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Phosphatic Fertilisers

Mineral Phosphates

Practically all the phosphorus in the ten-mile crust of the earth is present in the lithosphere, of which it forms 0.157 per cent., and is combined as phosphates of many bases. Compared with other acidic and basic oxides phosphoric anhydride is eleventh in order of abundance, falling immediately below titania (1.050 per cent.) and above carbonic anhydride (0.101 per cent.). On the average igneous rocks contain 0.299 per cent, of phosphoric anhydride, which is greater than the percentage in sedimentary rocks generally, and about three times the proportion found in average limestones. The mineral apatite, which is widely diffused in the deep-seated igneous rocks, is only slowly attacked by atmospheric weathering, but in the course of ages it and other phosphatic igneous rocks gradually dissolve as phosphates of calcium, iron and aluminium, and become changed into the carbonated or hydrated secondary phosphate rocks.

Phosphates occur in rocks of all the geological epochs. Apatite is associated with granites and gneisses.

Palaeozoic phosphorites (coprolites) have been found in England and Germany. In the Mesozoic epoch phosphorites occur in the Triassic, Jurassic and Cretaceous formations. The tertiary phosphate deposits in the United States of America and N. Africa are the most extensive and valuable in the world. Quaternary deposits include fossil bones, the guanos and phosphoguanos. The high-grade secondary deposits which are used as sources of fertilisers and of phosphorus generally have no doubt been segregated by processes in which animal life has played a large part. Thus the skeletons of marine animals and organisms collecting on the floor of the ocean are dissolved in areas containing a high concentration of carbon dioxide; their component phosphates are reprecipitated on shells, or by ammonia derived from the decay of nitrogenous matter, and form concretions of tribasic calcium phosphate.4, 5> 6 The small quantities of phosphates which are widely present in limestones 7 and dolomites may under certain conditions be concentrated by the leaching out of the calcium carbonate in the form of bicarbonate. All sedimentary deposits which consist of the remains of animals or plants must contain phosphate. The breaking down of these deposits disseminates the phosphatic material in the soil. These deposits are largely derived also from the gradual solution and subsequent deposition of widely diffused particles of apatite which are a constant constituent of igneous rocks. Granite contains 0.6 per cent., basalt 1 per cent, and gneiss 0.25 per cent, of phosphoric oxide on the average. Fertile soils contain 0.2 to 0.5 per cent, of phosphoric oxide, poor soils about 0.1 per cent. Some phosphate must be present in a soil which supports any flora. The plants use these supplies mainly in their seeds, which are eaten by animals, and the phosphorus subsequently segregated mainly in the nerves, brain and bone. It is then partly returned to the soil as animal remains. Phosphorus is not of course lost in the same way as combined nitrogen, by decomposition, since phosphorus compounds which are likely to be formed in nature are non-volatile. But considerable quantities of dissolved phosphates find their way to the sea, and this occurs especially under a system of water-borne sewage. The loss thus incurred is an additional argument in favour of the treatment of sewage with recovery of all fertiliser values.

Ordinary sea-water may contain from 0.06 to 0.07 milligram of P2O5 per litre, the amount varying with the season. This supply is drawn upon by diatoms and algae, and returned when they decay. The supply is also continually supplemented from rivers, etc. The algae are devoured by molluscs and crustaceans, which in their turn supply food to animals higher in the scale, until finally, as the bodies of fishes, the phosphorus is assimilated by sea-birds, who return some of it to the land in their excrement. This ultimately becomes guano, and the phosphate may then be converted into phosphatic limestone. The remainder of the phosphate from these and other sources accumulates on the bottom of the sea as a mud which contains 3Ca3(PO4)2.CaF2, as well as Ca4P2O9.4H2O and Ca3P2O8.H2O, derived from minerals and animal remains.

Assimilation by Plants

It is probable that plants obtain all their phosphorus from phosphates. Organic compounds containing phosphorus, like the phosphoproteins, are rapidly decomposed by soil bacteria, and the phosphoric acid combines with the bases in the soil. The assimilation of phosphorus from phosphates which are insoluble in water is probably aided by acid secretions from the root-hairs, and also perhaps by the carbon dioxide exhaled during the respiratory process. Soluble phosphates are quickly assimilated. The absorption of H2PO4- ion by a growing plant may be demonstrated by a fall of acidity. If the plant absorbs the base as well (i.e. lime), the acidity or hydrogen- ion concentration of the soil is maintained, and phosphate thus remains in solution. This is, of course, only one of the numerous reactions by which the acidity is regulated. A difference in the lime requirements may thus account for the difference in the availability of phosphatic fertilisers for different plants. This theory will also account for the greater availability of insoluble basic phosphates when applied to soils which lack lime, and thus confer on the phosphate a potential acidity which is due to the demands of vegetation for this base. The demands are greatest in the case of leguminosse (beans, etc.), brassicse (cabbages, etc.) and roots, and especially potatoes and beetroot.

The general effect of phosphates is to favour the formation and ripening of seeds, and in this respect it acts in the opposite direction to combined nitrogen, which favours the growth of stalks or straw at the expense of seed or fruit.

The concentration of phosphorus in vegetable matter is not high, being rather less than 0.1 per cent, in dry fodder, but much higher— about ½ per cent.—in grains. The phosphates assimilated by plants supply the loss of phosphorus eliminated in animal metabolism, and which, in the case of human adults, amounts to 3 or 4 grams of phosphoric anhydride a day. The reserve of calcium phosphate present in the bones weighs about 2 kilograms.

From the earliest ages the natural circulation of phosphorus has been altered and controlled by farmers. The systematic return of all kinds of excreta to the soil is still the basis of the intensive cultivation practised in densely populated areas of India and China, where the soil bacteria are so active at the favourable temperature prevailing that the nitrogen and phosphorus become available almost at once for another crop. The return of bones to the soil is a less obvious form of economy, partly because when in the massive form these disintegrate very slowly.

After the demonstration of the chemical basis of agriculture by Liebig in 1840, and others, bones were recognised as essential on account of their high phosphorus content. Great quantities of bones were imported into England for this purpose in the first quarter of the nineteenth century. Later, they were largely superseded by the highly concentrated Peruvian guano or by superphosphate, which are more readily available to the plants. Bones, however, when finely ground, and after the extraction of fat and of gelatin by steaming, still retain their place as a slow fertiliser in schemes of manuring. A typical analysis of raw bones shows—

Organic matter28%
Calcium and magnesium phosphates44%
Calcium carbonate, sand, etc5%

If a good deal of the organic protein (ossein) has been left in the bone, as is the case when the fat has been extracted by solvents, and not by steaming, the resulting bone-meal quickly decomposes in the soil, the phosphoric acid being made partly soluble by the decomposition products of the proteins. Such a bone-meal will contain (approximately) 45 per cent, of calcium phosphate, 1.5 per cent, of magnesium phosphate and over 30 per cent, of organic matter.

The question of availability is largely one of solubility. It was suggested by Liebig that bone-dust should be rendered soluble by treatment with sulphuric acid. But the action of the acid on the protein matter makes the product viscous and difficult to dry. Bone superphosphate is more easily made from a degelatinised bone-dust or from bone-ash. This latter product was formerly imported in large quantities from South America, and contained from 65 to over 80 per cent, of calcium phosphate. The ash may be completely dissolved in an excess of hydrochloric acid, and the calcium monohydrogen phosphate then precipitated by careful addition of lime. This process is also used to recover phosphate of lime from the acid liquid which is obtained as a by-product in the manufacture of glue from bones. The phosphate of lime so prepared is fairly free from impurities, and may of course be made soluble again by the addition of more acid:—

Ca3(PO4)2 + 4HCl = CaH4(PO4)2 + 2CaCl2
CaH4(PO4)2 + Ca(OH)2 = 2CaHPO4 + 2H2O

The chemical exploitation of bones thus led by a gradual transition to the artificial fertiliser or mineral phosphate industry which will now be described.

Basic Slag

This is a by-product produced in the manufacture of steel from pig iron which contains phosphorus (phosphide). The possibility of removing the phosphide dissolved in pig iron by carrying out oxidation in a Bessemer converter lined with a basic instead of a siliceous material was demonstrated by Snelus in 1872, by Thomas and Gilchrist at Blaenavon in 1878 and at the Eston works in 1879. The utility of the slag as a fertiliser was tested in the South and North of England from 1884 by Wrightson, Somerville, Middleton and Gilchrist.

The converters, and afterwards the open hearths, in or on which the pig iron containing phosphide was oxidised, are lined with lime or magnesia. The ferrous phosphate produced by oxidation, instead of being immediately reduced again by the excess of iron, is decomposed by the lime according to the equation

Fe3(PO4)2 + 4CaO = Ca4P2O9 + 3FeO

and the phosphate combines further with any calcium silicate or fluoride which is present. The hard slag is crushed, separated from inclusions of iron, and then ground in a ball mill.

In the open-hearth process the slag may be removed by ladling or tilting, when the phosphorus content of the iron has been reduced from 1 per cent, or over to about 0.2 per cent. This furnishes a high- grade slag. A " new " slag by means of which the phosphorus content is reduced to about 0.02 per cent, is of much poorer quality.

Open-hearth slag: high-grade and "new"

SilicaLimeIronTotal P2O5Soluble P2O5.Citrate - Soluble P2O5.

In the ordinary process the slag is allowed to flow off continuously. It then shows a diminution in phosphoric and silicic anhydrides and an increase in lime and total iron as the carbon and phosphorus in the metal diminish say from 1.77 and 1.30 per cent, respectively to 0.09 and 0.023 per cent.

Open-hearth slag: continuous flow

SilicaLimeIronTotal P2O5Soluble P2O5Citrate- Soluble P2O5.
At first20.3033.208.6017.0815.3689.92
After 6½ hours10.2047.8014.7010.851.6615.30

The solubility of basic slag has been found to increase with increasing calcium silicate. Some of the constituents other than phosphate have value on certain soils which happen to be deficient in these constituents.

Basic slag is a slow fertiliser; the phosphate is not immediately available as is that of calcium superphosphate. It is particularly valuable for fruit trees, and for heavy grass-land, on which it develops the growth of white clover and hence increases the amount of combined nitrogen. It neutralises acid soils, and its beneficial effects extend over many years.

Summary of Phosphatic Fertilisers

The chief varieties of naturally occurring or manufactured phosphatic fertilisers may be classified briefly as follows:—
  1. Natural nitrogenous phosphates such as guano, bone-dust.
  2. Finely ground phosphatic rock.
  3. Superphosphate of various grades—with 16 to 20 per cent. total phosphoric acid or with 8 to 10 per cent, of water-soluble phosphoric acid.
  4. Double superphosphate—with about 40 per cent, of soluble phosphoric acid.
  5. Precipitated dicalcium phosphate.
  6. Basic slag or " Thomas phosphate."
  7. " Wolter phosphate," obtained by decomposing phosphate rocks with calcium carbonate, sand, carbon and sodium sulphate in a furnace. " Rhenania phosphate," obtained by decomposing the rock with leucite or phenolite, potash or soda.
  8. Ammonium phosphates and superphosphates, which may contain also ammonium sulphates.
A scientific study of the various systems should determine the best conditions for the various reactions between salts and acids. The phosphates of calcium are the most important.

Mixed and Concentrated Phosphoric Fertilisers

The manufacture of fertilisers containing potassium or ammonium or both in addition to phosphoric acid has called for an accurate knowledge of the interaction of the salts concerned and also for the greatest refinements of chemical engineering in order to produce a material of uniform, dry and yet not dusty character.

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