SOIL ACIDITY 291 



same manuring had been continued for even longer periods, 

 was obviously due to the fact that the soil at Woburn 

 contained practically no carbonate of lime at the beginning 

 of the experiments. Analyses show less than one-tenth per 

 cent., and even this minute proportion was probably largely 

 over-estimated. 



Thus in the Woburn soil there is no base present to 

 maintain the neutrality should any agency arise to produce 

 acid, whereas in the E-othamsted arable soils, as we have seen, 

 there has always been sufficient carbonate of lime to keep up 

 a neutral condition and put out of action any acid as fast as 

 it was produced. However, it was observed later that one 

 of the Eothamsted fields did contain plots on which the soil 

 had become acid through the application of ammonium salts 

 year after year for a long period ; this was the Park grass field 

 which is cut for hay every year. Now there is no record of 

 the Park having ever carried anything but grass, and analyses 

 of the soil at the margins of the plots where no experimental 

 treatment had been given, showed that this was one of the 

 pieces of land which had never received the regular chalkings 

 to which allusion has been made earlier. The soil, therefore, 

 of the grass plots had started with but a small proportion of 

 carbonate of lime, an amount comparable with that present in 

 the Woburn soil at the outset of the experiments there, and 

 the acidity has developed itself on this soil just as it has 

 at Woburn. 



It is not clear at first sight how free acid can arise by the 

 interaction of ammonium - salts with any of the constituents 

 of the soil ; ammonium sulphate and chloride are both neutral 

 salts in which acid and base are firmly combined at ordinary 

 temperatures. Certain physical and chemical possibilities 

 had been suggested, and these were first examined in some 

 detail (see Hall and Gimingham, loc. cit.), using clay, sand, 

 humus, and other soil constituents separately, but without 

 detecting any process which would give rise to free acid. 

 On sand the ammonium - salts had no action ; with clay an 



