492 THE POPULAR SCIENCE MONTHLY. 



ing more vigorously at the expense of the other/' and that free 

 nitrogen is appropriated by the microbes. 



In 1883 Hellriegel began experiments with leguminous plants 

 in pots of washed quartz sand, to which no nitrogen was added. 

 Marked differences were observed in the growth of the plants un- 

 der these conditions, but tubercles were found on the roots of the 

 plants that made the best growth, while they were absent in 

 other cases. He was then led to attempt the production of the 

 root-tubercles by seeding or inoculating sterilized sand with a 

 water-extract of a soil in which leguminous plants were growing. 

 To some of the pots, in which peas and vetches were planted, from 

 twenty-five c. c. to fifty c. c. of a water-extract of a fertile soil 

 were added. When this soil-extract was not sterilized, there was 

 a luxuriant growth of the plants in the pots to which it was ap- 

 plied, with abundant formation of root-nodules ; but when the soil- 

 extract was sterilized, this result was not obtained. 



This soil-extract, however, was without effect on lupines and 

 some other plants ; but when the lupine pots were inoculated with 

 an extract of a soil in which lupines were growing, the plants made 

 a luxuriant growth, and root-tubercles were abundantly devel- 

 oped. In all cases the nitrogen supply of the plants was coinci- 

 dent with the development of root-tubercles, that were produced 

 by inoculation with the extract of a fertile soil. 



In 1888 a preliminary series of experiments, on the same lines, 

 were begun at Rothamsted by Sir John B. Lawes and Prof. J. H. 

 Gilbert ; and in 1889 they were continued, on a more extended 

 scale, with modified conditions suggested by the results of the pre- 

 ceding year. Their first experiments were made with peas, blue 

 lupines, and yellow lupines, in pots seven inches high and about 

 six inches in diameter. For our present purpose we need only 

 call attention to the experiments in 1888 with peas. 



Pots 1, 2, and 3 were filled with a washed yellow sand, to which 

 was added 0*5 per cent of the ash of pea plants to furnish the re- 

 quired mineral constituents. Pot 4 was filled with a rich garden 

 soil. Distilled water was used for watering the plants, and no 

 other application was made to pot 1. Care was taken to deter- 

 mine the nitrogen of the soils, and of the seeds planted, which we 

 need not describe in detail. 



An extract of a rich garden soil was prepared by shaking in a 

 stoppered bottle one part of soil with five parts of distilled water, 

 and, after the coarser particles had subsided, twenty-five c. c. of the 

 liquid was applied to each of pots 2 and 3. A chemical analysis 

 of this soil-extract showed that the amount of plant food con- 

 tained in it was so small that it could be safely neglected as an 

 element of plant growth, and that its effect must be attributed 

 solely to the soil microbes it contained. 



