CHAPTER XIV 
MAINTAINING THE SOIL FERTILITY 
Aux 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 quantity of the plant is, there- 
fore, drawn directly from the soil. Among the ash 
ingredients are many which are taken up by the 
plant simply because they are present 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 ingredients be absent, it is impossible for a plant 
to mature on such a soil. In fact, it is pretty well 
established that, providing the physical conditions 
and the water supply are satisfactory, the fertility 
of a soil depends largely upon the amount of avail- 
able ash ingredients, or plant-food. 
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