The organic matter of the soil has engaged the attention of soil investigators ever since the latter part of the eighteenth century. The volume of literature on the subject is enormous, and perhaps in a greater degree than in any other branch of soil work we find here examples of the tendency to draw definite conclusions from inconclusive evidence. This is not surprising when it is remembered that perhaps the most important part of the soil organic matter, the humic matter, is a high molecular organic colloid, the investigation of which takes us at once into the most difficult fields of organic and of physical chemistry—fields in which the worker often needs to use special methods of experimentation which have become available only during recent years. Moreover, the study of humic matter is rendered particularly difficult, since it is a secondary product derived from the complex organic constituents of plant residues, themselves of unknown or little-known constitution, and since it is not known from which of these plant constituents it is derived.

The alkali extraction of the organic matter from the soils of certain plots of the classical permanent experiments on Barnfield and Broadbalk at Rothamsted, receiving respectively organic manure, artificial fertilisers and no manurial treatment, has been studied, together with a colorimetric examination of the extracts for the comparison of the content of humic matter.

It is shown that, in spite of the different cultural and manurial treatments which the different plots have received, there is a marked similarity in the properties of the organic matter of these soils, with regard to its behaviour on extraction with cold and hot dilute caustic soda, and the colour intensity of the organic matter in the extracts.

The methods used for the determination of organic carbon in the soils and their extracts are noted in an Appendix.

The results discussed in this paper are further considered, along with those in the two next following Parts (III and IV), in Part V of this series, in their bearings on the origin of the humic matter of the soil.

Experiments are described in which the decomposition of various plant materials and of purified preparations of plant constituents, during rotting under neutral aerobic conditions in the presence of soil organisms, has been studied in relation to the formation of humic matter.

The formation of humic matter was more closely related to the change in the lignin content than to the change in content of the other groups of plant constituents estimated.

1. The fungicidal properties of solutions containing sulphur in polysulphide form, viz. sodium, potassium, calcium and barium polysulphides, were tested by application to the conidial stage of Sphaerotheca Humuli growing upon young hop leaves in the greenhouse.

2. Four samples of sodium polysulphide solutions, two prepared in the laboratory and two commercial products, were examined. When applied with 1 per cent. soft soap as the spreader, solutions containing 0·5 per cent. polysulphide sulphur proved fungicidal. With 0·5 per cent. gelatine as the spreader, fungicidal strength did not begin to be attained until the concentration of polysulphide sulphur reached 0·11 per cent. The results suggest that the nature of the spreader may influence the fungicidal properties of polysulphide sulphur.