Superphosphate application rate for pastures: how much super per hectare

What single super supplies

Single superphosphate (single super, SSP) supplies phosphorus (P), sulphur (S) and calcium (Ca). NSW Government pasture guidance calls it the most cost-effective way to apply both P and S to pasture.

Figures use Incitec Pivot's published analysis for SuPerfect. Other makers' single super, such as Impact SSP, Summit SuperPhosphate and Agfert Super, is similar, but check each label or compare two side by side.

Start with a soil test

NSW pasture guidance says to soil test first, paying attention to the Phosphorus Buffering Index (PBI, how strongly the soil locks up P). Two P tests are common:

• Olsen P: CSIRO and NSW DPI's Five Easy Steps guide puts the critical value at about 15 mg/kg for 95% of maximum pasture growth, on any soil type. MLA suggests a target of about 13–15 mg/kg for sheep or beef stocked near carrying capacity.
• Colwell P: the critical value rises with PBI. At a Bookham, NSW site with PBI 80 it was about 32 mg/kg. MLA gives about 20–30 mg/kg for lighter soils, higher on heavier soils.
• Results bounce around. Five Easy Steps suggests setting a target band (about 5 Colwell or 2–3 Olsen units wide) and watching the trend.

New to soil reports? Read how to read a soil test. If P is fine but growth is poor, Victorian guidance notes there is no point adding more P when potassium or sulphur is the limit. Check potassium options and sulphur options.

Maintenance rate: tied to stocking rate

Maintenance replaces the P that leaves the paddock in meat and wool, is dropped in stock camps or gets locked up in soil. Five Easy Steps works it out as:

• Maintenance kg P/ha = kg P per DSE × average stocking rate (DSE/ha).
• kg P per DSE ranges from 0.42 to 1.46. It is higher on high-P-fixing soils (acid sands, krasnozems), steep set-stocked hills, improved pasture and higher rainfall.
• MLA's simpler rules of thumb: about 1 kg P per DSE, or 1 kg P per 25 mm of annual rainfall. The old standard of 'a bag to the acre' was 125 kg super/ha (11 kg P).
• Victorian guidance suggests about 10 kg P/ha for 10–15 DSE/ha and 20 kg P/ha for 15–25 DSE/ha.

Capital dressings: lifting a low soil test

A capital dressing is maintenance plus extra P to raise the soil test. Five Easy Steps estimates the extra P needed for one unit of rise:

• Colwell P: 2.7–3.4 kg P/ha per unit, rising with PBI from 50 to 600.
• Olsen P: 8.6–13.1 kg P/ha per unit, over the same PBI range.

Lifting P only pays if you run more stock on the extra feed. NSW pasture guidance notes that if stocking is well below optimal, for example after drought, it may be wise not to fertilise.

Worked example

An illustration, not a recommendation: clay-loam (medium soil loss), set-stocked rolling country (low animal loss), improved pasture, 600 mm rainfall, 12 DSE/ha, PBI 100, aiming to lift Colwell P by 2 units this year.

1. Maintenance: Table 3 of Five Easy Steps gives 0.83 kg P/DSE. 0.83 × 12 = 10 kg P/ha.
2. Capital: 2 units × 2.7 kg P = 5.4 kg P/ha.
3. Total: about 15.4 kg P/ha.
4. Single super at 8.8% P: 15.4 ÷ 0.088 = about 175 kg/ha, which also supplies about 19 kg S/ha.
5. Once the target is reached, drop back to the maintenance rate of about 114 kg/ha.

When to spread

• NSW pasture guidance: autumn for legume-based pastures such as sub-clover, spring for grass-dominant paddocks, and time it with rain or irrigation.
• MLA: apply before the autumn break; avoid bare soil or when heavy rain is likely within a week.
• Incitec Pivot warns against spreading on bare, steep country when intense rain is forecast.

Re-test regularly. NSW pasture guidance says maintenance rates can only be set with confidence by tracking soil tests over time. Always check with your agronomist before acting.