Several such 
factors exist in nature. ution work of O'Gara 
Based on the air poll 
(1922), Thomas (1961 
» P. 236-237) ranked cultiv 
Although lumines- 
1 variability in 
» including conifers, but 
barley, and Cotton were most 
> elm, birch, Sumac, and poplar were 
mes more resistant, pine 7-15 times more resistant, and 
times more resistant. Thomas (1961, p. 253) also noted 
th old ponderosa needles were sensitive to hydrogen fluoride 
fumigation, but that old needles were resistant. 
others were relatively constant. Alfalfa, 
sensitive. Using alfalfa as unity 
about 2.5 ti 
live oak 12 
Another such factor i 
tions of one or more metallic elements may subje 
physiological changes. Copper and zinc are toxic if present in large 
quantities (Kramer and Kowlowski, 1960, p. 225-2 
26) and are reported to 
Produce symptoms closely resembling iron chlorosis (Sauchelli, 1969, 
P. 65). —J.E. McMurtrey (Sauchelli, 1969. p. 156) notes that copper in 
excess of 0.1 ppm stunted tobacco plants. .F.A. Gilbert (Sauchelli, 
1969, p. 158) states that growth of tomatoes in nutrient solution was 
reduced by as ittle as 1 ppm copper. Sauchelli (1969, . p. 112).cites 
zinc having a toxic effect in some cereal grains where levels exceed 
150 ppm, and in citrus where levels exceed 220 ppm. K.C. Beeson 
(Sauchelli, 1969, p..133 and 138) reports that concentrations of molyb- 
denum in plants may v ry from less than 0.1 Ppm to more than 300 ppm 
Without adverse effect on growth. Press (1974, p. 374-375) reports that 
S geochemical Stress, where toxic concentra- 
Sedimentary phosphate rock is an increasinly important raw material 
for production of agricultural fertilizer. A common problem in prospect- 
ing for this material, however, is that curs 
few physical properties which di 
ed ultraviolet lamps have been used to 
nd rocks, these methods of prospecting 
; effective range is limited 
ted at night because the 
Scence is obscured by bright sunlight. 
are limited because the lamps ar 
to a metre or less, 
low intensity lumine 
  
T 
rocks, 
geogra 
spectr 
within 
lumine 
a sedi 
gypsun 
values 
because 
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