HISTORY OF THE AMERICAN MENHADEN. 231 



be too small, a liglit yield is inevitable. For instance, potash is an csun- 

 tial ingredient of the food of plants. If all the other conditions for a 

 profitable crop of corn or potatoes, or other plants, are fulfilled in the 

 soil, except that potash is deficient, the crop will inevitably fail. But 

 if the potash be supplied the yield will be abundant. The chief use of 

 fertilizers is to supply the plant-food which the soil lacks. 



Vegetable and animal substances, and manures and soils as well, con- 

 tain, besides icatcr, two kinds of materials, the so-called organic matter 

 and the mineral matter or asli. 



The organic matter consists chiefly of the four chemical elements, 

 carbon, oxygen, hydrogen, and nitrogen. We do not need to trouble our- 

 selves about the first three of these in fertilizers, because they are sup- 

 plied to the plant in abundance by the atmosphere and the soil through 

 the leaves and through the roots. 



But the nitrogen is an important ingredient of fertilizers. It is, in its 

 pure state, a gas, and makes up about four-fifths of the air. Combined 

 with hydrogen it forms ammonia ; combined with oxygen it is known as 

 nitric acid. In these and other combinations it occurs in minute quanti- 

 ties in the atmosphere, and in considerable quantities in soils and manures. 

 Plants are unable to make use of the pure nitrogen of the air, though 

 some, if not all, absorb a very little combined nitrogen from the atmos- 

 phere. By far the largest part of the nitrogen of plants is absorbed 

 from the soil through the roots. From the facts that nitrogen is avail- 

 able to plants only in certain combinations, that it is slow to form and 

 easily leaves these compounds, that it readily escapes from manures 

 and soils into the air, and is leached away by water, it is one of the most 

 commonly deficient and hence the most costly ingredients of the food of 

 plants. 



The mineral matter or ash of plants is defived entirely from the soil. 

 It consists of several ingredients, known as potash, soda, lime, magnesia, 

 iron, silica, sulphuric acid, phosphoric acid, and chlorine. 



Essential ingredients of plant-food. 



302. The results of a vast amount of this sort of experimenting prove 

 that no agricultural plant can attain full growth without a suflBcient sup- 

 ply, through its roots, from the soil, of potash, lime, magnesia, iron, phos- 

 phoric acid, sulphuric acid, and some compound of nitrogen. Besides these, 

 chlorine, and perhaps silica, are sometimes, if not always, indispensa- 

 ble, though in very small proportions, to complete development. If any 

 one of these essential ingredients be lacking the plant will suifer in growth 

 and development. 



Exhaustion of soil by various crops. 



303. Crops take from the soil, then, the materials needful for their 

 growth; and these are rightly called "plant-food." Some soils yield 

 large crops many years in succession without manuring. They do this 



