28 



term "alkali," as ordinarily used, includes any soluble materials 

 present in sufficient quantity to be injurious to plants. The most 

 common materials are sodium chloride or common salt, sodium sul- 

 phate or glaubers salt, and sodium carbonate or washing soda. This 

 "alkali" is not necessarily brought into the soil from some other 

 location. It is merely a regrouping of the chemicals that existed in 

 the original rock, and the concentration of these compounds in the 

 surface soil because of excessive evaporation. 



If the soil has good natural drainage, any excess of water will 

 percolate through the soil and will seep out to the country drainage 

 channels, carrying with it in solution, small quantities of the soluble 

 salts. In such cases, the waters evaporated from the surface cannot 

 exceed the amount that passes down through the soil, and alkali 

 accumulations cannot occur. 



If the natural drainage conditions are not good, artificial drainage 

 will be necessary if the land is to be irrigated and farmed. In arid 

 -regions, the irrigation of poorly drained lands will produce conditions 

 that will ultimately bring about the accumulation of injurious amounts 

 of alkali. It is necessary to study the drainage conditions carefully, 

 noting the character of the soil with respect to permeability and the 

 penetration of water, the character of the substratum, the slope of 

 the land and the possible outlet for drainage waters. The possibility 

 of drainage waters seeping into the soil from higher lying lands 

 should also be investigated. 



Texture and Structure. The texture and structure of the soils 

 need considerable attention. Because of the low rainfall and absence 

 of leaching, the coarser soils, sands, and sandy loams are much more 

 productive in an arid region than are similar soils in a humid country. 

 If the soil is too loose and open, however, there is a great difficulty in 

 irrigating, owing to excess seepage and in addition the humus is readily 

 "burned out" and lost. On the other extreme, the clays and clay 

 adobes are so compact and impervious that they are very difficult 



to irrigate, the water penetrating 

 very slowly. Such soils are also 

 difficult to till and to maintain in 

 a proper state of firmness. 



The term "adobe" does not in- 

 dicate a specific kind of soil but 

 refers to the structure. There are 

 clay adobe, loam adobe, and even ' 

 sandy loam adobe, although the 

 latter is very rare. The name is 



Fig. 6. An adobe soil, on dry- 

 ing, shrinks markedly and breaks into 

 blocks, with wide cracks between. 



