52 THE STORY OF PLANT LIFE 



A small supply of sulphur is necessary, and is found 

 in the form of sulphates in rain and the soil. 



Silicon is not an essential element, but occurs in 

 plants. It is, however, required by grasses, and 

 obtained from sodium silicate. Silicates promote the 

 assimilation of phosphoric acid. 



Strontium, lithium, and caesium salts are dele- 

 terious. Some substances are injurious to plants in 

 small quantities. Too acid a solution is not beneficial 

 to some types of plants. The acids — nitric acid, 

 sulphuric acid — diffused by gases or fumes from 

 smoke, etc., are harmful to plants. 



Of metallic salts which are present in the soil in 

 some districts, ferrous, copper, and zinc salts are 

 harmful. Amongst other harmful substances are 

 sulphuretted hydrogen, often contained in springs, 

 and also nitrites, perchlorates, and arsenates. 



In large quantities the nutrient salts, such as 

 potassium and sodium, found in alkali soils in 

 deserts, are a source of infertility. Too chalky a 

 soil, with a superabundance of calcium carbonate, is 

 also harmful, and magnesian limestone in which 

 there is too much magnesium is generally barren for 

 the same reason. 



It is, therefore, apparent that there is a certain 

 optimum or degree above which percentage the 

 increase of nutrient salts is deleterious, and that a 

 perfectly fertile soil demands a perfect adjustment 

 of the proportion of each to the demands of any 

 particular plant. 



