66 SOILS. 



earthy masses or streaks, in the substrata of rich alluvial 

 ground (Louisiana, California). Dufrenite sometimes results 

 directly from the oxidation of the protoxid mineral, which then 

 turns greenish and finally brown. Unfortunately these miner- 

 als, rich as they are in phosphoric acid, cannot readily be util- 

 ized as sources of phosphate fertilizers, because of the difficulty 

 of getting rid of the iron. Their occurrence usually suggests 

 the presence of abundance of phosphoric acid in the soil. But 

 that which is actually combined with the iron oxids is prac- 

 tically unavailable to plants; especially so in the case of the 

 peroxid compound, the formation of which is a common 

 source of loss of phosphoric acid when soils rich in iron are 

 submerged for any length of time; a point which is discussed 

 below (chapt. 13). 



Among the iron phosphate minerals, may also be mentioned 

 " bog ore," which results from the reductive maceration of 

 swamped ferruginous soils, and accumulates in the subsoils 

 and in the bottom of swamps or moors, forming " moorbed- 

 pan " ; a dark brown, rather soft mass, which is sometimes used 

 as an iron ore, especially since the invention of the " basic 

 process " of iron smelting, one of the products of which is 

 the phosphate or Thomas slag. (See above). 



Nitrate of Soda or Chile saltpeter. This mineral being 

 (like all nitrates) easily soluble in water, can only occur in 

 regions nearly or quite destitute of rainfall. Such is the case 

 in the Plateau of Tarapaca in Northern Chile, where it occurs 

 in large quantities; it is likewise found, but to much smaller 

 extent, in Nevada, southern California, Egypt and India. By 

 far its most extended occurrence is that in Chile, where, to- 

 gether with common salt, it fills cavities and crevices in a 

 gravelly clay that forms the surface of a plateau from three to 

 six thousand feet above the sea. It is never pure, but always 

 mingled with a large proportion (up to $0% and over) of 

 common salt; also some Glauber's salt (sulfate of soda) and 

 some sodic perchlorate and iodid ; hence it forms an important 

 commercial source of iodine. 



The mixed mineral mass, called " Caliche," when taken out of the 

 ground is dissolved in water; and the solution boiled down, during 

 which process the common salt is first deposited and is raked out of 



