Three-component fertilisers predominate in the structure of manufactured compound fertilisers (Table 1).
Tab. 1. Structure of compound fertiliser production in % (in mass of fertilisers)
| Product range | % |
| NPK | 67 |
| NP | 26 |
| PK | 7 |
Source: Igras 2009
Fertilisers are estimated to be 54% in the form of single fertilisers and 38% in various compound fertilisers. The choice of the type of fertiliser depends on the manufacturing costs, where fixed costs, raw material costs, and the granulation or mixing technology used play the most crucial role.
Raw material costs generally account for 60-70% of production costs. High raw material costs are a disadvantage for the seemingly very economically attractive “bulk blending” system. Field blending of fertilisers is then very close in cost to granulating fertilisers from factory blends using steam granulation.
Multi-component fertilisers are mainly manufactured as complex fertilisers or mixed multi-component fertilisers containing constant proportions of essential fertiliser ingredients in each granule. The composition of these fertilisers is primarily tailored to the agronomic requirements of the plants.
Quality requirements for compound fertilisers
In Table 2, there are specified limit values for declared forms of phosphorus solubility. The table presents requirements in this respect for constant types of EC compound fertilisers produced in the country. These are fertilisers produced based on ammonium phosphate and superphosphates and based on soft phosphate rock or partly decomposed phosphate rock.
The presented requirements for compound fertilizers indicate the possibility of application by producers of many technological variants regarding phosphate decomposition into soluble and bioavailable forms of phosphorus in the fertiliser.
Moreover, Table 2 presents minimum and maximum quality requirements for solid compound fertilisers.
Tab. 2. Solid multi-component fertilisers – declared phosphorus forms and minimum and maximum values (EC fertilisers)
| Phosphorus solubility | Minimum, maximum P2O5 content, % | Declared form |
| Ammonium phosphate and superphosphate fertilisers | ||
| P2O5 water-soluble | < 2% | P2O5 soluble in ammonium citrate solution - rc |
| P2O5 water-soluble | min. 2% | P2O5 soluble in ammonium citrate solution and water - rc, rw |
| P2O5 soluble in mineral acids only | max. 2% | - |
| Fertilisers containing soft or partly decomposed phosphate rock | The fertiliser shall contain | |
| P2O5 water-soluble | min. 2.5% | P2O5 water-soluble - rw |
| P2O5 water-soluble and in ammonium citrate solution | min. 5% | P2O5 soluble in ammonium citrate solution and water - rc, rw |
| P2O5 soluble in mineral acids only | min. 2% | P2O5 soluble in acids |
| Soft phosphate fertilisers | ||
| P2O5 soluble in mineral acids only | 100% | P2O5 soluble in mineral acids |
| P2O5 soluble in formic acids only | 55% | P2O5 soluble in formic acid |
Source: Regulation (EC) No 2003/2003 of the European Parliament and of the Council, of 13 October 2003, relating to fertilisers. OJ L 304, 21.11.2003
The presented requirements for compound fertilizers indicate a potential for manufacturers to apply many technological variants regarding phosphate decomposition into soluble and bioavailable forms of phosphorus in the fertilizer. The superior value in classification for compound fertilizers is the sum of essential components and minimum basic components contained in the fertilizer (tab.3).
Table 3. Minimum contents of essential elements in compound fertilisers (% m/m) (EC fertilisers)
| Type of fertiliser | Fertilising component | Solid fertilisers % | Liquid fertilisers | |
| Solutions % | Suspension % | |||
| NPK | NPK | 20 | 15 | 20 |
| N | 3 | 2 | 3 | |
| P2O5 | 5 | 3 | 4 | |
| K2O | 5 | 3 | 4 | |
| NP | NP | 18 | 18 | 18 |
| N | 3 | 3 | 3 | |
| P2O5 | 5 | 5 | 5 | |
| NK | NK | 18 | 15 | 18 |
| N | 3 | 3 | 3 | |
| K2O | 5 | 5 | 5 | |
| PK | PK | 18 | 18 | 18 |
| P2O5 | 5 | 5 | 5 | |
| K2O | 5 | 5 | 5 |
Source: Regulation (EC) No 2003/2003 of the European Parliament and of the Council, of 13 October 2003, relating to fertilisers. OJ L 304, 21.11.2003
From a technological and agrochemical point of view, the differences in phosphorus solubility between the types mentioned above of declaration are fundamental. The effect of the ambiguity of both records is dangerous because of the tendency of manufacturers to introduce large quantities of chemically unprocessed phosphorus in the form of phosphate rock into compound fertilisers. This is particularly important when the recipient of the fertiliser does not have sufficient specialist knowledge to determine the quality of the fertiliser based on the phosphorus solubility forms declared by the manufacturer.