278 BOAKD OF AGRICULTURE. 



and Righ at Rothamstead, seem to show conclusively that 

 the free nitrogen of the atmosphere is not the direct source 

 of the nitrogen of plants. 



From later investigations it appears to be settled that 

 most of our farm crops, and particularly the cereals, take up 

 nitrogen only in the form of nitric acid ; but it is perhaps 

 probable that leguminous plants, and possibly Indian corn, 

 mav make use of nitrogen in organic combination, which is 

 not available for other plants. 



According to the latest report (1883) on the rainfall at 

 Rothamstead, the total amount of combined nitrogen in the 

 average annual rainfall, of twenty-nine inches, is estimated 

 by Lawes and Gilbert and Warrington at 4.5 lbs. per acre, 

 and they say, "As to the amount of gain by absorption by 

 the soil, there is unfortunately no direct or satisfactory 

 evidence at command. From such evidence as does exist, 

 we are disposed to conclude that with some soils the amount 

 will probably be greater, and with others less than that sup- 

 plied by the rainfall." 



It will not, therefore, be safe to estimate the atmospheric 

 supplies of nitrogen at more than 10 lbs. per acre, annually, 

 and this must be obtained through the medium of the 

 soil, the direct absorption of the ammonia of the air, 

 by plants, being so slight that practically it need not be 

 noted. 



As our farm crops contain on the average from twenty-five 

 to considerably more than one hundred pounds of nitrogen 

 per acre, we must look to the soil and to the manures ap- 

 plied as the principal source of available nitrogen. 



A large part of the nitrogen of the soil is in organic com- 

 bination, but nitrogen in this form, as well as that in nitro- 

 genous organic substances and ammonia salts that are 

 applied as manure, must be transformed into nitric acid 

 before it is available for plant growth. 



But little has been known in regard to this process of 

 nitrification until within a few years past, notwithstanding 

 the theories that have been formed to account for it. 



From the experiments of Schloessing and Muntz, which 

 were first published in 1877, it is evident that the nitrifi- 

 cation of soils is caused by a living organism, ■which acts as a 



