The cation exchange, that takes place between colloids and soil water, buffers (moderates) soil pH, alters soil structure, and purifies percolating water by adsorbing cations of all types, both useful and harmful. [113], In high rainfall areas, soils tend to acidify as the basic cations are forced off the soil colloids by the mass action of hydrogen ions from the rain against those attached to the colloids. Rapid salination occurs when the land surface is within the capillary fringe of saline groundwater. [124], The resistance of soil to change in pH, as a result of the addition of acid or basic material, is a measure of the buffering capacity of a soil and (for a particular soil type) increases as the CEC increases. Resistivity refers to the resistance to conduction of electric currents and affects the rate of corrosion of metal and concrete structures which are buried in soil. A gram of soil can contain billions of organisms, belonging to thousands of species, mostly microbial and largely still unexplored. [55] When reordered to climate, relief, organisms, parent material, and time, they form the acronym CROPT. A beach is a narrow, gently sloping strip of land that lies along the edge of an ocean, lake, or river. Peat soils are also commonly used for the sake of agriculture in nordic countries, because peatland sites, when drained, provide fertile soils for food production. The most common rock to form sand is granite, where the feldspar minerals dissolve faster than the quartz, causing the rock to break apart into small pieces. However, colloids are not the only regulators of soil pH. [105], Soil reactivity is expressed in terms of pH and is a measure of the acidity or alkalinity of the soil. Nutrients that enhance the growth of plants but are not necessary to complete the plant's life cycle are considered non-essential. The second most common type of sand is calcium carbonate, for example, aragonite, which has mostly been created, over the past half billion years, by various forms of life, like coral and shellfish. The particle size distribution of a soil, its texture, determines many of the properties of that soil, in particular hydraulic conductivity and water potential,[81] but the mineralogy of those particles can strongly modify those properties. Soil pH is a function of many soil forming factors, and is generally lower (more acid) where weathering is more advanced. CEC is the amount of exchangeable hydrogen cation (H+) that will combine with 100 grams dry weight of soil and whose measure is one milliequivalents per 100 grams of soil (1 meq/100 g). [166] Humus is less stable than the soil's mineral constituents, as it is reduced by microbial decomposition, and over time its concentration diminishes without the addition of new organic matter. Throughout history, civilizations have prospered or declined as a function of the availability and productivity of their soils. The world's ecosystems are impacted in far-reaching ways by the processes carried out in the soil, with effects ranging from ozone depletion and global warming to rainforest destruction and water pollution. The enrichment of soil with guano by the Incas was rediscovered in 1802, by Alexander von Humboldt. [165] Soils with humus can vary in nitrogen content but typically have 3 to 6 percent nitrogen. His 1857 book, Anfangsgründe der Bodenkunde (First principles of soil science) established modern soil science. The largest state in Mexico, Chihuahua is headquarters for the world’s fifth largest oil company, Pertróleos Mexicanos. [123] The addition of enough lime to neutralize the soil water solution will be insufficient to change the pH, as the acid forming cations stored on the soil colloids will tend to restore the original pH condition as they are pushed off those colloids by the calcium of the added lime. [42] Freed cations can be made available to plants but are also prone to be leached from the soil, possibly making the soil less fertile. [139][136], Soil organic matter is made up of organic compounds and includes plant, animal and microbial material, both living and dead. [161] The addition of organic matter to clay soils can render that organic matter and any added nutrients inaccessible to plants and microbes for many years,[citation needed] while a study showed increased soil fertility following the addition of mature compost to a clay soil. The mineralogy of the finest soil particles, clay, is especially important.[82]. [221] These practices help to control erosion and maintain productivity during periods when moisture is available. More precisely, it is a measure of hydrogen ion concentration in an aqueous solution and ranges in values from 0 to 14 (acidic to basic) but practically speaking for soils, pH ranges from 3.5 to 9.5, as pH values beyond those extremes are toxic to life forms. [204] However, wide areas of peat production, such as rain-fed sphagnum bogs, also called blanket bogs or raised bogs, are now protected because of their patrimonial interest. [220] Droughts are common in arid and semiarid lands. [24], Soils can effectively remove impurities,[25] kill disease agents,[26] and degrade contaminants, this latter property being called natural attenuation. Gold is often pinpointed in the ground, sand, iron ore fields, rivers/streams and even in the ocean water. Raw organic matter, as a reserve of nitrogen and phosphorus, is a vital component affecting soil fertility. Dirt: The ecstatic skin of the earth. [153], Humic acids and fulvic acids, which begin as raw organic matter, are important constituents of humus. [83] A colloid is a small, insoluble particle ranging in size from 1 nanometer to 1 micrometer, thus small enough to remain suspended by Brownian motion in a fluid medium without settling. 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. [78] Biogenic soil volatile organic compounds are exchanged with the aboveground atmosphere, in which they are just 1–2 orders of magnitude lower than those from aboveground vegetation.[79]. 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