312 SYSTEMS OF PERMANENT AGRICULTURE 



"It is scarcely necessary to remark that this excrementitious matter must 

 undergo a change before another season. During autumn and winter it begins 

 to suffer a change from the influence of air and water ; its putrefaction, and, at 

 length, by continued contact with the air, which tillage is the means of procur- 

 ing, its decay are effected ; and at the commencement of spring it has become 

 converted, either in whole or in part, into a substance which supplies the place 

 of humus, by being a constant source of carbonic acid. 



" The quickness with which this decay of the excrements of plants proceeds, 

 depends on the composition of the soil, and on its greater or less porosity. It 

 will take place very quickly in a calcareous soil ; for the power of organic ex- 

 crements to attract oxygen and to putrefy is increased by contact with the al- 

 kaline constituents, and by the general porous nature of such kinds of soil, 

 which freely permit the access of air. But it requires a longer time in heavy 

 soils consisting of loam or clay. 



"The same plants can be cultivated with advantage on one soil after the 

 second year, but in others not until the fifth or ninth, merely on account of the 

 change and destruction of the excrements which have an injurious influence on 

 the plants being completed in the one, in the second year; in the others, not 

 until the ninth. 



"In some neighborhoods, clover will not thrive until the sixth year; in others 

 not till the twelfth ; flax in the second or third year. All this depends on the 

 chemical nature of the soil ; for it has been found by experience that in those 

 districts where the intervals at which the same plants can be cultivated with 

 advantage, are very long, the time cannot be shortened even by the use of the 

 most powerful manures. The destruction of the peculiar excrements of one 

 crop must have taken place before a new crop can be produced. 



"Flax, peas, clover, and even potatoes, are plants the excrements of which, 

 in argillaceous soils, require the longest time for their conversion into humus; 

 but it is evident, that the use of alkalies and burnt lime, or even small quantities 

 of ashes which have been lixiviated, must enable a soil to permit the cultivation 

 of the same plants in a much shorter time. 



"A soil lying fallow owes its early fertility, in part, to the destruction or con- 

 version into humus of the excrements contained in it, which is effected during 

 the fallow season, at the same time that the land is exposed to a further disinte- 

 gration." 



In the first American edition of Liebig's book, published in 1841, 

 Doctor John W. Webster, then Professor of Chemistry in Harvard 

 University, inserted an appendix, in which he wrote as follows: 



"It should be stated that the accuracy of the experiments of Macaire : Princep 

 adduced by the author (Liebig) is not generally admitted. Other chemists 

 have been unable to obtain similar results, or, if they do, are inclined to ascribe 

 them to injury of the roots of the plants examined. Professor Lindley has in 

 his notice of Liebig's work remarked that he has no fixed opinion on the sub- 

 ject, it being a question of facts and not of induction." 



