rapidly across the globe only 10,000 to 12,000 years ago (Diamond 1999, Montgomery 2007, Price & Gebauer 1995, Smith 1995), during the Agricultural Revolution. This short, yet highly significant period of time, represents less than 0.3 % of the more than four million years of human evolution as bipedal hominids and ultimately Homo sapiens. In agriculturally-based societies during the last ten millennia, humans have developed complex, urban civilizations that have cycled through periods of increasing complexity, awe-inspiring intellectual achievement, persistence for millennia, and, in some instances, perplexing decline (Trigger 2003). In many cases, stressed, declining civilizations adapted, or reemerged, into new or similar complex cultures (Schwartz & Nichols 2006). Through such fluctuations, we have remained dependent on a relatively small number of crop and animal species for food, and on integrated soil-water systems that are essential for their production. There is no doubt that our modern human society has developed to the point that we cannot exist without agriculture.

It is clear that agriculture sustains and defines our modern lives, but it is often disruptive of natural ecosystems. This is especially true for plant communities, animal populations, soil systems, and water resources. Understanding, evaluating, and balancing detrimental and beneficial agricultural disturbances of soil and water resources are essential tasks in human efforts to sustain and improve human well-being. Such knowledge influences our emerging ethics of sustainability and responsibility to human populations and ecosystems of the future.

Although agriculture is essential for human food and the stability of complex societies, almost all of our evolution has taken place in small, mobile, kin-based social groups, such as bands and tribes (Diamond 1999, Johanson & Edgar 2006). Before we became sedentary people dependent on agriculture, we were largely dependent on wild plant and animal foods, without managing soil and water resources for food production. Our social evolution has accelerated since the Agricultural Revolution and taken place synergistically with human biological evolution, as we have become dependent on domesticated plants and animals grown purposefully in highly managed, soil-water systems.

Soil Fertility and Crop Growth

The early use of fire to flush out wild game and to clear forested land provided the first major anthropogenic influence on the environment. By burning native vegetation, early humans were able to gain access to herbivores grazing on the savanna and in nearby woodlands, and to suppress the growth of less desirable plant species for those easier to forage and eat (Pyne 2001, Wrangham 2009). These and other factors (e.g., population pressures, climate change, encouraging/protecting desirable plants), help to lay the groundwork for the Agricultural Revolution and caused a dramatic shift in the interactions between humans and the earth. The shift from hunter-gatherer societies to an agrarian way of life drastically changed the course of human history and irreversibly altered natural nutrient cycling within soils. When humans sowed the first crop seeds at the dawn of the Neolithic Period, the soil provided plant-essential nutrients and served as the foundation for human agriculture.

Throughout Earth's history, natural cycling of nutrients has occurred from the soil to plants and animals, and then back to the soil, primarily through decomposition of biomass. This cycling helps to maintain the essential nutrients required for plant growth in the soil. Complex nutrient cycles incorporate a range of physical, chemical, and — most importantly — biological processes to trace the fate of specific plant nutrients (e.g., N, P, C, S) in the environment. For a thorough analysis of these cycles, additional reference materials are available (Bernhard 2010, Brady & Weil 2008, Troeh & Thompson 1993). For the purpose of this article, a simplified version of nutrient cycling in natural and agricultural systems is shown in Figure 1.

It is generally accepted that there are 17 essential elements required for plant growth (Troeh & Thompson 1993). The lack of any one of these essential nutrients, listed in Table 1, can result in a severe limitation of crop yield — an example of the principle of limiting factors. Of the mineral elements, the primary macronutrients (N, P, and K) are needed in the greatest quantities from the soil and are the plant nutrients most likely to be in short supply in agricultural soils. Secondary macronutrients are needed in smaller quantities, are typically in sufficient quantities in soil, and therefore are not often limiting for crop growth. The micronutrients, or sometimes called trace nutrients, are needed in very small amounts and, if in excess, can be toxic to plants. Silicon (Si) and sodium (Na) are sometimes considered to be essential plant nutrients, but due to their ubiquitous presence in soils they are never in short supply (Epstein 1994, Subbarao et al. 2003).

Essential plant element		Symbol	Primary form
Non-Mineral Elements
	Carbon	C	CO2 (g)
	Hydrogen	H	H2O (l), H+
	Oxygen	O	H2O (l), O2(g)
Mineral Elements
Primary Macronutrients	Nitrogen	N	NH4+, NO­3-
	Phosphorus	P	HPO42-, H2PO4-
	Potassium	K	K+
Secondary Macronutrients	Calcium	Ca	Ca2+
	Magnesium	Mg	Mg2+
	Sulfur	S	SO42-
Micronutrients	Iron	Fe	Fe3+, Fe2+
	Manganese	Mn	Mn2+
	Zinc	Zn	Zn2+
	Copper	Cu	Cu2+
	Boron	B	B(OH)3
	Molybdenum	Mo	MoO42-
	Chlorine	Cl	Cl-
	Nickel	Ni	Ni2+
Table 1: Essential plant nutrient elements and their primary form utilized by plants.

Agriculture alters the natural cycling of nutrients in soil. Intensive cultivation and harvesting of crops for human or animal consumption can effectively mine the soil of plant nutrients. In order to maintain soil fertility for sufficient crop yields, soil amendments are typically required. Early humans soon learned to amend their fields with animal manure, charcoal, ash, and lime (CaCO₃) to improve soil fertility. Today, farmers add numerous soil amendments to enhance soil fertility, including inorganic chemical fertilizers and organic sources of nutrients, such as manure or compost, often resulting in surplus quantities of primary macronutrients. The efficiency of fertilizer application and use by crops is not always optimized, and excess nutrients, especially N and P, can be transported via surface runoff or leaching from agricultural fields and pollute surface- and groundwater (Moss 2008, Sharpley et al. 2002).

While soil is frequently referred to as the "fertile substrate", not all soils are suitable for growing crops. Ideal soils for agriculture are balanced in contributions from mineral components (sand: 0.05–2 mm, silt: 0.002–0.05 mm, clay: <0.002 mm), soil organic matter (SOM), air, and water. The balanced contributions of these components allow for water retention and drainage, oxygen in the root zone, nutrients to facilitate crop growth; and they provide physical support for plants. The distribution of these soil components in a particular soil is influenced by the five factors of soil formation: parent material, time, climate, organisms, and topography (Jenny 1941). Each one of these factors plays a direct and overlapping role in influencing the suitability of a soil for agriculture.

As one might expect, contributions from each mineral size fraction help to provide the physical framework for a productive soil. Loamy-textured soils are commonly described as medium textured with functionally-equal contributions of sand, silt, and clay. These medium-textured soils are often considered ideal for agriculture as they are easily cultivated by farmers and can be highly productive for crop growth.

The mineral components of soil may exist as discrete particles, but are more commonly associated with one another in larger aggregates that provide structure to soil. These aggregates, or peds, play an important role in influencing the movement of water and air through soil. Sandy soils have large pore spaces and increase water drainage, but do not provide soils with many nutrients. Clay-rich soils, on the other hand, increase water holding capacity and provide many plant essential nutrients. A common measure of soil fertility is obtained by measuring the cation exchange capacity (CEC). The CEC is a measure of a soil's ability to exchange positive ions between the soil particles and solution surrounding these particles.

Due to their high surface area, clay particles can exert a large influence on various soil properties (e.g., CEC, structure, water-holding capacity), even when the percent clay content is low. Clay minerals are colloidal particles, having high surface area, with charged surfaces; permitting binding of many essential plant nutrients. The most prevalent clay-sized particles in soils fall into the class of layer-type aluminosilicates (Sposito 2008) that commonly have permanent negative charge with a high CEC. Positively charged clay particles, which bind anions, include those which have pH dependent charge. The most common classes of these minerals in soils are the iron (Fe), aluminum (Al), and manganese (Mn) (hydr)oxides (Schulze 1989).

SOM comprises the partial or well-decomposed residues of organic biomass present in soil. SOM gives topsoil its deep black colors and rich aromas that many home gardeners and farmers of grassland soils are familiar with. Surface soils are composed of approximately 1 to 6% organic matter, with SOM decreasing with depth (Brady & Weil 2002). The ‘Great Plains' of North America and the ‘Bread Basket' of Europe are some of the world's most productive agricultural soils because they developed under grassland vegetation, whose root biomass and decomposition resulted in SOM accumulation. Figure 2 is a photograph of an organic matter-rich soil (mollisol) formed under prairie vegetation in the United States. The thick dark upper layers in this soil reflect the high SOM content. The presence of SOM is crucial for fertile soil as it provides essential plant nutrients, beneficially influences soil structure, buffers soil pH, and improves water holding capacity and aeration. The presence of organic, ionizable functional groups (e.g., carboxyl, alcoholic/phenolic OH, enol, quinone, and amine) impart charge to SOM (Sparks 1995), contributing high CEC, and pH buffering capacity.

Often referred to as the master variable of soil, pH controls a wide range of physical, chemical, and biological processes and properties that affect soil fertility and plant growth. Soil pH, which reflects the acidity level in soil, significantly influences the availability of plant nutrients, microbial activity, and even the stability of soil aggregates. At low pH, essential plant macronutrients (i.e., N, P, K, Ca, Mg, and S) are less bioavailable than at higher pH values near 7, and certain micronutrients (i.e., Fe, Mn, Zn) tend to become more soluble and potentially toxic to plants at low pH values (5–6) (Brady & Weil 2008). Aluminum toxicity is also a common problem for crop growth at low pH (<5.5). Typically, soil pH values from 6 to 7.5 are optimal for plant growth; however, there are certain plants species that can tolerate — or even prefer — more acidic or basic conditions. Maintaining a narrow range in soil pH is beneficial to crop growth. SOM and clay minerals help to buffer soils to maintain a pH range optimal for plant growth (Havlin et al. 2005). In instances where the pH is outside a desirable range, the soil pH can be altered through amendments such as lime to raise the pH. Ammonium sulfate, iron sulfate, or elemental sulfur can be added to soil to lower pH.

Soil Degradation and Crop Production

Soil forms from fresh parent material through various chemical and physical weathering processes and SOM is incorporated into soil through decomposition of plant residues and other biomass. Although these natural soil building processes regenerate the soil, the rate of soil formation is very slow. For this reason, soil should be considered a nonrenewable resource to be conserved with care for generations to come. The rate of soil formation is hard to determine and highly variable, based on the five factors of soil formation. Scientists have calculated that 0.025 to 0.125 mm of soil is produced each year from natural soil forming processes (Lal 1984, Montgomery 2007, Pimentel et al. 1987, Wakatsuki & Rasyidin 1992). Because of the time required to generate new soil, it is imperative that agricultural practices utilize best management practices (BMPs) to prevent soil erosion. The soil which is first eroded is typically the organic and nutrient enriched surface layer which is highly beneficial for plant growth. Thus, the primary on-site outcome is reduced crop yield as only the less fertile subsurface layers remain. Soil erosion also pollutes adjacent streams and waterways with sediment, nutrients, and agrochemicals creating serious off-site impacts.

Historically, conventional agriculture has accelerated soil erosion to rates that exceed that of soil formation (Table 2). Erosion is often accelerated by agricultural practices that leave the soil without adequate plant cover and therefore exposed to raindrop splash and surface runoff or wind (Singer & Munns 2006). Throughout human history, soil erosion has affected the ability of societies to produce an adequate food supply. Poignant examples of this can be seen in the eroded silt built up in the ancient riverbeds of Mesopotamia, making irrigation problematic (Hillel 1992), and the United States Dust Bowl of the 1930s where a devastating drought increased wind erosion, carrying fertile topsoil from the Midwest hundreds of kilometers to Washington, DC (Montgomery 2007). Figure 3 is a stunning photograph demonstrating the devastating effects of this severe wind erosion. The Dust Bowl made soil erosion a high priority in the American public consciousness of the 1930s, and it remains a top priority today.

Measurement type	Sample size n	Mean rate of soil erosion* (mm/yr)	Net rate of soil formation† (mm/yr)
Conventional Agriculture	448	1.54 (0.32)‡	-1.52 to -1.42
Conservation Agriculture	47	0.082 (0.022)	-0.057 to +0.043
Native Vegetation	65	0.013 (0.016)	+0.012 to +0.112
Geological	925	0.029 (0.029)	-4.00 x 10-3 to +0.096
* Literature values (Montgomery 2007). † Net soil formation estimated using literature values for the rate of soil formation of 0.025 to 0.125 mm/yr (Lal 1984, Montgomery 2007, Pimentel et al. 1987, Wakatsuki & Rasyidin 1992) ‡ Parenthesis represent standard error of the mean.
Table 2: Rates of soil erosion and net formation for various land use classes.

Today, agricultural fields are not immune to the forces of nature (e.g., moving water, blowing wind, extremes of temperature) that caused soil erosion in the past. Figure 4 shows the severe effects of surface runoff and soil loss in the northwestern United States. Implementation of agricultural best management practices (BMPs), and through the practice of conservation agriculture, the rate of soil loss can be reduced to approximately equal the rate of soil formation, although often still greater than that in natural systems (Table 2). In addition to soil erosion, intensive land use has resulted in deforestation, water shortages, and rapidly increasing desertification of vast areas of the globe, all of which threaten the sustainability of our agricultural systems.

Sustainable Soil Management

It is evident that, in order to maintain and increase food production, efforts to prevent soil degradation must become a top priority of our global society. Current population models predict a global population of between 8 and 10 billion in the next 50 years (Bongaarts 2009, Lutz et al. 2001) and a two-fold increase in food demand (Alexandratos 1999, Tilman et al. 2002). If mismanagement of soil resources continues to diminish the fertility of the soil and the amount of productive arable land (Pimentel et al. 1995), then we will have lost a precious and essential pillar of sustainable agriculture (Tilman 1999). Sustainable agriculture is an approach to farming that focuses on production of food in a manner that can be maintained with minimal degradation of ecosystems and natural resources. This sustainable approach to agriculture strives to protect environmental resources, including soil, and provide economic profitability while maintaining social equity (Brodt et al. 2011). The concept of sustainable agriculture is often misinterpreted to mean that chemical fertilizers and pesticides should never be used. This notion is incorrect, as sustainable agriculture should embrace those practices that provide the most beneficial services for agroecosystems and encourage long-term production of food supplies in a cultural context of the region. It cannot be overstressed that sustainable practices should not only consider crop production and profit, but must include land management strategies that reduce soil erosion and protect water resources. By embracing certain modern-day technologies, proven BMPs, and learning from the past, our society will be able to continue to conserve soil resources and produce food supplies sufficient to meet current and future population demands.

Agricultural Revolution: The shift from hunter-gatherer to agrarian societies occurring 10,000 to 12,000 YBP. The Agricultural Revolution is a key component of the Neolithic Revolution.

aluminosilicate: Class of clay minerals found in soils which are primarily comprised of silicon, aluminum and oxygen that are assembled into sheets of silica tetrahedral and aluminum octahedral.

cation exchange capacity: Operationally defined measurement of a soil's ability to exchange positive ions between the soil particles (e.g., clay, organic matter) and solution surrounding these particles.

chemical weathering: Process by which rocks, soil, and minerals are dissolved or broken down via a range of chemical processes including carbonation, hydrolysis, hydration, and redox reactions.

colloidal: Referring to very small (approximately 1 nm to 1 µm) inorganic or organic particles which tend to remain suspended in solution. These particles are ubiquitous in soil and, due to their high surface area, are highly reactive.

conservation agriculture: An approach to farming which minimizes degradation and/or loss of natural resources while providing sufficient crop yield and economic benefit.

deforestation: The cutting and clearing of forests and other vegetation.

desertification: Conversion and degradation of land which previously supported to plant growth, in arid or semi-arid regions, to desert land. This often occurs as a result of drought, deforestation, or other human induced land use changes.

erosion: The removal of soil from the land's surface by water, wind, ice, or gravity.

essential element (plant): Chemical elements required by plants for normal growth and reproduction.

lime: Solid material which contains carbonates, oxides, and/or hydroxides of calcium which is applied to agricultural fields to increase the soil pH (alkalinity).

mollisol: Soil order in USDA soil taxonomy, characterized by a thick organic matter enriched surface horizon, typically between 60–80 cm thick, which are commonly formed under grassland vegetation.

parent material: The geologic and organic material from which soil is formed through a variety pedogenic processes.

ped: Single unit of soil which is aggregated into granular, platy, blocky, prismatic, or columnar structure.

pH (soil): The negative log of the hydrogen ion concentration in a soil solution which gives a measure of a soils acidity or basicity.

physical weathering: Process by which rocks, soil, and minerals a broken down into smaller particles through physical processes such as heat, water, ice, and pressure.

soil organic matter: The organic components of soil which are comprised of living microbial biomass, fresh and partially decomposed biomass (plant and animal), and the well decomposed and stable biomass fraction (humus).

soil structure: The aggregation, or secondary shape, of soil particles which adhere together into structural units (peds).