152 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Nitrogen Fixation, Influence of Plant Residues on. By H. B. Hutchinson 



{Jour. Agr. Sci. vol. ix. Part i : Aug. 191 8). — In the experiments on which this 

 paper is based the author confirms and extends the conclusions arrived at by a 

 number of workers in the last twenty years on the processes involved when various 

 carbohydrates decompose in the soil. The work of Hall showed that land at 

 Rothamsted allowed to revert to prairie conditions showed considerable accumu- 

 lations of nitrogen. This accumulation might be due to a variety of causes, viz. 

 (1) leguminous plants in the herbage, (2) absorption of ammonia or nitric 

 acid from the air, (3) transference of nitrates by capillary uplift from the 

 permanent subsoil water, (4) activity of nitrogen-fixing organisms, particularly 

 Clostridium Pasteurianum and Azotobacter chroococcus, which are widely dis- 

 tributed in the soil and assimilate nitrogen energetically under suitable con- 

 ditions of aeration, of temperature, and of food supply. As these organisms 

 are not green they depend for their energy on the decomposition of carbohydrates 

 into simpler compounds. Such carbohydrates may be sugar and starch, or may 

 be derived from straw, green manures of all kinds, and so on. 



Koch and his collaborators, studying the action of sugar, found that doses 

 of the carbohydrate added to soil produced a proportionate increase of nitrogen 

 content in the soil up to 2 per cent, of the weight of soil treated. Beyond this 

 the rate was diminished, the reduction being probably due to concentration 

 of sugar and of available nitrogen compounds in the soil. In Mauritius, in 

 Java, and in the Leeward Islands molasses has long been used with good results 

 as a manure for the sugar cane, and it has been experimentally determined 

 that those soils, so manured, had a higher nitrogen content. On the other 

 hand, the application of saccharine matter to the soils of British Guiana and 

 elsewhere has not always promoted fertility. 



The author's experiments throw light on this discrepancy. He applied 

 sugar, at the rate of one ton to the acre, each year from 1906 to 191 1 to barley 

 ground. The application was made at Rothamsted on land which had been 

 continuously manured with complete mineral fertilizers (but not with nitrogen) 

 for fifty years. When the sugar was applied in the early autumn the yields of 

 straw and of grain showed increases of 20 to 50 per cent, as compared with the 

 controls ; but when applied in January, February, or March the yield was de- 

 creased. The essential importance of phosphates as an adjunct to carbohydrate 

 manuring was also confirmed. Pot experiments were made with barley, using 

 sugar, starch, or finely ground hay as a source of carbohydrate, and in these 

 experiments it was found that the carbohydrate caused a large increase in the 

 number of soil bacteria followed by a decrease (not permanent, however) in 

 nitrate nitrogen. Now this diminution in nitrate might conceivably be due 

 to ordinary denitrification (i.e. loss of nitrogen, in the gaseous form, to the 

 atmosphere), but it is much more likely to be due to increase of protein, in the 

 form of the substance of the bacteria, for it is known that many micro-organisms 

 can build up their own proteins from nitrate. Ultimately, of course, as the 

 bacteria die the protein reverts to nitrate. 



The interpretation of the results, then, appears to be that when sugar, starch, 

 straw, or green manures decay in the soil two sets of changes take place, one 

 set being favourable to nitrogen formation and fertility, the other detrimental. 

 Which of these two sets of changes predominates depends on the kind and 

 quantity of the material supplied, the temperature, the interval which elapses 

 before the crop is introduced, and the presence of specific organisms. Broadly 

 speaking, if the crop is introduced immediately after the incorporation of the 

 carbohydrate in the soil, and especially if the temperature remains low, a reduced 

 yield may be expected. The practical conclusion tc be drawn is that straw, 

 green manure, leaf-mould, turf, and so on should be applied in autumn rather 

 than in spring, and in any case well in advance of planting and further cnat when 

 large applications of such are made there should also be an application of 

 phosphate.—/. E. W. E. H. 



Odontoglossum chiriguense Reichb. By R. A. Rolfe (Bot. Mag. t. 8725 ; 

 September 19 17) .—Native of Central America. This plant thrives well in a 

 house where Miltonias are grown, the ordinary Odontoglossum house being 

 too cold in winter. Its nearest ally is O. coronarium, but is easily distinguished 

 by its larger and more undulate flowers. Flowers yellow conspicuously blotched 

 with brown. — L. C. E. 



Odontoglossum platychellum Weathers. By R. A. Rolfe (Bot. Mag. t. 

 8718 ; July 1917). — Native of Guatemala. A species which will grow well under 

 conditions suitable for O, crispum. Flowers showy, rose coloured, lip blotched 

 with carmine. — L, C. E. 



