70 SOILS AND FERTILIZERS 



The plant food of the second class is in a somewhat 

 more insoluble form, and consists of all those com- 

 pounds and silicates which are soluble in hydrochloric 

 acid of twenty-three per cent, strength, 1.115 sp. gr. 

 This class of compounds represents the limit of the 

 solvent action of the stronger solutions of organic 

 acids, such as are found in the roots of plants. 



The third class of silicates includes all of those com- 

 pounds which require the combined action of the 

 highest heat and the strongest chemicals and fluxes in 

 order to decompose them. 



The first and second classes of silicates and other 

 compounds are the only ones which possess any value 

 as plant food. The third class, after undergoing the 

 combined action of the various disintegrating agencies 

 of nature, may be changed into the second class, 

 called the zeolitic silicates. In this second class are 

 also included all of the mineral elements combined 

 with the humus. As a rule, not over fifteen or twenty 

 per cent, of the total soil is in forms soluble in hydro- 

 chloric acid ; and of the more important elements only 

 one to six per cent, form a part of this fifteen. In 

 two hundred samples of soil, the potash, nitrogen, 

 lime, magnesia, phosphoric and sulphuric acids, 

 amounted to 3.5 per cent. (Fig. 17). In many fertile 

 soils the sum of the nitrogen, phosphoric acid, potash, 

 lime, magnesia, and sulphuric and carbonic anhydrides 

 is less than 1.50 per cent. This means that in every 100 



