{"id":2045,"date":"2017-01-31T20:09:56","date_gmt":"2017-01-31T20:09:56","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/wm-biology2\/?post_type=chapter&#038;p=2045"},"modified":"2024-04-25T18:59:39","modified_gmt":"2024-04-25T18:59:39","slug":"nutritional-requirements","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/wm-biology2\/chapter\/nutritional-requirements\/","title":{"raw":"Nutritional Requirements","rendered":"Nutritional Requirements"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>List the elements and compounds required for proper plant nutrition<\/li>\r\n<\/ul>\r\n<\/div>\r\nPlants are unique organisms that can absorb nutrients and water through their root system, as well as carbon dioxide from the atmosphere. Soil quality and climate are the major determinants of plant distribution and growth. The combination of soil nutrients, water, and carbon dioxide, along with sunlight, allows plants to grow.\r\n<h2>The Chemical Composition of Plants<\/h2>\r\n[caption id=\"attachment_2049\" align=\"alignright\" width=\"300\"]<img class=\" wp-image-2049\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31200319\/Figure_31_01_01-e1485893042203.jpg\" alt=\"Illustration shows a root tip. The tip of the root is bare, and hairs grow further up. A cross section at the top of the root reveals xylem tissue interspersed by four ovals containing phloem at the periphery.\" width=\"300\" height=\"440\" \/> Figure 1. Water is absorbed through the root hairs and moves up the xylem to the leaves.[\/caption]\r\n\r\nSince plants require nutrients in the form of elements such as carbon and potassium, it is important to understand the chemical composition of plants. The majority of volume in a plant cell is water; it typically comprises 80 to 90 percent of the plant\u2019s total weight. Soil is the water source for land plants, and can be an abundant source of water, even if it appears dry. Plant roots absorb water from the soil through root hairs and transport it up to the leaves through the xylem. As water vapor is lost from the leaves, the process of transpiration and the polarity of water molecules (which enables them to form hydrogen bonds) draws more water from the roots up through the plant to the leaves (Figure 1). Plants need water to support cell structure, for metabolic functions, to carry nutrients, and for photosynthesis.\r\n\r\nPlant cells need essential substances, collectively called nutrients, to sustain life. Plant nutrients may be composed of either organic or inorganic compounds. An <strong>organic compound<\/strong> is a chemical compound that contains carbon, such as carbon dioxide obtained from the atmosphere. Carbon that was obtained from atmospheric CO<sub>2<\/sub> composes the majority of the dry mass within most plants. An <strong>inorganic compound<\/strong> does not contain carbon and is not part of, or produced by, a living organism. Inorganic substances, which form the majority of the soil solution, are commonly called minerals: those required by plants include nitrogen (N) and potassium (K) for structure and regulation.\r\n<h2>Essential Nutrients<\/h2>\r\nPlants require only light, water and about 20 elements to support all their biochemical needs: these 20 elements are called essential nutrients (Table 1). For an element to be regarded as <strong>essential<\/strong>, three criteria are required: 1) a plant cannot complete its life cycle without the element; 2) no other element can perform the function of the element; and 3) the element is directly involved in plant nutrition.\r\n<table id=\"tab-ch31_01_01\" summary=\"table 31.01.01\">\r\n<thead>\r\n<tr>\r\n<th colspan=\"2\" scope=\"col\" data-align=\"left\">Table 1. Essential Elements for Plant Growth<\/th>\r\n<\/tr>\r\n<tr>\r\n<th scope=\"col\">Macronutrients<\/th>\r\n<th scope=\"col\">Micronutrients<\/th>\r\n<\/tr>\r\n<\/thead>\r\n<tbody>\r\n<tr>\r\n<td>Carbon (C)<\/td>\r\n<td>Iron (Fe)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Hydrogen (H)<\/td>\r\n<td>Manganese (Mn)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Oxygen (O)<\/td>\r\n<td>Boron (B)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Nitrogen (N)<\/td>\r\n<td>Molybdenum (Mo)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Phosphorus (P)<\/td>\r\n<td>Copper (Cu)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Potassium (K)<\/td>\r\n<td>Zinc (Zn)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Calcium (Ca)<\/td>\r\n<td>Chlorine (Cl)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Magnesium (Mg)<\/td>\r\n<td>Nickel (Ni)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td>Sulfur (S)<\/td>\r\n<td>Cobalt (Co)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><\/td>\r\n<td>Sodium (Na)<\/td>\r\n<\/tr>\r\n<tr>\r\n<td><\/td>\r\n<td>Silicon (Si)<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n<h3>Macronutrients and Micronutrients<\/h3>\r\nThe essential elements can be divided into two groups: macronutrients and micronutrients. Nutrients that plants require in larger amounts are called <strong>macronutrients<\/strong>. About half of the essential elements are considered macronutrients: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. The first of these macronutrients, carbon (C), is required to form carbohydrates, proteins, nucleic acids, and many other compounds; it is therefore present in all macromolecules. On average, the dry weight (excluding water) of a cell is 50 percent carbon. As shown in Figure 2, carbon is a key part of plant biomolecules.\r\n\r\n[caption id=\"attachment_2050\" align=\"aligncenter\" width=\"652\"]<img class=\" wp-image-2050\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31200507\/Figure_31_01_02.jpg\" alt=\"Three cellulose fibers and the chemical structure of cellulose is shown. Cellulose consists of unbranched chains of glucose subunits that form long, straight fibers.\" width=\"652\" height=\"398\" \/> Figure 2. Cellulose, the main structural component of the plant cell wall, makes up over thirty percent of plant matter. It is the most abundant organic compound on earth.[\/caption]\r\n\r\nThe next most abundant element in plant cells is nitrogen (N); it is part of proteins and nucleic acids. Nitrogen is also used in the synthesis of some vitamins. Hydrogen and oxygen are macronutrients that are part of many organic compounds, and also form water. Oxygen is necessary for cellular respiration; plants use oxygen to store energy in the form of ATP. Phosphorus (P), another macromolecule, is necessary to synthesize nucleic acids and phospholipids. As part of ATP, phosphorus enables food energy to be converted into chemical energy through oxidative phosphorylation. Likewise, light energy is converted into chemical energy during photophosphorylation in photosynthesis, and into chemical energy to be extracted during respiration.\u00a0Sulfur is part of certain amino acids, such as cysteine and methionine, and is present in several coenzymes. Sulfur also plays a role in photosynthesis as part of the electron transport chain, where hydrogen gradients play a key role in the conversion of light energy into ATP. Potassium (K) is important because of its role in regulating stomatal opening and closing. As the openings for gas exchange, stomata help maintain a healthy water balance; a potassium ion pump supports this process.\r\n\r\nMagnesium (Mg) and calcium (Ca) are also important macronutrients. The role of calcium is twofold: to regulate nutrient transport, and to support many enzyme functions. Magnesium is important to the photosynthetic process. These minerals, along with the micronutrients, which are described below, also contribute to the plant\u2019s ionic balance.\r\n\r\nIn addition to macronutrients, organisms require various elements in small amounts. These <strong>micronutrients<\/strong>, or trace elements, are present in very small quantities. They include boron (B), chlorine (Cl), manganese (Mn), iron (Fe), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni), silicon (Si), and sodium (Na).\r\n\r\nDeficiencies in any of these nutrients\u2014particularly the macronutrients\u2014can adversely affect plant growth (Figure 3). Depending on the specific nutrient, a lack can cause stunted growth, slow growth, or chlorosis (yellowing of the leaves). Extreme deficiencies may result in leaves showing signs of cell death.\r\n\r\n[caption id=\"attachment_2051\" align=\"aligncenter\" width=\"1024\"]<img class=\"wp-image-2051 size-large\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31200922\/Figure_31_01_03-1024x221.jpg\" alt=\"Photo (a) shows a tomato plant with two green tomato fruits. The fruits have turned dark brown on the bottom. Photo (b) shows a plant with green leaves; some of the leaves have turned yellow. Photo (c) shows a five-lobed leaf that is yellow with greenish veins. Photo (d) shows green palm leaves with yellow tips.\" width=\"1024\" height=\"221\" \/> Figure 3. Nutrient deficiency is evident in the symptoms these plants show. This (a) grape tomato suffers from blossom end rot caused by calcium deficiency. The yellowing in this (b) Frangula alnus results from magnesium deficiency. Inadequate magnesium also leads to (c) intervenal chlorosis, seen here in a sweetgum leaf. This (d) palm is affected by potassium deficiency. (credit c: modification of work by Jim Conrad; credit d: modification of work by Malcolm Manners)[\/caption]\r\n\r\n<div class=\"textbox\">Visit this <a href=\"http:\/\/www.kscience.co.uk\/animations\/minerals.htm\" target=\"_blank\" rel=\"nofollow noopener\">interactive experiment on plant nutrient deficiencies<\/a>. You can adjust the amounts of N, P, K, Ca, Mg, and Fe that plants receive . . . and see what happens.<\/div>\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Hydroponics<\/h3>\r\n<div class=\"PageContent-ny9bj0-0 iapMdy\" tabindex=\"0\">\r\n<div id=\"main-content\" class=\"MainContent__HideOutline-sc-6yy1if-0 bdVAq\" tabindex=\"-1\">\r\n<div id=\"686bc1e6-9010-4523-826d-2a225701c3c8\" class=\"chapter-content-module\" data-type=\"page\" data-cnxml-to-html-ver=\"2.1.0\"><section id=\"fs-idp28642048\" data-depth=\"1\"><section id=\"fs-idp107564144\" data-depth=\"2\">\r\n<div id=\"fs-idp94539344\" class=\"everyday ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\"><section>\r\n<div class=\"os-note-body\">\r\n<p id=\"fs-idp1671552\">Hydroponics is a method of growing plants in a water-nutrient solution instead of soil. Since its advent, hydroponics has developed into a growing process that researchers often use. Scientists who are interested in studying plant nutrient deficiencies can use hydroponics to study the effects of different nutrient combinations under strictly controlled conditions. Hydroponics has also developed as a way to grow flowers, vegetables, and other crops in greenhouse environments. You might find hydroponically grown produce at your local grocery store. Today, many lettuces and tomatoes in your market have been hydroponically grown.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"603\"]<img id=\"8\" src=\"https:\/\/openstax.org\/resources\/95c9e9554a888fbb9a05d251e663c9e5667d8e30\" alt=\"This photo shows a NASA researcher examining a hydroponic array, including several types of plants. The onions grow in a tray with long, thin openings from which emerge their grass-like leaves. Next to the onions are lettuces in a try appearing to have holes from which their larger leaves can emerge.\" width=\"603\" height=\"401\" data-media-type=\"image\/jpg\" \/> Figure 4. Plant physiologist Ray Wheeler checks onions being grown using hydroponic techniques. The other plants are Bibb lettuce (left) and radishes (right). Credit: NASA[\/caption]\r\n\r\n<\/div>\r\n<\/section><\/div>\r\n<\/section><\/section><\/div>\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n<div class=\"textbox learning-objectives\">\r\n<h3>In Summary:\u00a0Nutritional Requirements<\/h3>\r\nPlants can absorb inorganic nutrients and water through their root system, and carbon dioxide from the environment. The combination of organic compounds, along with water, carbon dioxide, and sunlight, produce the energy that allows plants to grow. Inorganic compounds form the majority of the soil solution. Plants access water though the soil. Water is absorbed by the plant root, transports nutrients throughout the plant, and maintains the structure of the plant. Essential elements are indispensable elements for plant growth. They are divided into macronutrients and micronutrients. The macronutrients plants require are carbon, nitrogen, hydrogen, oxygen, phosphorus, potassium, calcium, magnesium, and sulfur. Important micronutrients include iron, manganese, boron, molybdenum, copper, zinc, chlorine, nickel, cobalt, silicon and sodium.\r\n\r\n<\/div>\r\n<div class=\"textbox tryit\">\r\n<h3>Try It<\/h3>\r\nhttps:\/\/assess.lumenlearning.com\/practice\/12761cba-c33f-4a1a-8cbb-481729e00787\r\n\r\n<\/div>\r\n<h2>The Soil<\/h2>\r\n<p id=\"fs-idm75787760\">Plants obtain inorganic elements from the soil, which serves as a natural medium for land plants.\u00a0<span id=\"term1354\" data-type=\"term\">Soil<\/span>\u00a0is the outer loose layer that covers the surface of Earth. <strong>Soil<\/strong> quality is a major determinant, along with climate, of plant distribution and growth. Soil quality depends not only on the chemical composition of the soil, but also the topography (regional surface features) and the presence of living organisms. In agriculture, the history of the soil, such as the cultivating practices and previous crops, modify the characteristics and fertility of that soil.<\/p>\r\n<p id=\"fs-idp92131984\">Soil develops very slowly over long periods of time, and its formation results from natural and environmental forces acting on mineral, rock, and organic compounds. Soils can be divided into two groups:\u00a0<strong><span id=\"term1355\" data-type=\"term\">organic soils<\/span><\/strong>\u00a0are those that are formed from sedimentation and primarily composed of organic matter, while those that are formed from the weathering of rocks and are primarily composed of inorganic material are called\u00a0<strong><span id=\"term1356\" data-type=\"term\">mineral soils<\/span><\/strong>. Mineral soils are predominant in terrestrial ecosystems, where soils may be covered by water for part of the year or exposed to the atmosphere.<\/p>\r\n\r\n<section id=\"fs-idm122071040\" data-depth=\"1\">\r\n<h3 data-type=\"title\">Soil Composition<\/h3>\r\n<p id=\"fs-idm86116256\">Soil consists of these major components (Figure 5):<\/p>\r\n\r\n<ul id=\"fs-idm20861600\">\r\n \t<li>inorganic mineral matter, about 40 to 45 percent of the soil volume<\/li>\r\n \t<li>organic matter, about 5 percent of the soil volume<\/li>\r\n \t<li>water and air, about 50 percent of the soil volume<\/li>\r\n<\/ul>\r\n<p id=\"fs-idp68595664\">The amount of each of the four major components of soil depends on the amount of vegetation, soil compaction, and water present in the soil. A good healthy soil has sufficient air, water, minerals, and organic material to promote and sustain plant life.<\/p>\r\n\r\n<div id=\"fs-idp92229216\" class=\"visual-connection ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\"><section>\r\n<div class=\"os-note-body\">\r\n<div id=\"fig-ch31_02_01\" class=\"os-figure\">\r\n<figure data-id=\"fig-ch31_02_01\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"399\"]<img id=\"4\" class=\"\" src=\"https:\/\/openstax.org\/resources\/94af46e2ee398a72f66ae10dbe69df2f1ecb1f63\" alt=\" Illustration shows a pie graph that outlines the composition of soil. Forty-five percent is inorganic mineral matter, 25 percent is water, 25 percent is air, and 5 percent is organic matter, including microorganisms and macroorganisms.\" width=\"399\" height=\"367\" data-media-type=\"image\/png\" \/> Figure 5.\u00a0The four major components of soil are shown: inorganic minerals, organic matter, water, and air.[\/caption]<\/figure>\r\n<div class=\"os-caption-container\"><\/div>\r\n<\/div>\r\n<div class=\"textbox exercises\">\r\n<h3>Practice Question<\/h3>\r\nSoil compaction can result when soil is compressed by heavy machinery or even foot traffic. How might this compaction change the soil composition?\r\n\r\n[reveal-answer q=\"778822\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"778822\"]The composition percentages would shift. A large amount of air would be lost and a small amount of water would be removed by soil compaction.[\/hidden-answer]\r\n\r\n<\/div>\r\n<\/div>\r\n<\/section><\/div>\r\n<p id=\"fs-idm88922480\">The organic material of soil, called\u00a0<strong><span id=\"term1357\" data-type=\"term\">humus<\/span><\/strong>, is made up of microorganisms (dead and alive), and dead animals and plants in varying stages of decay. Humus improves soil structure and provides plants with water and minerals. The inorganic material of soil consists of rock, slowly broken down into smaller particles that vary in size. Soil particles that are 0.1 to 2 mm in diameter are\u00a0<strong><span id=\"term1358\" data-type=\"term\">sand<\/span><\/strong>. Soil particles between 0.002 and 0.1\u00a0mm are called\u00a0<strong><span id=\"term1359\" data-type=\"term\">silt<\/span><\/strong>, and even smaller particles, less than 0.002 mm in diameter, are called\u00a0<strong><span id=\"term1360\" data-type=\"term\">clay<\/span><\/strong>. Some soils have no dominant particle size and contain a mixture of sand, silt, and humus; these soils are called\u00a0<span id=\"term1361\" data-type=\"term\">loams<\/span>.<\/p>\r\n\r\n<div id=\"fs-idp957920\" class=\"interactive ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\"><section>\r\n<div class=\"os-note-body\">\r\n<div class=\"textbox\">Explore this\u00a0<a href=\"http:\/\/openstax.org\/l\/soil_survey_map\" target=\"_blank\" rel=\"noopener nofollow\">interactive map<\/a>\u00a0from the USDA\u2019s National Cooperative Soil Survey to access soil data for almost any region in the United States.<\/div>\r\n<\/div>\r\n<\/section><\/div>\r\n<\/section><section id=\"fs-idm92717632\" data-depth=\"1\">\r\n<h3 data-type=\"title\">Soil Formation<\/h3>\r\n<p id=\"fs-idm92332624\">Soil formation is the consequence of a combination of biological, physical, and chemical processes. Soil should ideally contain 50 percent solid material and 50 percent pore space. About one-half of the pore space should contain water, and the other half should contain air. The organic component of soil serves as a cementing agent, returns nutrients to the plant, allows soil to store moisture, makes soil tillable for farming, and provides energy for soil microorganisms. Most soil microorganisms\u2014bacteria, algae, or fungi\u2014are dormant in dry soil, but become active once moisture is available.<\/p>\r\n<p id=\"fs-idm61091136\">Soil distribution is not homogenous because its formation results in the production of layers; together, the vertical section of a soil is called the\u00a0<strong><span id=\"term1362\" data-type=\"term\">soil profile<\/span><\/strong>. Within the soil profile, soil scientists define zones called horizons. A\u00a0<strong><span id=\"term1363\" data-type=\"term\">horizon<\/span><\/strong>\u00a0is a soil layer with distinct physical and chemical properties that differ from those of other layers. Five factors account for soil formation: parent material, climate, topography, biological factors, and time.<\/p>\r\n\r\n<section id=\"fs-idm89960448\" data-depth=\"2\">\r\n<h4 data-type=\"title\">Parent Material<\/h4>\r\n<p id=\"fs-idm1756192\">The organic and inorganic material in which soils form is the\u00a0<strong><span id=\"term1364\" data-type=\"term\">parent material<\/span><\/strong>. Mineral soils form directly from the weathering of\u00a0<strong><span id=\"term1365\" data-type=\"term\">bedrock<\/span><\/strong>, the solid rock that lies beneath the soil, and therefore, they have a similar composition to the original rock. Other soils form in materials that came from elsewhere, such as sand and glacial drift. Materials located in the depth of the soil are relatively unchanged compared with the deposited material. Sediments in rivers may have different characteristics, depending on whether the stream moves quickly or slowly. A fast-moving river could have sediments of rocks and sand, whereas a slow-moving river could have fine-textured material, such as clay.<\/p>\r\n\r\n<\/section><section id=\"fs-idp27656048\" data-depth=\"2\">\r\n<h4 data-type=\"title\">Climate<\/h4>\r\n<p id=\"fs-idm32351136\">Temperature, moisture, and wind cause different patterns of weathering and therefore affect soil characteristics. The presence of moisture and nutrients from weathering will also promote biological activity: a key component of a quality soil.<\/p>\r\n\r\n<\/section><section id=\"fs-idp73414688\" data-depth=\"2\">\r\n<h4 data-type=\"title\">Topography<\/h4>\r\n<p id=\"fs-idp77262848\">Regional surface features (familiarly called \u201cthe lay of the land\u201d) can have a major influence on the characteristics and fertility of a soil. Topography affects water runoff, which strips away parent material and affects plant growth. Steeps soils are more prone to erosion and may be thinner than soils that are relatively flat or level.<\/p>\r\n\r\n<\/section><section id=\"fs-idp22052656\" data-depth=\"2\">\r\n<h4 data-type=\"title\">Biological factors<\/h4>\r\n<p id=\"fs-idp91477184\">The presence of living organisms greatly affects soil formation and structure. Animals and microorganisms can produce pores and crevices, and plant roots can penetrate into crevices to produce more fragmentation. Plant secretions promote the development of microorganisms around the root, in an area known as the\u00a0<strong><span id=\"term1366\" data-type=\"term\">rhizosphere<\/span><\/strong>. Additionally, leaves and other material that fall from plants decompose and contribute to soil composition.<\/p>\r\n\r\n<\/section><section id=\"fs-idp3692064\" data-depth=\"2\">\r\n<h4 data-type=\"title\">Time<\/h4>\r\n<p id=\"fs-idm35248608\">Time is an important factor in soil formation because soils develop over long periods. Soil formation is a dynamic process. Materials are deposited over time, decompose, and transform into other materials that can be used by living organisms or deposited onto the surface of the soil.<\/p>\r\n\r\n<\/section><\/section><section id=\"fs-idm58959248\" data-depth=\"1\">\r\n<h3 data-type=\"title\">Physical Properties of the Soil<\/h3>\r\n<div id=\"fs-idm58951232\" class=\"visual-connection ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\"><section>\r\n<div class=\"os-note-body\">\r\n<div id=\"fig-ch31_02_02\" class=\"os-figure\">\r\n<figure data-id=\"fig-ch31_02_02\">\r\n\r\n[caption id=\"\" align=\"alignright\" width=\"285\"]<img id=\"7\" src=\"https:\/\/openstax.org\/resources\/0755a70bb4c7fe54ad67a1cba5b452a3dc1f4130\" alt=\" Illustration shows a cross-section of soil layers, or horizons. The top layer, from zero to two inches, is the O horizon. The O horizon is a rich, deep brown color. From two to ten inches is the A horizon. This layer is slightly lighter in color than the O horizon, and extensive root systems are visible. From ten to thirty inches is the B horizon. The B horizon is reddish brown. Longer roots extend to the bottom of this layer. The C horizon extends from 30 to 48 inches. This layer is rocky and devoid of roots.\" width=\"285\" height=\"549\" data-media-type=\"image\/png\" \/> Figure 6 This soil profile shows the different soil layers (O horizon, A horizon, B horizon, and C horizon) found in typical soils. (credit: modification of work by USDA)[\/caption]<\/figure>\r\n<\/div>\r\n<p id=\"fs-idm90918144\">Soils are named and classified based on their horizons. The soil profile has four distinct layers: 1) O horizon; 2) A horizon; 3) B horizon, or subsoil; and 4) C horizon, or soil base (Figure 6). The\u00a0<span id=\"term1367\" data-type=\"term\">O horizon<\/span>\u00a0has freshly decomposing organic matter\u2014humus\u2014at its surface, with decomposed vegetation at its base. Humus enriches the soil with nutrients and enhances soil moisture retention. Topsoil\u2014the top layer of soil\u2014is usually two to three inches deep, but this depth can vary considerably. For instance, river deltas like the Mississippi River delta have deep layers of topsoil. Topsoil is rich in organic material; microbial processes occur there, and it is the \u201cworkhorse\u201d of plant production.<\/p>\r\nThe\u00a0<span id=\"term1368\" data-type=\"term\">A horizon<\/span>\u00a0consists of a mixture of organic material with inorganic products of weathering, and it is therefore the beginning of true mineral soil. This horizon is typically darkly colored because of the presence of organic matter. In this area, rainwater percolates through the soil and carries materials from the surface. The\u00a0<span id=\"term1369\" data-type=\"term\">B\u00a0horizon<\/span>\u00a0is an accumulation of mostly fine material that has moved downward, resulting in a dense layer in the soil. In some soils, the B horizon contains nodules or a layer of calcium carbonate. The\u00a0<span id=\"term1370\" data-type=\"term\">C horizon<\/span>, or soil base, includes the parent material, plus the organic and inorganic material that is broken down to form soil. The parent material may be either created in its natural place, or transported from elsewhere to its present location. Beneath the C horizon lies bedrock.\r\n<div class=\"textbox exercises\">\r\n<h3>PRactice Question<\/h3>\r\nWhich horizon is considered the topsoil, and which is considered the subsoil?\r\n\r\na. O horizon\r\n\r\nb. A horizon\r\n\r\nc. C horizon\r\n\r\nd. B horizon\r\n\r\n[reveal-answer q=\"334561\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"334561\"]\r\n\r\n(a) The O horizon is the topsoil\r\n\r\n(d) The B horizon is the subsoil\r\n\r\n([\/hidden-answer]\r\n\r\n<\/div>\r\n<\/div>\r\n<\/section><\/div>\r\n<p id=\"fs-idm90711008\">Some soils may have additional layers, or lack one of these layers. The thickness of the layers is also variable, and depends on the factors that influence soil formation. In general, immature soils may have O, A, and C horizons, whereas mature soils may display all of these, plus additional layers (Figure 7).<\/p>\r\n\r\n<div id=\"fig-ch31_02_03\" class=\"os-figure\">\r\n<figure data-id=\"fig-ch31_02_03\">\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"305\"]<img id=\"8\" src=\"https:\/\/openstax.org\/resources\/710488c7a8a03db9a79b47a0d612605ec19a80ab\" alt=\" In the photo, soil has been cut away to reveal the soil profile. The O horizon is at the soil surface and is a rich black color. The brown A horizon starts beneath the O horizon and extends to about two-and-a-half feet beneath the surface. The B horizon is reddish brown and extends from the bottom of the A horizon to about two feet deep. The C horizon extends from the bottom of the B horizon to the bottom of the photo at a depth of four feet. The C horizon is light brown and has a coarser consistency than the A or B horizons.\" width=\"305\" height=\"567\" data-media-type=\"image\/jpg\" \/> Figure 7. The San Joaquin soil profile has an O horizon, A horizon, B horizon, and C horizon. (credit: modification of work by USDA)[\/caption]<\/figure>\r\n<div class=\"os-caption-container\"><\/div>\r\n<\/div>\r\n<div id=\"fs-idm61608448\" class=\"career ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\"><section>\r\n<div class=\"os-note-body\">\r\n<div class=\"textbox key-takeaways\">\r\n<h3>Soil Scientist<\/h3>\r\n<section id=\"fs-idm58959248\" data-depth=\"1\">\r\n<div id=\"fs-idm61608448\" class=\"career ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\"><section>\r\n<div class=\"os-note-body\">\r\n<p id=\"fs-idp17240768\">A soil scientist studies the biological components, physical and chemical properties, distribution, formation, and morphology of soils. Soil scientists need to have a strong background in physical and life sciences, plus a foundation in mathematics. They may work for federal or state agencies, academia, or the private sector. Their work may involve collecting data, carrying out research, interpreting results, inspecting soils, conducting soil surveys, and recommending soil management programs.<\/p>\r\n\r\n<div id=\"fig-ch31_02_04\" class=\"os-figure\">\r\n<figure class=\" \" data-id=\"fig-ch31_02_04\">\r\n\r\n[caption id=\"\" align=\"alignright\" width=\"275\"]<img id=\"10\" src=\"https:\/\/openstax.org\/resources\/22de3a6c18ef1f5d81010417a171b31b32760e52\" alt=\" Photo shows a man standing next to a wall of soil in a pit that is as deep as he is tall.\" width=\"275\" height=\"692\" data-media-type=\"image\/jpg\" \/> Figure 8. This soil scientist is studying the horizons and composition of soil at a research site. (credit: USDA)[\/caption]<\/figure>\r\n<div class=\"os-caption-container\"><\/div>\r\n<\/div>\r\n<p id=\"fs-idp43896176\">Many soil scientists work both in an office and in the field. According to the United States Department of Agriculture (USDA): \u201ca soil scientist needs good observation skills to analyze and determine the characteristics of different types of soils. Soil types are complex and the geographical areas a soil scientist may survey are varied. Aerial photos or various satellite images are often used to research the areas. Computer skills and geographic information systems (GIS) help the scientist to analyze the multiple facets of geomorphology, topography, vegetation, and climate to discover the patterns left on the landscape.\u201d[footnote]National Resources Conservation Service \/ United States Department of Agriculture. \u201cCareers in Soil Science.\u201d\u00a0<a href=\"http:\/\/openstax.org\/l\/NRCS\" target=\"_blank\" rel=\"noopener nofollow\">http:\/\/openstax.org\/l\/NRCS<\/a>[\/footnote]<sup id=\"footnote-ref1\" data-type=\"footnote-number\"><\/sup>\u00a0Soil scientists play a key role in understanding the soil\u2019s past, analyzing present conditions, and making recommendations for future soil-related practices.<\/p>\r\n\r\n<\/div>\r\n<\/section><\/div>\r\n<\/section><\/div>\r\n&nbsp;\r\n\r\n<\/div>\r\n<\/section><\/div>\r\n<\/section>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>List the elements and compounds required for proper plant nutrition<\/li>\n<\/ul>\n<\/div>\n<p>Plants are unique organisms that can absorb nutrients and water through their root system, as well as carbon dioxide from the atmosphere. Soil quality and climate are the major determinants of plant distribution and growth. The combination of soil nutrients, water, and carbon dioxide, along with sunlight, allows plants to grow.<\/p>\n<h2>The Chemical Composition of Plants<\/h2>\n<div id=\"attachment_2049\" style=\"width: 310px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2049\" class=\"wp-image-2049\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31200319\/Figure_31_01_01-e1485893042203.jpg\" alt=\"Illustration shows a root tip. The tip of the root is bare, and hairs grow further up. A cross section at the top of the root reveals xylem tissue interspersed by four ovals containing phloem at the periphery.\" width=\"300\" height=\"440\" \/><\/p>\n<p id=\"caption-attachment-2049\" class=\"wp-caption-text\">Figure 1. Water is absorbed through the root hairs and moves up the xylem to the leaves.<\/p>\n<\/div>\n<p>Since plants require nutrients in the form of elements such as carbon and potassium, it is important to understand the chemical composition of plants. The majority of volume in a plant cell is water; it typically comprises 80 to 90 percent of the plant\u2019s total weight. Soil is the water source for land plants, and can be an abundant source of water, even if it appears dry. Plant roots absorb water from the soil through root hairs and transport it up to the leaves through the xylem. As water vapor is lost from the leaves, the process of transpiration and the polarity of water molecules (which enables them to form hydrogen bonds) draws more water from the roots up through the plant to the leaves (Figure 1). Plants need water to support cell structure, for metabolic functions, to carry nutrients, and for photosynthesis.<\/p>\n<p>Plant cells need essential substances, collectively called nutrients, to sustain life. Plant nutrients may be composed of either organic or inorganic compounds. An <strong>organic compound<\/strong> is a chemical compound that contains carbon, such as carbon dioxide obtained from the atmosphere. Carbon that was obtained from atmospheric CO<sub>2<\/sub> composes the majority of the dry mass within most plants. An <strong>inorganic compound<\/strong> does not contain carbon and is not part of, or produced by, a living organism. Inorganic substances, which form the majority of the soil solution, are commonly called minerals: those required by plants include nitrogen (N) and potassium (K) for structure and regulation.<\/p>\n<h2>Essential Nutrients<\/h2>\n<p>Plants require only light, water and about 20 elements to support all their biochemical needs: these 20 elements are called essential nutrients (Table 1). For an element to be regarded as <strong>essential<\/strong>, three criteria are required: 1) a plant cannot complete its life cycle without the element; 2) no other element can perform the function of the element; and 3) the element is directly involved in plant nutrition.<\/p>\n<table id=\"tab-ch31_01_01\" summary=\"table 31.01.01\">\n<thead>\n<tr>\n<th colspan=\"2\" scope=\"col\" data-align=\"left\">Table 1. Essential Elements for Plant Growth<\/th>\n<\/tr>\n<tr>\n<th scope=\"col\">Macronutrients<\/th>\n<th scope=\"col\">Micronutrients<\/th>\n<\/tr>\n<\/thead>\n<tbody>\n<tr>\n<td>Carbon (C)<\/td>\n<td>Iron (Fe)<\/td>\n<\/tr>\n<tr>\n<td>Hydrogen (H)<\/td>\n<td>Manganese (Mn)<\/td>\n<\/tr>\n<tr>\n<td>Oxygen (O)<\/td>\n<td>Boron (B)<\/td>\n<\/tr>\n<tr>\n<td>Nitrogen (N)<\/td>\n<td>Molybdenum (Mo)<\/td>\n<\/tr>\n<tr>\n<td>Phosphorus (P)<\/td>\n<td>Copper (Cu)<\/td>\n<\/tr>\n<tr>\n<td>Potassium (K)<\/td>\n<td>Zinc (Zn)<\/td>\n<\/tr>\n<tr>\n<td>Calcium (Ca)<\/td>\n<td>Chlorine (Cl)<\/td>\n<\/tr>\n<tr>\n<td>Magnesium (Mg)<\/td>\n<td>Nickel (Ni)<\/td>\n<\/tr>\n<tr>\n<td>Sulfur (S)<\/td>\n<td>Cobalt (Co)<\/td>\n<\/tr>\n<tr>\n<td><\/td>\n<td>Sodium (Na)<\/td>\n<\/tr>\n<tr>\n<td><\/td>\n<td>Silicon (Si)<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<h3>Macronutrients and Micronutrients<\/h3>\n<p>The essential elements can be divided into two groups: macronutrients and micronutrients. Nutrients that plants require in larger amounts are called <strong>macronutrients<\/strong>. About half of the essential elements are considered macronutrients: carbon, hydrogen, oxygen, nitrogen, phosphorus, potassium, calcium, magnesium and sulfur. The first of these macronutrients, carbon (C), is required to form carbohydrates, proteins, nucleic acids, and many other compounds; it is therefore present in all macromolecules. On average, the dry weight (excluding water) of a cell is 50 percent carbon. As shown in Figure 2, carbon is a key part of plant biomolecules.<\/p>\n<div id=\"attachment_2050\" style=\"width: 662px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2050\" class=\"wp-image-2050\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31200507\/Figure_31_01_02.jpg\" alt=\"Three cellulose fibers and the chemical structure of cellulose is shown. Cellulose consists of unbranched chains of glucose subunits that form long, straight fibers.\" width=\"652\" height=\"398\" \/><\/p>\n<p id=\"caption-attachment-2050\" class=\"wp-caption-text\">Figure 2. Cellulose, the main structural component of the plant cell wall, makes up over thirty percent of plant matter. It is the most abundant organic compound on earth.<\/p>\n<\/div>\n<p>The next most abundant element in plant cells is nitrogen (N); it is part of proteins and nucleic acids. Nitrogen is also used in the synthesis of some vitamins. Hydrogen and oxygen are macronutrients that are part of many organic compounds, and also form water. Oxygen is necessary for cellular respiration; plants use oxygen to store energy in the form of ATP. Phosphorus (P), another macromolecule, is necessary to synthesize nucleic acids and phospholipids. As part of ATP, phosphorus enables food energy to be converted into chemical energy through oxidative phosphorylation. Likewise, light energy is converted into chemical energy during photophosphorylation in photosynthesis, and into chemical energy to be extracted during respiration.\u00a0Sulfur is part of certain amino acids, such as cysteine and methionine, and is present in several coenzymes. Sulfur also plays a role in photosynthesis as part of the electron transport chain, where hydrogen gradients play a key role in the conversion of light energy into ATP. Potassium (K) is important because of its role in regulating stomatal opening and closing. As the openings for gas exchange, stomata help maintain a healthy water balance; a potassium ion pump supports this process.<\/p>\n<p>Magnesium (Mg) and calcium (Ca) are also important macronutrients. The role of calcium is twofold: to regulate nutrient transport, and to support many enzyme functions. Magnesium is important to the photosynthetic process. These minerals, along with the micronutrients, which are described below, also contribute to the plant\u2019s ionic balance.<\/p>\n<p>In addition to macronutrients, organisms require various elements in small amounts. These <strong>micronutrients<\/strong>, or trace elements, are present in very small quantities. They include boron (B), chlorine (Cl), manganese (Mn), iron (Fe), zinc (Zn), copper (Cu), molybdenum (Mo), nickel (Ni), silicon (Si), and sodium (Na).<\/p>\n<p>Deficiencies in any of these nutrients\u2014particularly the macronutrients\u2014can adversely affect plant growth (Figure 3). Depending on the specific nutrient, a lack can cause stunted growth, slow growth, or chlorosis (yellowing of the leaves). Extreme deficiencies may result in leaves showing signs of cell death.<\/p>\n<div id=\"attachment_2051\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2051\" class=\"wp-image-2051 size-large\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/01\/31200922\/Figure_31_01_03-1024x221.jpg\" alt=\"Photo (a) shows a tomato plant with two green tomato fruits. The fruits have turned dark brown on the bottom. Photo (b) shows a plant with green leaves; some of the leaves have turned yellow. Photo (c) shows a five-lobed leaf that is yellow with greenish veins. Photo (d) shows green palm leaves with yellow tips.\" width=\"1024\" height=\"221\" \/><\/p>\n<p id=\"caption-attachment-2051\" class=\"wp-caption-text\">Figure 3. Nutrient deficiency is evident in the symptoms these plants show. This (a) grape tomato suffers from blossom end rot caused by calcium deficiency. The yellowing in this (b) Frangula alnus results from magnesium deficiency. Inadequate magnesium also leads to (c) intervenal chlorosis, seen here in a sweetgum leaf. This (d) palm is affected by potassium deficiency. (credit c: modification of work by Jim Conrad; credit d: modification of work by Malcolm Manners)<\/p>\n<\/div>\n<div class=\"textbox\">Visit this <a href=\"http:\/\/www.kscience.co.uk\/animations\/minerals.htm\" target=\"_blank\" rel=\"nofollow noopener\">interactive experiment on plant nutrient deficiencies<\/a>. You can adjust the amounts of N, P, K, Ca, Mg, and Fe that plants receive . . . and see what happens.<\/div>\n<div class=\"textbox key-takeaways\">\n<h3>Hydroponics<\/h3>\n<div class=\"PageContent-ny9bj0-0 iapMdy\" tabindex=\"0\">\n<div id=\"main-content\" class=\"MainContent__HideOutline-sc-6yy1if-0 bdVAq\" tabindex=\"-1\">\n<div id=\"686bc1e6-9010-4523-826d-2a225701c3c8\" class=\"chapter-content-module\" data-type=\"page\" data-cnxml-to-html-ver=\"2.1.0\">\n<section id=\"fs-idp28642048\" data-depth=\"1\">\n<section id=\"fs-idp107564144\" data-depth=\"2\">\n<div id=\"fs-idp94539344\" class=\"everyday ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\">\n<section>\n<div class=\"os-note-body\">\n<p id=\"fs-idp1671552\">Hydroponics is a method of growing plants in a water-nutrient solution instead of soil. Since its advent, hydroponics has developed into a growing process that researchers often use. Scientists who are interested in studying plant nutrient deficiencies can use hydroponics to study the effects of different nutrient combinations under strictly controlled conditions. Hydroponics has also developed as a way to grow flowers, vegetables, and other crops in greenhouse environments. You might find hydroponically grown produce at your local grocery store. Today, many lettuces and tomatoes in your market have been hydroponically grown.<\/p>\n<div style=\"width: 613px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"8\" src=\"https:\/\/openstax.org\/resources\/95c9e9554a888fbb9a05d251e663c9e5667d8e30\" alt=\"This photo shows a NASA researcher examining a hydroponic array, including several types of plants. The onions grow in a tray with long, thin openings from which emerge their grass-like leaves. Next to the onions are lettuces in a try appearing to have holes from which their larger leaves can emerge.\" width=\"603\" height=\"401\" data-media-type=\"image\/jpg\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 4. Plant physiologist Ray Wheeler checks onions being grown using hydroponic techniques. The other plants are Bibb lettuce (left) and radishes (right). Credit: NASA<\/p>\n<\/div>\n<\/div>\n<\/section>\n<\/div>\n<\/section>\n<\/section>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox learning-objectives\">\n<h3>In Summary:\u00a0Nutritional Requirements<\/h3>\n<p>Plants can absorb inorganic nutrients and water through their root system, and carbon dioxide from the environment. The combination of organic compounds, along with water, carbon dioxide, and sunlight, produce the energy that allows plants to grow. Inorganic compounds form the majority of the soil solution. Plants access water though the soil. Water is absorbed by the plant root, transports nutrients throughout the plant, and maintains the structure of the plant. Essential elements are indispensable elements for plant growth. They are divided into macronutrients and micronutrients. The macronutrients plants require are carbon, nitrogen, hydrogen, oxygen, phosphorus, potassium, calcium, magnesium, and sulfur. Important micronutrients include iron, manganese, boron, molybdenum, copper, zinc, chlorine, nickel, cobalt, silicon and sodium.<\/p>\n<\/div>\n<div class=\"textbox tryit\">\n<h3>Try It<\/h3>\n<p>\t<iframe id=\"assessment_practice_12761cba-c33f-4a1a-8cbb-481729e00787\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/12761cba-c33f-4a1a-8cbb-481729e00787?iframe_resize_id=assessment_practice_id_12761cba-c33f-4a1a-8cbb-481729e00787\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe><\/p>\n<\/div>\n<h2>The Soil<\/h2>\n<p id=\"fs-idm75787760\">Plants obtain inorganic elements from the soil, which serves as a natural medium for land plants.\u00a0<span id=\"term1354\" data-type=\"term\">Soil<\/span>\u00a0is the outer loose layer that covers the surface of Earth. <strong>Soil<\/strong> quality is a major determinant, along with climate, of plant distribution and growth. Soil quality depends not only on the chemical composition of the soil, but also the topography (regional surface features) and the presence of living organisms. In agriculture, the history of the soil, such as the cultivating practices and previous crops, modify the characteristics and fertility of that soil.<\/p>\n<p id=\"fs-idp92131984\">Soil develops very slowly over long periods of time, and its formation results from natural and environmental forces acting on mineral, rock, and organic compounds. Soils can be divided into two groups:\u00a0<strong><span id=\"term1355\" data-type=\"term\">organic soils<\/span><\/strong>\u00a0are those that are formed from sedimentation and primarily composed of organic matter, while those that are formed from the weathering of rocks and are primarily composed of inorganic material are called\u00a0<strong><span id=\"term1356\" data-type=\"term\">mineral soils<\/span><\/strong>. Mineral soils are predominant in terrestrial ecosystems, where soils may be covered by water for part of the year or exposed to the atmosphere.<\/p>\n<section id=\"fs-idm122071040\" data-depth=\"1\">\n<h3 data-type=\"title\">Soil Composition<\/h3>\n<p id=\"fs-idm86116256\">Soil consists of these major components (Figure 5):<\/p>\n<ul id=\"fs-idm20861600\">\n<li>inorganic mineral matter, about 40 to 45 percent of the soil volume<\/li>\n<li>organic matter, about 5 percent of the soil volume<\/li>\n<li>water and air, about 50 percent of the soil volume<\/li>\n<\/ul>\n<p id=\"fs-idp68595664\">The amount of each of the four major components of soil depends on the amount of vegetation, soil compaction, and water present in the soil. A good healthy soil has sufficient air, water, minerals, and organic material to promote and sustain plant life.<\/p>\n<div id=\"fs-idp92229216\" class=\"visual-connection ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\">\n<section>\n<div class=\"os-note-body\">\n<div id=\"fig-ch31_02_01\" class=\"os-figure\">\n<figure data-id=\"fig-ch31_02_01\">\n<div style=\"width: 409px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"4\" class=\"\" src=\"https:\/\/openstax.org\/resources\/94af46e2ee398a72f66ae10dbe69df2f1ecb1f63\" alt=\"Illustration shows a pie graph that outlines the composition of soil. Forty-five percent is inorganic mineral matter, 25 percent is water, 25 percent is air, and 5 percent is organic matter, including microorganisms and macroorganisms.\" width=\"399\" height=\"367\" data-media-type=\"image\/png\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 5.\u00a0The four major components of soil are shown: inorganic minerals, organic matter, water, and air.<\/p>\n<\/div>\n<\/figure>\n<div class=\"os-caption-container\"><\/div>\n<\/div>\n<div class=\"textbox exercises\">\n<h3>Practice Question<\/h3>\n<p>Soil compaction can result when soil is compressed by heavy machinery or even foot traffic. How might this compaction change the soil composition?<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q778822\">Show Answer<\/span><\/p>\n<div id=\"q778822\" class=\"hidden-answer\" style=\"display: none\">The composition percentages would shift. A large amount of air would be lost and a small amount of water would be removed by soil compaction.<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/section>\n<\/div>\n<p id=\"fs-idm88922480\">The organic material of soil, called\u00a0<strong><span id=\"term1357\" data-type=\"term\">humus<\/span><\/strong>, is made up of microorganisms (dead and alive), and dead animals and plants in varying stages of decay. Humus improves soil structure and provides plants with water and minerals. The inorganic material of soil consists of rock, slowly broken down into smaller particles that vary in size. Soil particles that are 0.1 to 2 mm in diameter are\u00a0<strong><span id=\"term1358\" data-type=\"term\">sand<\/span><\/strong>. Soil particles between 0.002 and 0.1\u00a0mm are called\u00a0<strong><span id=\"term1359\" data-type=\"term\">silt<\/span><\/strong>, and even smaller particles, less than 0.002 mm in diameter, are called\u00a0<strong><span id=\"term1360\" data-type=\"term\">clay<\/span><\/strong>. Some soils have no dominant particle size and contain a mixture of sand, silt, and humus; these soils are called\u00a0<span id=\"term1361\" data-type=\"term\">loams<\/span>.<\/p>\n<div id=\"fs-idp957920\" class=\"interactive ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\">\n<section>\n<div class=\"os-note-body\">\n<div class=\"textbox\">Explore this\u00a0<a href=\"http:\/\/openstax.org\/l\/soil_survey_map\" target=\"_blank\" rel=\"noopener nofollow\">interactive map<\/a>\u00a0from the USDA\u2019s National Cooperative Soil Survey to access soil data for almost any region in the United States.<\/div>\n<\/div>\n<\/section>\n<\/div>\n<\/section>\n<section id=\"fs-idm92717632\" data-depth=\"1\">\n<h3 data-type=\"title\">Soil Formation<\/h3>\n<p id=\"fs-idm92332624\">Soil formation is the consequence of a combination of biological, physical, and chemical processes. Soil should ideally contain 50 percent solid material and 50 percent pore space. About one-half of the pore space should contain water, and the other half should contain air. The organic component of soil serves as a cementing agent, returns nutrients to the plant, allows soil to store moisture, makes soil tillable for farming, and provides energy for soil microorganisms. Most soil microorganisms\u2014bacteria, algae, or fungi\u2014are dormant in dry soil, but become active once moisture is available.<\/p>\n<p id=\"fs-idm61091136\">Soil distribution is not homogenous because its formation results in the production of layers; together, the vertical section of a soil is called the\u00a0<strong><span id=\"term1362\" data-type=\"term\">soil profile<\/span><\/strong>. Within the soil profile, soil scientists define zones called horizons. A\u00a0<strong><span id=\"term1363\" data-type=\"term\">horizon<\/span><\/strong>\u00a0is a soil layer with distinct physical and chemical properties that differ from those of other layers. Five factors account for soil formation: parent material, climate, topography, biological factors, and time.<\/p>\n<section id=\"fs-idm89960448\" data-depth=\"2\">\n<h4 data-type=\"title\">Parent Material<\/h4>\n<p id=\"fs-idm1756192\">The organic and inorganic material in which soils form is the\u00a0<strong><span id=\"term1364\" data-type=\"term\">parent material<\/span><\/strong>. Mineral soils form directly from the weathering of\u00a0<strong><span id=\"term1365\" data-type=\"term\">bedrock<\/span><\/strong>, the solid rock that lies beneath the soil, and therefore, they have a similar composition to the original rock. Other soils form in materials that came from elsewhere, such as sand and glacial drift. Materials located in the depth of the soil are relatively unchanged compared with the deposited material. Sediments in rivers may have different characteristics, depending on whether the stream moves quickly or slowly. A fast-moving river could have sediments of rocks and sand, whereas a slow-moving river could have fine-textured material, such as clay.<\/p>\n<\/section>\n<section id=\"fs-idp27656048\" data-depth=\"2\">\n<h4 data-type=\"title\">Climate<\/h4>\n<p id=\"fs-idm32351136\">Temperature, moisture, and wind cause different patterns of weathering and therefore affect soil characteristics. The presence of moisture and nutrients from weathering will also promote biological activity: a key component of a quality soil.<\/p>\n<\/section>\n<section id=\"fs-idp73414688\" data-depth=\"2\">\n<h4 data-type=\"title\">Topography<\/h4>\n<p id=\"fs-idp77262848\">Regional surface features (familiarly called \u201cthe lay of the land\u201d) can have a major influence on the characteristics and fertility of a soil. Topography affects water runoff, which strips away parent material and affects plant growth. Steeps soils are more prone to erosion and may be thinner than soils that are relatively flat or level.<\/p>\n<\/section>\n<section id=\"fs-idp22052656\" data-depth=\"2\">\n<h4 data-type=\"title\">Biological factors<\/h4>\n<p id=\"fs-idp91477184\">The presence of living organisms greatly affects soil formation and structure. Animals and microorganisms can produce pores and crevices, and plant roots can penetrate into crevices to produce more fragmentation. Plant secretions promote the development of microorganisms around the root, in an area known as the\u00a0<strong><span id=\"term1366\" data-type=\"term\">rhizosphere<\/span><\/strong>. Additionally, leaves and other material that fall from plants decompose and contribute to soil composition.<\/p>\n<\/section>\n<section id=\"fs-idp3692064\" data-depth=\"2\">\n<h4 data-type=\"title\">Time<\/h4>\n<p id=\"fs-idm35248608\">Time is an important factor in soil formation because soils develop over long periods. Soil formation is a dynamic process. Materials are deposited over time, decompose, and transform into other materials that can be used by living organisms or deposited onto the surface of the soil.<\/p>\n<\/section>\n<\/section>\n<section id=\"fs-idm58959248\" data-depth=\"1\">\n<h3 data-type=\"title\">Physical Properties of the Soil<\/h3>\n<div id=\"fs-idm58951232\" class=\"visual-connection ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\">\n<section>\n<div class=\"os-note-body\">\n<div id=\"fig-ch31_02_02\" class=\"os-figure\">\n<figure data-id=\"fig-ch31_02_02\">\n<div style=\"width: 295px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" id=\"7\" src=\"https:\/\/openstax.org\/resources\/0755a70bb4c7fe54ad67a1cba5b452a3dc1f4130\" alt=\"Illustration shows a cross-section of soil layers, or horizons. The top layer, from zero to two inches, is the O horizon. The O horizon is a rich, deep brown color. From two to ten inches is the A horizon. This layer is slightly lighter in color than the O horizon, and extensive root systems are visible. From ten to thirty inches is the B horizon. The B horizon is reddish brown. Longer roots extend to the bottom of this layer. The C horizon extends from 30 to 48 inches. This layer is rocky and devoid of roots.\" width=\"285\" height=\"549\" data-media-type=\"image\/png\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 6 This soil profile shows the different soil layers (O horizon, A horizon, B horizon, and C horizon) found in typical soils. (credit: modification of work by USDA)<\/p>\n<\/div>\n<\/figure>\n<\/div>\n<p id=\"fs-idm90918144\">Soils are named and classified based on their horizons. The soil profile has four distinct layers: 1) O horizon; 2) A horizon; 3) B horizon, or subsoil; and 4) C horizon, or soil base (Figure 6). The\u00a0<span id=\"term1367\" data-type=\"term\">O horizon<\/span>\u00a0has freshly decomposing organic matter\u2014humus\u2014at its surface, with decomposed vegetation at its base. Humus enriches the soil with nutrients and enhances soil moisture retention. Topsoil\u2014the top layer of soil\u2014is usually two to three inches deep, but this depth can vary considerably. For instance, river deltas like the Mississippi River delta have deep layers of topsoil. Topsoil is rich in organic material; microbial processes occur there, and it is the \u201cworkhorse\u201d of plant production.<\/p>\n<p>The\u00a0<span id=\"term1368\" data-type=\"term\">A horizon<\/span>\u00a0consists of a mixture of organic material with inorganic products of weathering, and it is therefore the beginning of true mineral soil. This horizon is typically darkly colored because of the presence of organic matter. In this area, rainwater percolates through the soil and carries materials from the surface. The\u00a0<span id=\"term1369\" data-type=\"term\">B\u00a0horizon<\/span>\u00a0is an accumulation of mostly fine material that has moved downward, resulting in a dense layer in the soil. In some soils, the B horizon contains nodules or a layer of calcium carbonate. The\u00a0<span id=\"term1370\" data-type=\"term\">C horizon<\/span>, or soil base, includes the parent material, plus the organic and inorganic material that is broken down to form soil. The parent material may be either created in its natural place, or transported from elsewhere to its present location. Beneath the C horizon lies bedrock.<\/p>\n<div class=\"textbox exercises\">\n<h3>PRactice Question<\/h3>\n<p>Which horizon is considered the topsoil, and which is considered the subsoil?<\/p>\n<p>a. O horizon<\/p>\n<p>b. A horizon<\/p>\n<p>c. C horizon<\/p>\n<p>d. B horizon<\/p>\n<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q334561\">Show Answer<\/span><\/p>\n<div id=\"q334561\" class=\"hidden-answer\" style=\"display: none\">\n<p>(a) The O horizon is the topsoil<\/p>\n<p>(d) The B horizon is the subsoil<\/p>\n<p>(<\/p><\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/section>\n<\/div>\n<p id=\"fs-idm90711008\">Some soils may have additional layers, or lack one of these layers. The thickness of the layers is also variable, and depends on the factors that influence soil formation. In general, immature soils may have O, A, and C horizons, whereas mature soils may display all of these, plus additional layers (Figure 7).<\/p>\n<div id=\"fig-ch31_02_03\" class=\"os-figure\">\n<figure data-id=\"fig-ch31_02_03\">\n<div style=\"width: 315px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"8\" src=\"https:\/\/openstax.org\/resources\/710488c7a8a03db9a79b47a0d612605ec19a80ab\" alt=\"In the photo, soil has been cut away to reveal the soil profile. The O horizon is at the soil surface and is a rich black color. The brown A horizon starts beneath the O horizon and extends to about two-and-a-half feet beneath the surface. The B horizon is reddish brown and extends from the bottom of the A horizon to about two feet deep. The C horizon extends from the bottom of the B horizon to the bottom of the photo at a depth of four feet. The C horizon is light brown and has a coarser consistency than the A or B horizons.\" width=\"305\" height=\"567\" data-media-type=\"image\/jpg\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 7. The San Joaquin soil profile has an O horizon, A horizon, B horizon, and C horizon. (credit: modification of work by USDA)<\/p>\n<\/div>\n<\/figure>\n<div class=\"os-caption-container\"><\/div>\n<\/div>\n<div id=\"fs-idm61608448\" class=\"career ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\">\n<section>\n<div class=\"os-note-body\">\n<div class=\"textbox key-takeaways\">\n<h3>Soil Scientist<\/h3>\n<section id=\"fs-idm58959248\" data-depth=\"1\">\n<div id=\"fs-idm61608448\" class=\"career ui-has-child-title\" data-type=\"note\" data-has-label=\"true\" data-label=\"\">\n<section>\n<div class=\"os-note-body\">\n<p id=\"fs-idp17240768\">A soil scientist studies the biological components, physical and chemical properties, distribution, formation, and morphology of soils. Soil scientists need to have a strong background in physical and life sciences, plus a foundation in mathematics. They may work for federal or state agencies, academia, or the private sector. Their work may involve collecting data, carrying out research, interpreting results, inspecting soils, conducting soil surveys, and recommending soil management programs.<\/p>\n<div id=\"fig-ch31_02_04\" class=\"os-figure\">\n<figure class=\"\" data-id=\"fig-ch31_02_04\">\n<div style=\"width: 285px\" class=\"wp-caption alignright\"><img loading=\"lazy\" decoding=\"async\" id=\"10\" src=\"https:\/\/openstax.org\/resources\/22de3a6c18ef1f5d81010417a171b31b32760e52\" alt=\"Photo shows a man standing next to a wall of soil in a pit that is as deep as he is tall.\" width=\"275\" height=\"692\" data-media-type=\"image\/jpg\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 8. This soil scientist is studying the horizons and composition of soil at a research site. (credit: USDA)<\/p>\n<\/div>\n<\/figure>\n<div class=\"os-caption-container\"><\/div>\n<\/div>\n<p id=\"fs-idp43896176\">Many soil scientists work both in an office and in the field. According to the United States Department of Agriculture (USDA): \u201ca soil scientist needs good observation skills to analyze and determine the characteristics of different types of soils. Soil types are complex and the geographical areas a soil scientist may survey are varied. Aerial photos or various satellite images are often used to research the areas. Computer skills and geographic information systems (GIS) help the scientist to analyze the multiple facets of geomorphology, topography, vegetation, and climate to discover the patterns left on the landscape.\u201d<a class=\"footnote\" title=\"National Resources Conservation Service \/ United States Department of Agriculture. \u201cCareers in Soil Science.\u201d\u00a0http:\/\/openstax.org\/l\/NRCS\" id=\"return-footnote-2045-1\" href=\"#footnote-2045-1\" aria-label=\"Footnote 1\"><sup class=\"footnote\">[1]<\/sup><\/a><sup id=\"footnote-ref1\" data-type=\"footnote-number\"><\/sup>\u00a0Soil scientists play a key role in understanding the soil\u2019s past, analyzing present conditions, and making recommendations for future soil-related practices.<\/p>\n<\/div>\n<\/section>\n<\/div>\n<\/section>\n<\/div>\n<p>&nbsp;<\/p>\n<\/div>\n<\/section>\n<\/div>\n<\/section>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-2045\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology 2e. <strong>Provided by<\/strong>: OpenStax. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section><hr class=\"before-footnotes clear\" \/><div class=\"footnotes\"><ol><li id=\"footnote-2045-1\">National Resources Conservation Service \/ United States Department of Agriculture. \u201cCareers in Soil Science.\u201d\u00a0<a href=\"http:\/\/openstax.org\/l\/NRCS\" target=\"_blank\" rel=\"noopener nofollow\">http:\/\/openstax.org\/l\/NRCS<\/a> <a href=\"#return-footnote-2045-1\" class=\"return-footnote\" aria-label=\"Return to footnote 1\">&crarr;<\/a><\/li><\/ol><\/div>","protected":false},"author":17,"menu_order":22,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology 2e\",\"author\":\"\",\"organization\":\"OpenStax\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction\"}]","CANDELA_OUTCOMES_GUID":"50240e04-3e11-439e-9507-6619911437cb, 74fa1309-338c-4950-b0b2-d0fc761a8dd1","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-2045","chapter","type-chapter","status-publish","hentry"],"part":145,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/2045","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":19,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/2045\/revisions"}],"predecessor-version":[{"id":8384,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/2045\/revisions\/8384"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/parts\/145"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/2045\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/media?parent=2045"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapter-type?post=2045"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/contributor?post=2045"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/license?post=2045"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}