THE SOIL. 33 



is undergoing washing and drainage. Hygroscopic and 

 alkaline salts render the physical properties of the soil 

 unfavourable for cultivation and plant growth. When salt 

 soil is watered, the solution of salts is so dilute as to have 

 little effect on the plant, but as the soil dries, this solution 

 becomes more concentrated and more and more hurtful to 

 the plant. Hence, it is evident that salt soils require 

 much water to keep the salts in a harmless state, and to 

 permit of the survival of the crop. Seeds are usually sown 

 on moist or wet soil so that even on salt soil the germin- 

 ation is fairly good, although the seedlings may die later. 



The injurious salts found in the soil are principally 

 chlorides and sulphates of soda, and to a less extent, of 

 magnesia. However, when a soil contains soluble sodium 

 salts these react with other soil constituents, and a salt 

 soil will always contain at least certain small quantities 

 of soluble lime salts and other bodies. Soluble carbonates 

 are seldom found in Egyptian soil. Hygroscopic salts 

 (calcium and magnesium chlorides) in small quantities 

 keep the surface of some soils in a wet, sticky state. 



It is difficult to say definitely what percentage of salt 

 in the soil affects the growth of the crop. The harmful 

 effect of salts is different with different plants ; it depends 

 on the stage of growth of the plant ; it depends on the 

 character and distribution of salts in the soil ; and it 

 depends on the amount of moisture in the soil. Hence, 

 experiments to ascertain the effect of salts on plant 

 growth, give varying results depending on the special 

 conditions under which they are carried out. From the 

 cultivator's point of view, all salt must be considered un- 



