44 ■ CORN CROPS 



The dry substance is combustible, and when it is ignited, 

 about 90 to 95 per cent will be consumed, leaving a residue 

 of ash. The combustible part consists principally of the 

 elements carbon, hydrogen, and oxygen, with a smaller 

 quantity of nitrogen. The ash left is made up of mineral 

 substances taken from the soil. Thus, only about one 

 per cent of the weight of a green plant comes from the soil. 



34. The essential constituents. — There are ten essen- 

 tial elements necessary to plants, one of these coming from 

 the air, two from water, and six from the soil, while one 

 — nitrogen — comes indirectly from the air through the 

 soil. Carbon comes onlj^ from the carbonic acid gas of the 

 atmosphere, hydrogen and oxygen from water (oxj^gen also 

 from the air, and oxid salts), nitrogen from the soils, as 

 nitrates or ammonium salts. The other six essentials, 

 namely, sulfur, phosphorus, potassium, calcium, mag- 

 nesium, and iron, are taken from the soil. 



Plants do not find these elments in simple forms, but in com- 

 bination — for example, the hydrogen and oxygen from water, 

 where it is in combination as HoO, and carbon from carbon 

 dioxid (CO2). All the minerals, as phosphate, potassium, and 

 the others, are always found in combination. A demonstra- 

 tion of how plants can live on these minerals when in solution 

 may be made by taking pure distilled water and dissolving the 

 following mineral salts (after V. D. Crone) : — 



Distilled water 1-2 liters 



Potassium nitrate 1.0 gram 



Ferrous phosphate 0.5 gram 



Calcium sulfate 0.25 gram 



Magnesium sulfate 0.25 gram 



If properly handled, a cqrn plant may be grown to maturity 

 in this solution. 



In addition to the "essential" elements found in the ash of 

 plants there are also other elements, as sodium and silicon, 



