CHAPTER XIV 



MAINTAINING THE SOIL FERTILITY 



All plants when carefully burned leave a portion 

 of ash, ranging widely in quantity, averaging about 

 5 per cent, and often exceeding 10 per cent of the 

 dry weight of the plant. This plant ash represents 

 inorganic substances taken from the soil by the 

 roots. In addition, the nitrogen of plants, averaging 

 about 2 per cent and often amounting to 4 per cent, 

 which, in burning, passes off in gaseous form, is also 

 usually taken from the soil by the plant roots. A 

 comparatively large cjuantity of the plant is, there- 

 fore, drawn directly from the soil. Among the ash 

 ingrecUents are many which are taken up by the 

 plant simply because they are jjresent in the soil; 

 others, on the other hand, as has been shown by 

 numerous classical investigations, are indispensable 

 to plant growth. If any one of these indispensable 

 ash ingrechents be absent, it is impossible for a plant 

 to mature on such a soil. In fact, it is pretty well 

 established that, jDroviding the physical conditions 

 and the water supj^h' are satisfactory, the fertility 

 of a soil depends largely upon the amount of avail- 

 able ash ingredients, or plant-food. 



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