Soil test calculator
Adjust your dose to your soil test.
Enter your soil-test results — available P and K, organic carbon and pH — and a verified crop dose is adjusted up or down: build up low nutrients, maintain medium ones, draw down high ones.
Your adjusted dose will appear here
Pick a crop, enter your soil-test values, and hit calculate. Every adjustment step is shown.
Method
The method, in plain words
Build-up, maintenance, drawdown — the same logic soil labs use on health cards.
A blanket recommendation assumes an average soil. A soil test tells you what your field already holds: if phosphorus tests high, you can cut the P dose and save money; if it tests low, you raise it to build the soil up. This tool starts from a verified crop recommendation and applies exactly that logic — +30% for low-rated nutrients, base for medium, −25% for high.
Available phosphorus (Olsen method, mg/kg or ppm), available potassium (ammonium acetate method, kg/ha or ppm), organic carbon (%), and pH — the four values on a standard soil health card. You can enter P and K in either unit; the tool converts with the standard furrow-slice factor (×2.24).
Routine soil labs rarely report a mineral-nitrogen class, so extension systems use organic carbon as the proxy for native nitrogen-supplying capacity: more organic carbon means more nitrogen will mineralize during the season. It is a well-established proxy, but it is not a measured N value — the result is labeled as such.
No — pH never changes the calculated dose here. Above pH 7.5, phosphorus increasingly precipitates as calcium and magnesium phosphates (P fixation), so placement matters: band the full phosphorus dose near the seed at sowing instead of broadcasting. The tool flags this as a note, not a dose change.
The custom base option is generic guidance only — a starting point for crops with no verified recommendation in our database. If a verified crop×region dose exists, always use that instead; verified figures carry their published source.
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