PHOSPHATIC FERTILIZERS Four substances are in common use as phosphate fertilizers, namely, superphosphate, called acid phosphate in the United States; basic slag; mineral phosphate; and bones.
The mixture of calcium sulphate, monocalcic phosphate and some free phosphoric acid constitutes the superphosphate. No separation is attempted, and the calcium sulphate or gypsum is left in : it not only does no harm but has itself some fertilizing value and indeed was much used in the past : it also serves to get the superphosphate into a dry condition because it absorbs water very completely. The process has attained a considerable degree of perfection, and allows of the production of a high-grade prod uct, finely powdered and dry, free from many of the defects of the older samples.
British and continental makers largely use as raw material the North African mineral phosphates, but much of it comes also from the United States. In 1926 the imports of rock phosphate into Great Britain and the Free State of Ireland were :— On an average 1 o tons of rock phosphate give rise to about 18 tons of superphosphate instead of the theoretical It has been found convenient to standardize the various grades of superphosphate and sell them on a definite basis. The amount of soluble phosphate is determined by analysis as and the figure is then calculated as tricalcic phosphate. Thus the ordinary grade contains about 12% soluble in water; this figure is then multiplied by 2.18 to convert it into tricalcic phosphate, Both figures are conventional in that superphosphate consists neither of nor of but either figure does very well to express the amount of phosphate soluble in water.
The following grades are now in common use : "30% soluble" containing 13.6% (England and Wales) .
i6•o% "35% >> >> o » (Scotland).
"38% ,, ,, ,,The 30% super is made from Tunisian phosphates (58/63%, Gafsa or Dyr) ; the 35% from Algerian (Constantine 65/68%) or Florida Pebble phosphate (70%) ; and the 38% from Morocco phosphate (75%).
In Britain superphosphate is mainly used for swedes and pota toes, but some is used also for cereals. Its first effect is to hasten the rate of growth of the seedling. It causes swedes and turnips more rapidly to pass through the first leaf stage and acquire their true leaves, and cereals more rapidly to begin tillering. The root system also develops well. Later on the swelling of the root of the swede and turnip and the heading out and ripening of cereals are both hastened by superphosphate. The earlier heading out of cereals has certain incidental advantages; the head of the barley receiving superphosphate emerges from its ensheathing leaves a few days in advance of those insufficiently supplied, and therefore has a better chance of escaping the attack of the larvae of the goutfiies (Chlorops taeniopus, Meig) which, hatching from their eggs on the topmost leaf, crawl downwards, seeking the head for food. Addition of phosphate leads to no increase in the proportion of grain borne by the plant.
These effects are not observed in every season, nor are they always seen when farmyard manure has been applied. Fortunately for farmers, however, the years when superphosphate acts well are those when crops are low and when, therefore, the additional yield is worth more than is easily expressed in money terms. For if the farmer has insufficient food for his livestock in a bad year he must sell out at any price he can get, while if his food supply be adequate he can hold or sell whichever he pleases. As a bad root year is also commonly a bad grass year, he cannot usually meet the difficulty by changing from roots to hay. This valuable effect of superphosphate in bad seasons accounts for its popular ity among farmers; it can be regarded as an insurance against loss of crop at a time when loss would be peculiarly galling.
Actual phosphate starvation occurs on some soils and its symp toms are very marked. Swedes and turnips are among the first crops to suffer : they may fail entirely unless farmyard manure is given: it was because of the importance of these crops in British agriculture that the introduction of superphosphate pro duced so great a revolution in British agriculture 8o years ago. Cereals and grass may grow where swedes fail, but not well. Phosphate starvation occurs on certain soils in many countries: in Britain, on some of the boulder clays. Cereals suffering from phosphate starvation have a stunted root system, especially in their early days, and even more stunted leaf and stem; total number of tillers and the number of tillers bearing seed both fall off. Similar stunting of the root and leaf is seen in phosphate starved fruit trees.
Phosphate starvation markedly affects the composition of crops, lowering their nutritive value to animals and their special qual ity values to men. Over large areas of the world there are soils very deficient in phosphate. Such soils in parts of South Africa carry a natural herbage which causes deficiency diseases in cat tle; the affected animals .devour bones with great eagerness, even putrefying bones when the deficiency is pronounced, so that they become liable to a particular ptomaine poisoning. The obvious remedy is to feed the cattle with bone meal. Similar diseases occur in Australia, where also the arable lands show astonishing benefits from small dressings of phosphate. Great areas of land in the Middle West of the United States show marked response to phosphates. In the Rommey Marsh, the best fatting pastures are richer in phosphate than the poorer ones ; this is generally true of England and France. G. Paturel has shown that the best wines contain most (about 0•3 grm. per litre), the second and lower qualities containing successively less. Further, when the vintages for different years were ar ranged in order of their content, a list was obtained almost identical with the order assigned by the wine merchants. W. A. Davis has emphasized the importance of phosphate supply for the indigo crop.
Excess of phosphate over the amount required to increase plant growth is liable to cause injury; instances have been re corded of barley in Suffolk and potatoes in Lincolnshire, while W. F. Gericke in California has shown that the greatest growth is attained by supplying phosphate in the early life of the plant and withholding it later.
Certain soil conditions seem particularly to call for phosphatic manure. Heavy soils usually respond well in all parts of Britain : as do the loams of the eastern counties and many of the Boulder clays. The most striking increases in Britain, however, are ob tained on the fen soils of East Anglia. On the other hand sandy soils often show no response, and chalk soils may show but little.
In conclusion, superphosphate is used for:— Crops : fodder crops generally,—especially swedes and turnips in Britain ; and cereals in certain districts.
Soils: heavy soils, many Boulder clays, fen soils.
Seasons : hot and dry, cold and wet.
Two processes are in use for effecting this oxidation. The older one, adopted in 1879, is the Bessemer process; the molten pig iron is run into a pear-shaped vessel known as the converter, and air is blown through it. The necessary high temperature is obtained mainly by the combustion of the phosphorus, silicon, etc. in the pig iron, hence the need for sufficient of these sub stances : ores containing phosphorus are therefore used and the Bessemer slag contains some 18% phosphoric oxide, equivalent to 40% tricalcic phosphate. In recent years another process, the Open Hearth Process, has almost entirely displaced the Bessemer process in Great Britain but not in Belgium, the heat for the molten pig iron being supplied by a flame of producer-gas so that phosphorus in the pig iron is unnecessary : none is therefore added and the slags contain usually only 7 to 14% phosphoric acid equivalent to 15.4 to 31% tricalcic phosphate.
Further, the open hearth slags are not usually as readily soluble in citric acid as those yielded by the Bessemer process. This happens particularly when the pig iron contains much sulphur. An excess of lime is required to remove the sulphur, and as this makes the slag infusible, fluorspar or calcium chloride is added to increase the fusibility. This treatment converts the phosphate in the slag into a chlor- or fluorapatite insoluble in citric acid, hence the slags finally produced are more or less insoluble ac cording to the amount of lime and flux added.
There are theref ore three classes of basic slag available, namely, (I) Bessemer slag, containing phosphorus equivalent to 35-40% or more of tricalcic phosphate, and largely soluble in 2% citric acid. Usually 8o% of the total phosphate is guaranteed to be soluble.' (2) Basic open hearth slag containing less phosphorus, equivalent to to 30% tricalcic phosphate largely soluble (7o% of the total phosphorus) in 2% citric acid, the first pourings being richer than the last. (3) Basic open hearth slag made by the use of lime and fluorspar, containing as much phosphate as the poorer members of the preceding class but only slightly soluble (40% or less, usually only 2o% in 2% citric acid).
When basic slag was first obtained in the Bessemer converters in 1879 its fertilizing properties were not recognized, till John Wrightson in 1884 and 1885 made his field experiments at Ferryhill and at Downton, and Paul Wagner in 1885 began his systematic pot experiments at Darmstadt. Hereafter it gradually came into use and within four or five years could profitably be adulterated with mineral phosphates, to detect which Wagner devised the well-known citric acid test that, with certain modi fications, has remained in force ever since. Another important character of basic slag is the fineness to which it is ground. Gen eral experience shows that 80% of the slag should pass through a sieve having ioo meshes to the linear inch; the official specification of the Institute of Mining and Metallurgy is that this should have an aperture of side 0.12 7 mm.
The Bessemer slag achieved a high reputation for its marked effectiveness on poor pasture land, the experiments of W. Somer ville and D. A. Gilchrist at Cockle Park, Northumberland, and elsewhere, furnishing some striking, almost dramatic, examples. The open hearth slags of high solubility are equally effective. It has sometimes been urged that all grass land needs only treat ment with slag to transform it into something vastly more pro ductive, but further experience has shown that the improvement is strictly limited to certain types of soil conditions : improve ment has resulted because wild white clover has developed in the herbage ; there is no improvement, however, where the proportion of clover is already high, or where for any reason the clover can not grow well in spite of the presence of slag. In the Rotham sted experiments improvement could not be obtained if the grass land was already so good that one acre yielded 200 lb. live weight increase in sheep during the grazing season. In the Cockle Park experiments the increase had been only 20 to 40 lb. per acre, and here, therefore, improvement was possible : of ter slag treatment the live weight increase was 8o to zoo lb. per acre.
Basic slag is not confined in use to grass land : it has given good results also on arable land. It is not entirely like superphosphate in its action. On heavy clays, on downland pasture and in wet situations, slag is generally better than superphosphate. For roots, potatoes, hops and other short season crops superphosphate is usually better than slag. In J. Hendrick's swede experiments at Aberdeen there was little to choose between them, though for equal amounts of phosphate applied superphosphate gave on the whole slightly larger increase in crop, but where "finger and toe" was prevalent it required the addition of lime. In the Irish ex periments equal weights of basic slag and superphosphate gave approximately equal results.
The phosphorus is present in the slag as a silico-phosphate : R. Bainbridge adduces evidence that the chief constituent is silico carnotite, which is completely soluble in 2% citric acid. But when fluorspar has been used in the manu facture a fluorapatite, is formed, which is soluble only to the extent of 5% in 2% citric acid. It is shown at 'There is a confusing difference in meaning of the phrase "30% (or other amount) soluble" as applied to basic slag and to superphosphate respectively. For basic slag it means that 30%0 of the total phosphate present is soluble in 2% citric acid, while for superphosphate it means that the amount of soluble phosphate would, if expressed as tricalcic phosphate, be 30%0 of the weight of the superphosphate.
'The lower grades of slag containing less phosphorus than corre sponds with 15% tricalcic phosphate, are too poor to bear transport for agricultural purposes.
Rothamsted that fluorapatite has but little fertilizer value, and for a series of slags their effec tiveness decreased as more and more of the phosphate was in the fluorapatite form. These experi ments justify the use of 2% citric acid as a discriminating reagent in testing slags, though the action between the phosphate in the slag and the solvent is complicated by the presence of other soluble substances ex tracted from the slag. In conse quence the indications are not quite sharp, and solubility results do not altogether agree with the results of field tests.
In the Rothamsted experiments made in several parts of Britain, slag of which the phosphate is of high solubility (6o% or more) in 2% citric acid proved supe rior to those of low solubility (38% or less) ; slags of over 7o% solubility varied among them selves in effectiveness but the variations showed no relationship with solubility; slags of less than 70% solubility also varied among themselves in effectiveness but the variations are usually related to and expressible by the solubil ity. Medium and low soluble slags appeared to be more effi cient in the western counties of England than in the eastern, a difference which may be asso ciated with the higher rainfall or the longer growing seasons.
Of the other constituents of basic slag the calcium is valuable, as also is its oxide but this occurs only in small amounts. The manganese is considered valuable in France but not in Britain; it appeared, however, to act benefi cially on an acid millstone grit soil in Cheshire. Some of the con stituents may be actually harm ful to vegetation: a slag studied at Rothamsted lost in effective ness when it was more finely ground than usual, apparently be cause the harmful constituents came more into prominence.
In general the slags of low sol ubility differ from those of high solubility more than the figures indicate, showing that the dif ference does not consist simply in the amount of soluble phos phate present. The low soluble slags behave in the field differ ently in kind and not simply in degree from those of high solu bility and cannot be raised to their level of performance simply by increasing the dressing. These slags of low solubility still pre sent difficult problems to the Rock Phosphate.--In agricultural conditions where low costs of working are absolutely essential and high yields per acre are not so necessary, ground (but otherwise untreated) mineral or rock phosphate has often given sufficiently good results. On some of the acid soils of the United States, especially in the Middle West, notable results have been achieved by a combination of rock phosphate and clover: Cyril Hopkins in Illinois, and A. R. Whit son in Wisconsin have provided numerous demonstrations. In the north of England, north African phosphates ground so that 8o% shall pass a sieve showing i 20 meshes to the inch, and therefore finer than basic slag, have proved useful on arable land in increas ing the development of clover in the seeds ley. Elsewhere acid moorland soils have responded to these phosphates. In all com parative experiments basic slag has given better yields, but con siderations of transport may make the rock phosphate cheaper.
The following table gives the percentage of phosphate in some of the commoner minerals, in comparison with that in superphos phate and basic slag.
* Tunisia (Gafsa, Dyr). t Algeria (Constantine). ** Morocco. t S. Car olina. § Florida Pebble.
The chief use of rock phosphate is, however, for conversion into superphosphate and for this purpose it is highly desirable that the mineral should be as free as possible from iron and alumina. This consideration rules out many phosphatic deposits, which otherwise might have great value.
Bones.—Bones have long been applied as manure in isolated parts of the country, but they were not commonly used until the beginning of the 19th century. Such remarkable results were then obtained in certain districts, e.g., in Cheshire, that the demand became very great, and the rather large accumulations of the past in various parts of the world had to be drawn upon to satisfy it. The demand still continues the butchers' shops, meat markets and marine store dealers of the great cities are ransacked to keep up the supply. In modern practice the bones are sent to the works, put on to a perforated band and sorted clean shank bones are picked out for cutlery, hard bones for glue-making and the remainder for crushed bone: the separate batches are steamed at low pressure (15-20 lb.) to remove fat, nowadays a valuable commercial product. In some works the bones are degreased with benzene, and this process is more efficient than steam, so that the residual bonemeal is richer in nitrogen and in phosphate.
The bones intended for "bone meal" are then crushed and sorted into half-inch bones, quarter-inch bones and bone meal. The bones intended for glue, and the ends of the cutlery bones, are crushed and again steamed, but this time at a higher pressure (5o lb.), when most of the nitrogenous constituents are extracted as gelatine or glue. The residue can now be got into a very fine state of division and is sold as "steamed bone flour." Dissolved or "vitriolized bones" are made by treating bones with sufficient sulphuric acid to dissolve about half of the phosphate. The different products vary in composition, but typical analyses are: The bone manures can be used in any conditions in which superphosphate is effective : in general they are not as good but they are safer in the hands of the inexpert and therefore are probably better for use in gardens.