On this basis, some have suggested that it may have been of some importance in prehispanic agriculture. The importance of guano as a fertilizer prior to the nineteenth century is less well known, but is mentioned by Spanish chroniclers and in colonial administrative documents. In recent years, however, there has been a resurgence in its popularity (particularly in horticulture) as worldwide demand for organically grown produce has increased. The popularity of guano with European and North American farmers waned in the latter part of the nineteenth century for a number of reasons, including: increasing guano prices, irregular availability, unsuitability for particular crops (especially turnips), a dwindling supply, and the development of the chemical fertilizer industry. The trade in guano peaked during the middle of the nineteenth century, with 20 million tons being exported to Europe and North America between 18. The once thick deposits of seabird guano (>50 m in some cases) were mined extensively during the guano boom of the 1800 s, and today the islands rarely have more than several years worth of droppings accumulated. Because the region receives virtually no precipitation, the guano accumulates in sedimentary layers. The guano islands of Peru and Chile are typically composed of rocky cliffs essentially devoid of vascular plants, with a relatively small number of fauna (ants, spiders, scorpions, lizards) that are supported by allocthonous inputs from the guano birds (guano, carcasses, feathers, eggshells). The guano was mined from small, nearshore islands off the arid western coast of South America in the Peru-Humboldt upwelling region. Seabird excrement (guano) was arguably the most economically significant organic fertilizer in the world prior to the twentieth century.
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