The Importance Of Superphosphate

New Zealand Journal of Agriculture, Volume 62, Issue 2, 15 February 1941, Page 71

The Importance Of Superphosphate

By

I. L. ELLIOTT,

Supervisor of Fertiliser Supplies,

Wellington.

Improved Product There can be no doubt that most countries in the world are today obtaining a far better superphosphate than that produced even a few years ago. This is particularly so in New Zealand. In grade, in storage ability, in freedom from caking, and in ease of distribution, there is outstanding evidence of an improved material. In

lllllllllll Hilt 1111111 1111111 ■ 11111111111111111111111111111111111111111111111111111111111111111111111 111 1111111111 = In this article the author discusses the advantages E and disadvantages of superphosphate and the = methods that have been adopted in attempts to overcome these disadvantages. In view of the importance of superphosphate in ' New Zealand farming and the decision tn manufacture silica-su-her-hhns-hhate.

it is interesting to note that in a number of other countries there has been a trend away from the use of superphosphate on acid soils.

Methods of Combating Disadvantages On Acid Soils

r T~’HE present curtailment of supTHE present curtailment of supI plies of rock phosphate, temporarythough we hope it will be, is nevertheless of sufficient importance to the productivity of New Zealand to warrant some discussion on our main fertilisersuperphosphate some of the materials with which it is hoped that present supplies may be made to go further and at the same time maintain production. Although silico-super-phosphate is being turned out largely as a war measure, it is probable that some such development would in any case have come to this country. Judging by tendencies in other parts of the world and indications here, the use of this material could be of decided benefit to the farming community, particularly for topdressing many of the acid soils of New Zealand. Since the epoch-making work of Liebig and later of Lawes in 1842, when rock phosphate was first used for the manufacture of superphosphate, almost 100 years have passed. Essentially the same process initiated then is in use today in the four fertiliser. manufacturing companies in New Zealand. From the old method of hand mixing, the modern superphosphate industry has greatly progressed, mainly along the lines of improved industrial technique. Some of these improvements were concerned with the grinding and drying of the raw material, some with the manufacture of the sulphuric acid, and some with the chemical control necessary to produce high quality and uniform material.

spite of all these factors it is interesting to ask ourselves, “In what respects does superphosphate as a fertiliser suffer from disadvantages, and what appear to be the main directions in which fertiliser use is proceeding overseas?” As for the disadvantages from which superphosphate suffers, these can briefly be stated as variation in particle size, difficulty in obtaining

satisfactory distribution through machinery, caking, the rotting of the bag after any length of time in storage, and the facts that rock phosphates containing iron and alumina react to the detriment of the final product and that the water-soluble phosphoric acid tends to become • unavailable in acid-fixing soils. In the past it has been generally accepted that as the phosphoric acid of superphosphate exists in , the watersoluble form this must constitute a definite advantage over other forms of phosphatic fertiliser. It is safe to say that the time has now arrived when this statement must be critically reviewed. A Big Problem There are in many of the soils of New Zealand elements which cause the phosphoric acid or plant food of superphosphate to go into a form in which it is no longer available to the plant. To quote an overseas authority, “the quantity of phosphoric acid absorbed by the plant during the first year seldom exceeds 40 to 50 per cent, of the

total quantity used and may be as low as 15 to 25 per cent.” It is not an exaggeration to say that one of the greatest fertiliser problems of today is to increase the proportion of the phosphoric acid recovered by the plant. There are in general two approaches to this question. The first is a change in the physical form in which fertiliser is presented to the consumer. The second is by the method of converting the water-soluble acid into a form in which it can readily be taken up by plants and not converted into undesirable compounds by elements present in the soil. The first of these methods, which has commonly become known as “granulation,” has been . chiefly exploited in the United States, where the Oberphos process, in which decomposition is effected in an autoclave, produces a granulated superphosphate. The significance of granulation has not yet become fully known, but certain it is that a much-improved product has been developed. It is a definite advance so far as ease of spreading is concerned,

particularly where applications are made by hand. From the point of view of - the amount of plant food released and taken up by plants, there is considerable evidence to indicate that in this respect, too, it is superior to superphosphate. The reason advanced for this is that the soil in immediate contact with the particles becomes saturated with readily available plant foods which are only partially converted into a form which cannot be used by plants. This, then, is one of the approaches to the problem of increasing the proportion of phosphoric acid added to the soil and recovered by the plant. In New Zealand most of the superphosphate manufacturers are producing some type of granulated superphosphate, which may be purchased under various trade names. Making of Silico-Super The second method of approach to the problem, as previously indicated, is by effecting a chemical change in the

superphosphate itself. Notably in Europe and the United States the endeavour has been made to produce phosphate whose plant food content is readily available to plants by means of direct action 'on rock phosphate without the intermediate steps of superphosphate manufacture. Most of the methods have had for their central principle the application of heat in conjunction with silicic acid and water vapour. So far results on a commercial scale have not been really encouraging, but there is hope that technical difficulties may be overcome in the future. After this very brief review of development overseas away from the use of ordinary superphosphate on acid soils, it is both interesting and profitable to review developments in our own country. Even a glance at the figures of fertiliser applied in New Zealand shows that pride of place is given to superphosphate. In the - South Island, however, it is interesting to note the expanding use of basic or reverted superphosphate. The mixing of superphosphate and about 15 per cent, of lime has the effect of producing, a phosphatic fertiliser whose water-soluble phosphoric acid is converted to a form not soluble

in water but available to plants. This, then, represents a distinct departure from the usual New Zealand practice and shows a tendency to fall into line with overseas developments away from straight superphosphate. A second departure of great interest is the proposal in the North Island to manufacture silico-superphosphate on a large scale. This material is made by a hot mix of ground serpentine rock

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with superphosphate—three parts of superphosphate being used to one part of ground serpentine. The chemical process involved is the formation, with the magnesium oxide present in the serpentine to the extent of about 35 per cent., of a compound whose phosphoric acid is converted to the non-water-soluble form. World-wide Trend Both these departures, then, are mainly of interest in that, they portray a tendency to develop fertilisers from which it is hoped a higher proportion of the phosphoric acid added to the soil will be recovered by the plant. Judging by results abroad, it seems that these developments are not temporary measures to expand present inadequate supplies of superphosphate, but represent New Zealand’s contribution to a definite world-wide trend away from the use of superphosphate on acid soils. It must be remembered that superphosphate has given wonderful results in New Zealand, and it is not suggested that at the present state of our knowledge any great departure from its use should be made. It has rather been the purpose of this article to indicate that, undoubtedly useful as superphosphate. has proved itself, it still suffers from some drawbacks and that developments are afoot in New Zealand and elsewhere to improve the already high efficiency of this fertiliser. The concluding paragraph of an article which has been freely drawn

—“Possible Development of the Superphosphate Industry,” appearing in the “Chemical Age” of March, 1940 of such interest that it appears well worth reproducing. “That there should be more intimate co-operation and collaboration between manufacturers and agricultural chemists, experimental stations and farmers, is evident, and we wish to give renewed emphasis to . this view. The ideas of the customer should be sought and an endeavour made to meet his requirements, however vaguely and indefinitely these may be expressed. Only in this way can a favourable development of the superphosphate industry be expected. The manufacture of superphosphate was first conceived by the greatest agricultural chemist of his time, Liebig, and it is felt that the superphosphate manufacturer, having concentrated his interest during the last hundred years mainly on the economic, mechanical, and chemical aspects of the process of manufacture, should now devote some attention to the insistent task of agricultural research.”