26o MATERIALS OF INDIRECT VALUE [chap. 



phosphate, or aluminium phosphate ; and on soils 

 containing any reasonable amount of calcium carbonate 

 the former will predominate, the two latter on the sands 

 and clays where calcium carbonate is lacking. Now the 

 effective solubility of the two latter phosphates in the 

 soil water is very much below that of the precipitated 

 calcium phosphate, consequently their phosphoric acid 

 is much slower in reaching the plant, which may remain 

 short of this necessary constituent even though large 

 amounts have been applied to the soil. Similarly a 

 soil may contain considerable amounts of phosphoric 

 acid, which in the absence of lime is combined with 

 ferric oxide or alumina so as to be in a highly 

 insoluble condition; for example, a soil derived from 

 the marlstone has been known to contain 0-84 per cent, 

 of phosphoric acid but yet show great response to 

 phosphatic manures, because at the same time it con- 

 tained 28-16 per cent, of ferric oxide and no calcium 

 carbonate. Applications of lime or calcium carbonate 

 are of great value on these soils because they form a 

 certain amount of calcium phosphate by interaction with 

 the iron or aluminium phosphates, and so increase the 

 proportion of phosphoric acid in the soil water. 



The action of lime upon the potash compounds in 

 the soil is equally marked ; as the soil water carries 

 down the dissolved calcium bi-carbonate it attacks the 

 zeolitic double silicates in the clay and some of their 

 soluble bases, potash among them, change place with 

 the lime and come into solution. Thus lime is precipi- 

 tated and potash is found in the soil water. The action 

 is the converse of that which takes place when potash 

 salts are applied as manures ; whatever base is in excess 

 in the water reaching the soil will turn the others out 

 and be precipitated in the solid zeolite. When potash 

 salts are applied to the land the strong solution thus 



