384 Scientific Proceedings, Royal Dublin Society. 



From the foregoing it will have been seen that the acidity found in soil 

 is not necessarily due to humus or other organic acids. Of the actual 

 occurrence of acid derived from sulphur, records are not, however, numerous. 

 Beyond those cited by Hall and Alway, the writer has only met with (1] the 

 suggestion by Wherry (1918) that the cause of the acid reaction pH 4'5 of 

 certain spring waters in the United States is the presence of traces of 

 sulphuric acid derived from iron pyrites; (2) the statements of Fischer (1914) 

 and Kappen (1920), both cited from Oden (1921), in relation to the source 

 of acid in humus ; (3) the observations of acid water in Utah made by 

 Lee (1907) ; and (4) the surprisingly great acidity, amounting to 1,360 parts 

 per million of free sulphuric acid, found by Skeats (1902) in Texas. This 

 acidity is slightly greater than N/40, which contains 1,225 parts per million, 

 and corresponds to approximately pH 1'7. Clarke (1916) mentions no further 

 observations ; but it is highly probable that were spring waters in regions 

 free from calcareous strata to be tested, the existence of natural acidity due 

 to this acid would be found to be far from uncommon, even in districts 

 remote from known sulphur deposits and the neighbourhood of volcanoes. 



(e) Solubility of iron salts. — Soil reaction also regulates the solubility of 

 iron salts. The corrosion of iron pipes by moorland water is well known, 

 even though this water may be free from all acids other than atmospheric 

 carbon dioxide, giving a reaction of pH 6'5 only. This appears to suffice to 

 dissolve the metal with formation of ferrous bicarbonate, which is hydrolysed 

 very largely, or almost completely. When, however, water is alkaline, there 

 is much less solution of iron, and this is not without its effect on certain 

 crops. The deficiency of available iron is sometimes considered as a cause of 

 chlorosis, and it may be remarked that at Pusa, Gajanus indicus, a leguminous 

 crop, which is much grown, often shows a yellowing of the leaves, the 

 soil reaction being pH 8 '7. The soil is a highly calcareous silt, but here the 

 chlorosis may be due to causes other than the small quantity of available 

 iron, for available phosphate is greatly reduced, as shown by Davis (1918). 

 Gile and Carrero (1920) studied the cause of lime-induced chlorosis and 

 the availability of iron in Porto Rico soils with regard to the rice crop. 

 The ash of chlorotic plants was proved to be low in iron. They found that 

 rice became chlorotic in calcareous soils with the ordinary percentage of 

 water, yet would grow normally in certain calcareous soils when the soil 

 was submerged. They state that calcifugous plants become chlorotic on 

 calcareous soils, and advance as an explanation for the behaviour of rice in 

 submerged soil the view that a new kind of root is developed which is better 

 able to assimilate iron, the availability of which is decreased by the alkalinity 

 To the writer, however, it appears that the change in the hydrogen ion 



