CARBON AND NITROGEN CYCLES IN THE SOIL 97 



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. 84 and 126). 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 

 the action of this organism and that of azotobacter is that it gets its 

 carbohydrates from the plant, and is, therefore, independent of soil 

 organic matter. Thus, it operates perfectly well in the poorest soils 

 provided potassium salts, phosphates and calcium carbonate are present 

 in sufficient quantity for the host plant, while azotobacter (except where 

 it is associated with algae, a case that requires further investigation) 

 requires a supply of organic matter in the soil, and therefore only works 

 in fairly rich soils where its effects are more difficult to measure. 



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



are:- 



