132 SOILS AND MANUBES 



practice, the readiest means of oxidising the soil is to 

 fallow it ; in fact, the beneficial effects of fallow are 

 largely due to the processes of oxidation which it promotes. 

 It is not without interest to note in passing that the word 

 " fallow " is derived from the Saxon fealo a red or yellow 

 colour and originally meant to redden the land, i.e., to 

 oxidise the compounds of iron in the soil. 



Compounds of manganese are not very common in soils, 

 but in some cases they occur in considerable quantities. 



They undergo oxidation and reduction like those of iron, 

 but perhaps more readily, and produce similar effects. 



Dehydration. The hydrates of iron, alumina and silica, 

 to which reference has been made in several places, when 

 freshly precipitated, are obtained in a gelatinous condition. 

 They consist of the oxides united with large but indefinite 

 quantities of water. Much of the water, however, is so 

 loosely united that it evaporates on exposure to dry air at 

 ordinary temperatures, and the dry compounds of iron 

 and alumina finally obtained are definite hydrates, corre- 

 sponding to known salts, from which the remaining water 

 can be expelled only by heat, thus 



2 A1(OH) 8 = A1 2 3 + 3 H 2 

 2 Fe(OH) 3 = Fe 2 3 + 3 H 2 



The hydrates of silica corresponding to known salts con- 

 tinue to lose water at ordinary temperatures. 



Si(OH) 4 = Si0 2 + 2 H 2 

 SiO(OH) 2 = Si0 2 + H 2 



The gelatinous hydrates of indefinite composition are 

 produced in the soil by the decomposition of minerals and 

 other changes. On drying they shrink greatly in volume 

 and become much leps readily soluble in acids or alkalis. 



Other Reactions. The phosphatic minerals of the soil 

 probably consist chiefly of tricalcic phosphate Ca 3 



