358 IK^W CROPS FKEU. 



contact with tlie soil, the base may be withdrawn by tlie 

 absorbent silicate, and the acid may iniite to lime or mag- 

 nesia. The phosphates of lime and magnesia thus formed 

 arc, however, insoluble, and hence the acid as well as the 

 base remains fixed. Again, if the alkali-phosphate be 

 present in quantity so great that its base cannot all be 

 taken up by the absorbent silicate, then the hydrated 

 oxide of iron or alumina may react on the phosphate, chemi- 

 cally combining with the phosphoric acid, while the alkali 

 gradually saturates itself with carbonic acid from the air. 

 It is, however, more likely that organic salts of iron (cre- 

 nates and apocrenates) transpose with the phosphate. So, 

 too, carbonate of lime may decompose with phosphate of 

 potash, producing carbonate of potash and pliosphate of 

 lime (J. Lawrence Smith). Vadeker, in a number of ex- 

 periments on the deportment of the sohible superphosphate 

 of lime toward various soils, found that the absorption of 

 phosphoric acid was more rapid and complete with soils 

 containing much carbonate of lime tlian with clays or 

 sands. 



All observers agree that phosphoric acid is but slowly 

 fix;ed by the soil. Vcelcker found the process was not 

 completed in 26 days. Its absorption is, therefore, mani- 

 festly due to a different cause from that which completes 

 the fixation of ammonia and potash in 48 hours. 



As to silicic ac'id^ it may also, as solid hydrate, unite 

 slowly with the oxides of iron and with alumina (see Kers- 

 ten's observations, p. 352). When occurring in solution, as 

 silicate of an alkali, as happens in dung liquor, it Avould 

 be fixed by contact with solid carbonate of lime, silicate 

 of lime being formed (Fuchs, Kuhlmann), or by encoun- 

 tering an excess of solutions of any salt of lime, magnesia, 

 iron or ammonia. In presence of free carbonic acid in 

 excess, a carbonate of the alkali would be formed, and the 

 silicic acid would be separated as such in a nearly insoluble 



