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Physical Biology Of The Cell Phillips Solution Manual.rarl

Soil consists of a solid phase of minerals and organic matter (the soil matrix), as well as a porous phase that holds gases (the soil atmosphere) and water (the soil solution).[1][2] Accordingly, soil is a three-state system of solids, liquids, and gases.[3] Soil is a product of several factors: the influence of climate, relief (elevation, orientation, and slope of terrain), organisms, and the soil's parent materials (original minerals) interacting over time.[4] It continually undergoes development by way of numerous physical, chemical and biological processes, which include weathering with associated erosion.[5] Given its complexity and strong internal connectedness, soil ecologists regard soil as an ecosystem.[6]

Physical Biology Of The Cell Phillips Solution Manual.rarl


Most plant nutrients, with the exception of nitrogen, originate from the minerals that make up the soil parent material. Some nitrogen originates from rain as dilute nitric acid and ammonia,[44] but most of the nitrogen is available in soils as a result of nitrogen fixation by bacteria. Once in the soil-plant system, most nutrients are recycled through living organisms, plant and microbial residues (soil organic matter), mineral-bound forms, and the soil solution. Both living soil organisms (microbes, animals and plant roots) and soil organic matter are of critical importance to this recycling, and thereby to soil formation and soil fertility.[45] Microbial soil enzymes may release nutrients from minerals or organic matter for use by plants and other microorganisms, sequester (incorporate) them into living cells, or cause their loss from the soil by volatilisation (loss to the atmosphere as gases) or leaching.[46]

Humic acids and fulvic acids, which begin as raw organic matter, are important constituents of humus. After the death of plants, animals, and microbes, microbes begin to feed on the residues through their production of extra-cellular soil enzymes, resulting finally in the formation of humus.[160] As the residues break down, only molecules made of aliphatic and aromatic hydrocarbons, assembled and stabilized by oxygen and hydrogen bonds, remain in the form of complex molecular assemblages collectively called humus.[152] Humus is never pure in the soil, because it reacts with metals and clays to form complexes which further contribute to its stability and to soil structure.[159] Although the structure of humus has in itself few nutrients (with the exception of constitutive metals such as calcium, iron and aluminum) it is able to attract and link, by weak bonds, cation and anion nutrients that can further be released into the soil solution in response to selective root uptake and changes in soil pH, a process of paramount importance for the maintenance of fertility in tropical soils.[161]


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