22 



stances. They occur in soils combined with soda and lime, the 

 first as common salt or chloride of sodium, and the latter as 

 fluor spar, or fluoride of calcium. This is absorbed by plants and 

 is conveyed to animals chiefly to form the enamel of the teeth ; 

 it also exists in smaller quantity in the bones. Common salt 

 occurs in large beds, known as rock salt: it is also found in the 

 atmosphere near the sea, and also occasionally in the rain of 

 places far interior, where it is supposed to have been carried by 

 high winds. 



CHAPTER IV. 



PLANTS. 



All plants are composed as we have said, of two parts ; the 

 organic and the mineral; the latter we have treated of in the 

 preceding chapter, while the organic parts are those derived 

 from the atmosphere, and are carbon, hydrogen, oxygen and 

 nitrogen, and are built up into more complex forms by the vital 

 force of the plant. 



Some plants require more of one substance than others, and 

 on this is based the principal of rotation as has been said. 



Thus, thera are the potash plants, as corn, beets, turnips and 

 potatoes, whose ash contains more than half its weight of 

 potash. 



The lime plants, such as beans, peas, clover, tobacco, have 

 their ash chiefly composed of lime and magnesia. 



There are also the silica plants, such as wheat, rye, oats, 

 barley. 



In all these different ashes, of whatsoever class, phosphoric 

 acid forms a large proportion, and is usually united with the 

 predominant bases of the ash. 



Now as these ingredients are necessary for the plants to grow 

 and mature, they must be present in sufficient quantity, and in 

 a readily available form ; that is, there must be enough of them 

 soluble in water. 



These mineral constituents of plants form but a small pro- 

 portion of the weight of the plant, from three to six per cent., 

 and they were for a long time considered of no consequence, 

 but experience has proved their absolute necessity for the 

 growth of crops ; and as surely as crops are continually grown 

 and exported, so surely does the land diminish in productiveness, 

 unless restored by artificial means. This is manuring. 



To show how large is the quantity of these necessary sub- 

 stances which is removed, we transcribe several tables : 



