26 IRRIGATION FARMING. 



relative rate of flow. The forces which move the water 

 within the soil are gravity and the tension or contract- 

 ing power of the exposed water surface. The approxi- 

 mate extent of the water surface can be calculated from 

 the mechanical analysis of the soil. The surface tension 

 and effect of manures and fertilizers on the surface ten- 

 sion can be found by the ordinary method of the rise of 

 liquids in capillary tubes, using as a solvent pure water, 

 or extracts of the soil, representing as nearly as possible 

 the ordinary soil moisture. The different fertilizing 

 materials have a very marked effect on the pulling power 

 of the water. The same class of substances may differ 

 widely in their effect. 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 universal 

 occurrence and widest significance as influencing the 

 conditions of plant growth. Its manifestation is amona: 

 the most common processes of nature ; yet not till within 

 the present half century was it fully recognized or appre- 

 ciated 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 dis- 

 seminated more evenly throughout the topsoil, and are 

 left there on deposit, as it were, to be drawn upon 

 by the growing vegetation, and hence it is that irriga- 

 tion improves the mechanical condition of soils and 

 makes them the more readily subservient to the agricul- 

 turist. Some authorities claim that soils which have 

 been cropped until the soluble ingredients, organic ele- 

 ments and humus, have been materially decreased, retain 

 less water, and dry out more readily than when then- i> 

 a larger amount of organic matter present in the soil. 

 This depletion, however, may easily be obviated by the 



