246 



nodules are formed when lucerne is planted on new soils in South 

 Africa, as, for example, on freshly broken-up veld ; the con- 

 dition of the organisms within the nodule should be investigated, 

 so as to ascertain if improvement be possible by inoculation from 

 pure cultures, either imported or prepared de novo from lucerne 

 within the country. These and kindred questions connected with 

 the symbiosis of the nitrogen-fixing organism and the leguminous 

 plants must to a large extent be worked out afresh in each country, 

 and South Africa, with its special conditions of soil and climate, 

 cannot take on trust the results arrived at in Europe or America. 



SOIL ENRICHMENT BY CROP RESIDUES. 



The enrichment of the soil due to growing lucerne, caused 

 by the decay of the great root residues containing nitrogen 

 derived from the atmosphere, is quite independent of the amount 

 of similarly combined nitrogen taken away in the successive crops 

 of leafy growth. Some of the Rothamsted experiments show very 

 clearly how great the gain may be. In one particular case, when 

 an extra large crop of clover was grown, notwithstanding the fact 

 that the clover plots yielded between three or four tons per acre of 

 clover hay, yet the wheat crop which followed this growth of 

 clover was 15 per cent, better than the wheat crop following the 

 bare fallow. The swede turnip crop, which followed the wheat, 

 although similarly and heavily manured on both plots, continued 

 to be better where the clover had been grown two years previous- 

 ly ; and even the barley, which came next three years after the 

 clover, showed a decided superiority on the clover land. Thus a 

 clover crop, itself wholly removed from the land, exercised a 

 marked influence for good on at least the three succeeding crops 

 grown under the ordinary conditions of farming. In fact, the crop 

 residue supplies as well as nitrogen, also carbohydrates, which 

 are required for the development of the bacterial energy, and 

 where this supply of carbohydrates is deficient or not maintained 

 on arable land falling off of the crops ensues. In the case of grass 

 land the conditions are entirely difi'erent, especially when dealing 

 with wild prairie or forest where the annual growth of carbo- 

 hydrates falls back to the soil and is available for such organisms 

 as the Azotobactcr. The fixation of nitrogen is in fact an oxidizing 

 process, and hence in two experimental fields at Rothamsted it is 

 observed that the one with the smaller accumulation of nitrogen 

 has the larger quantity of carbohydrates in the soil and in bac- 

 teriological tests shows a much greater development of Azoto- 

 hacter than the soil from the other field. Henry has also shown 

 that the shed leaves of many forest trees during their decay may 

 bring about the fixation of nitrogen ; and this fact, which again 

 depends on the oxidation of the carbohydrates of the leaf to sup- 

 ply the necessary energy, has been confirmed in the Rothamsted 

 laboratory, as well as the presence of Azotohactcr on the decaying 

 leaf. It is obvious that one of the most interesting fields for the 

 study of these organisms must lie in the virgin lands of a country 

 like South Africa. We all know that virgin soil may, on the one 



