CARBON AND NITROGEN CYCLES IN THE SOIL 207 



organic acids, etc., and according to Olaru,^ manganese com- 

 pounds, have a marked effect in stimulating nodule formation 

 in soil cultures.'^ 



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 nitrates. 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 in 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. 314 and 184). 

 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. 



^ Compt. Rend., 1915, l6o, 280. 



2 J. K, Wilson, N.Y., Cornell, Agric. Expt. Sta. Bull., 386, 1917, W. A. 

 Albrecht, Soil Set., 1920, 9, 275, and various papers from A. L. Whiting's labora- 

 tory, Illinois. This may explain the action of farmyard manure on the clover 

 crop. (E. J. Russell, Journ. Bd. Agric, 1919, 26, 124.) 



