PROPERTIES OF MIXED EARTHS. 43 



I caused the two beds to be broken up and mixed well 

 together with the spade. The first year the harvest from 

 it was almost nothing, much less than before, though it 

 had never been fertile. The second year it was a little 

 more productive ; but it was not till the fifth year that it 

 recovered its usual degree of fertility. One of my friends 

 possessed a piece of ground of a moderate degree of pro- 

 ductiveness. The soil, which was sandy and very dry, 

 was much improved by the application of marl, which he 

 allowed to decompose upon it for two years before cultiva- 

 tion. As the same person had a bed of marl in one of his 

 fields at the depth of a foot, I advised him to break up a 

 piece of it, twelve or fourteen yards square, and to mix 

 the marl with the upper layer of earth, in a proportion 

 more considerable than in the other case. The portion of 

 the field thus operated upon was nearly barren for two 

 years, after which it was more decidedly fertile than 

 before. 



These two cases struck me very strongly. I sought for 

 the reason of the changes, and believe that I have found 

 it in the nature of the inferior layer of the earth at the 

 time of being mixed with the upper soil. 



In the first case the oxide of iron, which colored the 

 bed brown, was at the lowest state of oxydation ; but at 

 the moment that it was brought into contact with the at- 

 mospheric air, it began to combine with new portions of 

 oxygen, and the earth could not become fertile till the 

 iron was saturated. The progress of oxydation entirely 

 changed the color of the soil ; from black it became of a 

 deep lively yellow. This fact may admit of a diiferent 

 explanation. Is the oxide of iron, in its black state, de- 

 structive to vegetation? Does that oxide, which, by at- 

 tracting the oxygen from the atmosphere, decomposes it, 

 destroy by its action the necessary and salutary influence 

 of that fluid upon plants ? These are questions which can 

 only be answered by a long experience. 



In the second case the cause of sterility was differ- 

 ent, though it had a general relation to that of the first. 

 Marl is principally composed of sub-carbonate of lime and 

 alumina ; the proportions in which these are combined 

 constitute all its varieties. The lime contained in marl, 

 as it is taken from the bed, is never saturated with car- 

 bonic acid ; but after being exposed to the air, it becomes 

 at length saturated with the acid it receives from it, 



