NON-ACTION OP GYPSUM ON ACID SOILS. 149 



conclusion. If the subject were properly investigated, these facts, 

 apparently in opposition, might be explained so as no longer to 

 contradict this opinion, or perhaps might help to confirm it. Good 

 reasons, deduced from established chemical truths, may be offered 

 to explain why the acidity of our soils should prevent the operation 

 of gypsum ; though it may be deemed premature to attempt the 

 explanation of any supposed fact, before every doubt of the 

 existence of the fact itself has been first removed. 



One of the circumstances will be mentioned, which appears at 

 first glance most strongly opposed to the opinion which has been 

 advanced. On the poor acid clay soil, of such peculiar and base 

 qualities, which forms the subject of the 5th, 6th, and 7th experi- 

 ments, gypsum has been sufficiently tried, and has not produced 

 the least benefit, either before marling, or afterwards. Yet the 

 first growth of clover on this land after marling is fully equal to 

 what might be expected from the best operation of gypsum. Now 

 if it could be ascertained that a very small proportion of either 

 sulphuric acid, or of the sulphate of iron, exists in this soil, it 

 would completely explain away this opposing fact, and even make 

 it the strongest support of my position. The sulphate of iron has 

 sometimes been found in arable soil,* and sulphuric acid has been 

 detected in certain clays. f I have seen, on the same farm, a bed 

 of clay of very similar appearance to this soil, which certainly had 

 once contained one of these substances, as was proved by the form- 

 ation of crystallized sulphate of lime, where the clay came in con- 

 tact with a bed of marl. The sulphate of lime was found in the 

 small fissures of the clay, extending sometimes one or two feet in 

 perpendicular height from the calcareous earth below. Precisely 

 the same chemical change would take place in a soil containing 

 sulphuric acid, or sulphate of iron, as soon as marl is applied. The 

 sulphuric acid (whether free or 'combined with iron) would imme- 

 diately unite with the lime presented, and form gypsum (sulphate 

 of lime). Proportions of these substances, too small perhaps to be 

 detected by analysis, would be sufficient to form three or four 

 bushels of gypsum to the acre more than enough to produce the 

 greatest known effect on clover and to prevent any benefit being 

 derived from a subsequent application of gypsum ; because there 

 being already in the soil more gypsum than could act, no additional 

 quantity could be of the slightest benefit. J 



* Davy's Agr. Chem. p. 141. f Kirwan on Manures. 



[t Confirmatory testimony. Johnston has since fully sustained this rea- 

 soning, by chemical facts. Besides the sulphate of iron, he names the sul- 

 phates of alumina and magnesia as occasionally present in soils, and liable 

 to be hurtful to plants. He adds: "When soils which contain any of the 

 three salts I have named, have once been limed or marled, it is in vain to 



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