30 RESULTS OF EXPERIMENTS AT ROTHAMSTED, 



surface-soils had become considerably richer. If, for the sake of 

 illustration, we assume that 300 Ibs. of nitrogen were removed per 

 acre in the crops, and that 150 Ibs. were accumulated in the surface- 

 soil, we have 450 Ibs. of nitrogen to account for, as gathered by the 

 crops within a period of little more than two years. 



It is clear that we have in the experimental results themselves no 

 conclusive evidence as to the source of so large an amount of nitrogen. 

 As the surface-soil became determinably richer, it is obvious that it 

 must have been derived either from above or below it from the 

 atmosphere, or from the sub-soil ; and, if from the sub-soil, the question 

 arises whether it was taken up as nitric acid, as ammonia, or as 

 organic nitrogen ? I shall have to adduce evidence bearing on these 

 points further on ; but it must be admitted that there is nothing in 

 the experimental results themselves, to show that so large an amount 

 of nitrogen could have been available as nitric acid. 



VARIOUS LEGUMINOUS PLANTS GROWN AFTER RED CLOVER. 



I have now to adduce another, and even much more striking 

 instance, of successful growth, and of great accumulation of nitrogen, 

 by plants of the Leguminous family, on soil where another plant of the 

 same family had failed, and where the surface-soil had become very 

 poor in nitrogen. 



The experiments were made on the plots where it had been 

 attempted to grow Red Clover year after year on the same land; 

 where, in fact, Clover had been sown 12 times in 30 years, and where, 

 in 8 out of the last 10 trials, the plant had died off in the winter and 

 spring succeeding the sowing of the seed ; in 4 cases without any crop 

 at all, and in the other 4 yielding very small cuttings. 



In 1878, the land was devoted to experiments with various 

 Leguminous plants, differently manured, having regard, however, to 

 the previous manurial history of the plots. 



The object was to ascertain whether, among a selection of plants 

 all belonging to the Leguminous family, but of different habits of 

 growth, and especially of different character and range of roots, some 

 could be grown successfully for a longer time, and would yield more 

 produce, containing more nitrogen, as well as other constituents, than 

 others ; all being supplied with the same descriptions and quantities of 

 manuring substances, applied to the surface-soil. Further, whether 

 the success in some cases, and the failure in others, would afford 

 additional evidence as to the source of the nitrogen of the Leguminosse 

 generally, and as to the causes of the failure of Red Clover when 

 grown too frequently on the same land. 



Accordingly, 14 different Leguminosw were selected, and sown in 

 1878. These included 8 species or varieties of Trifolium, 2 species of 

 Medicago, Melilotus leucantha, Lotus corniculatns, Vicia sativa, and 

 Lathyrus pratensis. Of these, 6 of the 8 Trifoliums have already 

 failed, and been replaced by other plants ; as also have the Medicago 



