Rock Phosphate & Rate for P₂O₅ on Acid Soils

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.