POWER OF RETAINING WATER 79 



Weight of Water in 100 Drained Soil 



Coarse Sand 3-0 



Fine Sand 7-3 



Calcareous Sand ... 32-0 



Clay Soil 35-0 



Forest Soil 42-0 



We have here again examples of the increase of the water- 

 holding power as the particles become finer, or more porous 

 (e.g. calcareous sand). The forest soil consisted chiefly of 

 extremely fine sand, probably with some humus. Some 

 further illustrations of the amount of water held by drained 

 soils will be found in the next section. 



The state of consolidation of the soil, or in other words 

 the closeness of the packing of its particles, has a great 

 influence upon its power of retaining water. The operations 

 of tillage may thus supply a means of ameliorating the ex- 

 cessive dryness or wetness of a soil. Referring once more 

 to the examples of the loose and tight packing of soil particles 

 already given on p. I, it will be evident that in the system 

 of close packing the points of contact between the particles 

 are about twice as numerous as in the system of loose packing 

 (Fig. i), and the interstitial spaces are also much reduced 

 in size. The closely packed particles will in fact retain 

 when drained at least twice as much water per unit of 

 volume as the loosely packed particles. The reason of this 

 will become clearer when we have discussed the subject of 

 capillary action. In practice, the water-holding power of 

 a coarse sandy soil may be increased by consolidation with 

 a heavy roller, or by the treading of animals on the land. 

 On the other hand the water-holding power of a heavy soil 

 may be greatly reduced if the soil can be pulverized, and 

 brought into a loose state of aggregation. 



