74 RELATIONS OF SOIL TO WATER 



to only 30 per cent, of the saturation quantity, the proportion 

 was too low for the production of a maximum crop. The 

 largest crops were obtained when the proportion of water lay 

 between 40 and 60 per cent, of that required for full saturation. 

 When a soil is half saturated with water it of course implies 

 that the interspaces are half filled with air, and this is 

 apparently the condition to be aimed at. 



Power of Retaining Water. The utmost capacity of a soil 

 for water is a subject of comparatively little practical import- 

 ance, as most soils are fully saturated only when the level of 

 standing water is quite near the surface, or immediately after 

 long continued heavy rain. Soils usually occur in nature in 

 a more or less drained condition, and it is the quantity of 

 water which they retain when fully drained which determines 

 the supply which they are able to furnish to a crop. The 

 proportion of water held by a soil in a fully drained condition 

 is termed by Mayer its absolute water capacity. 



If a wide tube of sufficient length is filled with coarse sand, 

 consisting of particles of uniform size, the lower end of the 

 tube being closed by a piece of linen, and water then poured 

 on the top of the sand till it is fully saturated, and the whole 

 then allowed to stand till dropping ceases, there will be found 

 in the tube two distinct layers of wet sand, a short column at 

 the bottom fully saturated, and a long column above it fully 

 drained, and containing throughout a nearly uniform propor- 

 tion of water. In the lower layer the interstices are completely 

 filled with water. In the upper layer the water coats the 

 surfaces of the particles, and is held around their points 

 of contact, but the main interspaces are empty. 



If the particles of the sand are not uniform in size, but 

 consist of a mixture of large and small, as in a natural soil, 



