26 . Diseases of Truck Crops 



in soils have a directly poisonous effect on plants. 

 Soil acidity may be brought about by the loss of lime 

 and other bases ; and by the decomposition of organic 

 and inorganic matter. 



Crops are known to draw heavily on the lime of the 

 soil, and thus increase the proportion of acidity. 

 This then is one direct way of depleting the soil lime. 

 A ton of alfalfa, for instance, is known to take up 50 

 pounds of lime. With a yield of 6 tons per acre, the 

 annual loss of lime per acre would be 2100 pounds. 



Lime and other bases are further lost from the 

 soil by leaching. The soluble carbonates are but 

 slowly soluble in pure water. However, carbon 

 dioxide, nearly always present in soils, changes the 

 calcium carbonate into calcium bicarbonate, which 

 is rather soluble, and readily leaches out with the 

 drainage water. 



Soils which are heavily manured are apt to become 

 more acid. The decomposition of the organic matter 

 yields large quantities of carbon dioxide which act on 

 the carbonate in the manner above indicated. The 

 annual leaching of lime from soils varies from 100 to 

 1000 pounds per acre. 



In addition to these causes, poor drainage has a 

 tendency to increase the soil acidity. The application 

 of ammonium sulphate as a fertilizer leads to a devel- 

 opment of acidity by the production of sulphuric acid. 

 The same is true when muriate of potash is added. 

 In the process of nitrification in which nitrogen is 

 made more available for plants, acids are produced. 



Acidity in a soil is usually characterized by a Ian- 



