Agricultural Chemistry. 293 



development of plants, the atmospheric nutritive materials are 

 the indispensable conditions of the conversion of the terrestrial 

 nutritive materials into organic compounds ; and the terrestrial 

 nutritive materials are, in like manner, the indispensable condi- 

 tions of the conversion of the atmospheric nutritive materials into 

 corn and cattle, (See 'Agricultural Chemistry,' pp. 195, 187.) 



The amount of produce of a field in a given time, say in a 

 year, is proportional to that part of the sum of the terrestrial 

 nutritive constituents, which, during that year, has been trans- 

 ferred from the soil to the plants growing on it. In a double 

 crop there is double the quantity of terrestrial elements of 

 nutrition. 



These propositions are self-evident, and require no further 

 proof. 



Experience demonstrates that the produce of two fields in the 

 same district, or the quantities of corn or cattle raised on them, 

 are very unequal. One meadow yields twice, thrice, four times 

 as much hay as another meadow of equal surface, under the 

 same external circumstances. An acre of clover in one field 

 yields twice, thrice, or four times as much clover as an acre of 

 another clover-field. There are fields, nay entire districts, on 

 which clover either does not grow or grows but poorly. 



Wliat is the cause of this unequal fertility ? The surface of 

 the fertile, and that of the unfruitful field, are in contact with a 

 precisely equal volume of air : to both, therefore, are presented, 

 by the air and by the rain, precisely equal quantities of carbonic 

 acid and ammonia ; but on the surface of the so-called fertile 

 field, twice, thrice, or four times as much carbon and nitrogen 

 are condensed as on the equal surface of the other. It is plain 

 that the cause of the difference of produce must be sought for, 

 not in the atmosphere, but in the soil : this cause must be the 

 unequal quality of the soil, while the external conditions are the 

 same. 



In the fertile soil, twice, thrice, or four times as much of the 

 terrestrial elements of nutrition have entered into the plants, than 

 in the unfruitful one. There have tliorefore been more of these 

 terrestrial (onstitucnts present, either absokitely or as regards their 

 capacity of assimilation (their power of entering into the plant, 

 from their existing in available chemical forms) in the one soil 

 than in the other. 



The amount of produce, in these cases, is unquestionably 

 proportional to the quantity of mineral elements of nutrition 

 present in the soils, and not to the (juantity of carbonic acid and 

 ammonia, for the atmospliere has sujiplicd to both an equal 

 quantity of these materials ; but in the one soil the conditions ol 



