28 PEACTICAL BOTAISTY 



29. Effects of roots on the soil. If we dig up a spadeful of 

 earth from a well-grassed meadow, or from a little inside the 

 circumdEerence of the circle formed by the roots of a tree, we 

 shall find the soil bound together by the living roots or full of 

 little, crooked, tubular channels left by the decay of dead ones. 

 Thus the soil is in the one i-ase, held together so as to pre- 

 vent its becoming gullied and washed away by rains, and in' 

 the other case made more porous and easily penetrated by air 

 and water. The latter effect is a very important one in the 

 case of stiff clay soils, which when closely packed are almost 

 waterproof. 



The extensive washing away of soils when unprotected by 

 a covering of plants, such as grass, shrubs, or forest growth, is 

 one of the most serious calamities that can befall a country. 

 It is especially formidable in hilly regions, which may become 

 wholly uninhabitable if the forests are cut off and the turf on 

 the hillsides is destroyed by too constant grazing and tram- 

 pling of sheep or goats. Immense areas of land once valuable 

 for timber and for grazing have thus been ruined throughout 

 southern Europe, and the same process is under way in our 

 own country all the way from New England to the Pacific 

 coast region. One of the clearest ways in which the loss by 

 washing away of the soil can be presented is by considering 

 how the land is carried into the sea by great rivers. The delta 

 of the Mississippi covers an area of more than 12,000 square 

 miles. It consists of material brought down by the river in 

 the form of mud, now forming a deposit of unknown thick- 

 ness, probably averaging more than 500 feet. It is calculated 

 that the river e\ery year carries enough solid matter to form 

 a layer one foot thick over an area of about 268 square miles. 

 Remembering that this mud consists mainly of the choicest 

 part of the rich soil of the Mississippi basin, it is easy to see 

 that the land is robbed every year of the material to support 

 enormous harvests ^ (see Chapter XXIV). 



1 See "Forest Influences," Bulletin 7, Division of Forestry, U. S. Dept. 

 Agr., 1893. 



