18 



CIRCULAR 112, BUREAU OF PLANT INDUSTRY. 



equivalent, 1 in samples from different parts of the field of 24.4 to 31.4 

 per cent, corresponding to a wilting coefficient of 13.3 to 17 per cent. 

 The coarse sand had a moisture equivalent of only 2.8 per cent 

 (wilting coefficient, 1.5 per cent). The salt content of the soil in the 

 different samples was investigated by means of the electrolytic bridge 

 and was found to be nowhere sufficiently high to indicate that alkali 

 was a factor in bringing about these differences of growth. The case 

 was clearly one of the presence or absence of a sufficient depth of 

 soil having a high enough water-holding capacity to prevent the 

 plants from suffering as a result of drought between irrigations. It 

 is rather surprising, in view of the very low water capacity of the 

 underlying sand, that a depth of the silty layer of only 14 or 15 

 inches should have been sufficient to enable the plants to make a 

 strong growth and produce numerous large bolls. 



The results of this investigation are summed up in Table I. 



Table I. — Relations between the depth of silt and the growth of Egyptian cotton at 



Bard, Cat., in 1911. 



1 Unless otherwise specified, the silt rested directly upon the rather coarse, light-colored sand. This 

 sand extended to a depth of at least 4 feet at every boring which was carried to that depth (Nos. 1, 3, 5, 

 and G). 



2 Here the silt was underlain by 15 inches of a fine, reddish-colored sand (moisture equivalent, 7.6 per 

 cent), which in turn rested upon the above-described coarse sand. 



3 Here the surface silt was of lighter texture than elsewhere in the field (moisture equivalent, 24.4 per 

 cent). It rested upon G to 7 inches of very fine sand, which was in turn underlain by the coarser sand 

 above described. 



In 1912 similar observations were made on the same farm. A 

 series of soil samples were taken midway between two rows of 

 Egyptian cotton, in one of which the plants had been thinned to a 

 distance of 6 inches and in the other to a distance of 18 inches. 2 

 Borings were made at three points: (1) Where the plants were tall, 

 luxuriant, and dark green in color; (2) where the plants were smaller 

 and of a light yellowish green color; and (3) where the plants were 



1 The term "moisture equivalent" is defined and the value of this factor as a measure of the moisture- 

 holding capacity of the soil is pointed out by Briggs and Me Lane in Bulletin 45 of the Bureau of Soils (1907). 

 The moisture equivalent of a given soil being known, the wilting coefficient for plants growing in that soil 

 can be calculated by means of the formula given by Briggs and Shantz in Bulletin 230 of the Bureau of 

 Plant Industry, p. 58 (1912). 



All determinations of moisture equivalent referred to in this paper were made by Mr. J. W. McLane, 

 of the Biophysical Laboratory, Bureau of Plant Industry. 



2 The field had been planted under the direction of Mr. O. F. Cook in order to study the effect of dif- 

 ferent thicknesses of stands upon the development of the vegetative branches. 



[Cir. 112] 



