FERTILISING EFFECT OF BOG SOIL AND MUD. 105 



plants grow, the ash-constituents of which are carried 

 away by the rain-water, from their decaying remains. 

 Where the ground is rocky, covered with a thin coat- 

 ing of earth and a thick clothing of foliage, the water 

 which runs off will carry down to the lower layers all 

 the more fixed elements of vegetable food, in propor- 

 tion as the layer of earth itself retains less of them. 

 The finer earthy particles of such a soil, washed away 

 by heavy rains, are carried down by torrents to the 

 valleys and low lands, and form a soil of all degrees of 

 fertility according to their chemical condition, which 

 determines their power of absorbing dissolved nutritive 

 substances. But these layers of earth formed from the 

 mud borne down by the torrents will always either be 

 saturated, or gradually become saturated with the nu- 

 tritive substances contained in the water, from which 

 they are deposited. This, perhaps, explains the differ- 

 ence in the fertilising effects of the waters used for irri- 

 gating meadows, which must necessarily vary very 

 much according to the source of the water ; that which 

 has collected on hills covered with a rich vegetation, or 

 has been derived from overflowing stagnant pools, will 

 doubtless convey manuring matters to the meadow- 

 lands ; whilst water flowing from bare mountains can- 

 not, in this particular respect, exert any action upon 

 the increase of the grass crop. If such increase takes 

 place notwithstanding, the cause must be sought else- 

 where. 



In many places bog-soil, and the mud from ditches, 

 stagnant waters and ponds, are highly esteemed as fer- 

 tilising agents ; and their influence is explained by the 

 fact, that their smallest particles are saturated with 

 manuring matters, or elements of the food of plants. 

 The same remark applies to the fertility of many tracts 

 of cleared wood-land, w T here the soil for forty or eighty 

 years, or even longer, has received from the layer of 

 foliage and vegetable remains decaying on it, a certain 

 supply of ash constituents, drawn from a great depth, 

 which are retained by the upper layers of the porous 

 soil, and serve to enrich it. 



5* 



