Accounts have already been published by this department of investigations made as to the origin, nature and properties of the glacial drift soils of the north-east of Scotland; in particular, the soil as found at the Experimental Farm of Craibstone, near Aberdeen, has been studied by means of mechanical analysis and the determination of the chemical composition of the fractions resulting therefrom (4). The results obtained have brought out some characteristic features of the Craibstone soil, and have indicated that important differences may exist between a glacial drift soil, derived mainly from rocks of granitic nature, and such a soil as that of Rothamsted, arising from the “Clay with flints” formation.

1. The usual shaking methods have been examined and found unsatisfactory.

2. A method has been evolved for the estimation of the number of soil organisms by a direct count, and the numbers found to be very much higher than any previously reported.

3. A method for the disintegration and dispersion of the soil particles and bacteria has been devised, by means of which plate counts were made which were comparable with direct counts.

4. The dispersive actions of shaking and of vibrating have been compared. The former was found to give results which depended on the moisture content of the soil, whereas the trembling motion gave results in which the effect due to the moisture content was not apparent.

5. High counts made by previous observers can usually be correlated with one or more of the following factors:

(i) Greater dispersion of the soil as a result of the action of frost, etc.

(ii) An alkaline reaction, e.g. presence of ammonia, magnesia, etc.

(iii) Presence of a protective colloid, e.g. the effect of the addition of large quantities of straw, or traces of gelatine.

(iv) Absence of a flocculating agent, especially calcium compounds.

In conclusion the author wishes to express his gratitude to Dr G. S. Graham-Smith, F.R.S. at whose suggestion the investigation was started and under whose guidance and direction the bacteriological work was carried out in the Pathological Laboratory, Medical Schools, Cambridge; to Professor T. B. Wood, C.B.E., M.A., F.I.C., F.R.S., for facilities for carrying out the physico-chemical investigation at the School of Agriculture; to Messrs L. F. Newman, M.A., F.I.C., W. B. Hardy, M.A., F.R.S., W. H. Harvey, M.D., E. K. Rideal, M.B.E., M.A., G. Udny Yule, M.A., F.R.S., F. W. Foreman, M.A., F.I.C., and to the late Professor Sir German Sims Woodhead for their kindly advice and encouragement; to Messrs E. G. Staples, B.A., W. M. Davies, B.A., B.Sc, A.I.C. and A. W. R. Joachim, B.Sc. for assistance in the execution of the physico-chemical experiment; and to Mr W. A. Mitchell of the Medical School for his assistance in fitting up apparatus.

(1) The moisture equivalent of a normal soil diminishes as the weight of soil taken for the determination increases.

(2) Some soils are particularly impermeable to water in thick layers: in these cases the moisture equivalent increases with the weight of soil taken and may become very large owing to waterlogging. A soil may, however, have a very high moisture equivalent without showing waterlogging.

(3) Dilute solutions of flocculating salts such as calcium sulphate, or ammonium nitrate or sulphate, reduce the moisture equivalent, and sodium carbonate increases it.

(4) The effect of sodium carbonate is complicated: with gradually increasing concentration, the moisture equivalent first diminishes and then increases to a maximum, after which there is further diminution.

(5) The soil samples which easily showed waterlogging in the Briggs-McLane apparatus had a higher concentration of hydroxyl-ions (pH) than those which did not.

(6) Whilst the colloidal content of a clay seems related to its moisture equivalent, no such connection appears to exist for substances such as kaolin or aluminium hydroxide.

So many papers on the use of the phenol disulphonic acid method for estimating nitrates in soils have been published in this country and in America in the last few years, that some apology seems needed for adding yet another note to the literature of the subject. The method possesses great advantages as regards rapidity and convenience over the methods involving distillation and yet in the writers' experience, in spite of the numerous studies already made, it has failed to give entirely satisfactory results; in particular when small quantities of nitrate have to be estimated in soils containing much organic matter, the strong sulphuric acid in the reagent chars the organic matter and makes it exceedingly difficult to match exactly the tints from the soil extracts with those given by the standard nitrate solutions.

Several methods for estimating nitrates by reduction to ammonia were examined. The error involved in the use of acid reducing agents in presence of amino groups was emphasised.

A method based on the use of Devarda's alloy and magnesia is recommended.

In the estimation of nitrates in plant extracts, etc., by this method an equal volume of rectified spirit is first added to the solution to precipitate colloidal matter. The precipitate is filtered off and an aliquot part of the filtrate containing the equivalent in nitrate nitrogen of about 0.1 gm. of KNO3 is distilled with steam for 45 minutes with 1 gm. of Devarda's alloy and ½ gm. of freshly ignited magnesia. A blank determination is essential, in which the same volume of filtrate is distilled for the same length of time with ½ gm. of magnesia.

In Kjeldahl estimations in presence of nitrates, where the separate estimation of nitrates is not required, 2 c.c. of 25 per cent. NaOH may be substituted for the magnesia. Relatively this is very much less soda than Devarda recommends. The mixture in this case may be distilled directly for half-an-hour. The nitrates are thus reduced and the ammonia produced is collected before beginning the digestion with sulphuric acid.

This work was carried out under the supervision of Mr F. W. Foreman to whom the author's best thanks are due for much valuable criticism and advice.

Of the two available methods for the determination of hydrogen-ion concentration, the electrometric is the more fundamental and satisfactory but it requires somewhat elaborate and expensive apparatus. The colorimetric method, on the other hand, though not lacking in difficulties if accurate results are wanted, requires no special apparatus and the actual procedure is simple. It is based on the electrometric method, and involves the use of buffer solutions of known hydrogen-ion concentration determined originally electrometrically.

(1) The variation of the total yields according to the month of calving is given, and causes for this variation are suggested from the mean lactation curves of cows calving in each month.

(2) A curve showing the effect of pregnancy on the lactation yield has been prepared and corrections are given for different lengths of service periods after calving—“100 days” being chosen as a standard, as that was found to be the average of all service periods obtained, and as it gives a calf at about the same time in the following year.

(3) The variation in yield as the cow gets older has been studied and corrections are given. The maximum yield appears to be reached at the fifth calf; it was found that service periods varied in a definite way with age.

(4) The effect of the dry period before calving on the subsequent yield is shown; “85 days” has been taken as a standard and corrections given accordingly; the standard was the mean of all dry periods obtained, but is probably above the average throughout the country.

(5) By applying these corrections, the variation of individual totals is reduced by approximately 20 per cent, and the number of lactations showing less than 5 per cent, variation is increased by 27·3 per cent.

(6) A brief review of other literature on the subject is given, and the results obtained in this paper are compared with those of other investigators.

The thanks of the authors are due to the members of the Penrith Milk Recording Society for the loan of their registers, which made this investigation possible, and to Mr Garnet, the Live Stock Officer of the district, and the Live Stock Branch, of the Ministry of Agriculture and Fisheries for the collection of the data; the authors are also greatly indebted to Mr G. Udny Yule, M.A., F.R.S., for much assistance in the purely statistical part of the work, and to Dr F. H. A. Marshall, F.R.S., for his advice during the course of the investigation.