158 LECTURES ON 



the kind of material that forms the soil and to its state of division. 

 This absorptive power opposes evaporation. Again, different soils 

 manifest widely different capacities for imbibing liquid water — capa- 

 cities 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 peculiarities of dif- 

 ferent soils in these respects. The first column gives the per cents of 

 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 per cents of water that evaporated during the space of one 

 hour from the saturated soil spread over a given surface : 



Quartz sand 25 88.4 



Gypsum 27 71. 7 



Lime sand 29 75.9 



Slaty marl 34 68.0 



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



Loam 51 45.7 



Plough land 52 32.0 



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



Pure gray clay 70 31.9 



Fine carbonate of lime 85 28.0 



Garden mould 89 24.3 



Humus 181 25.5 



Fine carbonate of magnesia 256 10. 8 



It is obvious that these two columns express nearly the same thing 

 in different ways. The amount of water retained increases from quartz 

 sand to magnesia. The rapidity of drying in the air, diminishes in the 

 same direction. 



The want of retentive power for water in the case of coarse sand is 

 undeniably one of the chief reasons of its unfruitfulness. The best 

 soils possess a medium retentive power. In them, therefore, are best 

 united the conditions for the regular distribution of the soil-water under 

 all circumstances. In them this process is not hindered too much either 

 by wet or dry weather. The retaining power of humus is seen to be 

 more than double that of clay. This result might appear at first sight 

 to be in contradiction to ordinary observations, for we are accustomed 

 to see water standing on the surface of clay but not on humus. It 

 must be borne in mind that clay, from its imperviousness, holds water 

 like a vessel, the water remaining apparent; but humus retains it 

 invisibly, its action being nearly like that of a sponge. 



One chief cause of the value of a layer of humus on the surface of 

 the soil doubtless consists in this great retaining power for water, and 

 the success that has attended the practice of green manuring, as a 

 means of renovating almost worthless shifting sands, is in a great 

 degree to be attributed to this cause. The advantages of mulching 

 are explained in the same way. 



