What Are Soils? | Learn Science at Scitable

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Needelman (<span class="topicCitationItalics">Associate Professor of Soil Science, University of Maryland</span>) © 2013 Nature Education </div><div class="fleft"><a class='inlineLinks' href='javascript:show_inform("Terms of Use", "You may reproduce this material, without modifications, in print or electronic form for your personal, non-commercial purposes or for non-commercial use in an educational environment.")'><img height="14" width="14" class="nomargin noBackground" style="border:none !important;" src='/scitable/natedimages/info_icon.png'/></a></div></div><div class="clear"></div> <div class="clearfix posRelative"> <div class="fleft bold" style="width: 585px;">Citation: <span id="fullName" class="bold">Needelman, B. A.</span><span id="publishDate" class="bold"> (2013)</span> What Are Soils? <span id="publisher" class="topicCitationItalics">Nature Education Knowledge</span> <span id="volume" class="bold">4(<span id="issue" class="bold">3</span>)</span>:2</div> <div class="fright w180" style="position: absolute; right: -20px; bottom: -2px;"> <div class="clearfix w100p hideForPrint"> <div class="fright padleft3px"> <a rel="nofollow" id="redditShareLink" href="http://www.reddit.com/submit?url=" onClick="shareWithReddit();return false;"><img class="nomargin borderNone pad2px" title="Share Using Reddit" src="/scitable/natedimages/reddit-share-icon.png"/></a> </div> <div class="fright padleft3px"> <a rel="nofollow" id="stumbleUponLink" href="#" onClick="shareWithStubmleUpon();return false;"><img class="nomargin borderNone pad2px" title="StumbleUpon" src="/scitable/natedimages/stumbleupon.png"/></a> </div> <div class="fright padleft3px"> <a rel="nofollow" id="googlePlusLink" href="https://plus.google.com/share?url=" onClick="shareWithGooglePlus();return false;"><img class="nomargin borderNone pad2px" title="Share with Google+" src="/scitable/natedimages/gplus-16.png"/></a> </div> <div class="fright padleft3px"> <a rel="nofollow" id="twitterShareLnk" href="http://twitter.com/share?url=" onClick="normalShareWithTwitter();return false;"><img class="nomargin borderNone pad2px" title="Share with Twitter" src="/scitable/natedimages/icon-twitter.jpg"/></a> </div> <div class="fright padleft3px"> <a rel="nofollow" id="faceBookShareLnk" href="http://www.facebook.com/share.php?u=" onClick="normalShareWithFaceBook();return false;"><img class="nomargin borderNone pad2px" title="Share with Facebook" src="/scitable/natedimages/icon-facebook.jpg" /></a> </div> <div class="fright padleft3px"> <a name="shareEMailSplashAnc" id="shareEMailSplashAnc" href="#" onClick="sendMailToShare('lnkLBMdlLIID', event);return false;"><img class="nomargin borderNone pad2px" src="/scitable/natedimages/email_icon.gif" alt="Email" /></a> </div> <div class="fright padleft3px"> <a href="javascript:printReadingPage()"><img title="Print" class="nomargin borderNone pad2px" src="/scitable/natedimages/print_15.gif"/></a> </div> <div class="fright padleft3px"> <a onMouseOver="return displayOnMouseOverText(this);" href="#" id="bookmarkTRArttAnc" name="bookmarkTRArttAnc" onclick="addToLocker(); return false;"><img title="Bookmark" class="nomargin borderNone pad2px" src="/scitable/natedimages/icon_bookmark.gif"/></a> </div> </div> </div> </div> </div> </div> <input type="hidden" name="isCoverPage" id="" value="N" /> </div> <div class="clear"></div> </div> <div class="clear"></div> </div> <div class="px18 normalFontWeight margintop10px readPgHdng" style="line-height: 26px ! important;">In this article readers are introduced to the many facets of soils - their unique characteristics and diversity, the ecosystem services that soils provide, and their use and misuse.</div> <div class="fright padright5px hideForPrint"> <a href="javascript:resizeFontScitable('fontSize11')" class="padright10px"><span class="px11 mousePointerHand lh22" id="sizeNo1">Aa</span></a> <a href="javascript:resizeFontScitable('fontSize15')" class="padright10px"><span class="px15 mousePointerHand lh22" id="sizeNo2">Aa</span></a> <a href="javascript:resizeFontScitable('fontSize20')" class="padright10px"><span class="mousePointerHand lh22 px20" id="sizeNo3">Aa</span></a> </div> <div class="clear"></div> <div class="articleHR w98p"></div> <div id="orgCnt"> <div id="trOutLine"></div> <div id="fontSizeChange" class="fontSizeChange"><div id="mainCntArtPageDiv"><div id="trPage"> <div class="articleContent"> <div class="clear"></div> <div class="contentSection"> <div class="sectionTitle"><h2 style="display:none;"> </h2></div> <div class="clear"></div> <div class="sectionParas"> <div class="paraArticle"> <div> <p><a name="anchEmbedImg_sectionparatxt_1" ><input type="hidden" name="hidEmbedImg" id="hidEmbedImg_sectionparatxt_1" value="/scitable"/><input type="hidden" name="hidEmbedPlayerState" id="hidEmbedPlayerState_sectionparatxt_1" value="uninitedaudio"/></a><div style="width:632px;" class="imageSet"><div style="width:632px;"><IMG src="/scitable/content/ne0000/ne0000/ne0000/ne0000/100768650/final_1_2.jpg" title="" alt="Needelman banner" /><br/></div></div> </p> <p>Soils are dynamic and diverse natural systems that lie at the interface between earth, air, water, and life. They are critical ecosystem service providers for the sustenance of humanity. The improved conservation and management of soils is among the great challenges and opportunities we face in the 21st century.<br> </p> <h1>Soil is... a Recipe with Five Ingredients</h1> <p> <span style="font-weight: bold;">Soil </span>is a material composed of five ingredients — minerals, soil organic matter, living organisms, gas, and water. Soil minerals are divided into three size classes — <span style="font-weight: bold;">clay</span>, <span style="font-weight: bold;">silt</span>, and <span style="font-weight: bold;">sand </span>(Figure 1); the percentages of particles in these size classes is called <span style="font-weight: bold;">soil texture</span>. The mineralogy of soils is diverse. For example, a clay mineral called smectite can shrink and swell so much upon wetting and drying (Figure 2) that it can knock over buildings. The most common mineral in soils is quartz; it makes beautiful crystals but it is not very reactive. <span style="font-weight: bold;">Soil organic matter</span> is plant, animal, and microbial residues in various states of decomposition; it is a critical ingredient — in fact the percentage of soil organic matter in a soil is among the best indicators of agricultural <span style="font-weight: bold;">soil quality</span> (http://soils.usda.gov/sqi/) (Figure 3). Soil colors range from the common browns, yellows, reds, grays, whites, and blacks to rare soil colors such as greens and blues. </p> <p><a name="anchEmbedImg_sectionparatxt_2" ><input type="hidden" name="hidEmbedImg" id="hidEmbedImg_sectionparatxt_2" value="/scitable"/><input type="hidden" name="hidEmbedPlayerState" id="hidEmbedPlayerState_sectionparatxt_2" value="uninitedaudio"/></a><div style="width:632px;" class="imageSet"><div style="width:632px;"><IMG src="/scitable/content/ne0000/ne0000/ne0000/ne0000/101052458/figure1_v002-01_1_2.jpg" title="" alt="Relative sizes of sand, silt, clay." /><br/><div class="imageCaption pipeblack bold"><div class="imageCaption pipeblack bold nopad">Figure 1</div><div class="imageLegend pipeblack nopad">Relative sizes of sand, silt, clay.</div><div class="creditLine bold nopad">© 2013 <a target="_blank" href="http://www.nature.com/scitable" class="inlineLinks">Nature Education</a> All rights reserved. <a href='javascript:show_inform("Terms of Use", "You may reproduce this material, without modifications, in print or electronic form for your personal, non-commercial purposes or for non-commercial use in an educational environment.");' class="inlineLinks"><img class="nomargin infoImg nofloat" title="View Terms of Use" alt="View Terms of Use" height="13px" width="13px" src="/scitable/natedimages/info_icon.png" /></a></div><div class="clear"></div></div></div></div> </p> <p></p><a name="anchEmbedImg_sectionparatxt_3" ><input type="hidden" name="hidEmbedImg" id="hidEmbedImg_sectionparatxt_3" value="/scitable"/><input type="hidden" name="hidEmbedPlayerState" id="hidEmbedPlayerState_sectionparatxt_3" value="uninitedaudio"/></a><div style="width:341px;" class="imageSet"><div style="width:341px;"><IMG src="/scitable/content/ne0000/ne0000/ne0000/ne0000/101053900/Needleman_2_1.jpg" title="" alt="A Vertisol high in shrink-swell clay showing cracks formed during a dry period." /><br/><div class="imageCaption pipeblack bold"><div class="imageCaption pipeblack bold nopad">Figure 2</div><div class="imageLegend pipeblack nopad">A Vertisol high in shrink-swell clay showing cracks formed during a dry period.</div><div class="creditLine bold nopad">Photo courtesy of USDA. <a href='javascript:show_inform("Terms of Use", "This material is either public domain or Nature Education has received permission from the owner to make it available to all users without restrictions. You may reuse this material, with modifications, in print, electronic or any other form, for commercial or non-commercial purposes.");' class="inlineLinks"><img class="nomargin infoImg nofloat" title="View Terms of Use" alt="View Terms of Use" height="13px" width="13px" src="/scitable/natedimages/info_icon.png" /></a></div><div class="clear"></div></div></div></div><br> <br><a name="anchEmbedImg_sectionparatxt_4" ><input type="hidden" name="hidEmbedImg" id="hidEmbedImg_sectionparatxt_4" value="/scitable"/><input type="hidden" name="hidEmbedPlayerState" id="hidEmbedPlayerState_sectionparatxt_4" value="uninitedaudio"/></a><div style="width:341px;" class="imageSet"><div style="width:341px;"><IMG src="/scitable/content/ne0000/ne0000/ne0000/ne0000/68243525/2_1.jpg" title="" alt="This soil from Iowa is dark due to high organic matter content" /><br/><div class="imageCaption pipeblack bold"><div class="imageCaption pipeblack bold nopad">Figure 3</div><div class="imageLegend pipeblack nopad">This soil from Iowa is dark due to high organic matter content; combined with high plant-available, water-holding capacity due to a high silt concentration. Soils with these characteristics are present in parts of the United States, Russia, and South America, making them among the great grain-producing regions of the world.</div><div class="creditLine bold nopad">Courtesy of U.S. Department of Agriculture/NRCS. <a href='javascript:show_inform("Terms of Use", "This material is either public domain or Nature Education has received permission from the owner to make it available to all users without restrictions. You may reuse this material, with modifications, in print, electronic or any other form, for commercial or non-commercial purposes.");' class="inlineLinks"><img class="nomargin infoImg nofloat" title="View Terms of Use" alt="View Terms of Use" height="13px" width="13px" src="/scitable/natedimages/info_icon.png" /></a></div><div class="clear"></div></div></div></div><br> <h1> Soils are... Big</h1> <p>You may be surprised to hear "<span style="font-weight: bold;">dirt</span>" described as "big". However, in the late 1800's soil scientists began to recognize that <span style="font-weight: bold;">soils </span>are <span style="font-weight: bold;">natural bodies</span> with size, form, and history (Figure 4). Just like a water body has water, fish, plants, and other parts, a soil body is an integrated system containing soil, rocks, roots, animals, and other parts. And just like other bodies, soil systems provide integrated functions that are greater than the sum of their parts. </p> <p><a name="anchEmbedImg_sectionparatxt_5" ><input type="hidden" name="hidEmbedImg" id="hidEmbedImg_sectionparatxt_5" value="/scitable"/><input type="hidden" name="hidEmbedPlayerState" id="hidEmbedPlayerState_sectionparatxt_5" value="uninitedaudio"/></a><div style="width:632px;" class="imageSet"><div style="width:632px;"><IMG src="/scitable/content/ne0000/ne0000/ne0000/ne0000/101053091/figure4_v001-01_1_2.jpg" title="" alt="A soil body with contrasting and interacting soil horizons." /><br/><div class="imageCaption pipeblack bold"><div class="imageCaption pipeblack bold nopad">Figure 4</div><div class="imageLegend pipeblack nopad">A soil body with contrasting and interacting soil horizons.</div><div class="creditLine bold nopad">© 2013 <a target="_blank" href="http://www.nature.com/scitable" class="inlineLinks">Nature Education</a> All rights reserved. <a href='javascript:show_inform("Terms of Use", "You may reproduce this material, without modifications, in print or electronic form for your personal, non-commercial purposes or for non-commercial use in an educational environment.");' class="inlineLinks"><img class="nomargin infoImg nofloat" title="View Terms of Use" alt="View Terms of Use" height="13px" width="13px" src="/scitable/natedimages/info_icon.png" /></a></div><div class="clear"></div></div></div></div> </p> <p>One is usually able to distinguish different layers within soils, called <span style="font-weight: bold;">soil horizons</span> (Figure 5). These horizons interact with each other, and therefore cannot be considered as independent, although they can be very different from each other. There is great complexity and diversity in soil horizons, but in general the surface horizons are dynamic and rich in life and organic matter. Below the surface horizons, one often finds more stable horizons that are formed through a diverse suite of soil formation processes, such as bright white horizons formed through the removal of clays or deep-red, low-fertility horizons formed through millions of years of weathering (Figure 6). Below these horizons, soils transition into layers that are only partially affected by soil formation and ultimately into unaltered layers of parent material. <br> <br><a name="anchEmbedImg_sectionparatxt_6" ><input type="hidden" name="hidEmbedImg" id="hidEmbedImg_sectionparatxt_6" value="/scitable"/><input type="hidden" name="hidEmbedPlayerState" id="hidEmbedPlayerState_sectionparatxt_6" value="uninitedaudio"/></a><div style="width:341px;" class="imageSet"><div style="width:341px;"><IMG src="/scitable/content/ne0000/ne0000/ne0000/ne0000/68243652/2_1.jpg" title="" alt="A soil from Alaska showing distinct horizons" /><br/><div class="imageCaption pipeblack bold"><div class="imageCaption pipeblack bold nopad">Figure 5</div><div class="imageLegend pipeblack nopad">A soil from Alaska showing distinct horizons resulting from both soil formation processes and periodic deposits of volcanic ash.</div><div class="creditLine bold nopad">Courtesy of U.S. Department of Agriculture. <a href='javascript:show_inform("Terms of Use", "This material is either public domain or Nature Education has received permission from the owner to make it available to all users without restrictions. You may reuse this material, with modifications, in print, electronic or any other form, for commercial or non-commercial purposes.");' class="inlineLinks"><img class="nomargin infoImg nofloat" title="View Terms of Use" alt="View Terms of Use" height="13px" width="13px" src="/scitable/natedimages/info_icon.png" /></a></div><div class="clear"></div></div></div></div><br><a name="anchEmbedImg_sectionparatxt_7" ><input type="hidden" name="hidEmbedImg" id="hidEmbedImg_sectionparatxt_7" value="/scitable"/><input type="hidden" name="hidEmbedPlayerState" id="hidEmbedPlayerState_sectionparatxt_7" value="uninitedaudio"/></a><div style="width:341px;" class="imageSet"><div style="width:341px;"><IMG src="/scitable/content/ne0000/ne0000/ne0000/ne0000/68243683/2_1.jpg" title="" alt="A highly weathered soil that is red due to the high content of iron oxide minerals." /><br/><div class="imageCaption pipeblack bold"><div class="imageCaption pipeblack bold nopad">Figure 6</div><div class="imageLegend pipeblack nopad">A highly weathered soil that is red due to the high content of iron oxide minerals.</div><div class="creditLine bold nopad">Courtesy of U.S. Department of Agriculture. <a href='javascript:show_inform("Terms of Use", "This material is either public domain or Nature Education has received permission from the owner to make it available to all users without restrictions. You may reuse this material, with modifications, in print, electronic or any other form, for commercial or non-commercial purposes.");' class="inlineLinks"><img class="nomargin infoImg nofloat" title="View Terms of Use" alt="View Terms of Use" height="13px" width="13px" src="/scitable/natedimages/info_icon.png" /></a></div><div class="clear"></div></div></div></div><br>The lateral extent of a soil can be difficult to define because adjacent soils can have sharp to gradual transitions. Soil bodies can be conceptualized and mapped at different scales, for example for an individual property or an entire watershed. The characterization and delineation of soil bodies forms the basis of most soil mapping systems (Figure 7).</p> <p><a name="anchEmbedImg_sectionparatxt_8" ><input type="hidden" name="hidEmbedImg" id="hidEmbedImg_sectionparatxt_8" value="/scitable"/><input type="hidden" name="hidEmbedPlayerState" id="hidEmbedPlayerState_sectionparatxt_8" value="uninitedaudio"/></a><div style="width:632px;" class="imageSet"><div style="width:632px;"><IMG src="/scitable/content/ne0000/ne0000/ne0000/ne0000/101053360/figure7_v001-01_1_2.jpg" title="" alt="Block diagram showing distribution of soil bodies across a landscape." /><br/><div class="imageCaption pipeblack bold"><div class="imageCaption pipeblack bold nopad">Figure 7</div><div class="imageLegend pipeblack nopad">Block diagram showing distribution of soil bodies across a landscape.</div><div class="creditLine bold nopad">© 2013 <a target="_blank" href="http://www.nature.com/scitable" class="inlineLinks">Nature Education</a> All rights reserved. <a href='javascript:show_inform("Terms of Use", "You may reproduce this material, without modifications, in print or electronic form for your personal, non-commercial purposes or for non-commercial use in an educational environment.");' class="inlineLinks"><img class="nomargin infoImg nofloat" title="View Terms of Use" alt="View Terms of Use" height="13px" width="13px" src="/scitable/natedimages/info_icon.png" /></a></div><div class="clear"></div></div></div></div> </p> <h1>Soils are... Young to Very, Very Old</h1>It can be difficult to say exactly when some soils were born, but we can say that while some are young, many are very old. The oldest soils on earth may be in Australia, where stable land forms have allowed some soils to age several million years. New soils are born with every landslide, volcanic eruption, or glacial retreat. Soils change over time through a host of biological, chemical, and physical processes. Horizons form, minerals and rocks <span style="font-weight: bold;">weather</span>, nutrients <span style="font-weight: bold;">leach</span>, and plant communities change. Soil scientists have learned to predict the current stage of these processes if given five key pieces of information about the soil's history — the <span style="font-weight: bold;">five factors of soil formation</span> — climate, organisms, topography, parent material, and time (Jenny 1941). <br> <br> <h1>Soils are... Diverse</h1>The variety of soil formation processes operating on different parent materials under different climatic, topographic, and biological conditions over varying periods of time gives rise to the vast diversity of soils on earth. Soil formation creates a dizzying array of soil horizons; here are just a few examples:<br> <ul> <li>Plinthite — which hardens irreversibly upon repeated wetting and drying (Figure 8a).</li> </ul> <ul> <li>Sulfidic — a horizon containing pyrite which, upon exposure to oxygen, can produce so much sulfuric acid that it kills plants and can cause fish kills (Figure 8b).</li> </ul> <ul> <li>Petrocalcic — in which so much calcium carbonate is accumulated that it literally forms a rock-like layer in the middle of a soil (Figure 8c).</li> </ul>Soil scientists capture this vast diversity through systems of soil classification. The U.S. system, called Soil Taxonomy (soils.usda.gov/technical/classification/taxonomy/), groups soils into 12 broad orders at the most general level, and more than 19,000 soil series at the most detailed level (Ahrens & Arnold 1999). The International Union of Soil Sciences has developed a system called the World Reference Base, which has 32 Reference Soil Groups (http://www.fao.org/nr/land/soils/soil/en/).<br> <br><a name="anchEmbedImg_sectionparatxt_9" ><input type="hidden" name="hidEmbedImg" id="hidEmbedImg_sectionparatxt_9" value="/scitable"/><input type="hidden" name="hidEmbedPlayerState" id="hidEmbedPlayerState_sectionparatxt_9" value="uninitedaudio"/></a><div style="width:632px;" class="imageSet"><div style="width:632px;"><IMG src="/scitable/content/ne0000/ne0000/ne0000/ne0000/68243777/1_2.jpg" title="" alt="Plinthite in an eastern Nicaragua rainforest" /><br/><div class="imageCaption pipeblack bold"><div class="imageCaption pipeblack bold nopad">Figure 8</div><div class="imageLegend pipeblack nopad">(a) Plinthite in an eastern Nicaragua rainforest with distinct areas of red iron oxide concentrations. Soils with plinthite must be managed carefully because they harden irreversibly if they are exposed to repeated wetting and drying. (b) A soil showing subsurface horizons darkened by the black mineral pyrite (FeS<sub>2</sub>), which oxidizes to release sulfuric acid and iron — the iron has precipitated in the red layers. (c) Soil showing a petrocalcic horizon, in which so much calcium carbonate has precipitated that a soil horizon has become cemented.</div><div class="creditLine bold nopad">© 2012 <a target="_blank" href="http://www.nature.com/scitable" class="inlineLinks">Nature Education</a> (a) Courtesy of R. Vaughn. (b) Courtesy of B. Needelman. (c) Courtesy of U.S. Department of Agriculture/NRCS. All rights reserved. <a href='javascript:show_inform("Terms of Use", "You may reproduce this material, without modifications, in print or electronic form for your personal, non-commercial purposes or for non-commercial use in an educational environment.");' class="inlineLinks"><img class="nomargin infoImg nofloat" title="View Terms of Use" alt="View Terms of Use" height="13px" width="13px" src="/scitable/natedimages/info_icon.png" /></a></div><div class="clear"></div></div></div></div><br> <h1>Soils... Communicate</h1>These master horizons may then be further annotated to give additional information about the horizon. Horizons are first assigned to one of the following master horizons as designated by a single capital letter:<br> <ul> <li>O - Horizon containing a high percentage of soil organic matter.</li> </ul> <ul> <li>A - Horizon darkened by the accumulation of organic matter.</li> </ul> <ul> <li>E - Horizon formed through the removal (<b>eluviation</b>) of clays, organic matter, iron, or aluminum. Usually lightened in color due to these removals. </li> </ul> <ul> <li>B - Broad class used for subsurface horizons that have been transformed substantially by a soil formation process such as color and structure development; the deposition (<b>illuviation</b>) of materials such as clays, organic matter, iron, aluminum, carbonates, or gypsum; carbonate or gypsum loss; brittleness and high density; or intense weathering leading to the accumulation of weathering-resistant minerals. </li> </ul> <ul> <li>C - A horizon minimally affected or unaffected by the soil formation processes.</li> </ul> <ul> <li>R - Bedrock. </li> </ul> <p> </p> <p>These master horizons may then be further annotated to give additional information about the horizon. Lower case letters can be placed as suffixes following the master horizon letter to give additional information about soil characteristics or soil formation processes. For example, the lower case "t" on the B horizon in Figure 9 indicates that the horizon is characterized by illuvial clay accumulation. Multiple letters can be used — Figure 8c depicts a Bkm horizon meaning that it is cemented (m) by illuvial carbonates (k). Numbers placed before the master horizon name (e.g., 2Bt) indicate a difference in parent material; numbers placed at the end of a horizon name are used to subdivide horizons that have the same designation but are different in some way (e.g., a red Bt1 over a yellow Bt2). <br> </p><a name="anchEmbedImg_sectionparatxt_10" ><input type="hidden" name="hidEmbedImg" id="hidEmbedImg_sectionparatxt_10" value="/scitable"/><input type="hidden" name="hidEmbedPlayerState" id="hidEmbedPlayerState_sectionparatxt_10" value="uninitedaudio"/></a><div style="width:341px;" class="imageSet"><div style="width:341px;"><IMG src="/scitable/content/ne0000/ne0000/ne0000/ne0000/101053472/figure9_v002-01_2_1.jpg" title="" alt="Example soil with designations that communicate the soil formation processes occurring in each horizon." /><br/><div class="imageCaption pipeblack bold"><div class="imageCaption pipeblack bold nopad">Figure 9</div><div class="imageLegend pipeblack nopad">Example soil with designations that communicate the soil formation processes occurring in each horizon.</div><div class="creditLine bold nopad">© 2013 <a target="_blank" href="http://www.nature.com/scitable" class="inlineLinks">Nature Education</a> All rights reserved. <a href='javascript:show_inform("Terms of Use", "You may reproduce this material, without modifications, in print or electronic form for your personal, non-commercial purposes or for non-commercial use in an educational environment.");' class="inlineLinks"><img class="nomargin infoImg nofloat" title="View Terms of Use" alt="View Terms of Use" height="13px" width="13px" src="/scitable/natedimages/info_icon.png" /></a></div><div class="clear"></div></div></div></div><br> <h1>Soils are... Biological Bliss</h1>If you like life, you'll love soils. There are a host of small, medium, and large organisms that live in soils, including mammals, birds, insects, and protozoa. But the greatest biodiversity lies in the soil microbes — the bacteria, fungi, and archaea (Figure 10). A teaspoon of rich soil can contain one billion bacteria. We actually know very little about the diversity of soil microbes, partially because they are so diverse, but also because we have not been able to culture the vast majority of these organisms in the lab. Soil microbiologists are applying advanced molecular techniques to understand the diversity and function of soil microbes. It is an exciting field of exploration with new biological taxa frequently being discovered. <br> <br><a name="anchEmbedImg_sectionparatxt_11" ><input type="hidden" name="hidEmbedImg" id="hidEmbedImg_sectionparatxt_11" value="/scitable"/><input type="hidden" name="hidEmbedPlayerState" id="hidEmbedPlayerState_sectionparatxt_11" value="uninitedaudio"/></a><div style="width:341px;" class="imageSet"><div style="width:341px;"><IMG src="/scitable/content/ne0000/ne0000/ne0000/ne0000/68243999/2_1.jpg" title="" alt="Arbuscular mycorrhizal fungi are among the most critical soil organisms" /><br/><div class="imageCaption pipeblack bold"><div class="imageCaption pipeblack bold nopad">Figure 10</div><div class="imageLegend pipeblack nopad">Arbuscular mycorrhizal fungi are among the most critical soil organisms, they form symbiotic associations with roots that helps many plants species to thrive. This spore of the arbuscular mycorrhizal fungus <i>Acaulospora scrobiculata</i> has been broken open, allowing the lipid contents to spill out (the liquid in the lower right). The outermost layer of the cell wall is ornamented like a golf ball.</div><div class="creditLine bold nopad">© 2012 <a target="_blank" href="http://www.nature.com/scitable" class="inlineLinks">Nature Education</a> Courtesy of J. Wolf. All rights reserved. <a href='javascript:show_inform("Terms of Use", "You may reproduce this material, without modifications, in print or electronic form for your personal, non-commercial purposes or for non-commercial use in an educational environment.");' class="inlineLinks"><img class="nomargin infoImg nofloat" title="View Terms of Use" alt="View Terms of Use" height="13px" width="13px" src="/scitable/natedimages/info_icon.png" /></a></div><div class="clear"></div></div></div></div><br> <h1>Soils are... Fertile</h1> <p>Soils are the primary provider of nutrients and water for much of the plant life on earth. There are 18 elements considered essential for plant growth, most of which are made available to plants through root uptake from soils (Brady & Weil 2007). Soils retain nutrients by several mechanisms. Most nutrients are dissolved in soil water as either positively or negatively charged ions; soil particles are also charged and thereby are able to electrically hold these ions. Soils also hold nutrients by retaining the soil water itself.<br> </p> <p>Arguably the greatest of all the ecosystem services provided by soils is the retention of water — without soils our land would be little but rocky deserts. Plants use much more water than one might think because they are constantly releasing water into the atmosphere as a result of transpiration, which is a component of the process of photosynthesis. Clay and silt particles are the primary mineral components in soils that retain water — these small particles slow the drainage of water and, like a sponge, physically hold water through capillary forces. Clay provides such strong force that plants can't pull all the water away from it, which makes silt particles the ultimate ingredient for plant-available water storage — they hold large quantities of water but also release it to plant roots (Figure 3).<br> </p> <h1>Soils are... Clay Factories</h1>Among the most important functions performed by soils is to provide the ideal conditions for clay <b>synthesis</b>. Clays are important because they are often <b>active</b>, which is a general term soil scientists use to describe how chemically reactive a particle is with ions, water, and other particles. These reactions are critical for the provision of many ecosystem services. Clays are often the most active mineral particles because they have unique chemical characteristics and also because they have so much surface area — clays can have 10,000 times the surface area of sand of the same weight (Brady & Weil 2007). All this surface area makes clays a hot spot for chemical reactions. <br> <br> <h1>Soils are... Service Providers</h1>Soils are the among the great ecosystem service providers on earth (Haygarth & Ritz 2009) (Figure 11). They store and provide water for plants. They prevent floods by transferring water slowly to streams and groundwater. They filter and remediate pollutants. They cycle and recycle nutrients and wastes — transforming them into biologically available forms, storing them away for later use, and preventing their leaching to ground and surface waters. Soils provide habitat for a vast diversity of life. They take up and release important gases, including oxygen and greenhouse gases, a service called gas regulation. Many of these ecosystem services are being lost through the degradation and loss of soils. The conservation, restoration, and optimization of ecosystem services provided by soils is among the great challenges for humanity in the 21st century.<br> <br><a name="anchEmbedImg_sectionparatxt_12" ><input type="hidden" name="hidEmbedImg" id="hidEmbedImg_sectionparatxt_12" value="/scitable"/><input type="hidden" name="hidEmbedPlayerState" id="hidEmbedPlayerState_sectionparatxt_12" value="uninitedaudio"/></a><div style="width:632px;" class="imageSet"><div style="width:632px;"><IMG src="/scitable/content/ne0000/ne0000/ne0000/ne0000/101053552/figure11_v002-01_1_2.jpg" title="" alt="Soils as ecosystem service providers." /><br/><div class="imageCaption pipeblack bold"><div class="imageCaption pipeblack bold nopad">Figure 11</div><div class="imageLegend pipeblack nopad">Soils as ecosystem service providers.</div><div class="creditLine bold nopad">2013 Redrawn with permission </div><div class="clear"></div></div></div></div><br> <h1>Soils are... Degrading and Polluted</h1>Unfortunately many human activities degrade and pollute soils, lessening the ecosystem services provided by soils and making some soils and their runoff water harmful to our environment and human health. <b>Erosion </b>is among the great causes of soil degradation as essential topsoil is lost at rates far greater than it can be replaced (Figure 12a, 12b); this sediment is also among the greatest pollutants of water bodies. Salinization and desertification are major causes of soil degradation in arid areas. <b>Salinization </b>is the buildup of salts in soils to a point that they destroy the physical and chemical properties of soil and make it impossible for plants to take up water from the soil. Salinization is often associated with improper irrigation. <b>Desertification </b>is caused by a combination of climate changes and human-induced soil degradation (such as through overgrazing). <br> <br><a name="anchEmbedImg_sectionparatxt_13" ><input type="hidden" name="hidEmbedImg" id="hidEmbedImg_sectionparatxt_13" value="/scitable"/><input type="hidden" name="hidEmbedPlayerState" id="hidEmbedPlayerState_sectionparatxt_13" value="uninitedaudio"/></a><div style="width:632px;" class="imageSet"><div style="width:632px;"><IMG src="/scitable/content/ne0000/ne0000/ne0000/ne0000/68244087/1_2.jpg" title="" alt="(a) Soil in central Iowa with dark, organic-matter-rich topsoil." /><br/><div class="imageCaption pipeblack bold"><div class="imageCaption pipeblack bold nopad">Figure 12</div><div class="imageLegend pipeblack nopad">(a) Soil in central Iowa with dark, organic-matter-rich topsoil. Iowa's average topsoil depth decreased from 35-45 cm to 15-20 cm during the 20th century due to farming. (b) Severe gully erosion in western Iowa.</div><div class="creditLine bold nopad">Photos courtesy of U.S. Department of Agriculture/NRCS. <a href='javascript:show_inform("Terms of Use", "This material is either public domain or Nature Education has received permission from the owner to make it available to all users without restrictions. You may reuse this material, with modifications, in print, electronic or any other form, for commercial or non-commercial purposes.");' class="inlineLinks"><img class="nomargin infoImg nofloat" title="View Terms of Use" alt="View Terms of Use" height="13px" width="13px" src="/scitable/natedimages/info_icon.png" /></a></div><div class="clear"></div></div></div></div><br>Soils have a great capacity to filter and <b>remediate </b>pollutants, but applications of pollutants to soils often exceed this capacity. When we apply excess nutrients to soils, particularly nitrogen and phosphorus, we make these soils sources of nutrient pollution to water bodies. These nutrients can cause <b>eutrophication</b> — a process of excess algal growth that leads to oxygen depletion. We also apply many synthetic organic chemicals, metals, and radioactive materials to soils that can damage ecosystems and can have serious human health effects. The clean-up of soils is among the great efforts being conducted by soil scientists around the world.<br> <br> <h1>Soils are... Home</h1>The phrase "to be on a nation's soil" is used as a poetic means to express our connection to country, to home. Soils give us a sense of place in our environment, a sense of geography. Our language abounds with phrases such as being "grounded" and "down to earth". Although we seldom consider the role of soils in our day-to-day lives, we have an intrinsic understanding that soils are home. <br> <br> <h1>Soils are... a Profession</h1>While many people work with soils — from farmers, to gardeners, to construction workers — some people make soil science a career. Soil scientists work in a variety of fields — space exploration, archeology, insurance, defense, engineering, and yes, agriculture. Soil science professional societies have been established in nations throughout the world. The Soil Science Society of America is among the largest (http://www.soils.org). Many of these organizations are represented in the International Union of Soil Sciences (http://www.iuss.org). Soil scientists are trained to understand how the basic concepts of chemistry, biology, and physics operate within the diversity of soils, and to apply this knowledge to address problems related to soil behavior and management. People drawn to this profession tend to have a love and respect for soils rooted in appreciation of their complexity, importance, and beauty.</div> </div> <div class="clear"></div> </div> </div> <div class="clear"></div> <div class="contentSection"> <div class="sectionTitle"> <h2> <SECTIONTITLE-1>Glossary</SECTIONTITLE-1> </h2> </div> <div class="clear"></div> <div class="sectionParas"> <div class="paraArticle"> <div> <p> <span style="font-weight: bold;">activity </span>- A general term used to describe how chemically reactive a particle is with ions, water, and other particles. <br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">clay </span>- A mineral particle smaller than 0.002 mm.<br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">clay synthesis</span> - Clays are formed in soils through the transformation of existing clays or through the generation of entirely new clay particles from ions precipitating from solution.<br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">desertification </span>- The transformation of a non-arid landscape to an arid landscape, usually through a combination of climate changes and human-induced soil degradation. <br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">dirt </span>- 1. synonym for soil material; 2. soil out of place; 3. unclean material of any composition.<br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">eluviation </span>- The removal of materials such as clays, organic matter, iron, or aluminum from a horizon.<br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">erosion </span>- The surface removal of soil material from soils by the action of water or wind.<br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">eutrophication </span>- A process of excess algal growth that leads to oxygen depletion; often caused by excess nutrient inputs. <br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">factors of soil formation</span> - Factors from which soil scientists are able to predict the end result of soil formation processes: climate, organisms, topography, parent material, and time. <br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">gas regulation</span> - The absorption and release of gases that mediates the levels of these gases in the atmosphere.<br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">illuviation </span>- The deposition of materials such as clays, organic matter, iron, or aluminum into a horizon; generally the materials come from an upper horizon in the soil body.<br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">leaching </span>- The removal of dissolved ions from a soil.<br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">natural bodies</span> - Systems that form in nature with size, form, and history that act as in an integrated fashion to provide functions that differ from the sum of their parts. <br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">remediate </span>- To transform a chemical from a toxic form or state to a non-toxic form or state. <br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">salinization </span>- A build up of salts in soils to the point that they destroy the soil's physical and chemical properties and plants are not able to take up water due to the high salt concentration; often associated with improper irrigation. <br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">sand </span>- A mineral particle ranging in size from 0.02 to 2 mm.<br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">silt </span>- A mineral particle ranging in size from 0.002 to 0.02 mm.<br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">soil </span>- 1. A material composed of minerals, living organisms, soil organic matter, gas, and water. 2. A body composed of soil and other parts such as rocks, roots, and animals that has size, form, and history and provides integrated functions that are greater than the sum of its parts. <br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">soil horizon</span> - Layer present within soil bodies that are distinguishable from other layers; often generated through soil formation processes.<br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">soil organic matter</span> - Plant, animal, and microbial residues, in various states of decomposition.<br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">soil texture</span> - The percentages of sand, silt, and clay particles in a soil.<br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">soil quality</span> - The capacity of a soil to provide desirable ecosystem services.<br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">transpiration</span> - Evaporation of water from openings in plant tissues called stomata; associated with photosynthesis.<br> <span style="font-weight: bold;"></span> </p> <p> <span style="font-weight: bold;">weathering</span> - Physical, chemical, and biological processes that breakdown and transform rocks and minerals. </p> <span style="font-weight: bold;"></span> </div> </div> <div class="clear"></div> </div> </div> <div class="clear"></div> </div> <div id="divSources"> <h2 class="sourcesHead nopad">References and Recommended Reading</h2> <hr class="topFivebotTen"> <div class="articleSourcesList articleSourcesListBig nopad" id="sourcesDisp"> <p>Ahrens, R. J. & Arnold, R. W. "Soil taxonomy," in <i>Handbook of Soil Science</i>, ed. M. Summer (CRC Press, 2000) E117-E135.<br> </p> <p>Brady, N. C. & Weil, R. R. <i>T</i>he Nature and Properties of Soils, 14th ed. Upper Saddle River, NJ: Prentice Hall, 2008.<br> </p> <p>Food and Agriculture Organization of the United Nations (FAO). Guidelines for Soil Description, 4th ed. FAO, Rome, 2006. <a rel='nofollow' href='ftp://ftp.fao.org/docrep/fao/009/a0541e/a0541e00.pdf' target='_blank' title="ftp://ftp.fao.org/docrep/fao/009/a0541e/a0541e00.pdf" >ftp://ftp.fao.org/docrep/fao/009/a0541e/a0541e00.pdf</a><br> </p> <p>Haygarth P. M. & Ritz, K. The future of soils and land use in the UK: Soil systems for the provision of land-based ecosystem services. <i>Land Use Policy</i> 26S:S187-S197, 2009. </p> <p>Jenny, H. <i>The Factors of Soil Formation: A System of Quantitative Pedology</i>. New York, NY: Dover Press, 1941.<br> </p> <p>Soil Survey Division Staff. <i>Soil Survey Manual</i>. Soil Conservation Service, United States Department of Agriculture, Handbook 18, 1993. <a rel='nofollow' href='http://soils.usda.gov/technical/manual/' target='_blank' title="http://soils.usda.gov/technical/manual/" >http://soils.usda.gov/technical/manual/</a><br> </p> </div> </div> </div> </div></div> </div> <div class="hideForPrint"> <div class="clear"></div> <ul class="topicDetailsLeft hideForPrint "> <li><a href="#TB_inline?height=300&width=400&inlineId=trOutLine" title="Outline of this Article" class="thickbox inlineLinks">Outline</a></li> <li id="bar1">|</li> <li id="keywordHide"><a onMouseOver="return displayOnMouseOverText(this);" href="#url" id="keywordLnkTR" name="keywordLnkTR" onClick="loadContentTagsOnStartUp(event, 'keywordLnkTR'); return false;" class="inlineLinks">Keywords</a></li> <li id="fdBKLBMdlAncID"><a href="#" onClick="showPostCommentFdbk(event, 'fdBKLBMdlAncID'); return false;" class="inlineLinks"><img src="/scitable/natedimages/feedback.gif" alt="FeedBack" 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As a registered user you can have access to all content in Scitable, interact directly with community members, create a personal locker, start and join groups and discussions, and more"; var signINReqMsgNonLoggedINToolTip = "You have to be logged in to use this feature."; var collapseLabelPage = "View Fewer"; </script> <script type="text/plain" data-cc-script="C04"> var isTopicLandingPageFllg = ''; var topicID = '84826767'; var invToJoinGrpGrpID = ""; var currentPagerItem = 1; </script> <script type="text/plain" data-cc-script="C04"> var browserIs = ''; var topicID='84826767'; var contentPK = '67647639'; var clickedNodeType = ''; var clickedContentPK = ''; var topicPK = ''; var clickedTopicPK = ''; var defaultDisplay = ''; var fontSizeCookie = 'null'; var isUserLiggedIn = "" var currDate = ''; var currDay = ''; var currYear = ''; var currMonth = ''; var map = ''; var mon = ''; var viewMode = ''; var dateNumeric = ''; var today = new Date(currDate); var selected_day = currDay; var currWorkingDay; var yearviewcontext = 0; var printviewcontext = 0; var weekHref = ""; var monthHref = ""; var str_buffer = ""; var n_weekstart=0; var calMode = 'topic'; var previousMonthHref = baseURL+'calendar?action=viewCalendar&view='+viewMode+ '&calMode=' +calMode; var nextMonthHref = baseURL+'calendar?action=viewCalendar&view='+viewMode+ '&calMode=' +calMode; var dayHref = baseURL+'calendar?action=viewCalendar&view='+viewMode+ '&calMode=' +calMode; var isSameUser= ""; var zimbraIncludeFlag = ""; var isAuthTopicAdmin = ""; var isLMSReq = ""; var contentId = '67647639'; contentID = '67647639'; var isTopicRoom = true; var cleanUrlContextPath = "/scitable"; var cleanBaseURL = '/scitable'; forumId=''; var discussion = ''; topicID='84826767'; var urlLoc = 'soil-agriculture-and-agricultural-biotechnology-84826767'; var numMemberTopic = '141'; var artSubTypeRed = 'RED'; var numGroupTopics = ''; weblogId='84826767'; var articleCtxPath = '/scitable/nated'; var ratingLabel = ""; var rateLabel = ""; var elementPerPage = "4 "; var topicPageUrlPresent = "/scitable/knowledge/soil-agriculture-and-agricultural-biotechnology-84826767"; var urlParam = ""; var tykAsmtPk = ""; var tykAsmtVersion=""; var exceededAllowedRegisteredMsg = null; exceededAllowedRegisteredMsg = "The article you have selected is available only for signed in users. 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"https://ssl." : "http://www."); (function (s) { s.src = gaJsHost + 'google-analytics.com/ga.js'; s.type = 'text/javascript'; document.head.appendChild(s); })(document.createElement('script')); </script> <script type="text/plain" data-cc-script="C04"> try { var pageTracker = _gat._getTracker("UA-12063924-2"); pageTracker._trackPageview(); } catch(err) {}</script> </div> </body> </html>

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