THE BRAIN OF THE EARTH 



the dense black soil to an ashen gray, then 

 from gray to salmon, and finally to a dull red. 

 Every particle of the life-supporting nitroge- 

 nous substances of the soil had been burned 

 to death; there remained on the scoop only a 

 fine red dust. When it was photographed it 

 had only a vague resemblance to the rich black 

 particles which had held in their grasp the 

 marvelous secret of plant life. 



The dark vegetable or organic substance in 

 the rich wheat soil which, before burning, was 

 so hard to photograph the humus as it is 

 called is one of the main sources of nitroge- 

 nous food for the wheat plant, keeping it in 

 health from day to day and furnishing it with 

 strength against a day of disease. There are a 

 dozen main items on the plant's bill of fare, 

 but humus, to supply nitrogen, it must have, 

 whatever else it rejects, and it rapidly loses 

 strength when the nitrogen is low in the 

 larder. 



One agricultural chemist, in demonstrating 

 the importance of varying or rotating the crops 

 on a prairie farm, found out that a piece of 

 wheat land under continuous wheat culture 

 lost one hundred and seventy-one pounds of 



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