62 SOILS AND MANUKES 



by mixing with them substances of greater retentive power 

 such as clay, lime and organic matter. 



If the soil contain too much water, air is excluded and 

 the health of the plant Buffers ; if it contain too little, 

 growth is retarded. Hellriegel and Wollny concluded, from 

 certain experiments on this subject, that, in general, the 

 best results are obtained when the soil contains from 40 to 

 60 per cent, of the water required for complete saturation, 

 i.e., when about half the interspace is occupied by water 

 and the remainder by air. 



Coarse sandy soils, however, do not naturally retain more 

 than about 10 per cent, of the water required to saturate 

 them, but on the other hand they yield it up to the plants 

 much more freely than some other soils of greater reten- 

 tive power. Heinrich estimated the amount of moisture 

 left in different kinds of soil from which water had been 

 withheld until the plants growing in them wilted, and he 

 obtained the following results : 



Water left in 



Soil when 

 Kind of Soil. Plants Wilted. 



Coarse sand 1-5 per cent. 



Sandy garden soil 4'6 ,, 



Fine humus sand ...... 6'2 ,, 



Sandy loam ....... 7'8 ,, 



Calcareous loam 9'8 ,, 



Peat ... . 49-7 



The beneficial effects of farmyard manure on light land 

 are generally attributed, at least in part, to the fact that 

 it increases the water-retaining power of the soil. It may 

 be questioned whether the water so retained is ever of much 

 benefit to the plants. Heinrich's experiment would appear 

 to suggest that if there were any scarcity of water in the 

 soil the organic matter would not yield up to the plants 

 what it retains, but might even be in competition with 

 them for what is left. Long experience, however, has con- 



