POWER OF RETAINING WATER 83 



Before leaving this section we may sum up the chief 

 conclusions by saying that extreme fineness of the particles is 

 by itself capable of 'giving to a soil a maximum power of 

 retaining water, this condition alone sufficing to keep all 

 interstices full of water after percolation has ceased. The 

 quantity of water retained is thus in proportion to the 

 internal surface, and is doubtless increased by the presence 

 of colloid bodies in the soil. Of all soils peat has the greatest 

 capacity for retaining water, its porosity supplying an enormous 

 internal surface, the effect of which is heightened by the 

 affinity for water of its colloid constituents. At the other end 

 of the scale we have gravels and coarse sands, which have 

 hardly any power of retaining water. For swamp-loving crops, 

 as rice, soils retaining a maximum amount of water are desir- 

 able ; soils having a high retentive power may also have a special 

 value in climates of small rainfall. For ordinary farm crops 

 it has been already pointed out that a very large proportion 

 of water in the soil is distinctly injurious, and that the most 

 vigorous growth is obtained when the soil contains about one- 

 half of its saturation quantity. 



Sandy soils are by no means so inferior as suppliers of 

 water as is often supposed. We must recollect that the 

 quantity of water retained by a given volume of sand is 

 greater than would appear from the usual percentage deter- 

 minations made by weight. A moist sand containing 7 per 

 cent, of water by weight, contains 11-2 vols. of water per 

 100 vols. ; and this difference between weight and volume is 

 greater in the case of sand than in the case of a clay soil, 

 and still more than in the case of a soil containing humus, 

 owing to the higher weight per cubic foot of the sand. 

 The soils just named are in fact not so unequal when their 



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