CHAPTER IV 

 PLANT FOOD AND SOIL FERTILITY 



What plant food is. While the plant really makes its 

 own food from certain simple substances as was shown in 

 Chapter I, the expression " plant food," as commonly under- 

 stood, refers especially to those substances that are taken 

 from the soil and used by the plant. These are water and 

 certain soluble compounds containing elements necessary to 

 plant gr.owth. Some of these elements are actually used to 

 make food, and others though not used as food are quite as 

 essential, as shown by the fact that plants will not grow 

 without them. The following is the entire list of elements: 

 nitrogen, phosphorus, potassium, calcium, chlorine, mag- 

 nesium, iron, and sulfur. 



The last four of the list are used in such small quantities 

 that the natural supply in the soil is relatively abundant. 

 But one or more of the first four nitrogen, phosphorus, 

 potassium, and calcium may not be available in sufficient 

 amounts to afford plants their best development. For this 

 reason it will be worth while to consider them separately. 



NITROGEN 



Supply of nitrogen. There are three sources for the 

 supply of nitrogen in farm practice: the organic matter, or 

 humus, found in naturally fertile soils, in crop residues, and 

 in manure; the atmospheric nitrogen made available for 



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