350 THE PRINCIPLES OF SOIL MANAGEMENT 



Table IL 



Calcium salts may also increase greatly the rate at 

 which nitrogen becomes available by its effect upon 

 bacterial action, as before explained. 



220. Effect on toxic substances and plant diseases. 

 Free acids are toxic to most agricultural plants. Some 

 plants are much more sensitive than others. Clover and 

 alfalfa, for instance, should have a slightly alkaline 

 medium for their best growth, and any acid is very 

 injurious. Calcium salts in neutralizing acidity remove 

 this toxic condition. 



Certain toxic substances of an organic nature are 

 also said to be rendered innocuous by the presence of 

 calcium carbonate. Magnesium salts, when present in 

 excess, may exert a toxic action upon plants. The 

 relative proportion of calcium and magnesium, accord- 

 ing to Loew, determines whether or not magnesium is 

 toxic. The exact limits of the ratio of magnesium to 

 calcium beyond which the former is toxic depends upon 

 the combinations and solubilities of the two, and also 

 upon the crop grown. An actually greater amount of 

 magnesia, as shown by a strong hydrochloric acid diges- 



