SULPHUR and its Effect on SOIL ACIDITY

New Zealand Journal of Agriculture, Volume 101, Issue 3, 15 September 1960, Page 283

SULPHUR and its Effect on SOIL ACIDITY

By

A. W. HUDSON,

Professor of Field Husbandry, Massey Agricultural College

RESULTS from the use of sulphur applied to grasslandsin parts of the South Island in particular— recent years have been most impressive. Lobb (I) and Lobb and Bennetts (2) recently summarised the results of field experiments in which sulphur and a number of trace elements had been applied. These articles are very informative and include recommendations for the use of sulphur ano the trace elements concerned.

Though results from field experiments and the forms in which sulphur is used for agricultural purposes have been widely publicised, only brief reference has been made to the effect of uncombined sulphur on soil acidity. At no time has a statement been made indicating the extent of such effect.

THE main facts bearing on the point and stated simply are: Soils may be alkaline or acid. With a few exceptions New Zealand soils are acid in varying degrees which can be described as slightly acid, moderately acid, acid, and very acid. In their natural state most of our soils are in the moderately acid to acid range. The majority of plants grow best in a slightly acid soil, one important reason being that the availability of most nutrients present in the soil is at a satisfactory level in such a soil. Extremes of acidity or alkalinity may cause some elements to be soluble in toxic amounts while others may become so insoluble that plant growth is restricted. Since a large proportion of New Zealand soils are bordering on the “acid” range which, normally, is also Heading photograph by Whangarei Candid Photos.

a harmful degree of acidity, there is an element of danger in applying substances which increase acidity. A small increase in the acidity of a soil which is only slightly acid is not likely to influence its productivity appreciably. However, the same increase in a soil which is nearer the borderline of harmful acidity may bring undesirable consequences. Because sulphur can increase acidity markedly and is being used on soils which are already acid, a guide to proper precautions seems desirable. Forms Commonly Applied Though frequent reference has been made to the two common forms in which sulphur is applied, it is desirable to discuss these briefly. (For further details the reader is referred to an article by Lobb (1).) Gypsum The first commonly used sulphurcontaining substance, gypsum, is

present in superphosphate to the extent of 50 to 60 per cent of the total constituents. It is one of the products of the chemical changes which take place during the manufacture of superphosphate from phosphate rock and sulphuric acid. Pure gypsum contains 18.6 per cent of sulphur combined with calcium as a sulphate. Users of superphosphate therefore apply between 9 and 11 lb of sulphur in each 100 lb of superphosphate. Gypsum also occurs as natural deposits, the material from which is ground for application to the land. It is used in the manufacture of cement and plaster of paris also. Lobb has stated that the effect of gypsum on plant growth is more rapid than that of sulphur. Its important characteristic so far as this article is concerned is that it does not alter the intensity of soil acidity. Sulphur The second commonly used form of sulphur is the very fine flowers of sulphur or ordinary ground sulphur, which is considerably coarser and slower in effect than flowers of sulphur. Both are almost pure sulphur. Unlike gypsum, sulphur increases soil acidity, owing to the fact that it is oxidised by a variety of soil organisms and converted ultimately into sulphuric acid. The process is very gradual and the amount present at any time very small, but inevitably the acid displaces calcium from the soil in such a way that the latter is washed out in time. Under these circumstances soil acidity increases, calcium being the important element in the various forms of lime which are added to the soil to reduce acidity. Liming to Counteract Acidity The extent to which soil acidity will increase when a given amount of calcium is lost will vary according to the type of soil. What the farmer should know is how much lime is required to replace the lost calcium. Since the term “lime” is used to refer to calcium carbonate (carbonate of lime), to ground limestone, and to burnt or slaked lime, it is necessary

to be more specific. In New Zealand lime is applied to the soil in the form of ground limestone almost without exception. Good limestones may contain between 85 and 95 per cent of calcium carbonate. The important practical question is: How much ground limestone of good grade is required to correct the acidifying effect of, say, 100 lb of sulphur? While the answer will be influenced by a number of factors such as the purity and fineness of both the sulphur and the limestone, it is possible to make an estimate which will be a useful guide. A hundred pounds of pure sulphur can cause the loss of calcium which would be contained in 312 lb of pure calcium carbonate; 312 lb of pure calcium carbonate would be represented by between about 330 and 400 lb of a good grade of limestone. Since a single application of sulphur at 100 lb per acre would be unusually heavy, it is necessary to consider the problem in relation to what a farmer may use at each application and how frequently applications are likely to be made. Lobb (1) says: “A reasonable application of sulphur appears to be 30 to 40 lb per acre”. Of this about 10 lb would be supplied in the gypsum contained in 100 lb of superphosphate and the balance as added sulphur. Lobb also states: “For initial land development, rates of 20 to 60 lb of sulphur per acre appear necessary at present. For some areas a much higher rate may be needed”. Later in his article he draws attention to the very small increase in acidity likely to result from the use of recommended applications of sulphur-containing fertilisers and adds: “This may, or may not, indicate that lime will be needed in the future”. Use of sulphur must cause the loss of calcium and a consequent increase in acidity with its continued application. The small extra expense of adding sufficient ground limestone to make good the loss due to the sulphur must surely be a wise insurance against a too-acid soil at a cost which will not detract appreciably from the outstanding benefits from sulphur. In effect it means that an application of 20, 30, or 40 lb of pure sulphur will necessitate the addition of 80, 120, or 160 lb respectively of ground limestone on the basis of 400 lb of ground limestone per 100 lb of sulphur. Whether the limestone is added to the fertiliser mixture containing the sulphur or applied once in two or three years is likely to be of little consequence. Adding to the fertiliser mixture would obviate the lime being overlooked.

No Danger on Some Soils It must be emphasised that not all soils are in danger of being made harmfully acid quickly. Some because of the nature of their chemically active constituents are able to resist change even though they cannot prevent it entirely. Such soils will not be affected appreciably until a large number of small applications of sulphur—or any other treatment which may alter the intensity of acidityhas been used. Alternatively, some soils will have been limed recently or may be dressed fairly regularly. Others are far enough removed from the harmfully acid level to render special precautions unnecessary immediately. Officers of the Department of Agriculture Farm Advisory Division will be able to advise farmers whether or not steps should be taken to counter any possible acidifying effects of sulphur. However, except with a few soils having a surplus of lime in them, the best long-term policy will be to add the prescribed amounts of lime with the sulphur. The writer has no desire to create doubts in the minds of farmers about the wisdom of using sulphur on soils which respond to it, but seeks rather to make them aware of possible undesirable effects that may sometimes arise but which may be avoided with simple precautions. At least one fertiliser (sulphate of ammonia) gained unwarranted ill repute in New Zealand in the early 1930 s through the effects of its use on soils on the borderline of being harmfully acid. It is hoped that what has been said will provide a more precise answer than has been given hitherto to the question of what quantity of lime is required to correct the acidifying effect of sulphur. It may at least save the application of tons where hundredweights will suffice and insure against that small and harmful increase in acidity of soils which are already too close to the danger line. Acknowledgments The original draft of this article was perused by J. W. Woodcock, Director, P. B. Lynch, Senior Principal Scientific Officer, and W. R. Lobb, Superintendent, Winchmore Irrigation Research Station, all of the Farm Advisory Division of the Department of Agriculture. I am grateful to them for helpful criticism. The responsibility for what has been said must remain mine, however. References (1) Lobb, W. R., “Fertility Problems of Marginal Areas of the South Island”, “The New Zealand Journal of Agriculture”, September 1959, p. 207. (2) Lobb, W. R., and Bennetts, R. L., “Fertiliser Responses on Plains Soils of Canterbury”, "The New Zealand Journal of Agriculture”, October 1959, p. 317.

Ragwort Control

KIOT until the advent of hormone weedi--1 ” cides a decade ago did farmers have a really worth-while weapon to attack ragwort, but nowadays it is comparatively simple and inexpensive virtually to eradicate this dangerous weed wherever suitable preparations of 2,4-D can be used. ECENT large-scale demonstrations with various hormone weedkillers on hundreds of acres of badly infested pumice country have clearly shown that ragwort is best attacked with the ester of 2,4-D. This material, applied as a spray during late AugustSeptember at 1 lb acid equivalent (about 2| pints) per acre in 20 to 25 gallons of water through power sprayers using F. 32 jets at 30 lb per square inch pressure, has consistently accounted for 90 per cent and more of seedling and root-growth ragwort by late spring. Similar follow-up treatment of older ragwort in the rosette stage, generally about mid November, dealt effectively with these more mature plants. Thereafter control mainly centres around treating the odd plant missed in the first treatments or which may have since developed from seed. An essential requirement in the successful control of ragwort with the hormone weedkiller recommended is complete leaf coverage. This is fairly easy to accomplish at the time of first application (August-September), when pasture growth is short, but the second treatment in November is sometimes made more difficult by excess pasturage surrounding ragwort plants. However, this can be overcome by reasonably close grazing with cattle, which leaves the ragwort standing well exposed for spraying. In no circumstances should the recommended amount of hormone weedkiller be greatly exceeded, as not only could some clover damage result, but often the final kill of ragwort is not as good. A slow kill of top growth more efficiently kills the root system than does a quick knock-down resulting from excessively strong mixtures of hormone weedkiller.

—C. R. TAYLOR,

Instructor in Agriculture, Department of Agriculture, Rotorua