134 SOILS AND MANURES 



precipitated as phosphates of lime rather than as phos- 

 phates of iron and alumina. If the soil remained in an 

 acid condition, precipitation would not take place ; on the 

 other hand, if the soil contained enough lime to maintain 

 a neutral condition, very little soluble phosphate would 

 be formed. Quite perceptible amounts of phosphate are, 

 however, dissolved by water highly charged with carbonic 

 acid 



Ca 3 (P0 4 ) 2 + 4 C0 2 + 4 H 2 = CaH 4 (P0 4 ) 2 + 2 CaH 2 (C0 3 ) 2 



and the more dilute solutions in the soil have a similar if 

 slower effect. Small or infinitesimal quantities of phos- 

 phate are thus continually dissolved, and, in the absence 

 of lime, may be reprecipitated as phosphates of iron and 

 alumina. The gradual conversion of phosphate of lime 

 into phosphate of iron by prolonged action of carbonic acid 

 and water has actually been observed to take place, and, 

 in time, it must result in a marked reduction of the fertility 

 of the soil. 



Ca 3 (P0 4 ) 2 + 2 Fe (OH) 3 +6 C0 2 = 2 Fe P0 4 + 3 Ca H 2 (C0 3 ) 2 



It is possible that in the presence of an excess of lime 

 the reverse action may occur ; phosphates of iron and 

 alumina, if present, may be slowly converted into phos- 

 phate of lime, thus 



This action may be compared with the displacement of 

 potash from silicates by lime (p. 130), and with the general 

 tendency of lime to combine with the acid radicles and 

 set free the bases of soluble salts in the soil. 



The Air of Soils. The atmospheric air permeates the 

 soil and fills the interstitial space, i.e., that portion of the 

 total volume not occupied by solid particles and water. 

 The vast extent of internal surface of the soil causes the 



