120 SOIL CONDITIONS AND PLANT GROWTH 



the soil are, therefore, necessary : indeed, chalk and lime are 

 among the oldest of manures. Soils lying immediately above 

 chalk and limestone are no exceptions, and in wet regions they 

 may become thoroughly decalcified. 



On chalk soils the percentage of calcium carbonate may 

 rise very high, and then a wholly new set of properties comes 

 in. It is impossible to draw any exact line showing where 

 these properties begin to appear, but they entirely mask the 

 effects of the silica and silicate particles and obliterate the 

 distinctions between sands, loams, clays. Chalk soils, there- 

 fore, form a class by themselves to which the ordinary 

 laboratory methods of analysis and investigation do not apply : 

 unfortunately, appropriate methods ha-- not yet been worked 

 out. 



The Phosphorus, Potassium, and Calcium Compounds 



of the Soil. 



Part of the phosphorus in the soil is probably present in 

 organic combinations derived from plant or animal resi- 

 dues : this has been investigated by Vincent,^ by Potter and 

 Snyder ^ and by Schollenberger ; ^ none of whom, however, 

 were able to effect identification of the compounds. Part, 

 however, is in inorganic combination and Bassett has shown * 

 that it most probably occurs as hydroxyapatite — 



(Ca3P203)3Ca(OH)2, 



this being the solid phase stable over a range extending from 

 faintly acid to alkaline conditions : any phosphate such as 

 superphosphate or basic slag added to the soil as fertiliser 

 would tend to be converted into this substance. 



The potassium compounds in the soil are probably silicates. 

 Their distribution has been studied by Dumont* who found 

 that they may occur in the finer or coarser portions of the 



^ Compt. Rend., 1917, 164, 409. "^ Soil Sci., 1918, 6, 321-332. 



^Ibid., 365-395; 1920, 10, 127. In this case about one-third of the phos- 

 phorus occurred in organic combination. 



* Trans. Chen. Soc, 1917, III, 620-642. 



* Compt. Rend., 1904, 138, 215-217. 



