A spray-type irrigation system for young plants.

DRAE.
postura
3. f. Acción de poner o plantar árboles tiernos o plantas.
4. f. Planta o árbol tierno que se trasplanta.

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Note added at 5 mins (2015-10-26 12:15:42 GMT)
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NB: Am not quite sure if "por" is correctly used there.

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Note added at 7 mins (2015-10-26 12:17:45 GMT)
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AKA "sprinkler" irrigation.
http://www.wisegeek.com/what-is-spray-irrigation.htm

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Note added at 8 mins (2015-10-26 12:18:54 GMT)
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http://www.mca.am/files/Water management-Eng.pdf

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Note added at 2 hrs (2015-10-26 14:38:50 GMT)
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NB: Perhaps I should add that in over 20 years of horticultural/agricultural translations, I have never seen "postura" used like this, and the usual term for seedlings would be plantón or plántula:

http://www.linguee.es/espanol-ingles/search?source=auto&quer...