farmers' institutes. 241 



cannot produce crops. Only a very small per cent, of the humus of a 

 soil can be changed into nitrates during a single growing season. 



The ammonia of the air is not a very important source of soil nitrogen 

 when compared with the humus of the soil, but when compared with the 

 needs of plants the nitrogen obtained from this source is of considerable 

 moment. Ammonia is set free in various processes of decay and escapes 

 into the air in the form of a gas. This rapidly dissolves in water and is 

 brought back to the soil by rains. Bacteria then change it into nitrates. 

 The amount of nitrogen obtained in this way varies in different localities. 

 Measurements at various experiment stations have shown that an acre of 

 soil may receive in a single season as much nitrogen as is found in from 

 three to ten bushels of wheat. 



Tbe nitrogen obtained from "soil air" by the aid of certain bacteria is 

 of very great importance. These bacteria are found in nodules that grow 

 on the roots of certain plants belonging to the clover familv. They ex- 

 tract nitrogen from the air and leave it in a form available to the host plant 

 as food. Plants upon which such bacteria live are thus able to secure all 

 the nitrogen they need from the air and this suggests a definite and never 

 failing method of adding nitrogen to soils deficient in this element, which 

 will be more fully discussed further on. 



Potassium and phosphorous are found both in the humus and in the 

 mineral matter of the soil, but chiefly in the mineral matter. They are 

 spoken of as potash or potash salts and phosphoric acid or phosphates. 

 When exhausted from a soil it is necessary to add mineral matter to re- 

 plenish it. 



AN INVENTORY OF THE SOIL. 



But how much of these elements is found in soils and how does the 

 amount compare with the amount removed by crops? We will let the 

 chemist answer the question. I have before me a table giving the results 

 of the analyses of forty-nine dififerent soils selected from ten different 

 states ranging from Canada to Alabama and from New York to Colorado. 



The poorest one of these soils with respect to nitrogen contained 947 

 pounds of nitrogen per acre in the first foot of soil. The first foot con- 

 tains about as much nitrogen as is found in the next three feet of soil, so 

 in round numbers there would be 190D pounds of nitrogen in the first 

 four feet of soil, which represents the feeding field of the roots of our 

 average farm crops. A yield of 25 bushels of wheat per acre requires 47 

 pounds of nitrogen for both grain and straw allow;ing a ton and a half 

 of straw per acre. This farm would produce wheat at that rate only 40 

 years before all the nitrogen within the reach of plants would be exhaust- 



A — 10 



