A soil test tells you what the paddock is short of. It does not tell you what to buy. Turning one into the other is where most of the value gets lost — and where most growers default to whatever they bought last year.
The numbers that actually drive the decision
pH
Read this first, because it governs everything else. Phosphorus availability peaks around pH 6–7 and falls away sharply at both ends: locked by aluminium and iron in acid soils, locked by calcium in alkaline ones. On strongly acid ground, lime often returns more than extra phosphate does.
pH also decides which products suit. On alkaline and calcareous soils — much of South Australia — the mildly acidic reaction around a MAP granule can improve early uptake compared with DAP.
Phosphorus (Colwell or Olsen P)
Check which test was used before comparing anything — Colwell and Olsen values are not interchangeable, and Colwell figures run considerably higher for the same soil. Compare against the critical value for your soil type and crop, not against last year.
Phosphorus barely moves in soil, so placement matters more than timing. Banding near the seed generally beats broadcasting the same rate.
Potassium (Colwell K or exchangeable K)
Sandy soils hold little potassium and are the usual place deficiency shows up. Potassium removal at harvest is heavy in hay, silage and root crops — taking a cut off the paddock exports far more potassium than grain alone does.
Nitrogen
Mineral nitrogen is a snapshot, not a budget. It moves with rainfall, temperature and mineralisation, so a figure from March may be meaningless by June. Treat nitrogen as a decision about target yield and in-season conditions rather than a number to correct.
Sulphur, and the ones people skip
Sulphur deficiency looks like nitrogen deficiency and gets misdiagnosed constantly. If sulphur is low, ammonium sulphate or sulphate of potash addresses two things at once. Also read organic carbon, which tells you how much nitrogen the soil is likely to supply itself.
Turning a deficit into a product rate
This is the arithmetic that matters, and it is simpler than it looks.
| Step | Worked example |
|---|---|
| 1. Nutrient target, kg/ha | Soil test and yield target suggest 40 kg/ha of phosphorus as P2O5 |
| 2. Pick the product | DAP is 18-46-0, so 46% P2O5 |
| 3. Divide | 40 ÷ 0.46 = 87 kg/ha of product |
| 4. Check the passengers | 87 kg/ha of DAP also delivers about 16 kg/ha of nitrogen — subtract that from your nitrogen plan |
| 5. Multiply by area | 400 ha × 87 kg = 34.8 tonnes |
Every one of our 130 product pages carries a table showing exactly what one hectare receives at rates from 50 to 300 kg/ha, so you can work this without a calculator.
Where a standard grade stops fitting
Run the sum for each nutrient. If one product gets you to target on all three, buy it. More often the nearest standard grade over-supplies one nutrient to deliver enough of another — and across a whole property you pay for that surplus on every hectare.
That is the point at which a custom blend costs less than the standard grade, even at a higher price per tonne. The saving comes from not over-applying, not from a cheaper headline.
What to send us
- The full lab report, not a summary — and note which P test was used
- Crop and target yield
- Area, and whether the test is composite or paddock-specific
- What went on last season and at what rate
- Nearest town, so freight can be quoted
Send it through soil test matching and we will come back with the products that fit, or a blend specification if none do.
Email the lab report with your crop and area, and we will match it to our range and quote it delivered.