292 ELEMENTS OF GENERAL SCIENCE 



Water is one of the most important substances of the soil. 

 Plants use water directly as a raw material for food manu- 

 facture, and they also use other materials which are secured 

 from the soil only when they are dissolved in water. If water 

 were absent from the soil, plants would starve ; and if there 

 is too much water, some plants will drown from lack of air. 



293. 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 compounds 

 in the soil. The nitrogen compounds are absorbed from the 

 soil by the roots of plants, but they remain in the plants, and 

 when these die and decay, the compounds, at least the greater 

 part of them, are 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 in the form of protein and other 

 compounds usually does not return to the soil. In a grain 

 of wheat there is 12.2 per cent of protein, of which a large 

 part is nitrogen. 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 there would be a de- 

 ficiency of nitrogen compounds in a few years unless the loss 

 could be replaced in some way. 



We have spoken of the loss of nitrogen only; it may be 

 that other necessary minerals are exhausted in a similar man- 

 ner, but this matter is not well understood. 



294. The supply of nitrogen. It is a common practice to 

 return to the soil, in the form of manures, as much as pos- 

 sible of the straw and other materials which have come from 

 the fields, and in this way much may be done to maintain the 

 fertility of the soil. There is always a loss, however, in the 

 material that is sold and taken off the farm, and there are 

 certain other unavoidable losses of nitrogen compounds. 

 A very important source of nitrogen is found in the action 



