Ammonium polyphosphate (APP)

APP is produced by ammoniating superphosphoric acid, yielding a solution that contains both orthophosphate and longer-chain polyphosphate ions along with ammonium nitrogen. A grade of around 10-34-0 is a widely traded standard, though other liquid phosphate grades exist.

Composition

Ammonium salts of orthophosphate and polyphosphate

Phosphorus forms

A mixture of immediately available orthophosphate and polyphosphate that hydrolyses to orthophosphate

Nitrogen content

Ammonium nitrogen, around 10% in the common grade

Physical form

Clear liquid, handled and applied by volume

APP supplies phosphorus and nitrogen. Its orthophosphate is available at once, while its polyphosphate must first hydrolyse to orthophosphate, a step that is generally rapid in warm, moist, biologically active soils and slower in cold conditions. As with all soluble phosphates, some phosphorus is fixed by soil calcium, iron, or aluminium over time.

• Delivers both orthophosphate and polyphosphate phosphorus
• Supplies ammonium nitrogen alongside phosphorus
• Polyphosphates can hold metal micronutrients such as zinc in solution

APP is popular as a starter fertilizer banded near the seed at planting, where a concentrated, available phosphorus supply supports early root growth. Because it stays clear and stable, it is also used as a base solution into which compatible micronutrients such as zinc can be added for a single-pass application.

• Common as a banded starter fertilizer for row crops
• Serves as a stable liquid carrier for zinc and some other micronutrients
• Placement near the root zone improves phosphorus access in fixing soils

The benefit of the polyphosphate fraction depends on it hydrolysing to orthophosphate, which is slower in cold soils. APP also carries the same environmental concern as other phosphate fertilizers: phosphorus reaching surface water can contribute to eutrophication, so placement and rate matter for water quality.