64 THE PHYSIOLOQT OF, PLANTS 



This, table shows how many litres of air pass in one hour 

 through a depth of 50 cm. of soil under a pressure of .40 

 mm. of water — ;« 



Litres. 



Clay 1 |:,,i-6? 



Powdered lime mixed with humus j | ' 3-32 . . 



Chalk } '^ 3-78 



Pure broken limestone . . ."» . . .. , 1. ( 4-24 



pg^t I size of particles up *" J ■ 5 04 



Quartz sand! '.'.'.'.} °-^S >»". | ,g g^ 



Broken-up clay . . . .■! particles of 0.25-0.50 



.■» particles of 0.25-0.50 f 30.9 



Quartz sand / mm. \ ^11.04 



Quartz sand \ size of particles / 92.24 



Broken-up clay . . . ./ 0.5-1.0 mm. 1 1 33-75. 



Quartz sand \ size of particles / 287.5 7' 



Broken-up clay . . . ./ 1-2 mm. \42o,f6 



This table is interesting in showing the effect of breaking 

 up a soil which is not readily permeable. The broken-up 

 clay is shown to be more porous than the coarse-grained sand ; 

 but a small amount of powdered clay added to the quartz sand 

 will greatly reduce the porosity of the latter. In fact, upon 

 the thickness of a layer of clay which lies between two layers of 

 sand depends the porosity of the whole soil, as all the absorbed 

 air must pass through it. The moisture of the air and of the 

 soil, too, control the amount of air which passes through the 

 soil in a given time. Dry air moves more rapidly through a 

 moist soil than air containing a large amount of water vapour, 

 and so air of a given nature passes more rapidly through a dry 

 than through a wet soil; most rapidly, however, through a 

 slightly moistened soil. This latter phenomenon is probably 

 due to the fact that small particles of sand are broken up by 

 being moistened. 



How the various physical properties of a soil are changed 

 by the varying proportions of clay and sand in a field, and 

 how important the presence of stable manure is in this connec- 

 tion, will be seen from the following figures of Masure : — 



