Rock Phosphate & Rate for P₂O₅ on Acid Soils
Doses P₂O₅
Enter your target P₂O₅, the rock phosphate grade and the field area to get the rock phosphate in kg/ha, the total amount and the number of bags — apply the right slow-release rate.
Enter your field
Next: broadcast 300 kg and incorporate; on acid soils it builds P over 2–3 years — pair with organic matter or composting to speed availability.
Only the citrate-soluble fraction is plant-available; ineffective on neutral/alkaline soils — use DAP/SSP there.
Rock phosphate — key facts
- Material kg/ha
- P₂O₅ ÷ (grade% ÷ 100)
- Release
- slow, over seasons
- Best soil
- acid (low pH)
- Apply
- more than soluble P
- Available
- citrate-soluble fraction
- Grade
- ≈ 18–34% P₂O₅
- Wrong soil
- use DAP/SSP on neutral/alkaline
- Privacy
- Runs in your browser; nothing uploaded
A cheap, slow-release phosphorus source for acid soils
Rock phosphate is the patient farmer's phosphorus: cheap, mined straight from the ground, and released slowly over seasons rather than in a single soluble hit. Its quirk is chemistry — it dissolves through soil acidity, so it earns its keep on acid soils and stays largely locked up on neutral or alkaline ground. Because only the citrate-soluble fraction is available each year, you apply more total product than you would with a soluble fertiliser, banking the rest as a soil reserve.
This tool computes the rock phosphate in kg per hectare, the total product, the number of bags and the area covered from your target P₂O₅, the rock grade and your field size. Use it to dose acid soils, plantations and perennial crops, and to budget an organic-system phosphorus programme. Pair it with the Fertilizer (NPK) and Lime Requirement tools to balance the whole nutrient and pH plan.
Match the grade
Rate adjusts to your rock's P₂O₅ percent.
Suit acid soils
Slow release where acidity unlocks the P.
Plan the surplus
Apply more — only part is available yearly.
Order by the bag
Total product and bags for the field.
Frequently Asked Questions
What is rock phosphate?+
Rock phosphate is a naturally mined mineral fertiliser and a cheap, slow-release source of phosphorus. Ground to a powder, it releases phosphorus gradually over seasons rather than all at once, which suits perennial crops, plantations and soil-building programmes. It's also a common phosphorus input in organic systems where soluble fertilisers aren't allowed.
How is the application rate calculated?+
Material (kg/ha) = target P₂O₅ ÷ (grade% ÷ 100). You set the kilograms of P₂O₅ you want to supply per hectare and the grade of your rock phosphate (its P₂O₅ percentage), and the tool divides one by the other to give the kilograms of product, then scales it to your total area and into bags.
Why does rock phosphate work best on acid soils?+
Rock phosphate dissolves through soil acidity. On acid soils (low pH) the natural acids slowly break it down and release plant-available phosphorus over the seasons. On neutral or alkaline soils it stays largely insoluble and locked up, so very little reaches the crop — which is why it's the wrong choice there.
Why apply more rock phosphate than soluble fertiliser?+
Only a fraction of rock phosphate — the citrate-soluble portion — becomes available to the crop each year; the rest dissolves slowly over later seasons. So to match a given amount of available phosphorus in one season you apply more total product than you would with a fully soluble fertiliser, treating part of it as a long-term reserve in the soil.
What is the grade of rock phosphate?+
The grade is the total P₂O₅ content of the rock as a percentage — commonly in the range of 18–34% depending on the deposit. A higher grade means less product for the same P₂O₅, so always check your supplier's grade and enter it; the rate is very sensitive to this number.
What is citrate-soluble phosphate?+
It's the portion of the rock phosphate's phosphorus that is readily available to plants in a season, measured by a citrate-solubility test, as opposed to the total P₂O₅. Finely ground, reactive rock phosphates have a higher citrate-soluble fraction and act faster than coarse, hard rock.
When should I use DAP or SSP instead?+
On neutral and alkaline soils, or when a crop needs phosphorus quickly in its current season, use a soluble fertiliser such as DAP or single super phosphate (SSP) instead — rock phosphate is ineffective there. Rock phosphate is for acid soils and slow, long-term phosphorus building, not a fast fix.
How and when should I apply it?+
Apply rock phosphate well before or at planting and incorporate it into the soil so it contacts moisture and acidity and starts dissolving. Because it's slow-release, a single application can feed the crop over more than one season; it pairs well with organic matter, which helps mobilise the phosphorus.
Does this work in kg/ha or per acre?+
Enter your target P₂O₅ per hectare and your area, and the tool gives the rate per hectare, the total product and the bags for the whole field. If you plan in acres, use the per-area total and bag count it returns for budgeting and ordering regardless of the unit you think in.
Are the figures exact?+
They're solid planning figures based on your grade and target. The real availability of the phosphorus depends on soil pH, fineness of grind, rock reactivity, moisture and organic matter, so treat rock phosphate as a multi-season investment and confirm your target P₂O₅ against a soil test and crop recommendation.