INSTRUMENTS 



25 



WATER-CONTENT 



40. Value of different instruments. The paramount importance of water- 

 content as a direct factor in the modification of plant form and distribution 

 gives a fundamental value to the methods used for its determination. Au- 

 tomatic instruments for ascertaining the water in the soil are costly, in ad- 

 dition to being complicated, and often inaccurate. For these reasons, much 

 attention has been given to developing the simpler but more reliable methods 

 in which a soil-borer or geotome is used. The latter is simple, inexpensive, 

 and accurate. It can be carried easily upon daily trips or upon longer re- 

 connaissances, and is 



always ready for instant 

 use. In the determination 

 of physiological water-con- 

 tent, it is -practically indis- 

 pensable. Indeed, the 

 readiness with which geo- 

 tome determinations of 

 water-content can be made 

 should hasten the universal 

 recognition of the fact that 

 it is the available, and not 

 the total amount of water 

 in the soil, which deter- 

 mines the effect upon the 

 plant. 



Geotome Methods 



41. The geotome. In its 



simplest form, the geotome 

 is merely a stout iron tube 

 with a sharp cutting edge 

 at one end and a firmly 

 attached handle at the 

 other. The length is variable and is primarily determined by the location 

 of the active root surface of the plant. In xerophytic habitats, generally a 

 longer tube is necessary than in mesophytic ones. The bore is largely 

 determined by the character of the soil; for example, a larger one is neces- 

 sary for gravel than for loam. Tubes of small bore also tend to pack the 

 soil below them, and to give a correspondingly incomplete core. The best 

 results have been obtained with geotomes of 3^2-1 inch tube. Each geotome 



Fig. 1. Geotomes and soil can. 



