244 ELEMENTS OF GENERAL SCIENCE 



Proteins are most familiar to us in the form of the white 

 of egg, cheese, and lean meat 



The mineral salts, such as common table salt, are inorganic 

 chemical compounds. Many of them are of little importance 

 as food materials, and they are never present in large 

 quantities. 



261. Protein manufacture. Of the process of making pro- 

 teins in plants little is known except that the plant appears 

 to add nitrogen and other elements to the carbohydrates which 

 have been made by the process of photosynthesis. These sub- 

 stances have entered the plant in the form of chemical com- 

 pounds dissolved in soil water. They can be used only in the 

 form of compounds. For instance, the free nitrogen of the 

 air cannot be used by plants. The supply of some of these 

 compounds in the soil at any time is never great, and in case 

 they are exhausted plants cannot flourish. 



262. Loss of nitrogen from soil. Where the soil is covered 

 with a growth of wild plants and nature is allowed to take 

 its way unmolested, it is not probable that in the long run 

 there is any decrease in the amount of nitrogen in the soil. The 

 nitrogen is absorbed from the soil by the roots of plants, but 

 it remains in the plants, and when they die and decay, at least 

 the greater part of it is returned to the soil. On cultivated 

 ground the case is quite different. There the whole plant or 

 a part of it is harvested and taken away, and the nitrogen 

 which the removed part contains usually does not return to 

 the soil. We have seen that in a grain of wheat there is 12.2 

 per cent protein, of which a large part is nitrogen, and it is 

 calculated that in reaping a crop of 20 bushels of wheat 

 from an acre the farmer is removing 25 pounds of nitrogen. 

 At that rate in a few years there would be a deficiency of 

 nitrogen, unless it could be replaced in some way. 



We have spoken of the loss of nitrogen only, but it may 

 be that other necessary minerals are exhausted in a similar 

 manner, but this matter is not well understood. 



