34 PHYSICAL CONSTITUTION OF SOIL 



of nitrate of sodium produces a marked effect. After heavy 

 rain the water is observed to stand on such soil in puddles, 

 and after the water has disappeared the surface of the soil is 

 seen to be white and glistening. The compound particles of 

 the soil have in fact been disintegrated, a layer of fine sand 

 remains on the surface, and the clay lies under it in a puddled 

 condition. This effect of nitrate of sodium on clay soils results 

 in a sticky condition which greatly increases the difficulty 

 of tillage. The addition of superphosphate to the nitrate 

 would probably do much to prevent this evil, as the lime salts 

 of the superphosphate would tend to preserve the coagulated 

 condition of the clay. 



The action of the salts present in the surface soil of alkali- 

 lands is also very marked. The principal salts in the alkali- 

 lands of India and California are sodium sulphate and 

 carbonate ; the presence of the latter is particularly harmful. 

 Where the carbonate comes to the surface the wetting of the 

 soil by rain occasions a depression in the land ; the clay 

 becomes puddled and impervious to water, and the soil 

 finally dries into sheets of hard pan. This evil admits of 

 cure by treatment with gypsum, which converts the sodium 

 carbonate into the less injurious sodium sulphate. Sodium 

 carbonate has been already mentioned as a salt favouring the 

 diffusion of clay. 



Some agriculturists in England speak of the formation of 

 a pan in the subsoil in certain cases in which common salt 

 has been long used as a manure. In the absence of accurate 

 information, it seems probable that these so-called ' salt-pans ' 

 had their origin in the washing downwards of fine clay into 

 the subsoil due to the disintegration of the surface particles 

 by the salt. 



