20 IRRIGATION FARMIXG. 



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 alu- 

 minum. It is very seldom found pure, but contains pot- 

 ash, lime and ammonia, etc., mixed with it, and some 

 of these unite with it to form double silicates, which are 

 exceedingly valuable on account of the potash, 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, pot- 

 ash, 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 com- , 

 pounds. 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, pot- 

 a.-h. ammonia, etc., holds moisture well and is adapted to 

 withstand drouth, but is difficult to work and apt to bake 

 after having been irrigated in summer, and is cold and 

 wet in spring and fall. The amount of clay in soil varies 

 from ten to ninety per cent, but the quantity of pure 

 clay in heavy soils rarely exceeds thirty per cent. The 

 clay soils of the far west are locally called "adobe," lie- 

 cause it is of such soil that the adobe bricks are 

 made by the native Mexicans and used in their pimple 

 architecture. While iidobo soils are moro diUn-ult to 

 work they are well adapted t,. irrigation, and it is on 



