180 HOW CROPS FEED. 



circulation of water in it, no continuous upward movement 

 can take place without tvaporation. The ease and rapid- 

 ity of evaporation, while mainly depending on the condi- 

 tion of the atmosphere and on the sun's heat, are to a cer- 

 tain degree influenced by the soil itself We liave already 

 seen that the soil possesses a power of absorbing watery 

 vapor from the atmosphere, a power which is related both 

 to the kind of material that forms tlie soil and to its state 

 of division. This absorptive power opposes evaporation. 

 Again, diffeient soils manifest widely different capacities 

 for imhib'mg liquid water — capacities mainly connected 

 with their porosity. Obviously, too, the quantity of liquid 

 in a given volume of soil affects not only the rapidity, 

 but also the duration of evaporation. 



The following tables by Schiibler illustrate the peculi- 

 arities of different soils in these respects. The first col- 

 umn gives the percentages of liquid water absorbed by 

 the completely dry soil. In these experiments the soils 

 were thoroughly wet with water, the excess allowed to 

 drip off, and the increase of weight determined. In the 

 second column are given the pei-centages of vrater that 

 evaporated during the space of four hours from the satu- 

 rated soil spread over a given surface : 



Quartz sand 25 88.4 



Gypsum 27 71.7 



Lime sand 29 75.9 



Slaty m:ul 34 68.0 



Clay soil, (sixty per cent clay,) 40 52.0 



Loam 51 45.7 



Plough Luid 53 32.0 



Heavy clay, (ehhty per cent clay,) 61 34.9 



Pure ^irray clay 70 31.9 



Fine carbonate of lime 85 28.0 



Garden mould SI) 24.3 



Humus 181 25.5 



Fine carbonate of ma'.aiesia S^'jO 10.8 



It is obvious that these tAVO columns oxitress neai-ly the 

 same thiniz in different wavs. The am'unt of water re- 



