CARBON AND NITROGEN CYCLES IN THE SOIL 183 



soon covered itself with vegetation, the leaves and stems of 

 which go to enrich the soil in organic matter. The gain in 

 nitrogen is very marked, as shown in Table LI I. The gain 

 is much influenced by the amount of calcium carbonate in the 

 soil, and is considerably less on another plot in Geescroft field 



Table LII. — Gains in Nitrogen in Soils Permanently Covered with 

 Vegetation — Rothamsted Soils Left to Run Wild for 22-24 Years, 

 Hall (120/). 



Land Laid Down to Grass in 1856 and Mown Annually (Dr. Gilbert's 

 Meadow, Rothamsted). 



where only little calcium carbonate is present ; whether this is 

 due to any specific action, or to the changed physical con- 

 ditions brought out by decalcifying a soil, is not clear. Gains 

 of nitrogen also take place on land covered with perennial 

 grasses and clovers even when the crop is mown or grazed. 

 On clay pastures dressings of basic slag have been found to 

 increase the nitrogen content of the soil, whilst potassium 

 salts, such as kainit, have had the same effect on sandy soil. 

 In all these cases leguminous plants are present in greatest 

 extent where the gains in nitrogen are greatest, but they are 

 not necessarily the only nitrogen fixers. 



^ Estimated. 



