FORMS OF PLANT FOOD 8 1 



either citric or oxalic acid. The combined use of 

 dilute organic acids, as citric with hydrochloric acid of 

 i.i 15 sp. gr., will generally give an accurate idea of 

 the character of a soil. A fifth-normal solution of 

 hydrochloric acid has also been proposed as a measure 

 of the soil's active phosphoric acid, and has given 

 satisfactory results. 23 



The use of dilute organic acids renders it possible 

 to detect small amounts of readily soluble phosphoric 

 acid and potash. It has been stated that when a soil 

 has been manured a few years with a phosphate fertil- 

 izer and brought into good condition as to available 

 phosphoric acid, that a chemical analysis will fail to 

 detect any difference in the soil before or after the 

 treatment with fertilizer. In the case of hydrochloric 

 acid as a solvent, this is true because an acre of soil 

 to the depth of one foot weighs about 3,500,000 lbs. 

 and 500 lbs. of phosphoric acid would increase the 

 total amount of phosphoric acid about 0.015 per cent. 

 When a dilute organic acid is used, only the more 

 easily soluble phosphoric acid is dissolved, and this 

 readily allows fertilized and unfertilized soils to be 

 distinguished. The use of dilute organic acids and 

 salts have shown a decided difference between soils 

 fertilized and unfertilized with potash. 24 



91. Distribution of Plant Food. — In studying the 

 chemical composition of a soil, the surface soil and 

 the subsoil both require consideration. It frequently 



