Discussion 62 Determination of phosphorus in fertilizers

The analysis of phosphorus in fertilizers can be achieved in many ways, and due regard must be made both to the chemical form in which the phosphorus occurs, and its solubility, and hence relative rate of availability to the plant. Phosphorus can occur in many different molecular combinations and admixed with a variety of other substances, organic and inorganic, so that it is difficult to recommend one method in preference to any other. The type and amount of accompanying trace elements can also vary. The determination of total phosphorus is easier to achieve than available phosphorus, but the acid digest should not be highly coloured or else it could result in too high a colorimetric reading.

Phosphate rocks

Phosphate rocks usually contain apatite, and this is mainly virtually insoluble calcium fluorapatite. The molecular formula can be written either as 3Ca3(PO4)2.CaF2 or 9CaO.3P2O5.CaF2. It has been formed from partial or total replacement of the hydroxyl in hydroxyapatite, 3Ca3(PO4)2.Ca(OH)2, by fluoride. In addition, varying amounts of hydroxyl may also be replaced by carbonate, or chlorine. Thus there is also carbonapatite, 3Ca3(PO4)2.CaCO3; and chlorapatite, 3Ca3(PO4)2.CaCl2. Phosphate rocks with a high CO2 content generally have a very small grain size and high surface area, which facilitates their solubility. They are known as 'soft-earth' or reactive phosphate rocks (RPRs), such as Tunisian Gafsa phosphate. These have been formed on the sea floor in a different geochemical process to the hard variety. Their slow solubility is enhanced by acid soils, high rainfall and a warm climate. It is the presence of the carbonate that enables the finely ground fertilizer to dissolve slowly in the acid soil. Further details on RPRs may be found at the following websites:

http://www.latrobe.edu.au/www/rpr/what.htm http://www.fertico.com.au/rprbrochure.html The elemental composition of various formulations is available at:

http://www.fertico.com.au/rock.htm Another naturally occurring phosphate mineral is sombrerite (whitlock-ite), which is tricalcium phosphate, Ca3(PO4)2.


The 'hard-earth' coarsely crystalline forms of insoluble fluorapatite are virtually insoluble if the pH is above 5.5. Their very slow release rates mean that about four times the recommended rates for more soluble forms of phosphorus needs to be applied to correct an immediate deficiency. For the phosphate to be immediately available to plants, they need to be treated with acid to convert them to the soluble superphosphate. Sulphuric acid results in single superphosphate, and phosphoric acid produces triple superphosphate. In about 1840, Liebig proposed treating bone phosphate with sulphuric acid to increase the availability of P to plants. However, the conversion of insoluble hard rock phosphate to soluble superphosphate by treatment with sulphuric acid was patented in 1842 by Sir John Bennet Lawes, who opened the world's first artificial fertilizer factory the same year. He also founded Rothamsted Experimental Station, Harpenden, UK, the world's oldest agricultural research station still in existence. The chemical reaction of rock phosphate with sulphuric acid first forms free phosphoric acid, which reacts with more rock phosphate to give calcium dihydrogen phosphate (acid phosphate; monocalcium phosphate) and gypsum (anhydrite), together called single superphosphate (approximately 20% P2O5). Fluorine is removed as hydrogen fluoride, and the reaction equations are:

Thus one formula Ca is converted to dihydrogen phosphate, and the product is called single superphosphate (the super refers to its solubility/availability compared with rock phosphate).

With phosphoric acid, the main equation becomes:

Thus all three formula Ca atoms are converted to dihydrogen phosphate, and the product is therefore called triple superphosphate (approximately 46% P2O5 or 20% P).

Basic slags

Slags are usually obtained as a by-product from steel making, where phosphate is removed from the steel to prevent the level rising above 2% P when it becomes brittle. The process is carried out in a Thomas converter by adding lime and silicate to the molten metal, and blowing through it air previously heated to 1600°C. This oxidizes the phosphorus to calcium silicophosphate ((Ca3(PO4)2.x(Ca2SiO4)), which is called Thomas slag. This is then finely ground. They can contain varying amounts of phosphorus, but are preferred to contain at least 5% total P (11.5% P2O5) of which not less than 80% is soluble in 2% citric acid. Cadmium can sometimes cause a toxicity problem. Further details are viewable at:


Organic phosphorus

A common source of organic phosphorus is bone meal (approximately 9-14% P) and bone ash (approximately 18%). The bird excrement guano contains about 2-3% P as ammonium and calcium phosphates. Fresh solid dairy cattle manure has approximately 0.13% P (moisture = 81.7%), and solid swine manure has about 0.33% P (moisture = 71.8%), which will be in both organic and mineral forms.

Solubility of phosphates

Phosphatic fertilizers usually contain a mixture of phosphates exhibiting varying degrees of solubility, which also depend on the nature of the soil. It is therefore necessary to analyse for these various phosphate types. A list of various phosphate compounds, their molecular formulae and solubilities, where known, is given in Table 6.1.

The methods of analysis for phosphate solubility are not absolute, but empirical; that is, they are based on practical experience. For example, the neutral ammonium citrate method is favoured in the USA because it has received over 100 years of study and experimentation, and provides an index correlating the laboratory results with the fertilizing value of water-insoluble phosphates under the conditions prevailing in the principal farming regions of the country. The particular solvent is therefore not an attempt to accurately reproduce the properties of the soil solution in the immediate vicinity of the

Table 6.1. Chemical forms of phosphate with their solubilities, where known, in various solvents.

Solubility in solvent




Alkaline Neutral amm.

amm. citrate Citric Formic citrate (Petermann acid 2% acid (Fresenius) or Joulie) (Wagner)2%

Monocalcium phosphate (super/triplephosphate) Dicalcium phosphate Tricalcium phosphate Tetracalcium phosphate Apatite (fluorapatite) Basic slag (Thomas phosphates) Rhenania phosphate Soft earth phosphate

Ca(H2PO4)2 CaHPO4

Ca3(PO4)2.2/3 H2O Ca4P2O9





High High Low High V.low Low High Low

Insol. Insol. V.low



High plant roots, but to provide a standard measure of phosphate extraction roughly comparable to that available to the plant, which can be related to the observable plant growth. It should also be remembered that the continuing solubility of sparingly soluble calcium phosphates depends on the removal of the solution of the calcium and phosphate ions from the region around the fertilizer granule as soon as they have been formed. With certain tropical soils, the presence of significant amounts of aluminium and iron can fix the phosphate in an unavailable form. A further discussion of the use and analysis of phosphates is found in Sauchelli (1965); there are also useful details on the solubility of phosphate fertilizers in Finck (1982).

Superphosphate contains a mixture of monocalcium phosphate (soluble in water), plus dicalcium phosphate and calcium sulphate (both with low solubility in water) together with other mineral residues. Thus the solubility in water gives the monocalcium phosphate (sometimes abbreviated to monophosphate) content, and extraction of the residue with neutral ammonium citrate gives the dicalcium phosphate component.

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