98 Influence of Plant Residues on Nitrogen Fixation 



that although barley grown on this soil responds readily to an application 

 of phospha tic manures alone, it nevertheless does so to an equal or greater 

 extent when a simple nitrogenous dressing is given. It is possible there- 

 fore that the growth of the crop on 6. I. was not Kmited primarily by the 

 lack of phosphates, but b)' the absence of any appreciable fixation of 

 jiitrogen. This is perfectly compatible with other work, and amongst 

 others Koch has shown the great importance of an adequate supply of 

 available phosphates for the assimilation of nitrogen by free living 

 organisms, as is shown by the following table. 



Soil + 0- 1 % Soil + 0-6 % Soil + 1-0 % 

 Soil alone superphosphate basic slag basic slag 



After 4 weeks 11-2 mgrm. 19-2 mgrm. 16-7 mgrm. 16-9 mgrm. 



After 7 weeks 25-5 \, 33-8 „ 27-0 „ 28-2 „ 



Finally, it should be recognised that the soil of plot 6. I., which failed 

 to give any definite return on treatment with carbohydrates, had been 

 persistently starved for more than fifty years, and it must not be con- 

 cluded that equally unfavourable results would be obtained with any 

 normal soil. 



The experiments carried out by Boname, to which reference has 

 already been made, indicate the expediency of ensuring an adequate 

 supply of phosphates if an economic utilisation of the carbonaceous 

 material is to be expected. The same also applies of course to basic 

 compounds where the soil is naturally deficient in these constituents (29). 



B. The Utilisation of Plant Residiws for Nitrogen Fixation. 



Laboratory Experiments. The organic materials returned to field 

 soils are, in the main, the leaves of root crops, the stubble of cereal crops, 

 green manures, and the constituents of farmyard manure. In nature, a 

 similar position is taken by the remains of annual plants and the leaves 

 of perennials. A number of preliminary laboratory experiments were 

 therefore carried out to ascertain whether these substances could be used 

 for the assimilation of nitrogen. 



The chief disadvantages inherent in such laboratory work are that, 

 in order to limit experimental error in the determination of nitrogen, 

 the amount of soil must be kept as low as practicable, whilst in order to 

 get appreciable quantities of nitrogen assimilated, the source of energy 

 must be present in relatively high proportions. Hence in the case of 

 soluble compounds the concentration of the solution soon passes the 

 optimum for the process; in the case of plant residues, the decomposition 

 is liable to be strictly localised and the acids initially set up by bacterial 



