October, 1909.] 



325 



SCIENTIFIC AGRICULTURE, 



THE GROWTH OF LEGUMINOUS 

 CROPS AND SOIL INOCULATION. 



By W. Biffen, B.Sc. 



(From the West Indian Bulletin, Vol. 

 X., No. 1, 1909.) 



The fact that the growth of leguminous 

 crops, as peas, beans, clover, etc, instead 

 of diminishing the fertility of the land, 

 often result in an increase of its crop- 

 yielding capacity, was known and acted 

 upon by practical workers in agriculture 

 from early times, although it is only 

 within comparatively recent yeai's that 

 any reason was brought forward which 

 accounts satisfactorily for the matter. 



In the eighteenth, and greater part of 

 the nineteenth centuries, while legum- 

 inous crop:;? were frequently included in 

 farm rotations in European countries, 

 opinions differed as to the actual way in 

 which these plants were able to make 

 such good growth — often without the 

 aid of any nitrogenous manure — and at 

 the same time to leave the land in such 

 a condition that highly satisfactory 

 returns were obtained from a succeeding 

 cereal crop. By many it was thought 

 that this was explained by the deep- 

 rooting habit of the legumes, which 

 enabled them to tap resources of 

 moisture and food in lower strata of the 

 soil, untouched by other plants. Liebig, 

 a welll-known agricultural chemist, 

 brought forward the theory that clover 

 and other broad-leaved plants were able 

 to obtain considerable supplies of 

 nitrogen from the air, this nitrogen being 

 absorbed not in the elementary state, 

 but in the form of ammonia and other 

 compounds, which exist in small quantity 

 in the atmosphere. 



The means by which plants obtain the 

 nitrogen necessary for their growth, and 

 the proportion between the quantity of 

 nitrogen supplied in the manure and 

 that removed in the resulting crop, were 

 investigated by Boussingault, a French 

 experimenter, about the middle of 

 the nineteenth century. Boussingault 

 weighed and analysed the crops pro- 

 duced on his own farm during six separ- 

 ate courses of rotation (all of which 

 included a legume crop). As a result, he 

 found that, on the whole, from one-third 

 to one-half more nitrogen was removed 

 in the produce than was supplied in the 

 manure. He further observed that the 

 excess of nitrogen in the crop over that 

 provided in the fertilizer was especially 



great in the case of a leguminous crop. 

 This naturally pointed to the conclusion 

 that the known value of clover, peas, 

 and other plants of this family was due 

 to a power possessed by them of enrich- 

 ing the soil with nitrogen obtained from 

 some outside source, most probably 

 the air. 



Further experiments carried out by 

 Boussingault, however, and also experi- 

 mental work conducted by Lawes and 

 Gilbert at Rothamsted in England, not 

 only with legumes, but with plants ot 

 other families as well, failed to sub- 

 stantiate the growing belief that the free 

 nitrogen of the atmosphere was avail- 

 able as food for growing plants of any 

 family. As a result, the matter rested 

 for a time, until new investigations 

 carried on in Europe and America, re- 

 opened the question, and about the year 

 1883 evidence was brought forward 

 which was sufficient to prove that plants 

 of the legume family, under certain 

 conditions, are capable of utilizing the 

 free nitrogen of the air as a source of 

 food supply. 



Among the many investigators of 

 this question were two Germans, Messrs. 

 Hellriegel andWilfarth, to whom belong 

 the credit of clearing up the whole matter 

 of nitrogen assimilation by leguminous 

 plants. In experiments conducted by 

 these two scientists, the medium of 

 cultivation employed consisted of sterile 

 sand, in which were planted the seeds of 

 plants belonging to different botanical 

 families. The necessary plant food was 

 supplied in the form of nutrient solutions. 

 Mineral plant food was given in all 

 cases, but it was observed that when 

 combined nitrogen was withheld, all the 

 seedlings of plants belonging to orders 

 other than the Leguminosfe died from 

 starvation as soon as the nitrogen con- 

 tained in the seed was used up. By far 

 the greater number of the legumes (peas) 

 grown, died at the same stage, but it 

 was noted that one or two pea seedlings 

 recovered and made good growth, despite 

 the absence of combined nitrogen. In 

 such cases, examination always showed 

 that the roots of the plant were set with 

 little nodules, which, it was known, are 

 generally characteristic of the growth 

 of legumes under normal conditions in 

 the field. No nodules could be found on 

 the roots of the plants which died. A 

 further series of experiments was then 

 started, in which pea plants, grown in 

 sterile sand, were all fed with solutions 

 of mineral plant food, but to some were 



