Basic slags and rock phosphates

Preface

By E. J. Russell, D.Sc, F.R.S.

Director of the Rothamsted Experimental Station

JLhe utilisation of basic slag in agriculture is an excellent example of the help that modern science affords to the working farmer. A waste product of steel-making, resulting from a modification by Thomas and Gilchrist in 1878 of the Bessemer process, it was at first considered worthless and thrown on the refuse heap. The late Prof. John Wrightson made field experiments in 1884 and 1885 at Eerryhill and at Downton, and showed that the material had noticeable fertihsing value: this discovery was confirmed and developed by the systematic pot experiments of Paul Wagner at Darmstadt, which began in 1885 and continued for several years afterwards. Extensive field tests were made during the 'nineties by Sir (then Professor) J. J. Dobbie and Prof. D. A. Gilchrist at Bangor, and by Prof. W. Somerville and later on by Sir T. H. Middleton at Cockle Park, with the result that a considerable body of information was accumulated as to the effectiveness of basic slag under the various conditions obtaining in practice. This has already been summarised by Prof. Somerville in the Journal of the Board of Agriculture for 1911, 1918, etc.

Some ten years ago, however, it became evident that the basic open hearth process would be a serious competitor with the Bessemer process, and chemical examination showed that the slag, though correctly described as 'basic slag,' was altogether different from the material with which the agriculturist had become famihar. The upheaval caused by war and post-war conditions gave an enormous impetus to the open hearth process, and it is now extending to so many works that before long the older process will probably cease to be operated.

This result is of course distinctly awkward for the agriculturist who sees a valuable f ertihser disappearing, and being replaced by one which is more costly and at first sight seems to be nothing Hke as good.

Dr Scott Robertson has the great advantage of being in close touch with the steel-making industry, and at the same time of being able to carry out agricultural experiments. At the outset of his investigations he made a careful selection of the types of slag likely to viii PREFACE

be produced in the future and in 1915 laid out field experiments in Essex to compare these newer types with the old familiar Thomas or Bessemer slag. These experiments were continued for five years and they gave a mass of data so important in character as to deserve widespread circulation among farmers and agricultural experts. Separate publication was therefore advised and the Syndics agreed to its inclusion in the Cambridge Agricultural Monographs.

Fortunately Dr Scott Robertson had included also some typical mineral phosphates in the trials so that valuable information has been obtained in regard to a second problem which, while not pressing in 1915, has grown in importance since and is likely to be serious in the future.

This second problem arises as a direct consequence of the circumstance that basic slag is a by-product only, and not a primary object of manufacture. From the steel-makers' point of view it is relatively unimportant. Some 4 cwts. only are obtained for each ton of basic steel produced, and while the ton of steel has been worth anything from £27 in 1920 to £10 in 1921, the 4 cwts. of slag is worth less than 5s. to the steel-makers and only about 155. even after the slag grinder has graded, ground and bagged it. The steel-maker cannot afford to alter his processes in any way that would lengthen them or make them more costly or hazardous. The agriculturist must therefore take the slag as he finds it and cannot expect the consideration that would be shown him by the makers, say, of superphosphate, which is a primary object of manufacture and not a by-product. The practical result is that the composition of basic slag is determined by the conditions under which the steel-maker is working, and the total amount producible is regulated by the demand for steel ; neither of which factors is in any way within the control of the agriculturist or influenced to any appreciable extent by his demands.

It is important that this distinction between basic slag and other fertilisers should be recognised. If the farmers of this country demanded double their present supphes of superphosphate, of nitrates, of sulphate of ammonia or of potassic fertiHsers, the manufacturers could provide the additional material : if, however, basic slag were desired over and above the quantity determined by the demand for steel it could not be suppHed except perhaps by importation.

The position thus created is being explored by the Permanent Committee set up by the Ministry of Agriculture to advise on basic slag. On the agricultural side there is evidence that the farmers of the United Kingdom might with advantage to themselves and the com-

PREFACE ix

munity use no less than 890,000 tons per annum, equivalent to 33,820,000 units of tricalcicphosphate. On the other hand the 1920 output of British steel yielded about 560,000 tons of slag of 15| per cent, or higher content of phosphate, equivalent to 13,400,000 units of tricalcicphosphate. There is therefore a considerable gap between the farmers' potential demand and the visible supply. The difficult problems associated therewith are being fully and sympathetically studied by agriculturists and steel-making experts and no doubt various solutions will be devised. One obvious possibihty is to use ground mineral phosphates to stiffen out the supplies, and here Dr Robertson's experiments will prove helpful.

Dr Robertson has not confined himself to the practical demonstration of increased yields: he has gone further and endeavoured to ascertain why the increases have been obtained, thus giving the monograph a scientific as well as an empirical interest. He examines the change in herbage and he shows that the physical properties of the soil and the bacterial actions in the soil are much influenced by the phosphate in the slag, thus throwing important hght on the view now commonly held by experts that poor grassland should not be ploughed out till after it has been improved by slag.

The monograph contains a store of information about the new slags and is a model of thorough and systematic investigation. I have personally inspected the plots on several occasions and have seen much of the experimental work. It deserves close study by all who are interested.

E. J. R.

January, 1922.

Author's Preface

The main purpose of this book is to put on record the results of the field experiments with rock phosphates and open hearth basic slags conducted in Essex during the period 1915-20.

The field trials have been confined to grass land, and the results have been measured by increases in the weight of the hay crop, and by the improvement in the quality of the crop, as determined by botanical analyses. This plan has been adopted for two reasons: first, because it is on grass that the primary and secondary actions of phosphates are most apparent, and most readily measured ; secondly, because on permanent grass, in Essex at any rate, the issue is not comphcated by previous appHcations of artificial manures, and it is therefore easier to follow out the experiment year by year than under arable conditions.

The objection may be raised that increased weights of hay do not give a true test of the improvement which has taken place, and that such a test can only be obtained through the medium of the animal. While there is much to be said in favour of this contention, it may be safely assumed that, when botanical analysis shows the quahty of the herbage is similar, the increased weights of hay bear a definite relationship to the five-weight gains, and do afford a satisfactory method of comparing the efficiency of the various phosphates. Moreover, it must be remembered that hay is an important crop, and in Essex, as elsewhere, it is the prevalent custom to graze and mow the permanent grass in alternate years.

The Essex results with ground rock phosphates indicate that there are soil conditions under which these types of phosphates may be expected to give as good and as quick results as the more soluble types of phosphatic fertilisers. It is equally clear, however, that under other conditions the advantage is decidedly in favour of the more soluble types. Experiments in progress in the North of Ireland on the turnip crop strikingly bear out this conclusion, and it would seem probable that an explanation of the different results secured elsewhere might be obtained by means of an examination of soil and chmatic conditions.

I have to acknowledge my indebtedness to Dr E. J. Russell, F.R.S., for the great interest he has taken in the work here described. From xii AUTHOR'S PREFACE

the discussions which took place during his annual visits to the various experimental centres many helpful suggestions came.

For the method of estimating nitrates I am indebted to Mr D. J. Matthews, till recently of the Rothamsted Experimental Station, and, for the nitrate determinations, to my former colleague, Mr R. G. Baskett, For the bacterial counts in Tables XLV and XL VI I am indebted to Mr James Bryce, B.Sc; for the mechanical analyses in Table XXXIX to my former colleague Capt. H, H. Nicholson, M.A. (Cantab.) ; for the rainfall data to Mr Carle Salter, the Superintendent of the British Rainfall organization, and for the illustrations of Nauru and Ocean Islands to Mr A. F. Elhs, the Commissioner for New Zealand on the Board of the British Phosphate Commissioners,

I should also Uke to record my thanks to Messrs B. Smith, N. F. Miles, T. Wood, C. L. Petheybridge and A. Freshwater, on whose farms the more important of the field trials were laid down, for the care they have taken of the plots and for the ready help they have given through the whole period of the trials.

Finally, I have to express my keen appreciation of the kindness of the Agricultural Education Committee of the Essex County Council, who provided unique f acihties for the work and gave me a free hand in the carrying of it out. To the generosity of the members of this Committee is due, in no small measure, the opportunity of submitting this book to all who are interested in the progress of Agriculture.

G. S. R.

The Queen's University of Belfast, January, 1922.