Scientific Agriculture. 372 [June 1907 



RECENT PROGRESS IN THE PRACTICE OP GREEN MANURING. 



Though " green manuring " has been practised from very early times it is 

 only comparatively recently that advances in chemical, agricultural and bacter- 

 iological knowledge have afforded an explanation of how the beneficial results long 

 known to accrue from " green manuring " are brought about. 



The following are the principal ways in which green manures may improve 

 the soils to which they are applied :— 



(1) The addition of vegetable organic matter to soils deficient in this 

 constituent. 



(2) The improvement of the mechanical condition of the soil by the action 

 of the roots of the plants and of the gases evolved when the vegetable matter 

 decomposes in the soil. 



(3) The vegetable matter in decomposing gives rise to acids, which act as 

 solvents of the soil constituents, and thus render more material available for plant 

 nutrition. 



(4) The fixation of atmospheric nitrogen (i.e. its conversion into nitrogenous 

 compounds) by leguminous plants {e.g. clover, alfalfa and beans), a change which 

 cannot be as cheaply effected by any chemical or electro-chemical process yet 

 devised. 



Of these actions the last is probably the most important. Great improve, 

 ments haVe been made recently, however, in the production of nitric acid by 

 electrical means, and it is perhaps possible that in the future atmospheric nitrogen 

 may be " fixed " by this means even more cheaply than by leguminous crops (compare 

 Bulletin of the Imperial Institute, 1906, Vol. IV. p. 69). 



Many theories as to the actual mode of fixation of nitrogen by leguminous 

 plants have been advanced, but until 1886 the true explanation was not known. In 

 that year Hellriegel and Wilfarth found that while most plants, when grown in sand 

 free from nitrogen, ceased to flourish when the reserve nitrogen contained in the 

 plant itself had been absorbed, leguminous, plants sometimes overcame this " nitrogen 

 starvation " and grew well. In cases where growth did occur, nodules or swellings 

 were always found on the roots. It was further found that leguminous plants grown 

 in sterile sand soon ceased to grow well, but that if a little water extract of some 

 ordinary cultivated soil was added the plants recovered, formed nodules on the 

 roots and also became capable of absorbing nitrogen. These nodules upon examina- 

 tion were found to be full of organisms which could only have been derived from the 

 water extract of the cultivated soil which was added. From these results it is 

 obvious that the assimilation of free nitrogen by leguminous plants takes place 

 after the formation of root nodules which are caused by some organism present in 

 cultivated soil. 



Different species of organisms were at first thought to be associated with 

 different leguminous plants, but it has since been shown that the different forms 

 described are all physiological modifications of one organism to which the name 

 Pseudomonas radicicola, Beyerinck, has been asigned, and are produced by variations 

 in the conditions and environment. 



Various theories have been advanced as to the actual way in which the 

 organism cause leguminous crops to take up nitrogen. One of these theories was 

 that the bacteria fixed the nitrogen in the soil, from which the plant then assimilated 

 the nitrogenous matter through its roots. Another theory held that the bacteria 

 acted as a stimulus to the plant and caused the plant itself to assimilate the nitrogen 



