148 Food from the Soil 



of mineral matter, and of gases ; but they, too, need a 

 little organic matter as well, either animal or vegetable. 

 Some plants need more than others; but no soil is 

 really fertile which does not contain at least some 

 small amount. As has been shown, however, no soil 

 is absolutely lacking in this important ingredient, for 

 wherever plants have grown, or animals, however 

 lowly, have lived, there they have left their remains. 



Why plants should need organic matter is another 

 and more difficult question, which seems to be at 

 present unanswerable. All organic remains, of course, 

 contain some mineral matter ; but this the plant can 

 get from the soil. They all also contain much carbon ; 

 but this the plant can get without their help from the 

 air. And finally, they all contain nitrogen in some one 

 or more of its compounds; and it is this nitrogen 

 which the plant wants, and cannot apparently get, 

 in sufficient quantity, except from organic matter. 



There is an abundant supply of nitrogen in the air, 

 however, and why plants cannot help themselves to it 

 — when they can, and do, take up carbon dioxide from 

 the same source — one cannot say ; but such is tiie fact. 

 Both are gases; and, as nearly four-fifths of the air 

 consists of nitrogen, there is certainly no lack of it. 

 However, the plant takes the one up by its leaves, as 

 will be seen in the following chapter; and does not 

 take the other, much as it wants it. 



All animal and vegetable matter, then, contains 

 nitrogen ; and as all plants, whether lichens and mosses, 

 or oaks and palms, must have some amount of it, they 

 most of them get it from this source — the decayed 

 organic matter in the soil. 



But there are others which get it equally well from 



