THE RELATION OF SOILS TO IRRIGATION. 23 



also allows the assimilation of irrigating waters. The 

 amount of sand in the soil varies from eight to more 

 than ninety per cent. It absorbs very little moisture 

 or other fertilizing material in the air, but retains heat 

 much longer than does any other soil constituent. 

 From these facts, then, it is evident that a sandy soil 

 will be loose, easy to work, dry, warm, and free from 

 baking, but peculiarly apt to suffer from drouth when 

 irrigation is not available, and lose valuable plant food 

 by leaching, especially if the subsoil be sandy or 

 gravelly. 



Clay Soils. — Clay is a compound of silica and 

 aluminum. It is very seldom found pure, but con- 

 tains potash, lime, ammonia, etc., mixed with it, and 

 some of these unite with it to form double silicates, 

 which are exceedingly valuable on account of the pot- 

 ash, lime, or ammonia which they furnish to plants. 

 Clay is not a plant food. It is not taken up by plants 

 except by a few of the lower orders, but the impurities 

 in it — lime, potash, etc. — are absolutely essential to 

 vegetable growth, and these at once become soluble 

 under the influence of irrigating waters. Red clays 

 always contain iron, and most clay soils are rich in 

 potash, thus adding to their availability as plant food, 

 and rendering them peculiarly adapted to such plants 

 as require a liberal supply of compounds. Clay gives 

 body to the soil and absorbs moisture readily. It 

 absorbs heat much more readily than sand does, but 

 has not the same power of retention. A clayey soil, 

 then, is usually rich in phosphoric acid, potash, am- 

 monia, etc., holds moisture well, and is adapted to 

 withstand drouth, but is difficult to work and apt to 



