]'28 BULLETIN 1059, U. S. DEPARTMENT OF AGRICULTURE. 



ignition, suffers a loss of water. The experience of the writers in- 

 dicates that the sand filter devised by Grandeau, with also the paper 

 filter used by Hilgard, will come nearest holding the colloids in the 



il. Such as are likely to escape will have passed the filter by the 

 time the acid t reatment is complete. In the case of the coarser forest 

 soils the addition of sand is wholly unnecessary. 



Alwav suggests the recording of the humus ash percentage as 

 well as the humus as a means of detecting the errors which commonly 

 enter into this determination. 



CAPILLARY CONDUCTIVITY. 



As has been frequently pointed out in the discussion oi the mois- 

 ture problems, the rate at which a plant is able to obtain water from 

 the soil particles with which the roots are in actual contact may have 

 an important bearing on the wilting coefficient for t he whole soil mass, 

 and may, in turn, depend largely on the facility witli which the mois- 

 ture travels from one soil particle to another when there is unequal 

 distribution. Thus, a clean sand is generally understood to have the 

 highest conductivity (whether because of the close contact between 

 the particles or because of the clearness of the spaces between the 

 larger particles, is not known), while clay in the soil seems to impede 

 this movement, probably because of absorption, and humus appears 

 to retard the movement, possibly by breaking the contacts between 

 the mineral particles. 



The whole subject of capillary conductivity appears to have been 

 thrown into confusion in recent years by practical findings, especially 

 in connection with the study of moisture supplies in the arid farming 

 regions of the West. In brief, it has been found that in certain lo- 

 calities the soil is never moistened to a greater depth than, say, 10 

 feet, by precipitation; that the moisture which goes beyond the 

 depth of ordinary crop roots is never brought toward the surface by 

 capillary action, and hence is lost for practical purposes; that fallow- 

 ing with the object of storing moisture in the deep soil is therefore 

 useless. 



Buckingham (116) and McLaughlin (132) have apparently made 

 the most exhaustive studies of the movement of soil moisture: and 

 it may be said that these investigations confirm the practical con- 

 clusion that when the mean moisture content is very Low and the 

 difference in moisture between two points is slight, the rate of mot 

 ment from the moister to the drier point is negligible. These, of 

 course, are the conditions to be dealt witli as the wilting coefficient 

 is approached, and it is somewhat relevant to remark that there 

 is no evidence against the ordinary conception of capillary movi 

 ment when the amount of moisture in the soil is considerable, h La 

 true, however, that this movement is very slow upward- that Ls, 

 against the force of gravity. 



