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<br />82
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<br />CONGREBO GEOLOGICO INTERNACIONAL
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<br />As phreatophytes, depend on ground water, their occurrence under natural
<br />conditions indicates its presence. In addition, certain species will give a hint
<br />as to the depth to the water table. Some phreatophytes are shallow rooted and
<br />grow only where the water table lies a short distance helow the land surface;
<br />others are deep rooted and may send their roots several teus of feet in search
<br />of ground water. Saltgrass, greasewood, batamote, cottonwood, or willows ge-
<br />nerally iudicate a relatively shallow water table, Saltgrass commonly grows
<br />where the depth to the water table does not exceed about 8 feet, greasewood to
<br />thrive best where the depth to water does not exceed 15 feet, and willow, cotton-
<br />wood, and batamote grow here it is 10 feet or less. There are exceptions to this
<br />general rule. Saltgrass has been ohserved growing where the depth to water
<br />was as much as 12 feet, the growth being due to a high capillary fringe,
<br />grasewood where the depth to water was almost 60 feet, an cottonwood where
<br />it was as much as 20 feet. Generally, however, these greater depths to water
<br />are reflected in the plant by a decrease in its size and vigor. Areas in which
<br />the water table could lie at much greater depths may be indicated by alfalfa,
<br />mezquite, or saltcedar. Alfalfa roots have. been identified at a depth of 129
<br />feet below the land surface (Meinzer, 1927, p, 55), and mezquite is know to
<br />have a. deep and extensive root system that penetrates the soil many tens
<br />of feet in search of ground water. However, the presence of a plant capable
<br />of sending its roots deep is not an i.nfallible indicator of a deep water table~
<br />for they frow also where water may be only a few feet below the land surface,
<br />In such places, however, a shallow water table is generally indicated by an
<br />association of typically shallow rooted plants.
<br />flints as to the quality of the ground water also are given by phreatophytes.
<br />Species such as saltgrass, saltcedar, greasewood, and pickleweed (AllenTolfea
<br />occidentalii) have a high tolerance for mineralized water, and their presence
<br />is a warning that the water may be unsatisfactory for most ordinary uses.
<br />This is especially true for pickleweed, which occurs only where the water has
<br />a high mineral content and is usually saline, A pure stand of greasewood ge-
<br />nerally indicates black alkali, for it grows well where the ground water is
<br />high in sodium carbonate. Saltgrass and saltcedar are warnings that the water
<br />may be saline, for they grow well where the sodium chloride content is high.
<br />Both the date palm and the ornamental palm have a high tolerance for salt.
<br />In predictiug the quality of ground water by plant species, some degree of
<br />caution must be exercised, for many plants of high ""It tolerance will grow
<br />also where the mineral content of the water is low. Where this occurs, however,
<br />there usually is an association of plants of low tolerance. A pure stand of plants
<br />of high tolerance is a warning that the ground water is. too mineralized for
<br />most uses.
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