Phosphatic Fertilisers

The natural tricalcium orthophosphate of either mineral or organic origin has considerable fertilising value when finely ground, but its action is slow owing to its slight solubility. For intensive cultivation, where a quick-acting fertiliser is necessary, a much more soluble product, namely the monocalcium phosphate, must be employed, and this is obtained in superphosphate of lime.

As a result of the experiments of Lawes, who took out a patent in 1842, the manufacture of this compound was first started in England about the year 1845. It consists simply in the treatment of calcium phosphate with sulphuric acid in order to produce a more soluble phosphate, and the reaction is usually expressed by the equation

Ca₃(PO₄)₂ + 2H₂SO₄ = Ca(H₂PO₄)₂ + 2CaSO₄,

but it is probably more complicated. The presence of a certain amount of free phosphoric acid is necessary to prevent reversion to dicalcium phosphate, and Aita represents the main reaction as follows: -

5Ca₃(PO₄)₂ + 11H₂SO₄ = 4CaH₄(PO₄)₂ + 2H₃PO₄ + 11CaSO₄.

If properly mixed, the product dries by its own heat of reaction, giving an easily powdered residue. When too large an excess of sulphuric acid is used there is danger of obtaining a sticky mixture which cannot be spread satisfactorily upon the land. With too little sulphuric acid, on the other hand, there is danger of reversion to dicalcium phosphate on keeping, especially if much iron and aluminium are present. Pressure on the lower layers, when the superphosphate is stored in large heaps, also appears to cause reversion.

Superphosphate is tested by its content of water-soluble phosphate, although it is by no means certain that this is a true criterion of its availability to plants.

The system, lime: phosphoric acid: sulphuric acid: water, has been studied by Cameron and Seidell.

It has been suggested that a basic superphosphate, prepared by adding slaked lime to the ordinary superphosphate, might be used for acid soils.

Another artificial phosphatic manure is a by-product of the Bessemer process in the iron and steel industry, namely basic or Thomas slag. It was at first regarded as valueless because of the large amount of iron present, until it was shown that the phosphoric acid is all combined with calcium, and is therefore available. The phosphorus is, in fact, apparently more effective than in other phosphatic fertilisers. This may be due to the presence of silica. According to Dieckmann and Houdremont, there should be sufficient silica in basic slag to neutralise any excess of lime above that required for the formation of tricalcium phosphate, and so to form calcium silicophosphate, Ca₃(PO₄)₂.2CaO.SiO₂, of melting-point 1760°-1780° C., because this compound, unlike tetra- calcium phosphate, does not decompose during slow cooling to form insoluble oxy-apatite. It is also more soluble than tetraphosphate. Experiments with the finely ground slag began about 1882. Its value was first recognised by the Germans, who manufactured a similar compound, under the name of Wiborgh Phosphate, by heating mineral phosphates with soda. It is especially suitable for peaty and clayey soils deficient in lime, and for pasture lands. Its fertilising value is tested by the amount soluble in citric acid solution, but the same criticism applies to this test as to the water-solubility of superphosphate.

In order to reduce slag phosphates to the finely divided state necessary for their efficient utilisation, Plauson's colloidation process, carried out in the presence of small quantities of acid or alkali, may be employed.

Formerly slag manure contained as much as 40 per cent, of phosphate regarded as tricalcium phosphate, but now, owing to the use of the basic open-hearth process, the slag often contains only 20-30 per cent., and it has been suggested that mineral phosphates should be added in the furnace or to the molten slag.