110 ANNUAL REPORT OF NEW- YORK 



caD take place without evaporation. The rapidity of evaporation 

 depends upon several causes, which will be individually noticed. 

 One of the most important is : 



IV. The retentive powers of the soil for water. — The following 

 tables by Schiibler illustrate the peculiarities of diiierent soils in 

 this respect. 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, (80 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 w^et 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 

 ordinaiy 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, 



