C 5'3 ] 



acid has an absorptive power for free bases or carbonates, 

 (5) Humus also has the power of absorbing bases when they 

 are in free state, or as carbonates or silicates. When P 2 Os is 

 presented in a soluble form as CaHiP 2 O 8 (Super phosphate of 

 lime) it first acts on the CaCOs of soil to form Ca 2 H 2 P2O 8 

 (Dicalcium Phosphate) and afterwards CasPsOs (Tricalcium 

 Phosphate) ; possibly some Mg 2 P 2 Oz also is formed. These 

 combining with the iron and alumina of the soil become fixed 

 as Phosphates of iron and alumina. The reactions that take 

 place may be expressed by the following formulae : 



1 i ) sCaH 4 P 2 O 8 + 2Fe 2 O 3 = 2Fe 2 P 2 O 8 + Ca 3 P 2 O 8 4- 6H 2 O. 



(2) 3CaH 4 P 2 O 8 + 2 AI 2 O 3 = 2 A1 2 P 2 O 8 + Ca 3 P 2 O 8 + 6H 2 O. 



822. The absorption of P 2 Os is more rapid in calcareous 

 soils than in clays or sands. Clays and sands go on 

 absorbing P 2 O5 for several days. One of the functions of 

 CaCOs in soils is to supply lime with which acids of certain 

 salts may combine, so as to enable the bases to be absorbed. 

 The Calcium carbonate of the soil naturally present or added 

 as manure, helps to keep up proper equilibrium between 

 bases of the compound silicates. 



823. The three kinds. of absorption may be shortly illus- 

 trated thus : 



(1) Physical absorption^ e.g., when colouring organic 

 matter is removed from buffalo-dung (dung of buffaloes fed on 

 mango leaves), litter &c., and absorbed by the soil. 



(2) Absorption without exchange of bases } as in the case 

 of hydrates, carbonates, phosphates and silicates. Hydrates of 

 Fe and Al and humus take an active part in this absorption. 



(3) Absorption with exchange of bases in which the : by- 

 drated compound silicates are the active agents. For ordinary 

 fertile soils this kind of absorption plays the most impor^ 

 tant part. Those ash constituents of plants which are most 

 valuable and least abundant in the soil are those which 

 are fixed in large quantities ; e. g. } P 2 O 5 and K^O. These 

 when liberated in the soil by the action of weathering of 



