76 CHEMICAL PROPERTIES OF SOIL 



weight in the steam oven. The loss in weight, 

 representing the hygroscopic water, is highest in peat 

 (humus) and lowest in sand. 



56. Retention of Water by Soils. 



The amount of water which a soil can hold is 

 measured by the pore-space plus the quantity of water 

 capable of being absorbed by the particles without 

 disturbing their condition. The estimation of the water- 

 holding capacity of a soil is of importance. 



Apparatus. — The apparatus consists of a small brass 

 cylinder 6 cm. in diameter and lo cm. long, having a 

 perforated bottom. The capacity of the cylinder is 

 about 280 c.c, and is determined accurately by placing 

 while slightly hot on a slab of wax so as to fill up the 

 holes, and then running in water from a burette until 

 full. 



Procedure. — A disk of moist muslin is placed over the 

 perforated bottom, any excessive moisure removed, and 

 the cylinder weighed. It is then carefully filled with the 

 "fine earth" of the soil (64, p. 84) under examina- 

 tion, the sample being compacted by gently tapping on 

 the bench. The surface is then struck off level and 

 the cylinder again weighed, this giving the weight of 

 soil taken. The cylinder is then placed in a vessel of dis- 

 tilled water, the surface of the water being kept just below 

 the top of the cylinder. The whole apparatus is covered 

 over and allowed to stand for one hour, or until free 

 moisture appears on the surface of the soil, thus showing 

 that the maximum amount of water has been absorbed. 

 The cylinder is then removed, wiped dry, and again 

 weighed. The increase of weight gives the amount of 

 water absorbed. 



The moisture present in the " fine earth " must be 

 determined if this has not already been done. 



