30 IRRIGATION FARMING. 



which move the water and the conditions which 

 determine the relative rate of flow. The forces which 

 move the water within the soil are gravity and the 

 tension or contradling power of the exposed water sur- 

 face. The approximate extent of the water surface 

 can be calculated from the mechanical analysis of the 

 soil. The surface tension and effe(5l of manures and 

 fertilizers on the surface tension can be found by the 

 ordinary method of the rise of liquids in capillary tubes, 

 using as a solvent pure water, or extradls of the soil, 

 representing as nearly as possible the ordinary soil 

 moisture. The different fertilizing materials have a 

 very marked effedt on the pulling power of the water. 

 The same class of substances may differ widely in their 

 effedl. Kainit, for instance, increases the surface 

 tension of pure water, but nitrate of potash lowers it 

 very considerably. 



Nutritive Dissemination. — The absorption of 

 nutritive matter by the soil is a phenomenon of uni- 

 versal occurrence and widest significance as influencing 

 the conditions of plant growth. Its manifestation is 

 among the most common processes of nature; yet not 

 till within the past half century was it fully recog- 

 nized or appreciated in its bearings on plant nutrition. 

 Solutions, as a result of our modern irrigating methods, 

 are known to part with their solid constituents on 

 passing through any considerable quantity of soil. 

 They are thus disseminated more evenly throughout 

 the top-soil, and are left there on deposit, as it were, 

 to be drawn upon by the growing vegetation, and 

 hence it is that irrigation improves the mechanical con- 

 dition of soils and makes them the more readily sub- 



