208 MASSACHUSETTS HORTICULTURAL SOCIETY. 



soil along with the air. But there is not necessarily disagreement 

 here ; it is quite reasonable to suppose that the humus in tliis soil, 

 together with the oxygen of the air that was circulated freel}'' 

 through it, produced the same effect and in the same way as 

 was produced by the carbonic acid ready formed in the third soil. 

 The formation of carbonic acid from the humus can take place only 

 in the presence of oxygen, and the more liberal the suppl}- of 

 oxj'gen the larger will be the production of carbonic acid from a 

 given quantity of humus. In this second soil we had, as in all the 

 otliers, the ordinary* quantity of humus ; the supply of air, with its 

 one-fifth part of oxygen, was liberal ; carbonic acid must have been 

 produced freely ; and it would have been strange if there had 

 been no increase of crop. Such a result would have tended to dis- 

 prove just what we are seeking to prove, that the humus does a 

 good work for the farmer b}' the carbonic acid given off in tlie soil 

 as it decays, or oxidizes, which two terms mean much the same 

 thing. 



Another experiment shows in a no less striking manner the part 

 that humus may take in bringing plant food into solution : a sam- 

 ple of a sand\' loam was compared with another portion of the 

 same soil to which some humus had been added ; in the course of 

 the summer months, while a crop was growing vigorously on these 

 soils, the quantities of potash that became soluble in the two soils 

 were as 366 parts in the soil poor in humus to 574 parts in the 

 other ; the quantities of plant substance produced in the two cases 

 were 5,040 and 9,800 parts. 



That the presence of decaying vegetable matters or of humus in 

 the soil does increase the proportion of Carbonic acid there, is fully 

 shown by analysis of the air in the pores of the soil. Tlie air 

 above the soil contains 3 parts of carbonic acid in 10,000, while 

 that in the soil may contain ordinarily 100 parts in 10,000; and, 

 moreover, such richness in carbonic acid is found only in the air of 

 soils containing humus. A rich dressing of stable manure, or, in 

 other words, a large addition of decaying, humus-forming sub- 

 stance, largely increases the quantity of free carbonic acid in the 

 soil. An asparagus bed that had not been manured for a year con- 

 tained in the air in the pores of the soil 122 parts of carbonic acid 

 in 10,000, but when recently manured 233 parts. Anotlier, a sur- 

 face soil rich in humus, had 540 parts of carbonic acid, a newly 

 manured sand}* field 333 parts, and the same soil, in wet weather, 

 1,413 parts, of carbonic acid in 10,000 of its air. 



