Plant-Food and Use of Fertilizers. 153 



of loss by leaching from the soil. Soils with but few exceptions contain those ele- 

 ments which are necessary for the fixation of plant-food. Exeecdingly sandy 

 soils do not have the power of fixing or making insoluble the plant-food which 

 is applied in fertilizers. Most soils, however, contain enough of those materials 

 such as lime, iron, aluminum, zeolites and organic matter, which combine with the 

 soluble plant-food applied in fertilizers and make it insoluble so that it does 

 not leach from the soil. Even though it is rendered insoluble in water and hence 

 leaches from the soil only in very small quantities, it is still available to the 

 feeding rootlets of the growing crop. 



Generally speaking, all forms of phosphoric acid and potash are fixed in the 

 soil; nitrogen (with the exception of nitrates), which is contained in stable 

 manure and other organic substances, and also nitrogen in the form of ammonia 

 compounds is fixed in the soil. 



The fixation of the potash and ammonia compounds is brought about by the 

 so-called zeolites in the soil. The phosphoric acid compounds are fixed in the soil 

 by compounds of lime, iron, aluminum and possibly magnesium. The excep- 

 tions to the above are, first, that all forms of soluble plant-food tend to leach 

 out of very sandy soils ; second, that nitrogen in the form of nitrate tends to 

 leach out of all kinds of soils. 



FERTILIZERS. 



Fertilizers may be complete or incomplete ; direct or indirect. 



A Complete Fertiliser is one that contains each of the three elements of plant- 

 food, nitrogen, phosphoric acid and potash. 



An Incomplete Fertiliser is one that contains only one or two of the three 

 elements, nitrogen, phosphoric acid and potash. 



A Direct Fertiliser is one that contains any or all of the three plant-food ele- 

 ments, nitrogen, phosphoric acid and potash. A direct fertilizer is used for the 

 plant-food which it contains. 



An Indirect Fertiliser is one that does not contain nitrogen, phosphoric acid 

 or potash. An indirect fertilizer is not applied for the purpose of adding plant- 

 food to the soil, but in order to make some of the plant food already in the 

 soil more available. The chief indirect fertilizers are land-plaster, lime and 

 common salt. 



I^and-plaster, gypsum of calcium sulphate. These are different names for 

 the same compound. Gypsum or land-plaster is nothing more than the sul- 

 phate of lime rock which has been ground exceedingly fine. 



Quicklime and calcium carbonate. When limestone is burned the resulting 

 product is calcium oxide or quicklime, sometimes called stone-lime. When exposed 

 to the air it becomes air-slaked and is then calcium carbonate or carbonate of 

 lime. 



SOURCES OF FERTILIZING MATERIALS. 



NITROGEN SUPPLY. 



The following materials are used in this country as sources of nitrogen 

 supply. Soine of them, of course, are used much more than others : 



