EFFECT OF DRYING SOILS ON WATER-SOLUBLE CONSTITUENTS 197 



solution is probably far from saturated with respect to any of the salts. This 

 soil solution upon adding distilled water to make up the total 500 cc. becomes 

 a part of the 500 cc. and the final result is a more concentrated solution as 

 shown by table 6. When the saturated soil was placed in the oven, water 

 stood on its surface, so the oven-drying process required more than the usual 

 8 hours. This, it has been suggested, brings more organic matter and much 

 iron and alumina into solution. 



Experiment 5. Determination of hygroscopic coefficient 



In the hope of securing information which might shed a bit of light on the 

 cause of the increase in soluble material in soils due to heating, two small 

 pieces of experimental work were undertaken, viz. (experiment 5), the deter- 

 mination of the hygroscopic capacity, or coefficient, of the soils used in experi- 

 ment 4 and (experiment 6) a study of the retention of potassium nitrate by 

 different grades of sand. 



The soils are those used in experiment 4 which range from clay to sandy 

 loam. 



Procedure 



In order to have the soils as uniform as possible, they were passed through a 1-mm. sieve. 

 Duplicate samples of air-dry soil equivalent to approximately 2 gm. of water-free soil were 

 weighed directly into weighing bottles, 7 cm. in diameter. The soil was spread out in a uni- 

 formly thin layer over the bottom and exposed to a saturated atmosphere in a humidifier.* 

 Strips of filter paper were used as wicks, increasing the surface of contact between air and 

 water in order to insure complete saturation of the air. By means of a water-pump the air 

 pressure within the humidifiers was reduced by several centimeters in order to hasten satura- 

 tion and reduce condensation. To avoid marked sudden changes in temperature, the humidi- 

 fiers were placed in a thick-walled wooden box lined with asbestos. This was first located 

 in a cold room and heated electrically, a thermostat being used for maintaining a constant 

 temperature. It was found after 6 weeks' work, however, that the external temperature 

 of winter varied so much that the thermostat and heating apparatus were incapable of main- 

 taining a sufliciently constant temperature. In several instances there was a sudden drop in 

 outdoor temperature the day before weighings were to be made. This brought about con- 

 densation in the soil, giving hygroscopic coefficients too high and far from uniform. 



The next step was to place the entire apparatus in a deep, unheated basement room. 

 Here the temperature to which the soils were subjected remained fairly constant. The 

 maximum variation in room temperature during the first week was from 61 to 64°F., and 

 inside of the box from 15.5 to l^C; the second week the corresponding temperatures were 

 60.5 to 65.5°F. for the room and from 15 to 18°C. inside the box. There did not appear to 

 be any condensation and yet there is not an altogether satisfactory agreement of duplicate 

 determinations made the first week on soils 4, 5 and 6. 



The soils were kept in the saturated atmosphere for a period of 7 days when the humidifiers 

 were removed from the constant-temperature box. Before removing the lid of humidifiers, 

 the pressure was equalized by admitting air very slowly through the side tube. Upon remov- 

 ing the weighing bottles from the humidifier, the lids were immediately and tightly inserted 



^ The humidifier was a large desiccator whose dehydrating substance had been replaced 

 by a 10 per cent solution of sulfuric acid to furnish the water vapor. The desiccators are 

 supplied with side tube and stop cock. 



