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DUDLEY MEMORIAL VOLUME 



tral portion of the salt-spot he enters the polycarpa zone, where such non-salt 

 loving plants as are found there are along the washes only. Still nearer the 

 center of the spot A. nuttalli is met and with it A. elegans and dwarfed 

 specimens of Suceda suffrutescens. In places where there are evidently less 

 salts, species of Lycium and Prosopis occur along the washes. The inner 

 area is called here the nuttallii zone. At the very center of the spot there are 

 no plants and the surface of the ground usually shows white incrustations of 

 salts. 



CHARACTER OF THE SOIL. 



The soil of the Santa Cruz river bottoms is largely "adobe," but that of 

 the salt-spot is a fine sandy loam. Mechanical and chemical analyses made 

 by the Bureau of Soils, U. S. Department of Agriculture, of samples of soil 

 taken from the center of the spot are, for the upper 12 inches, as follows: 

 Clay, 33.3% ; silt, 21.5% ; very fine sand, 17.0% ; fine sand, 23.5% ; medium 

 sand, 2.6%; coarse sand, 2.0%; fine gravel 0.1%. 



Table 1. Analysis of soil from salt-spot. 1 



The salts are not uniformly distributed in the soil. The total soluble 

 salts in the first foot, as determined by resistance tests made by the Bureau 

 of Soils, is 1,72%; in the second foot, 1.0%, and in the third foot, 1.3%. 

 The salt content was not observed at a greater depth than three feet. Ob- 

 servations on the resistance of the soil solutions, which the writer made with 

 an electric bridge of the type used by the Bureau of Soils 2 , showed that the 



1 Analysis furnished by the Bureau of Soils, U. S. Dept. of Agriculture. The 

 soil was from 1-12 inches deep taken from the center of the salt-spot, Tucson. 



2 The instrument employed in these tests was kindly loaned me by Prof. 

 R. H. Forbes, Arizona Experiment Station. 



