258 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



Here also the carbonate of lime gave better results than the 

 quicklime. 



Salt. Salt is used to some extent as an application to the soil. 

 It acts as an indirect fertilizer. 



Gypsum or sulphate of lime CaSO 4 2HX), is used to a small 

 extent at present. Formerly it was held in high esteem, but at 

 present, preference is given to lime, limestone, or direct fertilizers. 



Toxic Substances. It is possible that some soils may contain 

 injurious substances besides alkali or acids. The theory of 

 Whitney and Cameron, that plants excrete toxic substances, has 

 already been discussed. 



Sulphur. The amount of sulphur 1 in plants has been for a 

 long time under-estimated. Recent work 2 has shown that many 

 plant products (especially seeds) contain much more sulphur than 

 was once thought. Most soils contain only small quantities of 

 sulphur. It is therefore quite possible that some soils are deficient 

 in sulphur. It is possible that the sulphates contained in acid 

 phosphate are often directly beneficial to plants. Direct experi- 

 mental evidence that such is the case has not yet been furnished. 



Alkali Soils. 3 Alkali consists of soluble salts. When present 

 in the soil in excessive quantities, these salts interfere with the 

 growth of plants, or prevent their growth entirely. 



The ordinary "alkali" salts are sulphate of soda, chloride of 

 soda, and carbonate of soda. The salts first named, when crys- 

 tallized in the surface of the soil, appear as white substances, 

 and generally form what is known as white alkali. Carbonate of 

 soda has a corrosive action upon vegetable matter (usually found 

 in the soil) producing a black solution or substance, and for this 

 reason is called black alkali. 4 Carbonate of soda is especially in- 

 jurious, for it causes the soil to become hard so that water will 

 not easily penetrate it. It is also more injurious to plants than 

 the less corrosive alkali salts. 



1 See, however, Arendt, Jahresber, Agr. Chem., 1858, p. 125. 



2 Withers and Fraps, Report North Carolina Exp. Sta., 1902-3, p. 53. 



3 Alkali Soils of the United States, Bulletin 35, Bureau of Soils. 



4 California Bulletin 128. 



