SOIL BUILDERS 9 



Swamp lands and meadows are the most con- 

 spicuous examples of soils built mostly of plants. 

 Lakes, ponds, streams and swamps are being 

 filled in, not only with soil washed from surround- 

 ing higher land, but also with plants. The little 

 pond that I skated upon as a boy is reduced to a 

 mere mudhole now; the lilies, sedges, reeds, cat- 

 tails and other aquatic and semi-aquatic plants 

 have encroached upon it from the edges year by 

 year, until now hay is cut where I used to catch 

 bullheads. Most of the rich valleys and meadows 

 of northern United States were once water-courses 

 or glacial lakes. The weedy water's edge of to- 

 day may be a sphagnum bog a century hence and a 

 cabbage field in another hundred years. The 

 mangrove swamp of this century, reaching trunk- 

 like roots into the sea, may be the tilled land of a 

 future generation. 



Steins and Roots Split Rocks. Plants also aid 

 in soil building, to a considerable extent, by the 

 pressure they exert upon rocks. The roots of 

 trees often follow the crevices of rocks to a consider- 

 able depth, and by the force of growth help to 

 widen them. Even on top of the ground one may 

 see many examples of rocks that have been rent by 

 the growth of trees. Among greenhouse plants it 

 is quite common to find pots that have been split 

 apart by the growth of roots. But in many 

 of the cases where rocks are split apart, and 

 a tree is growing in the crevice, the rock was 

 was first split open by weathering and the tree then 

 widened the crevice. The acids secreted by the 

 roots of plants dissolve ,a small portion of plant 

 food from the rocks that the roots embrace. Rocks 

 that have been etched by root acids may be found 

 in almost any tree-covered ledge. In these various 



