232 SOILS. 



The first effect of the stagnation of water in such lands (as 

 already explained in a former chapter (3, p. 45) is to set up a 

 reductive (bacterial) fermentation of the organic matter of 

 the soil, transforming the ferric into ferrous hydrate, which 

 in the presence of the carbonic acid simultaneously formed, be- 

 comes ferrous carbonate, readily soluble in carbonated water. 

 That this compound is poisonous to plant growth, has been 

 stated (chap. 3, p. 46). The carbonates of lime and magnesia 

 are simultaneously dissolved by the same, as is also calcic phos- 

 phate, the usual form in which phosphoric acid is present in 

 the soil. Under the influence of partial aeration from the 

 surface, the ferrous carbonate is slowly re-transformed into 

 ferric hydrate, aggregated in the form of spots or concretions 

 of "bog ore" (see chapter 5, p. 66). In this process the 

 greater part of the phosphoric acid of the soil is also abstracted 

 from its general mass and concentrated in the bog ore (chap. 5, 

 p. 65), in which it is wholly unavailable to vegetation, and 

 cannot be made available while in the ground, by any known 

 process. The soil is therefore permanently impoverished in 

 phosphoric acid ; it is also deprived of its content of ferric hy- 

 drate, and is transferred from the class of " red " to that of 

 " white " soils, well known everywhere to be unthrifty and to 

 require early fertilization. Not only is this true, because of 

 their almost invariable poverty in phosphoric acid, but also 

 usually in lime, which like the iron, if not leached out, is aggre- 

 gated into concretions in the subsoil, leaving the surface soil 

 depleted of this important ingredient. The humus, also, is 

 either destroyed or at least " soured " at the same time. 



Reduction of Sulfates. Should such a soil contain any considerable 

 amount of sulfates, especially in the form of gypsum or calcic (or 

 magnesic) sulfate, the reductive process results in the formation of 

 iron pyrites (ferric sulfid, chap. 5, p. 75) ; while at the same time the 

 soil is often sufficiently impregnated with sulfuretted hydrogen as to 

 be readily perceived by the odor, or by the blackening of a silver coin. 

 This is very commonly the case in seacoast marshes, where a hole made 

 with a stick thrust into the mud will be found to give forth both car- 

 buretted and sulfuretted hydrogen, while a careful washing of the soil 

 will reveal the presence of minute crystals of iron pyrites. Hence the 

 need of prolonged aeration of marsh soils, effecting the peroxidation of 



