SOIL REACTION 417 



crops as oats or wheat when grown immediately after the liming. 

 This is apparently due to temporary conditions which soon 

 disappear, when the lime will benefit these crops as well as other 

 subsequent ones. A similar temporary condition has been 

 noticed in that the addition of lime will sometimes apparently 

 retard the immediate effectiveness of insoluble phosphatic 

 fertilisers. Such a temporary condition may possibly be 

 connected with the time required for the reversion of oxide or 

 hydroxide into carbonate under the conditions prevalent in 

 soils ^ ; until such conversion is completed it may well happen that 

 the OH-ion-concentration of the soil solution due to the solution 

 and ionisation of Ca(0H)2 may be too high to be consistent with 

 fertility and it is not until the OH-ion concentration is dimin- 

 ished by the conversion of Ca(0H)2 in CaCOg or CaH2(C02)3 

 that suitable soil conditions are obtained. This point of view 

 seems to be supported by the work of Morse and Curry, who 

 showed that the concentration of several elements in the soil solu- 

 tion of certain acid soils was diminished by the addition of lime. 

 Iron is a conspicuous and interesting example of this : depress- 

 ing the concentration of iron in the soil solution is advantageous, 

 as it decreases the likelihood of formation of ferric phosphate to 

 replace the calcium phosphate of normal soils ; if carried too 

 far, however, it may result in injury to the crop, as the ferric-ion 

 concentration may, through over-liming, be reduced beyond 

 the limit of availability to the plant, which becomes chlorotic in 

 consequence. Instances of this have been recorded in the case 

 of certain pine-apple soils and with some kinds of lupins.* 

 This behaviour, however, is somewhat exceptional, and other 

 plants are not known definitely to be affected, although one or 

 two instances in this country have been brought to the writer's 

 notice. 



Soil Acidity a Phase of the Broader Question of 



Soil Reaction 



In view of the above considerations, it is permissible to doubt 

 the value of the usual laboratory methods as means of measuring 

 soil acidity. At the best they can give some rough empirical 

 information to the effect that certain soils examined will or 

 will not be benefited by liming. They give — and can give^ — no 

 quantitative information as to the degree of acidity in its strict, 



^ For a comprehensive treatment of the conditions governing the 

 carbonation of burnt Hme in soils see Maclntire, Soil Set., 7 (1919), pp. 325- 

 446 ; per contra, see Hager, Journ. Landw. 65 (1917), p. 245. 



- For a somewhat difierent interpretation of such chlorosis, see E. J. 

 Russell, Soil Conditions and Plant Growth (4th ed,), pp. 86 and 304. 



