176 HOW CROPS FEED. 



be regarded for the most part as j^urely pliysical. There 

 are other resuUs of the action of the soil on saline solu- 

 tions, which, though j)erhaps influenced hy simple physical 

 action, are prepondcratiugly chemical in their aspect. 

 These effects, which manifest themselves by chemical de- 

 compositions and substitutions, will be fully discussed in 

 a subsequent chapter, p. 333. 



PERMEABILITY OF SOILS TO LIQUID WATER. IMBIBITION. 

 CAPILLARY POWER. 



The fertility of the soil is greatly influenced by its de- 

 portment toward water in the liquid state. 



A soil 1?, permeable to water when it allows that liquid 

 to soak into or run through it. To be permeable is of 

 course to be porous. On the size of the jjores depends its 

 degree of permeability. Coarse sands, and soils which 

 have few but large pores or interspaces, allow water to 

 run through them readily — water ^:»ercota#es them. When, 

 instead of runni-ng through, the water is largely absorbed 

 and held by the soil, the latter is said to possess great 

 capillary power ; such a soil has many and minute pores. 

 The cause of capillarity is the same surface attraction 

 which has been already under notice. 



When a narrow vial is partly filled with water, it will 

 be seen that the liquid adheres to its sides, and if it be not 

 more than one-half inch in diameter, the surface of the 

 liquid Avill be curved or concave. In a very narrow tube 

 the liquid will rise to a considerable heiglit. In these 

 cases the surface attraction of the glass for the water neu- 

 tralizes or overcomes the weight of (earth's attraction for) 

 the latter. 



The pores of a sponge raise and hold water in them, in 

 the same way that these narrow (capillary *) tubes sup- 



* From capilli/s, the Latin word fur hair, because as fine as hair; (but a hair la 

 no tube, as is often supposed.) 



