242 



Green Manuring. 



[June, 



mentioned as used in Essex, namely, to fallow daring the dry 

 summer months, and sow a green manure in early autumn to 

 «ave the accumulated nitrates from leaching. 



It is not, however, certain that the nitrogen added to soil 

 by green manures is always as readily available to the follow- 

 ing crop as that of farmyard manure. This is a point upon 

 which further investigation is needed; the results of the 

 Woburn experiment already quoted illustrate this aspect. 

 Although the amount of nitrogen added to the soil by vetches 

 was found to be markedly superior to that added by mustard, 

 and although analysis of the soil showed that after vetches it 

 w r as indeed licher in nitrogen than after mustard, yet the wheat 

 after mustard was always a bigger crop than after vetches. 

 Evidently there is seme factor operating in the light land at 

 "Woburn to limit the availability of the nitrogen buried with 

 the green crop, a factor w T hich is apparently not operative in 

 Bothamsted soil. 



Although the nitrogen question is one which undoubtedly 

 bulks large in the value of farmyard manure, and of 

 green manures, it is not the indispensable fac'or in 

 either. There is no reason to suppose that the require- 

 ments of a crop for nitrogen, as for minerals, cannot be 

 adequately met by an enlightened use of artificials. As stated 

 before, it is as a source of organic matter — " humus " — that 

 farmyard manure must be chiefly prized, and it is similarly as 

 a source of humus and by their effects on the physical pro- 

 perties of the soil that green manures must stand or fall. 



(3) Effect on the Supply of Moisture and on the Physical 

 Properties of the Soil. — We do not know definitely whether, bulk 

 for bulk of dry matter, green manures are as efficient as farm- 

 yard manures as sources of humus, nor whether the humus 

 produced from both is of the same character. These are ques- 

 tions which can only be answered after much more work has 

 been done on the general question of humus formation and the 

 nature and properties of humus, and in the meantime we can 

 only assume that humus can be equally well derived from 

 either, and that once formed it will have the same effects in 

 both cases in improving the physical condition of the soil. It 

 is evident then that the difference between farmyard and green 

 manures will be due to the difference in their mode of prepara- 

 tion and application. The essential difference is of course that 

 the farmyard manure is made off the land, and is usually applied 

 only when decomposition is well in hand, whereas the green 



