220 Fertilisers [pt. hi 



attributed to the conversion of the deflocoulated or 

 sticky clay into the flocculated form. While in practice 

 chalk brings about this change it is not the actual agent 

 concerned in the deflocculation, indeed in the presence 

 of clay alone it is inert. It is effective only in presence 

 of a little carbonic acid which always occurs in the soil 

 and which causes it to go into solution as calcium bicar- 

 bonate (p. 25); this appears to be the effective agent. 



Of course where the bad physical condition is due to 

 other causes chalk may be unable to put it right. The 

 silty clays form a case in point (p. 23). 



In common with many other substances calcium bi- 

 carbonate is absorbed from its solution by certain con- 

 stituents of the soil, and displaces some of the substances 

 previously absorbed. Thus chalk causes the liberation 

 of a certain amount of potash from the soil so that a 

 dressing of chalk is often equivalent to a dressing of 

 potassic fertiUser. 



Chalk increases the amount of bacterial action and 

 therefore the rate of nitrate production in the soil. If 

 much organic matter is present, it increases also the 

 amount of phosphate available for the plant^. 



Limestone has the same chemical composition as 

 chalk, but it is much harder and does not readily dis- 

 solve in the soil water so that it cannot be used direct 

 in agriculture until it is ground. Mills were set up for 

 this purpose' as long ago as the eighteenth century, but 

 only recently have the mechanical difficulties been com- 

 pletely overcome and ground limestone put on the 

 market as a regular article of commerce. 



It acts in precisely the same manner as chalk, and 



^ British examples are common. An instance from Brittany is given 

 in Compt. Bend., 1917, clxiv. 409. 



