104 THE PRINCIPLES OF AGRICULTURE 



130c. Phosphoric acid and potash are not elements, but com- 

 pounds. The elemental forms are phosphorus and potassium. It 

 is customary, however, to speak of nitrogen, phosphoric acid 

 and potash as the elements of plant-food. Here the word ele- 

 ment is not used in the chemical sense, but rather as the sim- 

 plest form in which plants can use these substances. 



131a. Roots have the power of dissolvins: plant-food (30, 

 30a), but this is only a process of making it soluble. Substances 

 which are not soluble in rain water may be soluble in soil water. 

 for the water in the soil contains various acids. Even when a 

 substance is in solution, the plant has the power of rejecting it : 

 it is thereby not available as plant-food. For example, nitrogen 

 in the form of nitrites (as nitrite of soda, NaN02) is not availa- 

 ble, although it is soluble ; but nitrogen in the form of nitrates 

 (as nitrate of soda, NaNOs) is available. Charcoal is not availa- 

 ble plant-food, although it is carbon, and carbon enters more 

 largely than any other element into plant tissue. But when the 

 charcoal is burned, it forms a gas called carbon dioxid or carbonic 

 acid (CO2), from which the plant can get carbon. 



140a. The black or blue head of an old-fashioned sulfur 

 match is a paste containing the element phosphorus, P. On 

 igniting the match, this phosphorus unites with the element 

 oxygen, O, in the air to form a small white cloud, which is the 

 compound phosphorus pentoxid. Its symbol is PjOs, which 

 means that it is made by the union of two parts of phosphorus 

 and five parts of oxygen. Phosphorus pentoxid is known in 

 agriculture as phosphoric acid. 



143a. The term superphosphate is sometimes used in the 

 same sense as acid phosphate ; that is, to designate available 

 phosphates, or those which are made up of monocalcic and 

 dicalcic phosphates. A fertilizer containing available phosphoric 

 acid, but no nitrogen or potash, is often called a plain superphos- 

 phate. Complete fertilizers contain all three of the important 

 plant-foods. 



153«. Moisten a strip of blue litmus paper with vinegar or 

 sour milk, and note the change in color. Then add to the milk 

 or vinegar some lime water till it no longer tastes sour, and 



