5 NATALIE BUMGARNER Extension Specialist University of Tennessee This chapter includes content from the chapter on soil science written by Hugh Savoy in the previous TEMG handbook.
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CONTENTS INTRODUCTION TO SOIL 5-5 The Importance of Soil...5-5 How Soils Are Formed...5-6 OVERVIEW OF SOIL 5-6 General Descriptions of Soil...5-6 Soil Components...5-6 Soil Horizons...5-6 Soil Classification...5-7 Physical Aspects of Soil...5-7 Soil Texture...5-7 Determining Soil Texture...5-8 Soil Structure...5-8 Presence of Rock Fragments...5-9 Soil Landscape Position...5-9 Slope...5-11 Depth...5-11 Color...5-12 Drainage...5-12 Chemical Aspects of Soil...5-12 Why Is Soil Chemically Active?...5-12 What Nutrients Do Plants Need from Soil?...5-14 What Is Soil ph?...5-14 Biological Aspects of Soil...5-14 Organic Matter...5-14 Organisms in Soil...5-15 Nutrient Cycling in Soil...5-15 The Nitrogen Cycle...5-15 SOIL MICROBE - PLANT INTERACTIONS 5-16 Soil, Water, and Air...5-16 Porosity...5-16 Available Water-holding Capacity...5-17 Microorganisms in the Rhizosphere...5-17 Plant Growth Promotion Activities...5-18 Activity of Mycorrhizal Fungi...5-18 Nitrogen Fixation...5-19 Association with Legumes...5-19 Other Nitrogen-Fixing Organisms...5-20 Soil, Nutrients, and Plants...5-20 How Does the Soil Supply Plant Nutrients?...5-20 Soil ph and Plant Nutrition...5-20 Recognizing Signs of Nutrient Deficiency...5-21 SUMMARY 5-22 5-3
The ability of soil to sustain the growth of plants by providing a reservoir for water and nutrients taken up through roots is the basis for much of agriculture and horticulture. 5-4
In gardens and landscapes, we often focus primarily on plants rather than the soil beneath. As gardeners, however, we should always keep in mind the many ways that soils support plants and subsequently humans. Soils are truly the basis for life as we know it. Charles Kellogg, a former chief of the USDA Bureau of Chemistry and Soil, said it most clearly: Society has its roots in the soil. Important connections between water and soil are introduced in Chapter 4. This chapter and the next one address the specific composition, structure, and function of soil. Agriculture and food production, as well as the environmental and quality-of-life aspects of ornamental horticulture and landscape management, often require the presence and proper functioning of soil to support plants and perform many other roles. Soil, by definition, is the unconsolidated mineral or organic matter on the immediate surface of the earth that serves as the natural medium for the growth of land plants, according to Soils.org. This definition alludes to the physical, chemical, and biological factors affecting plants as discussed in this chapter. The intent is to introduce basic concepts of soil science that will connect the stewardship concepts of Chapter 4 with the discussions on soil management in Chapter 6 and then with the plant management practices covered in the many chapters that follow. INTRODUCTION TO SOIL The Importance of Soil Developing a greater understanding of soil functions, the sources of plant nutrients, and the role those nutrients play in plant growth has been the key to propelling world food production to its current level. Developing improved plants along with better methods for managing soil and fertility have been essential in enabling our planet to support its current population. Soil management practices have certainly not reached perfection. There will always be ways to improve management and enhance soil function while preserving the ability of soil to support plants and humans. As we look 5-5
to the future and an ever-increasing human population, the management of soils will be at the forefront of sustainable agriculture and horticulture. As Extension Master Gardeners, we must realize that responsibility for sustainability in coming years extends beyond large agricultural producers and landholders. Careful soil management is highly important at both large and small production levels, and we have a responsibly to educate citizens and residents of this need. Stewardship of soil and water resources on a residential scale is increasingly important as populations increase and urban and suburban land areas expand. (See Chapter 4.) Homeowners and their decisions about soil, fertility, and water are an important part of attaining sustainable and beautiful landscapes and gardens. This chapter provides basic information on soil science and the key properties of soils. It focuses on the physical, chemical, and biological aspects of soil, providing descriptions of soil and why soil attributes are important to plants. The subject of plant nutrition is introduced because of its close connection to soil management. Chapter 6 places additional emphasis on soil quality and management in the landscape. Proper use of fertilizers and the application of organic matter are presented there as a foundation for plant management in the home garden and landscape. Many of these topics are referenced in later chapters. How Soils Are Formed Soils exist because parent material, which can be thought of as the raw material for soil formation, was weathered and changed to form soil. General similarities in soil are often related to the type of parent material present in different regions. (See Chapter 2.) This raw material may be bedrock or it may be materials moved by wind, water, gravity, or glaciers from another location. Soil genesis, the study of soil formation, investigates how the forces of climate, organisms, landscape (topography), parent material, and time affect soil formation. All of these factors turn raw parent material into the dynamic, living, breathing body that we call soil. OVERVIEW OF SOIL General Descriptions of Soil SOIL COMPONENTS Soil is often referred to as a three-phase system. To function well, soil needs to have an appropriate balance of solids, liquids, and gases. The solid portion of soil contains both minerals (sand, silt, and clay) plus organic matter (previously living material) that together should make up about 50 percent of the soil volume. The proportion of organic matter varies by soil and location but is generally between 1 and 5 percent. Water and air in the soil should each make up around 25 percent of the soil volume. Each of these three components or phases is essential for proper functioning of soil. SOIL HORIZONS The vertical arrangement of soil provides important information about how the soil was formed as well as its capacity for plant productivity. Most soils show at least three distinct layers, called horizons. (See Figure 1.) The principal horizons (collectively called the soil profile) are the surface soil (A), the subsoil (B), and the underlying material that has been slightly affected by soil-forming processes (C). Other possible horizons can include a layer of organic material (O horizon) above the A horizon, or a layer of material that that has been leached, meaning soluble materials have been lost from this layer as water has moved them down the profile. Hard bedrock found below the C horizon is referred to as R. Horizons with capital letters are called master horizons and are combined with specific lowercase letters that provide more detail. Development is the term used to describe how clear and distinct the horizons are in the soil profile. Undisturbed, well-aged soils are more likely to show strong development than those recently disturbed or in areas with frequent erosion, flooding, or soil movement. In those areas, the processes of soil formation do not have the opportunity or time to fully act. In landscapes with severe erosion or 5-6
construction disturbance, horizons may have been removed or may be buried. On sites with previous development or construction, it is quite common for native A horizons to have been removed or to be quite shallow or buried. Agriculture and other human or environmental impacts also cause horizons to be different from those of native soil. SOIL CLASSIFICATION Across the wide variations in soils, certain common traits can be used to group soils by type. These groupings form a hierarchy that is similar to that used to classify plants and animals. The largest groups are called orders and are followed by suborders, great groups, subgroups, families, and, finally, series. This system makes it possible to describe soils using features pertaining to parent material, formation, location, climate, or management. Those descriptions can be used to indicate a soil s potential productivity and to suggest appropriate management practices for soils around the world. FIGURE 1 Common soil horizons. Physical Aspects of Soil Physical properties of soil describe the type, size, appearance, and configuration of soil particles. They also relate to potential plant productivity and manageability. The physical aspects provide the basis for many of the chemical and biological aspects of a soil as well as its water and nutrient interactions with plants. Many physical soil properties are referred to as permanent to indicate the difficulty in changing these aspects of soil that are often inherited from the parent material. More information on soil physical properties and management is presented in Chapter 6. SOIL TEXTURE Sand, silt, and clay are the three mineral particles found in soil, and they all differ in size. (See Figure 2.) The relative proportions of these three minerals determine the soil texture. Sand is the largest particle and can be seen by the naked eye. It has a high mass per unit of volume (bulk density). The larger size means there is Dickson The State Soil Series of Tennessee Dickson soils are very deep, moderately well-drained soils that formed from a deep silt layer over limestone. Tennessee contains over 400,000 acres of Dickson soils, occurring mainly in the central and western portion of the state in the Highland Rim region. They are generally nearly level or gently sloping and are commonly used for row crops (corn and soybeans) as well as grass and legume pastures. 5-7
FIGURE 2 Relative size of sand (0.05 2.0 mm), silt (0.002-0.05 mm), and clay (<0.002 mm) minerals in soil. Sand Clay Silt less surface area and larger open spaces between particles (pores) per unit of volume than in silt or clay. These factors are important in terms of relationships with plants because sand holds little water or nutrients. It is not slick or sticky when wet and gives soil a gritty feel. The medium-sized silt particles are too small to be seen without a microscope. The bulk density of silt is between that of sand and clay. Silt has the feel of talcum powder and is moderately sticky when wet. Silt holds a moderate amount of water and nutrients, but these are held less strongly than in clay soils. This often means that a larger portion of the water and nutrients are available to plants. The smallest soil particles, clay, are the most chemically and physically active of the minerals. A small particle size means that clay has a large surface area for its volume and many small pores. As a result, clay can hold large amounts of water and nutrients but sometimes retains them so tightly that they cannot be used easily by plants. Soils that are high in clay are sticky when wet and hard when dry. Clay can also act as a binder between sand and silt particles and can be important in soil structure. The size and relative percentage of these three minerals affects water movement in the soil and the extent to which the soil can provide water and nutrients to plants. Loamy soils are often described as ideal because of their combination of sand, silt, and clay (around 40, 40, and 20 percent, respectively) that provide not only nutrient and water-holding ability, but also drainage and aeration. The soil s ability to hold water, nutrients, and other compounds has an environmental aspect because this ability determines how easily excess nutrients or pesticides might leach into groundwater and how frequently a gardener needs to lime, fertilize, or irrigate. Soil texture also affects ease of tillage, ease of erosion, fertilizer management, and root penetration. Determining Soil Texture Soil scientists have defined 12 classes of soil texture, each encompassing soils with specific percentages of sand, silt, and clay. Precise descriptions of soil texture can be determined in laboratory assays. Gardeners may not need to determine the percentages of these minerals precisely, but knowing the general composition of the soil can be an asset in management. With some practice, accurate estimations can be made rubbing a moist sample of soil between the thumb and forefinger. That method will be described here, but additional methods are described in the supplemental materials for this manual. Three groups (coarse or sandy, medium or loamy, and fine or clayey) can be defined based on textural class identified by hand. Start by wetting a sample of soil until it has the consistency of putty or modeling clay. Make a ball of the wetted soil about ½- to ¾-inch in diameter. Press the ball between the thumb and finger and try to make a thin ribbon. Estimate the texture using the guidelines in Table 1. SOIL STRUCTURE Soil structure refers to the arrangement of individual sand, silt, and clay particles into clusters or aggregates, which are called peds. These soil peds vary in shape, arrangement, size, distinctness, and durability. There are several different types of soil structures. Two of the most common in Tennessee are granular and blocky. Granular structure has small aggregates (usually less than ¼ inch in diameter) that are weakly held together. They may be roughly spherical with many irregular surfaces. Granular aggregates often have the greatest amount of pore space between soil aggregates, so it is usually the most desirable for gardening. It is 5-8
common for surface horizons in Tennessee to have a weak, fine, granular structure. Blocky structure has peds that are angular or subangular in shape and often range from ½ to 2 inches in diameter. Most subsoils in Tennessee have an angular or subangular blocky structure. The grade may vary between peds that are distinct, strong (easily seen), and well developed and those that are weak and difficult to discern. Other structures less common in Tennessee soils are platy, prismatic, columnar, massive, and single-grained. (See Figure 3.) Soil structure is affected primarily by the proportions of clay and organic matter in the soil as well as by the activity and presence of organisms, microorganisms, vegetation, and specific ions. Structure is important in determining pore space, which is important for water movement, water holding, and aeration. More pore space in soils means that the soil weight per volume (bulk density) is lower. (See Figure 4.) Soil structure and the presence of pores also affects rooting depth and percolation rates, the latter being the amount of water (in inches per hour) that is able to move through a soil. Well-structured soil is easier to manage in a productive manner because it has free air TABLE 1 Estimation of Soil Textural Groups and Classes Using the Hand-feel Method TEXTURAL GROUP TEXTURAL CLASSES DESCRIPTION Coarse (Sandy) Sand Loamy sand Will not form a ribbon. Feels very gritty because the soil is mostly sand. A ball of moist soil formed by hand is loosely held together and falls apart easily when handled. Medium (Loamy) Sandy loam Will usually not form a ribbon. Feels gritty, but contains considerable silt and clay. Ball will hold together when handled gently, but will break apart easily when pressed. Fine (Clayey) Silt Silt loam Loam Sandy clay loam Clay loam Silty clay loam Sandy clay Silty clay Clay and water movement. Well-structured soil is also more resistant to erosion because it enables water to infiltrate the soil rather than run off, as discussed in Chapter 4. PRESENCE OF ROCK FRAGMENTS Rock fragments are defined as any loose pieces in soil larger than sand particles (fine fragments), and they influence water and nutrient movement and availability in soil. Rocks are assessed as a percentage of the total soil rooting volume. The proportion of rock fragments present can be determined by taking a sample of the soil and separating it into stacks of fragments and fine particles (also called fines). If the stack of fragments is one-fourth as large as the stack of fines, then the soil has a 25 percent volume of rock fragments. The percentage of rock fragments results in an identical percentage reduction in the amount of water that can be held by the soil. Rocks also reduce the ability of soil to hold nutrients, and they interfere with tillage. SOIL LANDSCAPE POSITION Landscape position (Figure 5) is important because it affects how soils develop and their potential for use. Upland soils are developed Forms a very short ribbon that breaks easily. Feels smooth like talc in a silt loam, but with slight grittiness in a loam. Ball will compress only slightly before cracking when pressed. Slightly sticky when wet. Will form a short ribbon that breaks easily. Ball will compress somewhat without cracking when pressed. When smoothed with fingernail or knife blade, will not leave a shiny surface. Sticky when wet. Will form a ribbon easily and holds together well. Ball of moist soil can be molded into various shapes with little cracking. Will leave shiny surface when smoothed out with knife or fingernail. Very sticky when wet. 5-9
FIGURE 3 Various types of soil structures. FIGURE 4 Examples of soil particle aggregation and its relationship to the amount of water and air in the soil. Granular Blocky Prismatic Granular Blocky Prismatic Columnar Platy Single Grained Columnar Platy Single Grained Illustration remade from http://www.ctahr.hawaii.edu/mauisoil/a_factor_ts.aspx Aggregation and Impacts of Water and Air in Soil Lower bulk density Lower weight Lower More pore weight space More pore space Higher bulk density Higher weight Higher Less pore weight space Less pore space Illustration remade from International Society of Arboriculture, International Society of Arboriculture, Bugwood.org.aspx 5-10
primarily from the underlying rock, marine deposits, loess, or other materials not deposited by streams, as is the case of residual soils. (See Chapter 2.) Soils on uplands and terraces usually have more clear and distinct horizons in the profile than soils from other areas. Terrace soils are developed in materials deposited by previous streams and were former bottomlands. The footslope is at the base of a slope where material that has washed or slid downslope accumulates, producing colluvial soils. (See Chapter 2.) The soil above the footslopes is steeper than the slopes below them. These sideslopes are more susceptible to erosion and can be a combination of residual and colluvial soils. Footslopes generally have slight to moderate profile development. They often look like floodplain soils but are usually found on more sloping areas and do not usually flood. Footslope soils receive runoff from higher areas on the landscape, giving them a higher water-supplying capacity for plants than the surrounding areas. They are often very productive garden sites if not too wet. Bottomland or floodplain soils developed in materials deposited by recent stream action and they are identified as alluvial soils. (See Chapter 2.) These soils are in a current floodplain or overflow area and are at risk unless protected by dams or levees. Soils in floodplains usually show little development of the soil profile. If welldrained, these soils can be productive garden sites, but they are at risk of flooding. A depression refers to an area surrounded on all sides by higher land. It has no natural outlet for surface water flow and is therefore subject to flooding during heavy rains. Frequency and duration of flooding can vary considerably and could be a problem when locating a garden or landscape area. Soils developed in depressions have slight to moderate profile development. SLOPE Soil slope is the change in vertical elevation of the soil surface over a given horizontal distance. Slope is expressed as a percentage, which is equal to change in elevation divided by distance times 100. Slope can be accurately determined using survey instrumentation, but that level of precision is generally not necessary in home Upland Sideslope Footslope Illustration remade from http://www.fs.fed.us/psw/ publications/documents/gtr-155/06-fig06.gif landscapes and gardens where estimates can be made using a tape measure and string. (See guide in the supplemental materials.) Slope is important because steeper areas usually have more water runoff and are drier as a result. They also often lose soil with this water runoff (erosion). On steeper slopes, conservation practices like mulches, winter cover crops, grass strips, and possibly even terracing will be needed. Slope classes of soils have been defined as a means of mapping soils and making recommendations about their use. Slope can also be related to aspect because southern slopes often warm up and dry out faster than northern slopes. This characteristic can impact soil development, and it is common for topsoil to be thinner on southern slopes. DEPTH Rooting depth is the vertical distance in which root penetration and growth occurs. Layers that stop or severely limit root growth include bedrock, continuous fragipans (dense, subsurface layers that restrict water and root growth), unconsolidated or partially weathered parent material, structureless clay, and compact layers of rocks or gravel. If root growth is confined to cracks several inches apart, the layer is considered unfavorable for roots. Deeper soils can provide more volume in which roots can grow and take up nutrients and water. Shallow soils may be used for gardens and landscapes, but irrigation will likely be required for healthy, productive plants because the plants will be less tolerant of drought conditions. Shallow soils, however, may not be suitable for some long- FIGURE 5 Primary landscape positions, from upland to floodplain. Terrace Floodplain 5-11
term or deep rooted crops, such as fruit trees. Rooting depth can be divided into four classes: Deep 36 inches or more Moderately deep 20 to 36 inches Shallow 10 to 20 inches Very shallow less than 10 inches COLOR Color is commonly the first observed soil property and is an important indicator of soil organic matter content, mineral composition, weathering, development (age of the soil), and drainage. Surface soil colors vary from almost white to shades of brown, red, gray, and black. Light or pale colors can indicate low organic matter content, the presence of salt, relatively coarse texture (more sandy), and high leaching. Dark colors often indicate high organic matter content, poor drainage, low annual temperatures, or conditions such as profuse plant growth and low decomposition rates. However, since similar colors can indicate differing properties, always interpret the soil color within the context of landscape and climate. In general, subsoil colors indicate aeration and drainage. Red, brown, and other brightly colored subsoils indicate free movement of air and water. Some soils with mottled subsoil (areas of mixed, variable colors), especially where the colors are shades of red and brown, are also well aerated. The red-to-brown rust color of the subsoil comes from iron coatings on soil particles under well-aerated conditions. In frequently wet soils with low oxygen levels and fluctuating wet-dry conditions, the iron coatings may be removed, leaving the gray color of background soil minerals. Most soils that have gray mottling or a predominant gray color have too much water and too little air during parts of the season. DRAINAGE A lack of good soil drainage can also be due to a high water table, a slowly permeable layer within the soil profile, seepage of water from outside the soil profile, or some combination of these conditions. Poor drainage causes delays in soil warming and land preparation since the soil is too wet to work without causing compaction. Plant roots are also suffocated by the lack of oxygen. Excessive drainage is also a challenge to optimum plant growth if water cannot be held by the soil long enough for plants to use it. Soil drainage influences land use, plant selection, and garden management. Wetter soil can be improved by drainage tactics (see Chapter 6), while droughty soils benefit from irrigation. However, both of those practices can be expensive and time-consuming. Chemical Aspects of Soil WHY IS SOIL CHEMICALLY ACTIVE? The chemical activity of soil is based primarily on the properties of clay and the characteristics of stable organic matter called humus. Silt and sand are less of a factor in chemical soil interactions because of their mineral structure and low external surface area compared to clay and humus. Clay can have 1,000 times more surface area than an equivalent mass of sand. By their structure and composition, clay and organic matter largely provide the charge found on soil particles. This charge is important because the nutrients that plants need are generally taken up as ions (charged particles). (See Table 2.) When all the small clay and humus particles in soil are combined, there is a large charged surface area to interact with these ions. The movement of ions between charged clay and humus and the water in the soil is called ion exchange. Negative charges in soil interact with positively charged ions in a manner similar to that of the opposite poles of a magnet; that is, they attract each other. Since negative charges are predominant in clay and humus, cation (positively charged ions) exchange capacity (CEC) is an important soil property. The interaction between the CEC and nutrient ions in soil water form the basis for soil fertility management. 5-12
TABLE 2 Essential Mineral Elements Required by Plants ELEMENT SYMBOL IONS ABSORBED BY PLANTS MOBILITY IN PLANTS MOBILITY IN SOIL MACRONUTRIENTS Macro/Primary Nitrogen N NO 3 -, NH4 + Mobile Mobile, Immobile Phosphorus P H 2 PO 4 -, HPO4 2- Somewhat mobile Immobile Potassium K K + Mobile Somewhat mobile* Macro/Secondary Calcium Ca Ca 2+ Immobile Somewhat mobile* Magnesium Mg Mg 2+ Mobile Immobile Sulfur S SO 4 2- Somewhat mobile* Mobile MICRONUTRIENTS Micro/Trace Boron B H 2 BO -, HBO 3 2-, BO3 3- Immobile Mobile Chloride Cl Cl - Mobile Mobile Copper Cu Cu 2+ Immobile Immobile Iron Fe Fe 2+ Immobile Immobile Manganese Mn Mn 2+ Immobile Immobile* Molybdenum Mo MoO 4 2- Mobile* Somewhat mobile* Nickel Ni Ni 2+ Immobile* Immobile* Zinc Zn Zn 2+ Somewhat immobile Immobile * There are some discrepancies in published resources regarding these mobility descriptors, and yes/no classifications can be difficult. Many are currently the subject of research. 5-13
WHAT NUTRIENTS DO PLANTS NEED FROM SOIL? The study of plant nutrition examines the basic chemical elements needed for proper growth and reproduction. Plants require 17 nutrient elements to survive and produce seed. (See Table 2.) Air and water are the sources of the first three carbon (C), hydrogen (H), and oxygen (O) so these are not commonly included in an essential-elements table as they are rarely limited. The soil usually supplies the remaining 14 essential mineral elements nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), boron (B), chlorine (Cl), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn), and nickel (Ni). Nitrogen (N), P, and K are classified as primary macronutrients based on the large amounts required by plants. Calcium, Mg, and S are categorized as secondary macronutrients. Primary macronutrients are most commonly added through fertilizers, but it may be necessary to add secondary macronutrients as well. The micronutrients are B, Cl, Cu, Fe, Mn, Mo, and Zn. Micronutrients are just as essential as the primary and secondary macronutrients but are needed in much smaller quantities. In Tennessee, native soils often contain enough of these micronutrients to support healthy plants. Deficiencies are possible, but soil tests should be conducted before applying micronutrient supplements because there is a narrow range between sufficiency and oversupply (toxicity). WHAT IS SOIL PH? The term ph is a measure of acidity or alkalinity. Soil ph refers to the ph of the water in the soil (that is, in the soil solution). The numerical value refers to the ratio of free hydrogen (H + ) to hydroxyl (OH - ) ions. Values of ph below 7.0 indicate more H + in the solution, while ph values higher than 7.0 mean more OH - is in the solution. Soil ph values can range from 0 to 14, and 7 is considered neutral. Anything above 7 is alkaline, or basic, and anything below 7 is acidic. The ph scale is logarithmic rather than linear. This means that a ph of 8.0 is 10 times more alkaline than 7.0, while a ph of 5.0 is 100 times more acidic than 7.0. The ph of the soil solution is very important because of the impact it has on nutrient availability for plants. Keep in mind that soil acidity values are not good or bad ; they just need to be managed in a way that is best for the plants they support. Management is necessary only if the soil ph does not support the growth and productivity of the desired food or landscape plants. Soils may become acidic for a number of reasons, such as the soil s parent material is acidic rainfall has leached away soluble soil calcium plant roots have excreted hydrogen ions (which are acid-forming) to assist in nutrient uptake plant roots have produced carbonic acid to break down organic matter for nutrient release the soil has been exposed to acid rain nitrogen fertilizers containing ammonium or urea have been applied. Biological Aspects of Soil ORGANIC MATTER Organic matter is an important component of soil; its roles are described in Chapter 4 and earlier in this chapter. The organic fraction of the soil includes plant and animal residues, cells and tissues of soil organisms, and substances synthesized and secreted by soil organisms. Within this definition, soil organic matter (SOM) can consist of material currently breaking down and slowly becoming available for plant use or material that has highly decomposed and is nearly stable. The material currently breaking down will be discussed in terms of nutrient cycling. The highly decomposed, fine, and generally stable portion of organic matter is commonly referred to as humus, and it delivers many benefits in terms of water- and nutrient-holding capacity and enhanced structure. 5-14
Cultivated soils in Tennessee may have less than 1 percent SOM by weight, but this proportion varies with soil type and management practices. In the South, heat and humidity increase microbial activity, breaking down organic matter quickly. Chapter 6 provides more detailed information and practical guidance on organic matter management and soil quality. ORGANISMS IN SOIL Living organisms play important roles in nutrient cycling, nutrient availability, soil structure development, and soil permeability. Animals such as earthworms, beetle grubs, and ants burrow into the soil, increasing its permeability to air and water. Microscopic organisms such as bacteria, fungi, actinomycetes, and animals (such as protozoa and nematodes) play important roles in decomposition of both plant and animal matter and nutrient cycling. An important contribution of fungi is the physical structures (hyphae) they form that spread through the soil in a network. The compounds they produce help bind mineral soil particles into aggregates, improving soil structure. The hyphae of certain species (mycorrhizae) form a mutualistic, symbiotic association with plant roots, whereby the plant provides carbon compounds to the fungus in exchange for inorganic soil nutrients. Some specific types of bacteria (such as Rhizobium) associate with legumes, such as clovers or beans, and play an important role in turning atmospheric nitrogen into forms that can be used by plants. NUTRIENT CYCLING IN SOIL Nutrient cycling is basically the breakdown of previously living organisms to provide energy and nutrients for currently living organisms. Organic matter contains about 5 percent nitrogen and 0.5 percent phosphorus and sulfur. These nutrients are said to be in the organic form because they contain proteins and cellular structures. (This designation does not mean that they are certified for organic food production.) Microorganisms are needed to convert nutrients from organic forms to inorganic forms that are available for uptake and use by plants. This process is called mineralization. In tilled systems, about 1 to 2 percent of this organic matter mineralizes each year. For example, it is commonly estimated that the surface 6 inches of soil in 1 acre (referred to as an acre furrow slice) weighs about 2 million pounds. If that soil contains 1 percent organic matter and 2 percent is decomposed in one year, then 20 pounds of nitrogen and 2 pounds of phosphorus and sulfur will be released to the mineral soil within that year. For this process to be sustainable, the yearly release has to be replaced by new additions of organic matter or supplied by inorganic nutrient sources. The Nitrogen Cycle While microorganisms have a role in multiple nutrient cycles, the nitrogen cycle is especially important and is a good example of the crucial contribution of microorganisms to soil biology and plant nutrition. Nitrogen transformations are some of the most important biological changes that occur in the soil. They have significant effects on soil fertility and nitrogen uptake by plants because nitrogen is usually the most limiting plant nutrient in Tennessee soils. Transformation of nitrogen from organic to inorganic forms relies on specific soil bacteria and environmental conditions. Before plants can use organic material, various microorganisms must convert the organic material into forms the plant can use, primarily ammonium (NH 4+ ) and nitrate (NO 3- ) ions. The transformation from organic nitrogen to plant-available nitrogen, as shown in Figure 6, occurs in two major steps: mineralization (conversion of organic matter N to solution ammonium) nitrification (transformation of ammonium to nitrites and then to nitrates). When microorganisms use the mineralized nitrogen to form biomass, the nitrogen is returned to an organic form in a process called immobilization. Nitrogen transformations also occur when fertilizers containing inorganic ammonium or nitrate forms of nitrogen are added to the soil. All nitrogen transformations, as well as levels of ammonium and nitrate in soil, are influenced by soil ph, temperature, aeration, 5-15
FIGURE 6 Transformations of nitrogen in soil. Soil Organic Matter Microbial Uptake Microbial Turnover Decomposition Immobilization Illustration remade from http://ohioline.osu.edu/sc186/sc186_14b.html and moisture. If the soil has a low ph (less than 5.5), most bacteria cannot thrive, and transformations are slowed. If air is lacking, denitrification occurs and nitrogen is lost to the atmosphere as N 2 gas. Under cool temperatures, little ammonium is changed to nitrate. Since ammonium (NH 4+ ) is a positively charged cation, it is held by the soil's cation exchange capacity and is not as readily leached as nitrate (NO 3- ), an anion. If the soil is warm, moist, and well aerated during the growing season, most of the plant-available nitrogen is in the nitrate form. Nitrogen added as ammonium sulfate, ammonium nitrate, urea, or anhydrous ammonia is rapidly converted by soil bacteria into the nitrate form in warm, moist soils. Nitrates are subject to leaching because they are negatively charged and are not held by negatively charged clay and humus. Periods of high rainfall can result in the loss of these ions from the rooting zone. For this reason, most Tennessee soils are low in nitrogen and require fertilizer, manures, or compost to provide sufficient nitrogen. Organic N Available Nitrogen Pool Mineralization Plant Uptake Fertilizer SOIL MICROBE - PLANT INTERACTIONS Soil, Water, and Air POROSITY An important concept in understanding how soil actually functions is porosity. Pores are the spaces between soil particles that are filled with either water or air. Large openings, such as those made by earthworms or decayed plant roots, are called macropores. Smaller openings are called micropores. Pore spaces not occupied by water contain air, which is necessary to supply oxygen to the roots of plants. As water enters the soil pores, air is pushed out and the water becomes known as the soil solution. The soil solution contains dissolved nutrients and other chemicals. It acts as a medium for the movement of plant nutrients into the plant. Soil porosity is affected by soil texture. Sandy soil tends to have larger pores, while clay soil has many smaller pores. Silt and loam soils are intermediate. Soil structure is also directly connected to porosity. The amount of organic matter in the soil as well as management factors such as tillage, compaction, and plant rooting 5-16
depth affect total porosity, which is the size and connectedness of the pores. Soil porosity affects several aspects of plant productivity, such as access to water and nutrients by roots. Soil pores are important because soil sites with good access to water and oxygen are also important in supporting the living organisms in soil. Soil porosity also has a large influence on environmental factors such as water infiltration, runoff, and water-holding capacity, as discussed in Chapter 4. AVAILABLE WATER- HOLDING CAPACITY Available water-holding capacity (AWHC) is the amount of water a soil can store and release for use by plants. Three terms are used to describe water held in the soil. Saturation indicates that all pore spaces are filled with water. Field capacity refers to the amount of water a soil can hold after gravity has drained the water away from a saturated field. Permanent wilting point refers to water that is held so tightly by soil that plants cannot use it. The actual water content that each of these levels represents varies with soil texture. Sandy soils typically have the lowest AWHC, silt has the highest AWHC, and clayey soils have an intermediate AWHC. (See Figure 7.) Available water-holding capacity is estimated using data on texture, rooting depth, and coarse fragment content. The average AWHC of soils with different textures is shown in Table 3. To estimate the available water-holding capacity of a soil: 1. determine the rooting depth. If it is 36 inches or more, use 36 inches 2. within that vertical space, determine the thickness, in inches, of each layer of the soil having different textures (coarse, medium, or fine) 3. determine the AWHC for each layer by multiplying the thickness of the layer by the AWHC shown in Table 3 (inches/ inch) for the texture of the layer 4. if any of the layers contains 15 percent or more rock fragments, reduce the AWHC for that layer by a percentage equal to the percentage of rock fragments because the space occupied by the fragments will not hold water. If the fragments make up less than 15 percent of the soil, make no deduction 5. add the available water-holding capacities for each layer within the rooting depth and use Table 4 to interpret the results. For example, consider a soil that is 24 inches deep. The first 4 inches has a medium texture and the lower 20 inches has a fine texture. Ten percent of the soil consists of rock fragments. (4 inches soil x 0.20 inch water/inch soil) + (20 inches soil x 0.15 inch water/inch soil) = AWHC No adjustment need be made for rock fragments because they comprise less than 15 percent of the soil. After performing the arithmetic operations: (0.8) + (3.0) = 3.8 inches AWHC This result indicates that the soil may be prone to droughty conditions. Microorganisms in the Rhizosphere Sometimes in focusing on soil testing, ph, and nutrient management, the functions and support of microorganisms is overlooked. The role of microorganisms in nutrient cycling and their functions as decomposers of organic matter and transformers of nutrients is essential. However, they play many other roles as well. The area of the soil immediately surrounding roots is called the rhizosphere essentially, the root s sphere of influence. Here microorganisms can flourish because of the beneficial effect of the roots on microbial communities. Roots release soluble organic metabolites in the soil water and dead surface cells as they grow in the soil, providing a rich food source for microbes and dramatically increasing their number and activity close to plant roots. Bacteria and actinomycetes are plentiful in that region, but various types of fungi and soil 5-17
FIGURE 7 Available water as related soil texture. Inches of water/ft. soil depth 4 3 2 1 0 Illustration remade from http://estore.osu-extension.org/ohio-agronomy-guide-14th-edition-p1.aspx TABLE 3 AWHC of Soil Textural Groups TEXTURE OF SOIL Coarse 0.05 Medium 0.20 Fine 0.15 AVERAGE AWHC (INCHES/INCH OF SOIL) TABLE 4 Possible Implications for Garden Management of Various AWHC Levels AWHC (INCHES) Less than 4.0 4.0 to less than 6.0 6.0 or more Sand Loamy sand Field Capacity Sandy loam INTERPRETATION Low: May be droughty Medium: Intermediate water supply High: Good available water supply Available Water Permanent Wiltiing PointUnavailable Water Fine sandy loam Loam Silt loam Silty clay loam Clay loam animals are also present. Because there are a large number of microbes near the root, many plant-microbe interactions take place in the rhizosphere. The following descriptions provide an overview of some of the key interactions. PLANT GROWTH PROMOTION ACTIVITIES Examples of microbes that foster plant growth include specific bacteria, called plant-growthpromoting rhizobacteria (PGPR). They live close to roots and seeds and can improve germination and plant growth. Some of these bacteria have been found useful in biocontrol efforts to provide protection against pathogens. Other PGPRs can produce plant hormones that promote plant growth. Other examples include those that can chelate nutrients to make them more accessible to plant roots. This area of research is very active. Much remains to be understood about PGPRs and how they can be used to support healthy and productive soils and plants. Silty clay Clay ACTIVITY OF MYCORRHIZAL FUNGI Many fungi in soil function as decomposers of organic matter. But some fungi can live 5-18
and grow in a mutually beneficial relationship (symbiosis) with plant roots. These fungi, called mycorrhizae, colonize roots of many native and crop plants, both woody and herbaceous. The fungi attach themselves to the root and help expand the soil area from which plants can obtain nutrients and water. Mycorrhizal symbiosis can improve phosphorus nutrition of plants, especially those growing in phosphorus-limited soils. These fungi are also thought to increase plant resilience in the face of environmental challenges such as drought, salinity, certain pollutants, herbicides, and infection by pathogenic fungi. In exchange for enhancing acquisition of phosphorus and potentially other nutrients the fungi receive vital carbon compounds from the plant for energy. One of the most important agricultural consequences of encouraging healthy populations of mycorrhizal fungi in soils may be for the soil itself. The fungi s tiny but prolific hyphae spread out into the soil and exude organic substances, which help promote good soil structure and prevent soil erosion, as described in Chapter 6. NITROGEN FIXATION Association with Legumes The key roles of microorganisms in the nitrogen cycle have already been discussed. However, some important microbes can also form a mutually beneficial relationship with plants and thereby affect nitrogen nutrition. These microbes transform atmospheric nitrogen (N 2 ) into nitrogen forms more usable by plants and microorganisms, a process called fixation. Fixation is accomplished by bacteria called Rhizobia living in association with the roots of legumes. The atmosphere is about 78 percent nitrogen gas (N 2 ), but higher plants cannot take up that form of nitrogen. Rhizobia bacteria take the nitrogen from the air and convert it into an amine (R-NH 2 ) form that they and their host plant can use for building amino acids and proteins. Only legumes, such as peas, beans, clovers, and vetches, can support these nitrogenfixing bacteria. Some trees (such as mimosa, redbud, and locust) and many weeds (including SOURCE: Natalie Bumgarner, University of Tennessee beggarweed, sicklepod, and kudzu) are also legumes. When the legumes die or are tilled into soil, the nitrogen in the plants is mineralized by other microbes so other plants can use it. Most garden soils contain an abundant supply of Rhizobia bacteria. However, on uncultivated land, inoculating legume seed with live Rhizobia bacteria specific to the plants being grown may be a good practice. Keep in mind, though, that if legumes are fertilized with nitrogen, the Rhizobia may remain inactive. The presence of active Rhizobia on legume roots is apparent by observing the presence or absence of small, round nodules on the roots. Nodules that are actively fixing nitrogen display a pink color Some plants, including many native orchids, have mycorrhizal associations. In fact, the disruption of these symbiotic relationships is why it is so difficult to transplant orchids successfully from the wild. 5-19
when broken. Table 5 shows how much nitrogen per acre a good crop of legumes can fix. Other Nitrogen-Fixing Organisms Certain blue-green algae and other organisms are also known to fix atmospheric nitrogen. However, the amount of nitrogen fixed by these microorganisms may not be significant in relatively well-managed systems. It is not uncommon to see microbial fertilizer products sold with the claim that when applied, they inoculate the soil with these microorganisms or stimulate their growth by adding some natural material. In most cases, the soil already contains these microorganisms, but the conditions for optimum growth of garden plants do not favor optimum growth of the microorganisms. Adding these products to the soil may not improve conditions beyond those attained through good gardening practices alone. Soil, Nutrients, and Plants HOW DOES THE SOIL SUPPLY PLANT NUTRIENTS? The 14 essential nutrients that plants acquire from the soil must be absorbed by the roots in order to be used. The majority of nutrients are absorbed by the roots after they have been dissolved in the soil solution, whereas a very small percentage of nutrients are absorbed from soil that is directly contacted by the roots. This fact makes clear the importance of the soil solution in plant nutrition. As shown in Figure 8, plant nutrients dissolved into the soil solution TABLE 5 Potential Nitrogen Content of Crops of Different Legumes (lb N/acre) CROP Alfalfa 130-220 Soybeans 45-130 Clover 90-130 Vetch 45-130 Dry Beans 25-45 POUNDS OF N/ACRE SOURCE: Brady, N.C. and Weil, R.R. The Nature and Properties of Soil, 13th edition. Prentice Hall, NJ. 2002. are available to the plant for uptake and use in growth and development. As the plant takes up nutrient ions from the soil solution, the nutrients are replenished by rock and mineral fragments, secondary minerals, organic materials, and undissolved nutrients (the solid phase of soil). The rate of replenishment depends on many factors, including temperature, microorganisms, the type and quantity of other ions held by clay and organic matter, and the nature of the solid phase. A soil system that can quickly replenish plant nutrients in the soil solution is described as highly buffered. Not all ions stay in solution. Some combine with other ions to form new salts and become unavailable to the plant. When there is sufficient moisture in the soil for leaching to occur, the percolating water can carry away dissolved nutrients that will consequently be lost from the soil profile. The nutrients that are easily leached are usually those that are less strongly held by soil particles. These nutrients are also referred to as the more mobile nutrients in soil. (See Table 2.) It is common for the mobile nutrients to be applied more often to soil because they are more prone to leaching, while immobile nutrients are more likely to be retained in the soil. SOIL PH AND PLANT NUTRITION Ornamental plants generally prefer a ph range of 5.2 to 6.5, but most vegetable crops prefer a ph of 6.0 to 6.5. The ph of soil, or more precisely the ph of the soil solution, is very important because most nutrient uptake is from the soil solution, and ph determines the availability, as shown in Figure 9. Most plant nutrients can be found in forms available for plant uptake at certain ph levels, but they may combine with ions and become unavailable at other ph levels. Figure 9 shows in detail how ph affects the availability of different nutrients, with the highest availability at the various ph values represented by thicker line width. For example, P is most readily available to plants in the ph ranges of 6.5 to 7.5. Soil ph influences nutrient availability and exchange because H + ions can be held by soil particles instead of essential nutrient cations. Hydrogen is held most strongly by clay 5-20
Plant Root H + Negatively charged soil particle Root hairs + + + SOURCE: Revised from Image 5395574. International Society of Arboriculture, International Society of Arboriculture, Bugwood.org and humus, so soil with a low ph and many H + ions has fewer spaces to store other ions. In general terms, if the soil is too acidic (with a ph less than 5.5), plants cannot use N, P, K, S, Ca, Mg, and Mo as well. In addition, nutrients like aluminum (Al) and iron (Fe) become very available and potentially toxic. If the soil is too basic, deficiencies of Fe, Mn, Cu, and Zn can occur. Very alkaline soils are low in N, Fe, Cu, Zn, and Mn and are often high in soluble salts, which can create additional stresses. These soils are more common in arid regions. RECOGNIZING SIGNS OF NUTRIENT DEFICIENCY Linked with any discussion of soil nutrient availability is an understanding of the roles these nutrients perform and the indications of nutrient deficiency in plants. For the 14 key plant nutrients, Table 6 describes their role in plant growth and signs of nutrient deficiency or excess. In plant production, the appearance of deficiency symptoms can be an aid in + + + _ + + + + + _ + + + Cations Plant Tissue Testing Soil testing is one of the most common methods of addressing plant nutrition issues (as described in Chapter 6), but plant tissues can also be tested to determine if nutrients are present in appropriate quantities. Young, recently mature leaves are commonly air dried and sent to a lab for testing. The percentages of different nutrient elements in the leaves can be compared against crop-specific standard optimum levels to diagnose or confirm nutrient deficiencies or toxicities. + FIGURE 8 Magnified view of particles in soil as they interact with the plant roots and soil water to provide nutrients. 5-21
FIGURE 9 Nutrient availability as influenced by ph. determining if the appropriate nutrients are present or available in soil. Depending on the characteristics of the nutrient, too little or too much nutrient may present specific symptoms in certain locations that are helpful in diagnosing plant issues. Caution should be used, however, when diagnosing nutrient deficiencies from plant symptoms since those indicators can mimic disease symptoms and vice versa. The mobility (or immobility) of nutrients within the plant can provide useful clues when diagnosing deficiency symptoms. (See Table 2.) If the deficiency symptom appears first in the old growth, we know that the deficient nutrient is mobile. On the other hand, if the symptom appears in new growth, the deficient nutrient is immobile. Although the movement of N, K, and Mg varies in soils, once inside a plant, all three may be mobilized, moving from older plant tissue to newer tissue. However, Ca, S, Fe, Mn, and Zn are rarely transported from old SOURCE: University of Tennessee leaves to support new plant growth. This is why deficiency symptoms occur first on new leaves. SUMMARY Soil has three distinct facets: physical, chemical, and biological. The role of each facet is related to its effect on plants. Physical soil properties relate to soil structure; they affect water and nutrient movement and therefore affect the growth of plants. The chemical aspects of soil relate to the ability to hold and release nutrients that are essential for plant growth. The nutrientsupplying functions of soil are linked with the study of plant nutrition. Finally, soil biology and the effects of organisms and microorganisms in soil are highly important. All of these facets of soil are interrelated. The management of the physical, chemical, and biological properties of soil as well as their interconnections is described in more detail in Chapter 6 on soil management. 5-22