FORMS OF PLANT FOOD 73 



portion of the phosphoric acid of the soil is soluble in 

 hydrochloric acid. In some soils, however, from 20 

 to 40 per cent, is present in the third class of com- 

 pounds. When a soil is digested with hydrochloric 

 acid, the insoluble residue is usually a fine gray 

 powder. Some clay soils retain their red color even 

 after treatment with acids showing that the iron is in 

 part in chemical combination with the more complex 

 silicates. 



In order to decompose the insoluble residue obtained 

 from the treatment with hydrochloric acid, fluxes, as 

 sodium carbonate and calcium carbonate, are employed 

 which act upon the complex silicates at a high tem- 

 perature, and produce silicates soluble in acids. 

 Plants, however, are unable to obtain food in such 

 complex forms of chemical combination. 



88. How a Soil Analysis is Made. — A sample is 

 obtained from a field by taking several small samples 

 to a depth of 6 to 9 inches, from different places, and 

 uniting them to form one sample. All coarse stones 

 and roots are removed and a record is made of the 

 amount of these materials. The soil is air dried, the 

 hard lumps are crushed, and the materials passed 

 through a sieve with holes 0.5 mm. in diameter. Only 

 the fine earth is used for the chemical analysis. Ten 

 grams of soil are weighed into a soil digestion flask, 

 and 10 cc. hydrochloric acid of 1.115 sp. gr. are added 

 for every gram of soil used. The digestion flask is 



