l86 PRINCIPLES OF AGRICULTURAL CHEMISTRY 



lime. The amount of phosphoric acid and potash brought into 

 action through the solution of the lime in the soil first named, may 

 not be large, but in the case of the second soil, 10 per cent, of the 

 soil enters into solution, and all the phosphoric acid and potash 

 protected within this 10 per cent, is exposed to the action of the 

 solvent. This action is further emphasized, in the case of the soil 

 just mentioned, by the fact that a second treatment with acid dis- 

 solves 43,400 parts per million of lime, and a third treatment dis- 

 solves 46,360 parts, making a total of about 14 per cent, of lime 

 dissolved from the soil, corresponding to about 25 per cent, car- 

 bonate of lime. 



This soil, of course, represents an extreme instance, but it 

 emphasizes the difference between a calcareous and a non- 

 calcareous soil. In a non-calcareous soil, the phosphoric acid and 

 potash inclosed within the soil particles are protected from the 

 solvent, while in a calcareous soil, that portion of the phosphates 

 and potash minerals included in the calcareous matter dissolved 

 by the acid is exposed, and may be dissolved. 



Since the plant food dissolved from a non-calcareous soil is 

 present on the external surface of the soil grains, and accessible 

 to the roots of the plants and the action of soil moisture, while 

 that dissolved from calcareous soils is, without doubt, in part 

 included within the soil grains, and not accessible to plant roots, 

 it is obvious that calcareous soils may contain a larger quantity of 

 seemingly active plant food than non-calcareous soils, and yet 

 require fertilization on account of the phosphoric acid being pro- 

 tected. 



Two calcareous soils may also contain the same amount of 

 active plant food, and yet differ in the amount plants can take 

 from them. In one the plant food may be on the extreme surface 

 of the soil grains, in the other it may be disseminated through 

 them. 



Calcareous soils are more durable than non-calcareous soils. 

 This may be explained by the fact that the gradual weathering of 

 such soils continually exposes fresh surfaces of plant food. 



