COMPARISON OF MANURES. 207 



every )'ear, may go far towards producing these crops for the 

 fanner or gardener. If humus by its decay lielps to bring about 

 the sokition of these supplies, then it must be useful, since all such 

 plant food must be taken up in solution. 



That the carbonic acid, which is one of the main products of the 

 decay of humus, does in some wa}' favor vegetable growth is very 

 neatly illustrated by an experiment preformed many years ago by 

 Stoeckbardt. Tliree deep glass vessels, two of which had holes 

 pierced through the bottom, were filled with soil, and peas were 

 planted ; through the bole in the bottom of one of the vessels and 

 up through the soil there was passed ever}' da}- a certain quantity 

 of air, and up tlirough the soil of another of the vessels a mixture 

 of air and carbonic acid ; the third vessel was left to itself, and the 

 condition of things in it was about the same as in an ordinary soil 

 with a ver}' compact and impervious subsoil. The weight of dried 

 crop produced in the third vessel without any circulation of air was 

 about 90 grains, in the soil through which air was circulated, 162 

 grains, and in the soil through which air and carbonic acid were 

 circulated, 190 grains. In some wa}- the carbonic acid along with 

 the air helped the crop amazingly, more than doubling it. This was 

 not necessaril}' because the plant fed on this carbonic acid directly 

 to supply itself with that most important element, carbon. It has 

 been proved over and over again that vegetation supplies itself 

 with carbon, at least mostly, if not entirely, from the carbonic acid 

 of the atmosphere. The fair presumption is that the carbonic acid 

 passed through the soil brought more plant food into solution, and 

 so the crop was more liberal!}' provided with this means of growth. 



In support of this presumption we have the fact, demonstrated 

 also by Stoeckbardt, that the very soil which produced the largest 

 crop, and therefore had yielded up the largest amount of dissolved 

 plant food, still contained the largest amount of soluble plant food, 

 ready for the next crop. The quantity of such soluble food was, in 

 the soil of the closed cylinder without any circulation of air, 22 

 grains ; in the cylinder furnished only with air, 43 grains ; and in 

 the soil to which both air and carbonic acid were supplied, 60 

 grains. 



You will have noticed that the second soil, receiving only air, 

 also gave a notable increase both of crop, and of soluble plant food 

 left for the next crop. On first thought this result would appear 

 to conflict with the explanation given of the increased crop in the 

 third soil, that it was due to the carbonic acid passed through that 



