106 



Green Manuring. 



[May, 



which those soluble substances are conveyed to the plant, and 

 without which this " plant food," however plentiful it may be 

 in the soil, cannot be obtained by the plant. A light soil is 

 given more ' ' body ' ' and rendered better capable of withstand- 

 ing drought, while a heavy soil is made more open and work- 

 able. This is not the occasion to discuss the exact mode of 

 action of humus in bringing about these effects, nor for that 

 matter are we in a position to do so with any degree of 

 certainty;* but from the practical point of view the important 

 thing is that these effects undoubtedly exist, and are of great 

 significance. 



Alternative Sources of Organic Matter. — It is therefore as a 

 source of humus that farmyard manure must be chiefly prized, 

 and in the face of a growing scarcity the agriculturist is faced 

 with the problem of finding an alternative source of organic 

 matter, that is to say, of keeping part of his soil in good heart 

 without the assistance of animals as manure-makers. What are 

 the possibilities of such alternative supplies? Apart from 

 purely local or undeveloped sources, such as seaweed, which is 

 used in maritime districts like the Channel Islands and many 

 coastal districts of Scotland,! or such as activated sewage 

 sludge, + there are at least three possible sources of general 

 applicability. These are (1) The ploughing of raw straw into 

 the soil; (2) The use of artificial straw-manure made by the 

 process of Hutchinson and Richards as worked out at 

 Rothamsted;§ (3) The use of green manures. 



With regard to the first method, although the practice of 

 ploughing in raw straw in the autumn is being adopted in some 

 parts, notably on the heavy land in Essex, it is as yet of un- 

 proved value. One great danger of such a practice is that the 

 addition of a large bulk of non-nitrogenous, carbohydrate matter 

 to the soil, may cause a temporary locking up of nitrogen by 

 biological agencies in an insoluble form not available to the 

 plant; such an effect, if sufficiently transitory, might be all 

 to the good, as for example, in preventing loss of nitrates by 

 leaching during the winter months, but in other circumstances 

 much harm might result. I! The question needs careful in- 



* For a discussion of this aspect of the part played by humus in the soil, 

 see a paper by the writer in the " Transactions of the Faraday Society" 

 17, 272 (1922) (General Discussion on Physico-chemical Problems relating to 

 the Soil, held on 21st May, 1921). 



f See Ministry of Agriculture Leaflet No. 254. 

 % See Journ. Soc. Chem. Inch, 39, 177, 41, 62 T. 

 § See this Journal, 28, p. 898, (1921). 

 See Journ. Agr. Sci. 9. 92. 



