Food from the Soil 119 



besides. For they give to it, also, that food which they 

 draw from the air, of which we have yet to speak. 



The roots of a tree are constantly bringing up supplies 

 from the deep subsoil, which, when the leaves fall, are 

 added to the surface-soil; and the ancient forests of North 

 America, after flourishing for ages, and producing enor- 

 mous quantities of timber, left the soil, not impoverished, 

 but so rich that it was hardly exhausted by a whole cen- 

 tury of wasteful farming. 



The ''yellow earth" of China, a deposit of very great 

 extent, is believed to consist very largely of the ashes of 

 plants, accumulated during more generations than one can 

 attempt to realize, for in some parts it is more than fifteen 

 hundred feet thick. 



It is the long-continued course of this green-manuring 

 which has so largely contributed to produce the extraordi- 

 nary fertility of the ** black earth" of Russia and the region 

 of Manitoba. And so, too, with the pampas of South 

 America, a still more interesting example, because the 

 process is going on under our eyes. 



In the winter Captain Head found the ** thistle" part 

 of this region looking something like a rough turnip-field 

 intermixed with clover, so large and luxuriant were the 

 leaves of the thistles — really wild artichokes. In the 

 spring, the thistle-leaves had spread, and had overgrown 

 the clover, but still had the appearance of a rough crop 

 of turnips. Less than a month later, however, they had 

 shot up in the most surprising manner, and were in full 

 bloom. They were now ten or eleven feet high, and 

 formed such a close, impenetrable barrier on each side of 

 the track that nothing whatever could be seen in any 

 direction. The growth was so amazingly rapid that an 



