CARBON AND NITROGEN CYCLES IN THE SOIL 99 



occur in the plant (277). The fixation of nitrogen rapidly comes to a 

 stop unless the resulting compound is removed, as in the plant. Golding 

 has attained this end by an ingenious filtering device, and has thus 

 succeeded in fixing considerable quantities of nitrogen. He has also 

 shown that the reaction of the medium during actual fixation is alka- 

 line, but changes to acid when fixation is stopped by the accumulation 

 of nitrogen compounds. An actual loss then seems to set in (108, 109). 



The chemistry of the process is unknown ; even the changes in the 

 carbohydrates of the culture medium have not been worked out. Nitro- 

 gen fixation is known to take place in the nodule, which thus becomes 

 richer in nitrogen than the rest of the root, 1 and its final product is sup- 

 posed to be a soluble protein which is passed on to the plant. 



But the amount of nitrogen fixed in this way is so large that it is 

 easily measured on the field. When the host plant dies, or is ploughed 

 into the ground, the nitrogen compounds speedily change into plant 

 food. A uniform piece of ground at Rothamsted was divided into 

 two parts : on one a crop of clover was taken, on the other barley was 

 grown. After the crops were removed samples of soil were taken for 

 analysis, and then barley was grown on both plots. The analytical 

 results were : 



These facts are well known to the practical man, and are utilised 

 for increasing the nitrogen supply of cultivated soils and for reclaiming 

 barren sands and clays (pp. 85 and 146). Leguminosae are among 

 our commonest plants, both wild and cultivated. Wherever they grow 

 they lead to enrichment of the soil in organic nitrogen compounds 

 through the operation of the nodule organisms. The difference between 



1 Stoklasa's analytical results with yellow lupines (Landw. Jahrb., 1895, xxiv., 827 

 are: 



