Much of the modern work on the physical properties of soil has been interpreted on a colloidal basis and more recently this hypothesis has been extended by the thermodynamical studies of Wilsdon and also by the investigations of two of us on an indirect measurement of the vapour pressure of capillary systems. There is evidence that the colloidal portion of the soil can be regarded as possessing a reticulate structure, possibly analogous to that shown by Zsigmondy to exist in silica gels. The pore space in soils is therefore an assemblage of voids and irregular capillaries ranging from ultramicroscopic dimensions in the colloidal portions to the macroscopic interstices between adjacent compound particles and the larger mineral fragments. Whereas in studies of evaporation and movement of water the total intersticial space is operative, the vapour pressure of soils at different moisture contents is very largely controlled by the minute pores associated with the colloidal portion and the larger voids have comparatively little influence. Vapour pressure measurements therefore afford a promising line of attack on the physical relations between the colloidal soil material and water, especially when the measurements are made on soils subjected to a variety of preliminary treatments, known to have a considerable effect on other physical properties. The effect of successive wetting and drying, heating and addition of salts are of especial interest in this connection.
1. The work described in the present series of papers was undertaken with the object of ascertaining the best conditions for the use of calcium cyanamide and of comparing it against older nitrogenous fertilisers. Earlier work has in part been repeated so as to ascertain whether the recent improvements in manufacture have overcome the difficulties encountered with the pre-War product.2. The decomposition of cyanamide to urea can be brought about by a number of minerals likely to occur in the coarser fractions of the soil. The decomposition of cyanamide by soil proceeds according to a logarithmic law such that the rate of disappearance is proportional to the concentration of cyanamide in the soil water.
SUMMARY. As susceptible varieties of potato are still widely cultivated and sporadic outbreaks of Wart Disease are a serious menace, it is imperative to find a method whereby the winter sporangia of Synchytrium endobioticum in contaminated soil may be killed. Previous studies and the unusual difficulties presented by the problem are discussed. Results of experiments extending over four years are recorded. During 1920–2 pot experiments were carried out to test various chemicals alone and in conjunction with steam. Steaming the soil proved effective. but offered little hope of being economically possible as a field treatment. The amount of disease was reduced by sulphur. calcium and potassium polysulphides. formaldehyde, dichlorcrrsol. chlordinitro‐benzene and nitrobenzene. Satisfactory infection was not obtained in pot experiments; this method was therefore abandoned in favour of field experiments. The incorporation of chemicals with the soil in the field was carried out with the Simar Rotary Tiller, great care being taken to ensure very thorough and even distribution. Results suggest that the efficiency of the treatment depends on this thoroughness of incorporation. During 1922 a selection of the chemicals tried in 1921 and others were tested. From these sulphur was selected in 1923 for more extensive study as being the most hopeful, because of its efficiency and cheapness. In 1924, a year of very heavy disease, it was proved at Ormskirk that when the dose of ground sulphur was increased through 1, 2, 3, 4, 5, 10 cwts. per acre the degree of infection was reduced in direct ratio from 73 per cent., the value for untreated soil, to 8 per cent. for an application of 10 cwts. per acre. Doses greater than the latter did not completely eradicate the disease; but there are reasons for thinking that the small amount of disease in certain of the plots was due to recontamination of those plots later in the season. When the results are represented in graphical form the straight line of nearest fit to the experimental values cuts the horizontal axis at a point representing 11.2 cwts. per acre of sulphur; and, in the absence of recontamination, this quantity of sulphur should be slightly more than the minimum necessary to free the Ormskirli soil of disease. On the heavy clay soil at Hatfield it was found necessary to use much heavier applications of sulphur (about 40 cwts. per acre) to ensure absolutely clean plots. Gasworks spent oxides, tried as an alternative source of sulphur, proved rather less effective than ground sulphur when equal quantities of sulphur were applied in each case. The result was probably due to the unsatisfactory state of division of our sample of spent oxides. Sulphur inoculated with Thiobacillus thiooxydans showed no increased efficiency over uninoculated sulphur on Ormskirk soils and appeared less effective than the latter on the Hatfield clay. The elimination of Wart Disease in the field by sulphur and sulphur compounds is not a simple function of the final soil reaction and it would appear that so...
IN the two preceding papers of this series Fisher 1 gave a general re'sume' of the early work on soil reaction, and outlined the principles underlying the colorimetric method of determining the hydrogen ion concentration of soil suspensions and extracts. Considerable difficulties in the use of indicators arise with turbid soil suspensions and it would appear that the method is primarily adapted for rapid and approximate determinations, rather than for precise measurements. Very useful results can be obtained by "spotting out" small quantities of soil and indicator solution, as in Spurway's 2 ingenious method of using paraffined paper strips instead of test tubes. The indicator method of measuring hydrogen ion concentration is, of course, a useful addition to other studies on the composition of the "soil solution," as obtained by percolation methods 3 , and such measurements probably afford the most useful method of following seasonal changes in soil reaction, though the time and labour involved in the preparation of the percolates are too great to render the method of general utility.The electrometric method of measuring hydrogen ion concentration is the most fundamental and has proved the most satisfactory on almost all types of solutions and materials. It is not proposed to review here the principles underlying this method, since these are fully discussed in Clark's well-known monograph 4 . A hydrogen electrode apparatus developed in these laboratories four years ago will be briefly described and 1 E.
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