THE SOIL AND ITS IMPROVEMENT. 97 



As these are conditions of soil completely under the farmer's 

 control, it is evident that if this theory is correct the farmer has 

 it in his power to add to the nitrogen in his soil without the 

 purchase of fertilizers from outside his farm ; and as these condi- 

 tions of soil are desirable on all accounts it is perfectly safe for 

 the farmer to endeavor to add to the nitrogen in his soil in this 

 manner. On the other hand, as this is as yet only an opinion 

 and has not been demonstrated, it is perfectly safe for the 

 farmer to be very careful about his farm and stable and allow 

 no waste of this substance (nitrogen) which is one of the most 

 essential for the growth of plants and one of the most expensive 

 if it has to be purchased. 



Purposes of Soil. The soil serves a threefold purpose in 

 the economy of nature. 



1st. A bed to support the plant and afford protection and 

 moisture to the roots. 



2d. It furnishes a supply of food for the plants. Just as 

 truly as animals eat and live upon the food set before them, so 

 truly do plants live upon the soil. Although they get compara- 

 tively a small portion of their food from the soil (the greater 

 part being obtained from the air), yet this portion is essential to 

 their existence, and without it they have no power to take food 

 from the air. 



3d. The soil is a great chemical laboratory, in which mate- 

 rial that is in itself inert and valueless is changed into forms 

 capable of sustaining plant life. In any fertile soil this pro- 

 cess goes on continually, except when stopped by frost. 

 Through these changes a soil that seems barren and infertile 

 may become fertile without any thing being added to it, and if 

 this process is stopped or proceeds wrongly, a fertile soil may 

 lose its fertility without any thing being taken from it. 



Varieties of Soil. The substances we have mentioned, 

 phosphoric acid, potash, and nitrogen form but a small percent- 

 age (usually less than one per cent) of even a fertile soil. The 

 remainder is composed of sand, clay, and the partially decom- 

 posed remains of plants, called humus. According as sand or 

 clay predominate in the soil it is called a clay, a clay-loam, a 



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