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Humus
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<p>In classical <a href="/facts/Soil_science/nJ9SxpEE">soil science</a>, humus is the dark organic matter in soil that is formed by the decomposition of plant and animal matter. It is a kind of <a href="/facts/Soil_organic_matter/WrAuRGp4">soil organic matter</a>. It is rich in nutrients and retains moisture in the soil. Humus is the Latin word for "earth" or "ground". </p><p>In <a href="/facts/Agriculture/TGztqzJs">agriculture</a>, "humus" sometimes also is used to describe mature or natural <a href="/facts/Compost/q6kmXGWU">compost</a> extracted from a woodland or other spontaneous source for use as a <a href="/facts/Soil_conditioner/35DWU3Cc">soil conditioner</a>. It is also used to describe a <a href="/facts/Topsoil/BDClBQrs">topsoil</a> <a href="/facts/Soil_horizon/GcXhdams">horizon</a> that contains <a href="/facts/Organic_matter/t17tVCvk">organic matter</a> (<i>humus type</i>, <i>humus form</i>, or <i>humus profile</i>). </p><p>Humus has many <a href="/facts/Nutrient/34bn4skK">nutrients</a> that improve the health of soil, <a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a> being the most important. The ratio of <a href="/facts/Carbon/ukrgQexo">carbon</a> to <a href="/facts/Nitrogen/Nf1QpGDz">nitrogen</a> (<a href="/facts/C%EF%BC%9AN_ratio/yauDuwUB">C:N</a>) of humus commonly ranges between 8:1 and 15:1 with the median being about 12:1. It also significantly improves (decreases) the <a href="/facts/Bulk_density/qoPXMQQy">bulk density</a> of soil. Humus is <a href="/facts/Amorphous_solid/YTIzmrR0">amorphous</a> and lacks the cellular structure characteristic of <a href="/facts/Organism/j8JkL09u">organisms</a>. </p><p>The <a href="/facts/Biosolids/KJnIs9yM">solid</a> residue of <a href="/facts/Sewage_sludge_treatment/MygYNh5Y">sewage sludge treatment</a>, which is a secondary phase in the <a href="/facts/Wastewater_treatment/b5bJ6aJm">wastewater treatment</a> process, is also called humus. When not judged <a href="/facts/Contamination/RC0L7KRy">contaminated</a> by <a href="/facts/Pathogen/axMXtV1M">pathogens</a>, toxic <a href="/facts/Heavy_metals/Cblmy8qI">heavy metals</a>, or <a href="/facts/Persistent_organic_pollutant/80UtnJEp">persistent organic pollutants</a> according to standard tolerance levels, it is sometimes <a href="/facts/Compost/q6kmXGWU">composted</a> and used as a <a href="/facts/Fertilizer/ysh3r6NJ">soil amendment</a>. </p>