Along with water, sunlight, and the earth's atmosphere, the soil is one of the key resources underlying food production by humans. In terms of the coupled human-natural systems we use as a way to understand food systems, we can say that human systems organize landscapes and manage soils, along with agricultural biodiversity and other parts of natural systems, to produce food. Soils exert an influence on this coupled system because they vary in terms of properties such as depth and nutrient content, which alters their response to human management. Soils also have great importance as the site of many nutrient and carbon transformations within the biosphere. They are a storehouse of beneficial soil organic matter that benefits the earth system in many ways. Also, by understanding soils and the earth's surface and ecological processes that occur there, human management is able to maintain and improve them, as well as overcome initial limitations or past degradation.
The purpose of this module is to give you as a learner a basic grounding in the nature of soils and soil nutrients. Module 5.1 provides the foundation for understanding soils, soil nutrients, and their connection to food. We will also focus on ways that soils are vulnerable to degradation that impairs their role in food production. In module 5.2 we will deepen our understanding of how soil management can protect soils in their role of supplying nutrients to crops and protecting other valuable resources such as surface water. To accomplish this we will focus on nitrogen (N) and phosphorus (P) as key nutrients for food production in module 5.2.
Detailed instructions for completing the Summative Assessment will be provided in each module.
Action | Assignment | Location |
---|---|---|
To Read |
|
|
To Do |
|
|
If you have any questions, please send them through Canvas e-mail. We will check daily to respond. If your question is one that is relevant to the entire class, we may respond to the entire class rather than individually.
If you have any questions, please post them to the discussion forum in Canvas. We will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.
In this course, we describe food systems as a coupling between human societies and natural earth systems and environments. This coupling is especially clear in the activities of food production that rely on crop and livestock raising. Crops and livestock production (and to a similar extent, fisheries, and aquaculture) require food producers to bring together human management with soil conditions and soil nutrients (this module), water (next module), as well as sunlight for energy and adequate climate conditions (temperature, humidity, adequate growing season). To understand these human-natural interactions across the entire course, and to build your capacity to understand natural factors as part of your capstone projects and other chapters of your education, this module describes basic soil properties and the role of soils in creating adequate conditions for crops to grow, which underlies most aspects of food production. It’s therefore very important that we understand soils as the “living skin of the earth” in their properties and history, the global patterns of soil fertility and soil limitations, and then its role in supplying nutrients to plants, and how soil fertility is regenerated by the human societies and management knowledge that allows them to continue supporting food production. Our goal is not to condense an entire course in soil science, although we hope that many of you will go on to take such a course. Rather, we want to sketch out major factors and determinants of the opportunities and limitations posed by soils to a human food production system.
We may be used to referring to soil as “dirt”, as in “my keys fell in the dirt somewhere” or “after planting the garden we had dirt all over our hands” but the way in which soil supports food production far more complex than a smear of clay on our hands. One way to define this difference in perspective is to think about the biological and chemical complexity in soil, and the fact that soils are not just brown, powdery handfuls of dirt but occupy a grand scale in the natural systems that underlie food systems. Soil is the "skin of the earth", layers that ascend from bedrock and supply water and nutrients to the fields and forests that make up the terrestrial biosphere. Soils are ecosystems in their own right, within mineral layers that form part of the earth’s surface. Soils can be as shallow as ten centimeters and as deep as many tens of meters.
An interesting exercise is to think of a single term or concept that describes how soils work and what they are. For example, if we were seeking an acronym to describe soil and market it as the marvelous thing that it is1 —and if we lacked time to think of a catchier name – we might think up the acronym “PaBAMOM” which nevertheless is a pretty good summary of what soil is: a “Porous and Biologically Active Mineral-Organic Matrix”. It’s a good summary because it defines the unique properties of soils (see Figure 5.1.1 below):
So, soil is not dirt. It is porous and complex, it covers almost every land surface on the planet (ice caps, glaciers, and bare rock are exceptions), and it is a ubiquitous, critical resource that is heavily coupled to human societies for their food production and in need of protection. It’s not dirt, it’s a PaBAMOM!
1. We don’t have to do this marketing job (phew!) because the existence and value of soils are so often taken for granted. Recently, economists have been working on estimating the implicit worth of the services performed for society by a single hectare (100 m x 100 m) of soil, and the amounts can range into tens of thousands of dollars per year depending on soils’ properties and the way they are used.
Before examining other basic soil functions, it is helpful and will avoid possible confusion, to understand the basics of how soils support the needs of crops, which in turn support the food needs of humans and their livestock. Firstly, soils provide a physical means of support and attachment for crops – analogous to the foundation of a house. Second, most water used by plants is drawn up through roots from the pores in soils that provide vital buffering of the water supply that arrives at crops either from rainstorms or applied as irrigation by humans. Third, as crops grow and build their many parts by photosynthesizing carbon out of the air (see module 6, next, for more on this) they gain most of the mineral nutrients they need (chemical elements) they need2 from soils, for example by taking up potassium or calcium that started out as part of primary minerals in earth’s crust, or nitrogen in organic matter that came originally from fertilizer or the earth’s atmosphere. The adaptation of crop plants domesticated by human farmers (and other plants) to soils, and the adaptation of the soil ecosystem to plants as their primary source of food mean that soils usually fulfill these roles admirably well.
2 The elements needed by plants other than Carbon (from the air) and Hydrogen/Oxygen (from water) in rough order of concentration are Potassium, Nitrogen, Phosphorus, Calcium, Magnesium, Sulfur, Iron, Manganese, Zinc, Boron, Copper, Molybdenum, and Cobalt (for some plants). Other elements are taken up into plants in a passive way without being essential, such as Sodium, Silicon, or Arsenic.
Soils around the world have different properties that affect their ability to supply nutrients and water to support food production, and these differences result from different factors that vary from place to place. For example, the age of a soil -- the time over which rainfall, plants, and microbes have been able to alter rocks in the earth's crust via weathering-- varies greatly, from just a few years where soil has been recently deposited by glaciers or rivers to millions of years in the Amazon or Congo River Basins. A soil's age plus the type of rock it is made from gives it different properties as a key resource for food systems. Knowing some basics of soil formation helps us to understand the soil resources that farmers use when they engage in food production. Below are some of the most important factors that contribute to creating a soil:
These four factors along with the vegetation, microbes, and animals at a site, create different types of soils the world over. A basic global mapping of these soil types is given below in Fig. 5.1.2 We've attached some soil taxonomic names (for soil orders, categories used by soil taxonomists) to these basic soil types for those who are familiar with some of the terminology of soil classification. We should emphasize that understanding these orders is not essential to your understanding of food production and food systems, as long as you understand how the basic processes of soil formation described above, and the properties of soils described on the next page, contribute to the overall productivity of a soil. You should think about how the soil formation processes affect crop production in your capstone regions of your final project, and you should be able to find resources on how soils were formed in any place in the United States and around the world.
Another important point is that soil formation processes described above largely determine only the initial state of a soil as this passes into human management as part of a coupled human-natural food system. Human management can have equally large effects as soil formation on productivity, either upgrading productivity or destroying it. The best management protects the soil from erosion, replenishes its nutrients and organic matter, and in some ways continues the process of soil formation in a positive way. We'll describe these best practices as part of a systems approach to soil management in module 7. Inadequate human management can be said to "mine" the soil, only subtracting and never re-adding nutrients, and allowing rainfall and wind to carry away layers of topsoil.
The next page adds to this description of soil formation by focusing in on the basic properties that affect food production on soils, like acidity and pH which is discussed above.
In growing crops for food, farmers around the world deal with local soil properties that we started to describe on the previous page. These properties can either be a positive resource for crop production or limitations that are confronted using management methods carried out by farmers. The first of these, a soil's nutrient status, is described in more detail in module 5.2. Regarding nutrients is only important to emphasize here that most nutrients taken up by plants (other than CO2 gas) come to plant roots from the soil, and that the supply of these nutrients often has to do with the amount of dead plant remains, manure, or other organic matter that is returned to the soil by farmers, as well as fertilizers that are put into soils to directly boost crop growth. Here are the other major soil properties that farmers pay attention to in order to sustain the production of food and forage crops:
Most crops prefer soils that have a pH between 5 and 8, mildly acidic to mildly alkaline (to understand these pH figures, remember that water solutions can be either acidic or basic (alkaline), and that pH 7 is neutral, vinegar has a pH of about 2.5, and baking soda in water creates a pH of about 8). As discussed above under the climate and parent material sections describing soil formation, soils in rainy regions tend to become more acidic over time.& Soils with too low a pH will have trouble growing abundant food or feed for animals. Farmers manage soils with low pH by adding ground up limestone (lime) and other basic (that is, acid-neutralizing) materials like wood ash to their soils. As an alternative, farmers sometimes adapt to soil pH by choosing or even creating crops or crop varieties that have adapted to low pH, acidic soils. For example, potatoes do well in high elevation, acidic soils of the Andes and other areas around the world. Alfalfa for livestock does better in neutral and alkaline soils while clovers for animal food grow better in more acidic soils.
Module 4 described the importance of water for food production and the way that humans go to great lengths to provide irrigation water to crops in some regions. Soil properties also play a role in the amount of water that can be stored in soils (for days to weeks) that is then available to crops. A soil that holds more water for crops is more valuable to a farmer compared to a soil that runs out of water quickly. Among the properties that create water storage in soils is soil depth or thickness, where a deep soil is basically a larger water tank for plant roots to access than a thin soil. The proportions of fine particles (clay) versus coarse particles (sand) in a soil, called soil texture, also influence the water available to plants: Neither pure clay nor pure sand hold much plant-available water because clay holds the water too tightly in very small pores (less than 1 micron or 0.001 mm, or smaller than most bacteria) while sand drains too rapidly because of its large pores and leaves very little water. Therefore an even mix of sand, clay, and medium-sized silt particles hold the maximum amount of plant-available water. This soil type is known as loamy, which for many farmers is synonymous with “productive”. In addition to these soil properties, farmers try to maintain good soil structure (also called "tilth"), which is the aggregation of soil particles into crumb-like structures, that help to further increase the ability of soils to retain water. Soil aggregation or structure, and its multiple benefits for food production are further described in Module 7 on soil quality.
Clayey soils, and soils that have been compacted by livestock or farm machinery ("tight" vs. "loose" soils), can also have problems allowing enough water to drain through them (poor drainage), which can lead to an oversupply of water and a shortage of air in soil pores (refer back to figure 5.1.1 and the roughly equal proportion of air and water in pores of an agricultural soil). Too much water and too little air in a soil lead to low oxygen in the soil and an inability for roots and soil microbes to function in providing nutrients and water to plants. Part of good tilth, described above, is maintaining a loose structure of the soil.
In the face of these important soil properties for water storage, farmers seek out appropriate soils with sufficient moisture (e.g. deep and loamy, see Figs. 5.1.3 and 5.1.4) but also adequate drainage. Food producers also modify and maintain the moisture conditions of soils, through irrigation but also through maintaining good soil aggregation or tilth (see modules 5.2 and 7), and by avoiding compaction of soils that also leads to poor drainage and soils that are effectively shallower because roots cannot reach down through compacted soils to reach deeper water.
Dry climate soils have less rainfall to leach them of minerals. They can, therefore, be high in nutrients, but also carry risks of harmful salts building up as rainfall does not carry these away either. Salt-affected soils may either be too salty to farm at all or may carry a risk that 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 until crops will not grow. The way that arid soils are managed is a key part of the human knowledge of food production in dry regions.
Soil slope and relief are described on the previous page as creating higher risks of erosion (Fig. 5.1.5). To address this limitation food producers have either (a) not farmed vulnerable sloped land with annual crops, leaving them in the forest, tree crops, and year-round grass cover and other vegetation that holds soils on slopes; (b) built terraces and patterned their crops and field divisions along the contours of fields (Fig. 5.1.6). Terracing and terraced landscapes can be seen from Peru to Southeast Asia to Greece and Rwanda. Nevertheless, while sloped soils have been seen as the Achilles heel of environmental sustainability in mountain areas, the extreme elevation differences present in mountain areas can also be seen as a benefit to these food systems. The benefits arise because soils with very different elevation-determined climates and soil properties in close proximity, which allows for the production of a greater variety of crops. The simultaneous production in the same communities of cold- and acid soil tolerant bitter potatoes and heat-loving maize and sugar cane in lower, more neutral soils in the Peruvian Andes is an example of this benefit in high-relief mountain regions.
We hope that you are beginning to appreciate that appropriate management of soils is emphatically about integrating management principles like the ones presented here as human responses, along with an understanding of the basic properties of soils, and also the nutrient flows presented next in module 5.2. Soils are very much a complex system, and managing them for food production and environmental sustainability means that we must understand the multiple components and interactions of this system. The way in which this is accomplished has been summarized as the concept of Soil Health, which involves multiple components that are more fully addressed in module 7. Soil health is an aspiration of effective management and means that management has maintained or promoted properties like nutrient availability, beneficial physical structure, and diversity of functionally important and 'health-promoting' microbes and fauna in soils along with sufficient organic matter to feed the soil ecosystem. These integrated properties then allow production to avoid soil degradation, produce sufficient amount of food and livelihoods, and preserve biodiversity in soils as well as other significant ecosystem services like buffering of river flows and storage of carbon from the atmosphere.
3 This is not always true; Molybdenum, Sulfur, Boron and other micronutrients are sometimes found to limit plants, but the complexity of analyzing these is beyond the scope of this survey course.
Soil scientists have done an enormous amount of work in mapping the patterns of soil at a global level. The most current and detailed effort comes out of mapping work from the Food and Agriculture Organization of the United Nations, now an independent agency that is known as the International Soil Resource Information Centre (ISRIC), and is based on classifying a set of diagnostic types of topsoil layers that occur in different climates, landscape ages, and vegetation types. The details of this system5 are beyond the scope of this course, however, and to summarize the introduction to global soil fertility in this unit we present a simplified version of the United States Department of Agriculture (USDA) system that is still in wide use by soils practitioners in the United States. The USDA system lines up very well with the ISRIC system at this simplified level and allows understanding of the broad strokes of soil nutrient geography in the way we have presented it (Figure 3.8).
This simplified map is intended to serve as a resource for your other learning in the course on how food systems may respond to the opportunities and limitations of soils, and also summarizes the learning in this module about how soils result from an interaction of parent material, time, climate, vegetation, and other factors. For example, you’ll notice that just four very broad summarized types (See section 1 of the soils key, “Dominant global soils” in Fig. 3.8) cover the vast majority of the earth’s surface, and can be organized into a rough typology of precipitation from wet to dry, along with their age and vegetation types (e.g. tropical and subtropical forests; other forest types; grasslands, and desert vegetation). Soils formed by temperate grasslands have been hugely important in recent history because once humans developed steel plows that were sufficiently strong to til prairie soils, these Mollisols could be farmed and became the breadbaskets of the modern era (e.g. the U.S. and Canadian Great Plains, the Ukraine, the Argentinian pampas). There are also small pockets of soils globally that depend strongly on their original parent material. Andisols or volcanic ash soils are an excellent example of this: although their global extent is minuscule and even invisible on our map (Fig. 3.8) at this scale, they often occur in areas with high population densities such as Ecuador, Japan, and Rwanda. The high densities of population are not an accident but occur exactly because these soils have high fertility potential and have become extremely important in these local food systems. The simplified global soils map is also a way to spatially conceptualize a number of key limiting factors in soils that food producers must face: acidic, P-retaining soils in highly weathered tropical and subtropical soils, P retention in volcanic soils, and the risk of salinization of soil in dry climate soils.
In addition, it is worth noting that the broad swaths of soil of young to moderate age and with moderate to high fertility (light green in our map) may be the dominant type of soil in the world and also includes many areas that are critical in terms of the sustainability outcomes for human-natural systems in relation to soils. Because these tend to be “medium-everything” soils (medium age, medium fertility, medium depth, medium pH, medium moisture, etc.) they do not actively dissuade human systems from occupying them with high population densities or intensity of management and production, especially as the global population increases. However these soils are often easily degraded, and so sustainable methods are especially important to guarantee future food production.
Soil taxonomy is an enormous classification system that can initially be confusing. But knowing the first level of classification can be very useful, just like knowing whether an animal is a whale or a beetle is extremely helpful compared to not knowing anything. To classify soils broadly as to their limitations and productive potential, we can use the soil orders of the USDA system (see the order names in parentheses, in Fig. 3.8).
The key below will help you to use the last few letters of a USDA soil name, along with the ISRIC world soil mapping resource to query what types of soil are present around the world or specifically in your capstone regions. The categories are the same as what is presented in Figure 3.3, and you can use the query function in the ISRIC world soil mapper to find out what USDA soil names are present in each area, and draw conclusions about the potential fertility and properties of the soils at a broad level.
First, see the ISRIC resource is at SoilGrids [6]. This was also used in the formative assessment for Module 3.1.
In the ISRIC mapper you will need to click on layers icon in upper right and set the layer to “Soil Taxonomy: TAXOUSDA” and select the “All TAXOUSDA subclasses” -- when you query the map using a right click of the mouse, you’ll get a percent breakdown of the different soil orders at that location.
Soil name ending | Meanings | Example |
---|---|---|
-Epts -Ents -Alfs |
Entisols : soils of recent deposition, no soil development.
Inceptisols: the beginning of soil formation – medium to high fertility soils Alfisols: broad class of medium age, medium to high fertility soils |
Glossoboric hapludalfs
Orthents |
-Ols | Mollisols: prairie soils, high organic matter, generally neutral pH, fertile, deep | Dystric haplustolls |
-Ids | Aridisols – dry region soils, generally high pH | Argids |
-Ods | Spodosols – coniferous forest soils with acid needle litter leaching features | Orthods |
-Ults -Oxes |
Ultisols – warm region, old, leached soils
Oxisols – oldest tropical soils formed only of weathering remnants, metal oxides |
Udults |
-Ands | Andisols- volcanic ash soils | Vitrands |
-Erts | Vertisols – highly weathered limestone, with shrink-swell clays. | Uderts |
5 Nevertheless, you may peruse this impressive global resource and the soil horizon definitions at ISRIC [7].
You will complete an activity on mapping trends in soil properties using an online soil mapping resource. The emergence of tools such as this to visualize global and national soil data easily and with full public access is revolutionizing information about soils and management constraints in different regions of the world. Please download the worksheet so that you can fill it in (either on paper or preferably just by writing in your responses in MS Word).
The two web resources you will need for this worksheet are placed here so you can access them while you fill in the worksheet.
Mainly you will need the International Soil Resource Information Centre's soil mapping resource of the world, Soil [6]Grids [6]. Click past the intro window that will appear in the center of the screen and then pan the map to the area of interest as identified in the worksheet.
This is a mapping portal that resembles google earth - you have the ability to pan, zoom in, drag the map with the cursor and mouse (Fig. 5.1.7). When you enter you should see a toolbar in the top right corner. More instructions on the portal are given on the formative assessment worksheet.
You will also need briefly, this online map showing global annual total precipitation [8].
Download the Worksheet [9] to complete your assessment.
Please submit your assignment in Module 5 Formative Assessment in Canvas.
In module 5.2, we present a basic account of nutrient cycling and nutrient management in food production systems. When we talk about nutrients in this context, we are referring to the nutrients that are needed to grow crops which are taken up from soils by the roots of crop plants. These include the important nutrients nitrogen (N) and phosphorus (P) which will form the focus of this module. We refer to N and P as "important" nutrients because they are needed in large quantities, relative to the amounts that are readily available in many soils. In agricultural and ecological terms, we say that crops and food production are especially responsive to N and P abundance: a shortage of N or P causes dramatic declines in production of food, while sufficiency and abundance will raise yields so that N and P supply have been a focus of human management to maintain food production. We will begin by talking about the way that N and P move around in cycles in all ecosystems, including the agroecosystems that are managed by humans to produce food. Human management systems in agriculture thus play a major role in altering the cycles of these nutrients in order to maintain, and in some cases increase the production and supply of food from farmland (farmed soils). This management can also negatively impact water quality in watersheds, as you saw in module four. We will also understand the way that soil organic matter (SOM) relates to these two major nutrients and soil productivity, as well as the general concept of soil depletion and soil regeneration as these relate to strategies of soil management in food production.
In module four, and in your education previous to this course, you've learned about the water cycle, in which water evaporates from bodies of water, condenses into clouds, and then is returned as rain to drain again into groundwater, lakes, and oceans. Each of the major crop nutrients, and most chemical elements on the earth's surface, has a similar cycle in which the nutrient is transported and transformed from one place to another, spending time in different 'pools', analogous to the division of water into lakes, rivers, clouds, rain, and the ocean. Just as rainwater and groundwater may be of more immediate use to crop plants than the ocean, different pools of the same nutrient differ in availability to plants. For example, most soils hold a tremendous amount of nitrogen in large organic molecules, but only the smaller soluble pool, and some smaller molecular forms of N, are directly available to plants. The way that soil nutrients move through the earth system, including within food production systems, is called nutrient cycling. The objective of this module is for you to understand the main features of nitrogen (N) and phosphorus (P) cycling in human-managed soils. Earth scientists sometimes use the term "biogeochemical cycling" to emphasize that each nutrient’s cycle represents the geological and atmospheric sources of the nutrients, the biology of organisms that often transform nutrients from one form to another, and the chemical nature and interactions of each element.
As an example of biogeochemical cycling, think of the important element carbon (C). Carbon has a chemical nature that allows it to be a fundamental molecular building block for all living things. In addition, there is an impressive atmospheric pool (a sort of geologic pool) of non-organic carbon dioxide. Interacting with this atmospheric pool, green plants and algae play a fundamental role in turning atmospheric CO2 into biological organic carbon in living things and the remains of living things, such as plants, that fall back into the soil. Scientists refer to this large set of interacting parts with geological, biological, and chemical attributes, earth's system that "processes" and recycles carbon in a certain sense, as the biogeochemical C cycle. Another example is phosphorus (P), which will be described in more detail on the following pages: The earth’s crust is the primary source of all P, which is then weathered by geological and biological processes and also in human fertilizer factories, held or retained strongly by soil clay minerals after application by farmers, and eventually occupies a key role in every living thing as one of the elements within the DNA molecules encoding our genes. It’s essential to realize that humanity and human systems are now major players within these nutrient cycles including C, P, and nitrogen. We can see this in activities such as mining (and eventually threatened depletion) of phosphorus sources for fertilizers or fixing of large amounts of nitrogen for fertilizers with a massive expenditure of energy and emission of carbon dioxide through the use of oil and gas.
The proper management of soil nutrients in soils for human food production boils down to a simple requirement: the need to replace nutrients that are "subtracted" from soil during production. These subtractions occur as nutrients are taken up by crops from the soil and then exported as food products in crops and livestock. Nutrients can also be lost to soil erosion and in dissolved forms, by drainage of water from the soil (called leaching). The goal of incorporating manure, plant material, and chemical fertilizers by farmers is to add back these subtracted nutrients. In the case of soil erosion, the idea is to avoid such losses completely by protecting soils. Human-managed fields and farms can be compared to nutrient bank accounts, where withdrawals must be balanced by deposits, and where it is better to have a substantial balance than a minuscule balance. Natural systems like forests or prairies lose some nutrients as does a farm field, but to a comparatively minor degree (fig 5.2.1 below). The need for humans to replenish nutrients is much greater in any managed system like a crop field or pasture than in unmanaged forests or grasslands. This is especially true in intensive production systems of crops or animal forages, for example, the corn, vegetable, and hay fields and pastured rangelands that are typical in agriculture of the United States and around the world. In systems where soils are tilled to grow annual crops on hillsides, the combined exported nutrients in food and those lost to erosion can quickly rob a soil of most of its nutrients. Protecting a soil from these losses, and regenerating the nutrients lost by adding crop residues (straw, cornstalks, other stems, and roots), manure, and fertilizer materials (ash, phosphate rock, bone, chemical fertilizers) are therefore important strategies used by food producers to sustain production. We’ll devote more focus to the important role of crop species, crop rotations, tillage, and soil erosion as part of agroecosystems in modules 6 and 7. For now, we want to understand the basics of these principles of soil regeneration.
In addition to individual nutrients like N, P, potassium (K) and calcium, an overarching aspect of soil depletion and regeneration by human food producers is the important role played by soil organic matter (SOM) and the potential to either to deplete or sustain organic matter in soils (recall figure 5.1.1 and the fact that organic material is one of the key solid components of soil). In particular, concerns about soil organic matter (SOM) center on the large amounts of organic carbon in large molecules of SOM. This soil organic carbon (SOC) both feeds microbes in soil, allowing them to perform nutrient cycling functions and also contributes positively to soil properties. SOC is not a plant nutrient that comes from soil. In fact, it actually comes originally from the atmosphere in the form of plant remains that contain carbon fixed by plants (roots, leaves, manure, rotting wood, etc.) and accompanies N, P, and other nutrients that were in the plants. SOC within soil organic matter plays so many important roles in soil function and soil fertility that it should be considered a “master variable” explaining soil productivity, along with soil pH, soil depth, and soil drainage. Among its other functions, SOM promotes soil storage of crop-available water, is a major food source for soil microbes that perform beneficial roles in soil, and fosters the availability of many nutrients by holding them in moderately available form or decomposing to release them in soils. In addition, by far the largest pool of nitrogen in soils is held in N atoms within many types and sizes of soil organic molecules, and also within the bodies of soil microbes.
In many food production systems where the soil is plowed (also called tilling or tillage), SOM is in fact depleted by oxidation (a “slow burn”, like iron rusting) when soils are broken apart by plows, hoes, and other implements. Therefore, an important part of soil regeneration by human food production systems is not just replacing nutrients in a pure chemical form like fertilizers, but also maintaining overall soil function with soil organic matter. Therefore, in most parts of the world farmers have developed ways of reincorporating the roots and stems of plants (crop residues) as well as manure made by animals from the forage crops fed to them. These sources of plant carbon sustain SOM over the long term and feed microbes. These ways of sustaining the nutrients and organic matter of soils are depicted with a coupled human-natural systems diagram below (Fig 5.2.2) as a type of feedback loop in which human systems respond to soil degradation by incorporating organic matter like residues, compost, and manure.
The following brief reading assignment further illustrates the important functions of organic matter.
Building Soils for Better Crops, pages 9-17 in Chapter 2: Organic Matter: What It Is and Why It’s So Important. [1] (Free e-book as downloadable PDF). This chapter and book will be used in modules 7 and 9.
The following exercise asks you to use graphical data based on real soils to make conclusions about the important role of SOM in the water-holding capacity of soils. Along with the materials in module 4 on water and food production, and the systems approach to soil management in module 7, these concepts should help you to appreciate the role of SOM in fostering the environmentally sustainable production of food, as well as resilient systems (see module 10) that can deal with drought stress.
Examine Fig. 5.2.2, which draws on about sixty soils analyzed in a publication that related the water-holding capacity of soils to their organic matter content. The graph summarizes that data as the height of three columns on a bar graph. The height represents the amount of water stored in each soil, imagined as a depth of water in mm covering the soil at its surface (this is also how irrigation managers imagine applying water to soils, as the mm of rainfall they have replaced with irrigation). Each column represents a type of soil, from a coarse-textured sand on the left to a "heavy" or clayey soil on the right. The stacked colors on the graph represent the way that organic matter is able to improve the water-holding capacity of soils. Answer the following questions.
Nitrogen (N) is one of the most important nutrients for plant growth and crop production, along with phosphorus (P) considered on the next page. Nitrogen is important because it is used by plants to create proteins, which include the enzymes and building blocks of their photosynthetic "machinery". In fact, nitrogen in some ways underlies the green color of plants and vegetated areas on the earth's surface, because of the green, N-containing chlorophyll proteins (enzymes) used in photosynthesis (see module 4), which along with the other photosynthetic enzymes is one of the major uses of nitrogen within plants. These plant proteins become animals protein when plants are fed to livestock, or when we eat plants. The ubiquitous nature of nitrogen for the protein needs of the earth's biosphere explains why N is such an important nutrient for plant growth. Nitrogen is, therefore, a key element in the entire food system and interacts very strongly with human management. One indication of nitrogen's importance to the food system is that humans currently expend more energy on creating N fertilizers for food production by taking N2 out of the atmosphere in fertilizer factories (Fig. 5.2.3) than is spent on any other nutrient.
This module focuses on the subject of nutrient cycling, and below in figure 5.2.3, we present a basic diagram of the nitrogen cycle. Your initial impression of the diagram may be its relative complexity compared to the water cycle, for instance. This is true: the N cycle is complex, starting with the fact that it involves gas, solid, and liquid forms: gaseous N in the atmosphere, solid forms of N in soils and plants, and N dissolved in water in the soil and in earth's waterways (you may remember the problem of N pollution in waterways from module 4). To simplify this and take away the key concepts which should be your goal in this module (entire courses can be taught on the N cycle), we will present the basic pathway of N from the atmosphere into plants, soils, and water, which will complement the caption for the N cycling diagram below. Please refer to Figure 5.2.3 throughout this description. First, N exists in an enormous reserve as 78% of the earth's atmosphere (top left of Fig. 5.2.3). Creating usable forms of nitrogen requires that this N2 gas is "fixed" in the same way that plants fix carbon into their carbonaceous stems and leaves. Legume plants like beans, peas, and alfalfa host bacteria in their roots in nodules that are able to fix N2 gas (more on legumes as an important crop family in module 6). Nitrogen then moves directly into legume plants' tissues as proteins. In parallel to this biological fixation of N, humans have designed industrial methods to fix N in factories, using energy from petroleum and natural gas, and creating soluble nitrogen chemicals that are applied to soil, where they dissolve in soil water to become part of the pool of soil soluble N that is available to plants. This pool of soluble N (light green oval within the soil N pool below) is also called inorganic N to contrast it from organic N in proteins, crop residues, and soil organic matter. Inorganic N taken up by plants, plus the N fixed by legumes, is then used to grow crops and eventually produce crop- and livestock-based food products. Meanwhile, organic "waste" products from growing crops like straw, cornstalks, and roots, plus animal manures which are undigested plants, are not "waste" at all but are a hugely important organic source of N and other nutrients that are recycled to soil (brown arrows in Fig. 5.2.3). These organic soil inputs applied by farmers help to maintain soil organic matter (SOM; see previous pages and the assigned reading on soil organic matter) including the largest pool of soil N within SOM and soil microbes. Soil organic matter can be decomposed by microbes, liberating additional amounts of N to the inorganic N pool. µbes also can take up soil inorganic N, reversing the effects of SOM decomposition.
So far the N cycle may appear a relatively neat and ingenious system (albeit quite complex!). However, it is important to highlight the ways that it can become problematic under human management, indicated by the red "loss" arrows in Fig. 5.2.3. First, when the soluble N pool in soil is large, for example after fertilizer or manure is applied, and abundant water moves through the soil, like during a rain event, excessive soil N can move into waterways causing pollution and coastal dead zones (this is covered in some detail in module four, and again in this module's summative assessment). This process is called leaching of soil soluble N. Second, when erosion occurs, soils can also lose large amounts of their N "bank account" through erosion, because solid organic matter particles are rapidly eroded from soils in hilly areas when soil is not protected by plant cover or stabilized by plant roots. Lastly, soils can lose nitrogen back to the atmosphere through the processes of gaseous loss, where dissolved nitrogen becomes N-containing gases that diffuse back to the atmosphere. If you have ever caught a whiff of ammonia from a bottle of ammonia cleaning solution (dissolved ammonium that becomes ammonia gas) you know how N can move from a solution like that in a wet soil into the air. The most serious of these gas loss pathways is nitrous oxide (N2O) which is of concern because it is a potent greenhouse gas that contributes to global warming.
All of these loss pathways create the impetus for farmers and the food systems that support them, to manage nitrogen in an efficient and non-polluting way. The idea that highly productive farming systems with annual crops, manures, and fertilizers can completely eliminate N losses is actually quite challenging. This is because the N cycle has so many participants (humans, plants, microbes, livestock) interacting in complex ways (note: a complex system!), and because nitrogen is inherently "flighty" and "leaky", never staying put and always in transformation, with some forms so easily lost from soils to rivers, lakes, and the atmosphere. Nevertheless, there is much room for improvement that can also serve to save money and energy for food producers, and avoid the pollution costs to downstream ecosystems and food producers (for example, fishing communities affected by dead zones, see module 4). Two of these are (1) increasing the efficiency of timing and amounts of N fertilizer and manures to better match only what is needed by crops and (2) including crops and other plant components on farms that help to recycle soluble N from deeper in the soil and in downslope areas before it reaches waterways. Both of these strategies are addressed in the following modules on crops and systems approaches to soil management (modules 6 and 7). In addition, if N is not replenished in soils after it is exported as food products or suffers these losses, crops can face N insufficiency, which is a major issue for poorer farmers around the world. The summative assessment for this module focuses on these twin issues of nutrient deficiency and excess.
In an analogous way to the nitrogen (N) cycle on the previous page, we will present the basics of the phosphorus (P) cycle related to food production (refer to figure 5.2.4 below in this section) You'll note that the P cycle is a good deal simpler than the N cycle. For example, there is no gaseous form of P as there is for N, so the atmosphere does not participate in the P cycle. Also, leaching of soluble P is not a major issue as it is for soluble soil N. To begin the description of the P cycle, the large reserve of "primary" P that is accessed by plants and fertilizer production for agriculture is not the atmosphere (as it is for N), but rather so-called phosphate rocks (or rock phosphate) in the crust of the earth, which are mined like other minerals. These rocks are ground up and treated in fertilizer factories to make the phosphate (PO4-) in them water-soluble so that phosphate can be directly taken up by plants from the small pool of soluble phosphorus in soils. In addition to this industrial process that supplies P to plant roots, there are small amounts of soluble P that are continually released by weathering (see Module 5.1) of grains of rock phosphate that form a small part of most soils. These plant-available forms of P from fertilizers and weathering are taken up by plants and pass into the food system when crops are harvested for food products or are fed to livestock. Just as for N (figure 5.2.3), crop residues and manures with organic P are recycled to the soil and are an essential way of replenishing soil organic P supplies. Also, decomposition of soil organic P that liberates soluble P, and uptake of P into the bodies of microbes, link the organic P pool in soil organic matter (SOM) with the small amount of soluble P in soils.
One difference between the cycling of P vs. N in soils is the fact that most soils have ways of chemically capturing and holding soluble P in forms that can become very unavailable to plants. The clay mineral fraction of soils is especially active in retaining P, especially so for the clays that occur in tropical soils (you may be familiar with rusty or yellow-colored clays, made from iron oxides, in warmer areas of the United States and the world). This is called soil retention or fixation of P. In a soil that retains P strongly, less than five percent of the P in applied fertilizer, which enters in a soluble form very suited for plant uptake, is ever available for crops. The rest is quickly locked away by reactions with soil clay minerals. Soil scientists call this process P fixation or P retention, and a global map of estimated P retention has been made (Figure 5.2.5) that summarizes how phosphorus can become limiting to food production, which is a serious problem in many tropical soils. One comparison that may be helpful in remembering the way that soil locks away phosphorus is to contrast it to the behavior of soil N. While soil N is "flighty" or "leaky" with multiple forms and loss pathways, soil P tends to be the "clingy" opposite of soil N -- the issue is not that it is held too loosely in soils but rather that it is held too tightly.
To address the challenge of retained P, farmers may resort to continually supplying fertilizers and manures to crops, often in quantities that greatly exceed crop demand. Nevertheless, additions of organic matter also tend to make retained or fixed P more available, combined with the use of crop species that can better take up fixed forms of P, so that P is moved from the retained, unavailable fraction of P to organic forms in crop and microbial biomass that are eventually recycled into available soluble forms. Certain plant-symbiotic soil microbes, especially mycorrhizal fungi, are particularly efficient at helping plants to access these less soluble forms of soil P. In addition to these soil management measures, first farmers, and now formal plant breeders have developed crop varieties that are more efficient in taking up some of retained P that is locked away in soil.
As can be seen in Fig. 5.2.4, erosion of particles of soil that contain organic and retained P is the major pathway of phosphorus loss from soils (red arrow in Fig. 5.2.4), in contrast to P export for useful purposes in crop- and livestock-based foods. Along with maintaining the availability of soil P with regard to P retention, protecting soils against erosion is an excellent way to protect the ability of soils to supply P for food production. This main message will be taken up in the summative assessment for this module, and again in Module 7.
One of the important factors in deciding how much P must be added to soils to replenish them is the amount of P that is exported by typical crops and food products. This exercise will guide you in calculating the amounts of P that leave farm fields on a per area basis, and also at the level of a "phosphorus use footprint" for typical products, analogous to a water footprint in module 4. Consider the table below which reports the use of P to produce unit quantities of a few representative foods. The first column (A) reports very approximately how much a single hectare of soil (100 by 100 m area, about 2.5 acres) will support. The second column (B) is the content or concentration of phosphorus in the food, which means that multiplying A x B gives the kg P exported from the soil by the crop or animal product, which is shown in C. Columns D and E take a slightly different approach: in D the amount of the product eaten by an average U.S. person is reported. In column E, that per-person amount is turned into a per-person consumption of phosphorus in grams (per year)
Food crops |
(A) kg of fresh product or animal weight sustained per hectare (100 m x 100 m) |
(B) Phosphorus content of the fresh food (g P/ kg fresh wt.) | (C) kg P exported from soil, per Ha (100 m x 100 m) | (D) Per person consumption of product in the U.S. (kg per person per year) | (E) Per capita consumption of soil P resources (g P per person per year) |
---|---|---|---|---|---|
carrots | 10000 | 0.35 | 3.5 | 3.2 | 1.1 |
wheat | 3500 | 7.6 | 27 | 61 | 464 |
beef | 250a | 7 | 1.8 | 50 | 350 |
milk | 10000b | 3.6 | 36 | 20 | 72 |
Table assembled by the author from publically available data on typical yields and nutrient content of agricultural products. For example for yield data see National Agricultural Statistics Service (NASS) of the USDA [10] for crop nutrient content see National Resource Conservation Service's Crop Nutrient Database [11]. For nutrient values of foods such as beef and milk see the USDA food composition database [12].
aAbout the equivalent weight of half a beef cow/steer
bVery roughly a single production cycle (about 12 months) in liters for a single, lactating cow of a high-production variety
Both N and P are distinctive in possessing extremes of surplus and shortages across the variety of food production systems around the globe. For poorer small-scale farmers, who number more than two billion globally, the means to effectively replenish the nutrients exported by crops, or detain the nutrients removed by erosion on sloping land can be beyond the reach of their financial means or labor power, or simply not sufficiently part of their knowledge systems. Deficits of nitrogen and phosphorus in soils ensue, complicated by soils that may have a high degree of P retention, and low organic matter levels that decrease the overall soil quality by retaining less water and crusting easily, aspects that will be emphasized in the following modules. Applying the "bank account" analogy of soil nutrients introduced at the beginning of this module, after constant withdrawals the "soil bank account" begins to run such a low balance that overall functioning of soil productivity, and with it the livelihood of a smallholder household, are impaired. This can lead to a downward spiral of soil productivity (see the assigned reading for this module) that links issues of environmental, social, and economic sustainability.
Another feature of human-natural interactions for soil nutrients is the aspect of surplus exhibited by a "leaky" or "flighty" nutrient like nitrogen. This has been compounded by the development of the large-scale human capacity to add surplus nutrients to farm fields for food production. It's important to realize that prior to N fertilizers, bacterial nodules on the roots of legume crops (see Fig. 5.2.3 and the coverage of legumes in module 6) were the major way that N entered soils from the atmosphere, including the soils used for food production. Farmers before about 1900 relied exclusively on legume crops, as well as animal (and human!) manures derived from legumes and other crops as the principal way of regenerating the nitrogen in soil organic matter. These materials incorporated to soils decompose and release N that was used by crops. Since 1913, when N fertilizer production from the atmosphere was developed as a factory process, humanity has deployed greater and greater amounts of fossil fuel energy to fix greater and greater amounts of atmospheric N2 into soluble forms to feed crops. A startling fact is that humans now fix more atmospheric nitrogen than do legumes. This has buoyed the overall productivity of human food systems beyond what might have occurred without such fertilizers and is credited by many with avoiding widespread hunger (or dramatically expanding the population carrying capacity of earth’s human-natural systems, depending slightly on the perspective that is taken).
As has been noted in module 4, there have been unforeseen consequences of this trend towards greater fertilizer use that have become more evident in recent years. First, the share of CO2 greenhouse gas emissions from fertilizer production has become a primary contributor to the overall impact of agriculture on global warming. Another is that fertilizers, in combination with a profit-minded vision of soil fertility that did not incorporate a view of the whole human-environment system, bred a highly “chemical” vision of soils that neglected the important role of soil organic matter and the physical and biological qualities of soil. This resulted in unforeseen negative impacts as farmers over-applied nutrients at a local scale to guarantee the highest yields possible, thereby polluting watersheds, and allowing farmers to lose sight of the important role of soil organic matter outlined in this module. In a more subtle way, there has been an increasing focus in plant breeding and globalized seed systems on varieties that respond well to soluble fertilizers, which many argue have favored the expansion of more industrial modes of food production to the financial detriment of smaller and more sustainable food producers. If you recall the narration of agricultural history in module 2, you will recognize that this is an example of niche construction, in which a modern, chemical-intensive niche has been created for specially bred modern varieties along with fertilizers and other chemical inputs. Nevertheless, many of these problems associated with an exclusive reliance on nitrogen fertilizers and chemical fertilizers are now recognized by researchers and policymakers. Current approaches to soil the world over have placed renewed emphasis on the importance of organic matter and a more economical use of nitrogen fertilizers.
In the summative evaluation for this module, you will explore these “surplus and shortage” issues of sustainability for Nitrogen and Phosphorus, which are emblematic of present-day and future sustainability challenges in the area of nutrients cycling.
The last page of module 5.2 mentions the twin issues of deficit and surplus that are principal challenges in the management of soil nutrients. The exercise in this summative assessment requires you to use real data on nutrient inputs and outputs from two systems to create nutrient balances, and then analyze the situation of nutrient balance or surplus. These systems are the Ohio River sub-basin of the Mississippi River basin and measurements of nutrient flow from hillside farming in the Bolivian Andes. You should do this activity with a partner or small group in class, and prepare to discuss your results with the class. You will use data from a table to answer questions on the assessment worksheet (download below).
In analyzing the twin issues of nutrient surplus and nutrient shortage in soils and food production systems, you'll be practicing a geoscience "habit of mind" of systems thinking. In other words, to examine the wider impacts of nutrient management or the causes of soil infertility, we need to expand our focus from a single field to a landscape or river basin and think about a web of linkages between farmers, nutrient supplies, economic factors, and watersheds, among other system components. This allows us to contemplate these challenges in the proper frame and over the right timescale.
Download the worksheet [13]to complete and use for the graded quiz. The worksheet contains information in a table that you will need to complete the assignment.
You do not need to submit the worksheet; you instead will be using it to complete a summative quiz.
In this module, we have introduced the basics of soil properties and the nature of soil as a key resource for food production, which following modules will build upon to show how soils can be managed sustainably. We hope that you have understood the fundamental composition of soil as minerals, organic matter, water, and air as an essential part of earth's natural systems. We also have tried to illustrate the way in which key properties of soil, like its pH, nutrient content, and retention of water, affect how plants grow and produce food. On the human system side, we also presented the way in which human efforts have managed soil for sustained production of food, including the addition of nitrogen and phosphorus to replenish soil stores that are removed by crop harvests, and the protection of soils from erosion losses. However, a surplus of soil nutrients generated by over-applying N and P is also a problem, as illustrated in the nutrient balances in this module's summative assessment. We will continue to deepen your knowledge of sustainable soil management, as it supports sustainable food systems, during the next modules.
You have reached the end of Module 5! Double-check the to-do list in the Module 5 Roadmap [14] to make sure you have completed all of the activities listed there before moving on to Module 6!
Links
[1] https://www.sare.org/wp-content/uploads/Building-Soils-For-Better-Crops.pdf?inlinedownload=1
[2] http://www.sare.org/content/download/841/6675/Building_Soils_For_Better_Crops.pdf?inlinedownload=1
[3] https://www.e-education.psu.edu/geog3/node/875
[4] https://www.flickr.com/photos/soilscience/
[5] https://www.nrcs.usda.gov/wps/portal/nrcs/site/soils/home/
[6] http://soilgrids.org
[7] http://www.isric.org/
[8] https://nelson.wisc.edu/sage/data-and-models/atlas/maps/anntotprecip/atl_anntotprecip.jpg
[9] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod3/Formative5.1_soilTransects_Amended_May2017.docx
[10] https://www.nass.usda.gov/
[11] https://plants.usda.gov/npk/main
[12] https://nal.usda.gov/legacy/fnic/food-composition
[13] https://www.e-education.psu.edu/geog3/sites/www.e-education.psu.edu.geog3/files/Mod5/module5_SU22.docx
[14] https://www.e-education.psu.edu/geog3/node/583
[15] https://academic.oup.com/bioscience/article/55/7/552/306721