Total P2O5 Content Is Not A Good Index For Direct Application of Phosphate Rock
It has been shown that direct application of phosphate rock (PR) may be more cost-effective than the use of expensive water-soluble phosphate (P) fertilizers such as triple superphosphate (TSP) and diammonium phosphate (DAP), under certain soil, crop, and agro-climatic conditions.
Major factors affecting the agronomic effectiveness of PR for direct application are sources of PR, soil properties, and crop species.
In Malaysia, use of PR for direct application is ideal because of the acidic nature of soils, relatively high temperature and high rainfall.
These conditions are favourable to the use of PR for crop production of long-term plantation crops such as rubber and oil palm.
Selection of a proper source of PR for use, however, requires some understanding of the PR’s properties which vary widely among PR sources in chemical and mineralogical compositions. A good choice of an effective PR source is the first step to maximize the benefit of using PR for direct application.
First, it should be pointed out that PR, like soil, is not a pure compound. It consists of P mineral (apatite) and other associated materials such as calcite, silica, clay minerals, etc.
Because PR is water-insoluble, dissolution of PR in acid soil must take place first before the plant’s roots absorb P nutrient in soil solution.
Dissolution of PR greatly depends on the reactivity of apatite mineral that is controlled by the degree of isomorphic substitution of carbonate for phosphate in apatite structure.
The higher the substitution, the higher is the reactivity of PR. However, the higher substitution also lowers P2O5 content in PR.
For example, two highly reactive PRs (Gafsa, Tunisia and North Carolina, USA) have lower P2O5 content (29-30%) than that in low-reactive Kaiyang PR in China and Araxa PR in Brazil (both with 37% P2O5).
The P2O5 content in PR also greatly depends on the amounts of materials associated with apatite mineral in PR. For example, two Colombian PRs (Huila and Pesca) are low in P2O5 (20-21%) because of dilution by associated impurities in PRs.
Thus, there is no correlation between P2O5 and PR’s reactivity as shown in the following examples:
| PR Source | Isomorphic Substitution | Reactivity | P2O5 % |
| North Carolina, USA | High | High | 30 |
| Gafsa, Tunisia | High | High | 29 |
| Kaiyang, China | Low | Low | 37 |
| Araxa, Brazil | Low | Low | 37 |
| El-Hassa, Jordan | Medium | Medium | 30 |
| Christmas Island, A-Grade | Medium | Medium | 35 |
| Huila, Colombia | Medium | Medium | 21 |
| Pesca, Columbia | Low | Low | 20 |
In summary, total P2O5 content in PR is not a good index for PR’s reactivity and agronomic effectiveness.
Solubility of PR as measured by chemical extractants, e.g., neutral ammonium citrate, 2% citric acid, and 2% formic acid, can be used to estimate the PR’s reactivity which is related to the agronomic effectiveness when different sources of PR are compared for direct application.
S. H. Chien
Senior Scientist-Soil Chemistry
Research and Development Division
International Fertilizer Development Center (IFDC)
P.O. Box 2040
Muscle Shoals, Alabama 35662, U.S.A
This article was first published in Soil Tech Volume 6(1), 1998, Malaysian Soil Science Society and is re-published here with the permission of the author.