GYPStM ON ACID SOILS. 153 



one or more of the other substances named. Let us now suppose 

 gypsum to be applied to this soil. The substance (sulphate of lime) 

 is composed of sulphuric acid and lime. It is applied in a finely 

 pulverized state, and in quantities from half a bushel to two bushels 

 the acre generally not more than one bushel. As soon as the 

 earth is made wet enough for any chemical decomposition to take 

 place, the oxalic acid must let go its base of iron or alumina, and 

 seize upon and combine with the lime that formed an ingredient 

 of the gypsum. The sulphuric acid left free, will combine with 

 the iron, or the alumina of the soil, forming copperas in the one 

 case, and alum in the other. The gypsum no longer exists' 

 and surely no more satisfactory reason can be given why no effect 

 from gypsum should follow. The decomposition of the gypsum has 

 served to form two or perhaps three other substances. One of them, 

 oxalate of lime, like all salts of lime, is probable valuable as manure ; 

 but the very small quantity that could be formed out of one or even 

 two bushels of gypsum, might have no more visible effect on a 

 whole acre, than that small quantity of calcareous earth, or farm- 

 yard manure. The other substance certainly formed, copperas, is 

 known to be a poison to soil and to plants and alum, of which the 

 formation would be doubtful, I believe is also hurtful. In such 

 small quantities, however, the poison would be as little perceptible 

 as the manure ; and no apparent effect whatever could follow such 

 an application of gypsum to an acid soil. So small a proportion of 

 oxalic axid, or any oxalate other than of lime, would suffice to de- 

 compose and destroy the gypsum, that it would not amount to one 

 part in twenty thousand of the soil. 



Why gypsum sometimes acts as a manure on acid soils when 

 applied in large quantities for the space, is equally well explained 

 by the same theory. If a handful, or even a spoonful of gypsum 

 is put on a space of six inches square, it would so much exceed in 

 proportion all the oxalic or other vegetable acid that could speedily 

 come in contact with it, that all would not be decomposed, and the 

 part that continued to be gypsum would show its peculiar powers 

 perhaps long enough to improve one crop. But as tillage served 

 to scatter these little collections more equally over the whole space 

 or even as repeated soaking rains allowed the extension of the 

 attractive powers* applications like these would also be" destroyed, 

 after a very short-lived, limited, and rarely profitable action. 



Soils that are naturally calcareous, or even neutral, cannot con- 

 tain oxalic acid combined with any other base than lime. Hence, 

 gypsum applied there continues to be gypsiLm, and exerts its great 

 fertilizing power, as in the counties of Loudoun and Frederick. 

 But even on these most suitable soils, this manure is said not to 

 be certain and uniform in its effects ; and, of course, more certain 

 results are not to be looked for with us, I have not undertaken 



