SUBSTANCES REQUIRED BY PLANTS 143 



subjects of future treatises on the interrelationships between soils 

 and plants. Nevertheless, we may, with profit, consider briefly a 

 few essentials to a more rational conception of the fertilizer and 

 fertilization problem than the simple and attractive, but wholly in- 

 adequate, one mentioned above. 



THE CHEMICAL ELEMENTS ESSENTIAL TO PLANT 



GROWTH 



Careful investigation has shown that there are at least ten of the 

 eighty-odd chemical elements known, without any one of which 

 green plants cannot live. These elements are carbon, hydrogen, 

 oxygen, nitrogen, phosphorus, potassium, calcium, magnesium, sul- 

 phur, and iron. The first three of these are supplied from the 

 carbonic acid gas of the air (carbon and oxygen) and from the water 

 in the soil (hydrogen). The air is never short of all the carbonic 

 acid gas which may be needed by plants. A proper supply of water 

 to soils can be readily insured in most cases. The problem of the 

 supply of the essential elements for plant growth in soils, therefore, 

 is limited to the other seven elements named above. Nitrogen is 

 supplied very largely from the soil's organic matter supply and is 

 transformed from the organic, insoluble, and complicated form 

 therein, to a simple soluble, inorganic form (principally nitrates), 

 by the action of certain micro-organisms (bacteria and fungi) which 

 live in the soil. The other six elements are to some degree also 

 furnished by the decomposition resulting in simplification and 

 mineralization of the soil organic matter, but are chiefly derived 

 from the mineral particles of the soil which take their origin in turn 

 from the rocks and minerals originally disintegrated by weathering 

 agencies to form the more or less powdery mass making up a primi- 

 tive soil. Even the small amounts of minerals contained in the de- 

 caying organic matter were derived originally from the purely 

 inorganic, mineral sources upon which the plants initially composing 

 it obtained them. This is also true of nitrogen. In fact, the original 

 rock from which the first soils were formed probably contained no 

 organic matter, and hence the primitive plants which first made their 

 appearance on the rock surfaces of the earth from some unknown 

 source must have subsisted, as do our green plants today, on the 

 mineral elements from which they synthesized organic compounds 

 and, on their death, left organic residues from which more resistant 

 portions have accumulated such organic matter supplies as we find 

 in our soils today. 



Now, while all of the ten chemical elements named above are 

 indispensable to the life and normal growth of our green plants, 

 every one of them is not needed in the same quantity as every other 

 one for the constitution of plant tissue. Very nearly the entire 

 weight of a plant consists of three elements, viz., carbon, hydrogen, 

 and oxygen, derived as above explained, from an ever abundant 

 supply of carbonic acid gas and water. For example 97.4% of the 

 corn kernel consists of carbon, hydrogen and oxygen, and in timothy 

 hay the same three elements make nearly 95% of the total substance. 



