416 EXPERIMENT STATION RECORD. 



intermittent sand filtration, contact beds, sprinkling or continuous filters, with 

 preliminary or auxiliary treatment by sedimentation, septic tanks, and aeration 

 are explained. 



SOILS— FERTILIZERS. 



The moisture equivalents of soils, L. J. BKiotis and J. W. McLane ( IJ. H. 

 Dept. Agr., Bur. Soils Bid. .'i-J. pp. 23, pi. 1. fig. I). — Moisture equivalent is 

 defined as " the percentage of water retained by a soil, when the moisture con- 

 tent is reduced by means of a constant centrifugal force until it is brought into 

 a state of capillary equilibrium with the applied force," i. e., until the water 

 held in the larger capillary spaces is removed. 



The centrifugal machine devised for the work is described and the results 

 of preliminary studies of various factors — duration of test, initial water con- 

 tent, speed, etc. — upon the accuracy of the determinations are reported. 



The method adojited and used in the determination of the moisture equiva- 

 lents of over 100 samples of soils is essentially as follows : " The soils under 

 investigation are first thoroughly moistened, and are then placed in the per- 

 forated cups of a centrifugal machine, where they are subjected to a constant 

 centrifugal force until they cease to lose moisture. The percentage of water 

 remaining in the soil is then determined." 



It is held that by. this means it is possible to reduce the moisture content of 

 a soil until it is no greater than the moisture content of the soil inider favorable 

 field conditions, and thus to determine the retentive power of different soils for 

 moisture when acted upon by the same definite force, comparable in magnitude 

 with the pulling force to which the soil moisture is subjected in the field. 

 " Furthermore, this method of comparing tlie relation of soils to moisture avoids 

 to a large extent, if not entirely, the errors due to differences in i);iclcing, since 

 the soils are packed by centrifugal force, which acts"nipt)n each individual parti- 

 cle. This is further safeguarded l)y the high sjteed employed, which is suffi- 

 cient to I'emove the moisture from any large capillary spaces that may possibly 

 be formed. . . . 



"A series of soils which have thus been brought into equilibrium with the 

 same force will be in capillary equilibrium with one another when brought into 

 •contact, and no capillary movement of moisture will take place between them. 

 In other words, the moisture ecjuivalents of a series of soils represent the 

 moisture contents which tliose soils must have in order to make it equally ditti- 

 cult to remove a very small additional amount of moisture from any of the 

 soils. It is from this point of view that the determination of the moisture 

 equivalent becomes of special importance in the comparison of the moisture 

 contents of different soils under growing croi)S." 



The moisture equivalents of over 100 samples f)f type soils, using a centrifu- 

 gal force about 3,000 times the force of gravity, were found to vary from 3.0 

 per cent in the coarser sandy soils t6 4G.5 per cent in the case of a heavy clay 

 subsoil. 



When " these observations were reduced by the method of least squares to 

 determine the influence of the sand, silt, and clay groups, and of the organic 

 matter, upon the retention of moisture, it was found for the whole series that 

 each per cent of clay or organic matter in the soil corresponded to a retention 

 of 0.62 per cent of moisture when the soil was subjected to a force 3,000 times 

 that of gravity. Each per cent of silt, under similar conditions, corresponded 

 to a retention of 0.13 per cent of moisture, and the coarser grades show practi- 

 cally no retentive action against this force. The ' j^robable error ' for these 

 coetficients was rather high, and better results were obtained for smaller series 



