ALKALI PROBLEM IN IRRIGATION SCOFIELD. 219 



Thus, if the figures apply to the first 3 feet of soil it would mean 

 that there would be 30 tons of salt per acre dissolved in the soil 

 water within the root zone of crop plants. For purposes of com- 

 parison it may be said that in the use of chemical fertilizers it is not 

 customary to apply more than 400 or 500 pounds of soluble material 

 per acre and often much smaller quantities. 



THE EFFECT OF ALKALI ON THE SOIL. 



In the practice of irrigation where water is applied artificially to 

 the soil it is important that the water so applied shall soak into the 

 ground within a reasonable time. In other words the soil must be 

 readily permeable to water. When the soil is permeable to water and 

 water can be used freely without swamping the land there need be no 

 anxiety about danger from alkali salts. These will be carried away 

 by the irrigation water just as in humid climates the direct rainfall 

 leaches the major portion of the soluble material from the soil. 



The most critical and dangerous feature of the alkali problem 

 on irrigated land lies in the fact that under certain conditions 

 the alkali salts affect the soil in such a way that it becomes rela- 

 tively impermeable to water, or, as the saying is, it does not take 

 water well. When this condition is encountered the alkali problem 

 becomes immediately acute. Such conditions are found to occur 

 in certain desert soils that have been in times past subjected to the 

 action of salt, or they may develop in irrigated land as the result of 

 the accumulation of salts through improper irrigation or the lack of 

 adequate drainage. This effect of alkali salts on the soil is manifested 

 in several ways. It may cause the surface soil to run together or be- 

 come " puddled " when wet, forming a compact and gelatinous mass 

 which holds the water from soaking downward. It may cause the 

 formation of a layer a little below the surface of the soil which 

 checks the downward movement of the water. This formation is 

 known as a hardpan. 



When the conditions are such that the surface soil becomes puddled 

 by irrigation it usually becomes very hard on drying out and " bakes " 

 so that it is difficult to work into good tilth. This condition is ag- 

 gravated if the drying is rapid, as it frequently is in hot weather in 

 a desert country. An example of this baking of a puddled soil is 

 shown in plate 3. The formation of such a hard crust as a result of 

 irrigation is particularly serious when it occurs after a crop has been 

 seeded and before the seedlings have emerged. 



These conditions of impermeability to water and of hardness on 

 drying most commonly occur on land that is not very salty. For that 

 reason these conditions have not always been regarded as a phase of 

 the alkali problem. In general, land that is very salty takes water 



