Rock phosphate.
Understanding P fertilisers
Several forms of phosphorus (P) fertilisers are available. As this article explains, they are all comparable. You should base your decisions on price and availability.
Manufacture of P fertilisers
Most commercial P fertilisers are derived from rock phosphate, which contains >20% P. Rock phosphate mined in several parts of the world is the result of millions of years of sedimentary deposition of eroded minerals. Guano-derived rock phosphate (fossilised bird droppings) used to be mined on Nauru and Christmas Island, but these sources are now exhausted.
Rock phosphate per se is little used as a fertiliser on account of its very slow mineralisation rate. The process of making rock phosphate more available to plants occurs via several chemical processes.
The first step is the conversion of the rock to phosphoric acid (H3PO4), through either a dry (furnace-based) process or a wet (acid-based) process. The latter is less expensive and produces the bulk of P fertiliser. When sulphuric acid is used, single and double superphosphate (“super”) is produced, depending on the ratio added.
The phosphoric acid can be combined with ammonia to produce monoammonium phosphate (MAP) and diammonium phosphate (DAP); or with rock phosphate to produce triple super. The phosphate in all these forms is in the chemical form (orthophosphate or PO4) taken up by plants.
P contents of common P fertilisers.
|
Fertiliser |
P content (%) |
P solubility (%) |
|
Single super |
9 |
80 |
|
Double super |
17.5 |
80 |
|
Triple super |
20 |
80 |
|
MAP |
22 |
90 |
|
DAP |
20 |
90 |
|
Ammonium polyphosphate |
15 |
100 |
|
Rock phosphate |
20 |
0 |
Since all rock phosphate contains some cadmium, additional expense is incurred in removing the cadmium to acceptable levels for agriculture.
Organic P sources
Animal manure contains useful amounts of P: typically around 1%. Around 45% to 70% of this is in inorganic form and is therefore immediately available for plant uptake. The remainder, in organic form, is broken down by soil microorganisms at a rate that depends on temperature and moisture, supplying up to 80% of the total P in the first year.
Blood and bone contains about 5% P, which is released slowly.
Orthophosphate or polyphosphate?
P in regular phosphate fertiliser is present in the orthophosphate form, the form that plants take up. Heat-processed orthophosphate becomes polyphosphate. Almost immediately on application to soil, in the presence of soil moisture, the polyphosphate molecules break down again to orthophosphate. So chemically there is no difference after application. And yields have been shown to be identical.
Soil pH
Phosphate binds tightly to iron and aluminium in the soil and becomes unavailable to plant roots, especially at a soil pH below 5 (acidic). If your soil has a low pH, treating it with lime will raise the pH and lessen the tendency for the P to become unavailable. Conversely, at high pH (alkaline soils), calcium binds to P and reduces its availability. P is most available at a pH between 6 and 7.
This diagram shows how P availability changes with pH.
Contrary to popular belief, the addition of phosphate to soil, including superphosphate, does not acidify soils. The reason soils acidify when P is added is that the P stimulates nitrogen fixation (see Nitrogen cycle), whereby atmospheric nitrogen is eventually converted to nitric acid in the soil.
A note on P terminology
Most US publications refer to P2O5 (diphosphorus pentoxide). This is misleading and confusing, as fertilisers do not contain P2O5, which explodes on contact with water. Consequently, stated P contents are misleadingly high, because the P content of P2O5 is 43.64%. To get the actual P content, you must multiply the P2O5 value by 0.4364. For example, a US fertiliser stated to contain 21% P2O5 really contains 0.4364 × 21% = 9.16%, which makes it single super.
Further reading
Lines-Kelly R. 2002. Why phosphorus is important. NSW Agriculture, Wollongbar, NSW.
Bolland M. 1996. Choosing phosphate fertilisers for cereals. Farmnote 31/96, Department of Agriculture Western Australia.
Duncan MR. 2002. Soil acidity and P deficiency. Acid Soil Action leaflet 9. NSW Agriculture, Armidale, NSW.