VRA Prescription & Map the Right Rate

Each zone's soil-test rating sets a multiplier on the crop's nutrient removal at your target yield. Very Low builds heavily at 1.5 times removal, Low builds at 1.25, Medium maintains at 1.0 (just replacing removal), High draws the surplus down at 0.5, and Very High applies a starter-only 0.15. So zone rate (kg/ha of nutrient) = crop removal × the rating multiplier, done separately for N, P and K.

A flat rate applies one fertilizer rate across the whole field, usually the maintenance rate. Variable rate gives each management zone its own rate based on its soil test — more where fertility is low, less where it is already high. The tool computes both and shows the product and cost saved by matching the rate to each zone instead of over- or under-applying.

Nutrient rate is divided by the carrier's analysis fraction, then multiplied by the zone area in hectares. Urea is 46 percent N, so 100 kg/ha of N needs 100 ÷ 0.46 ≈ 217 kg/ha of urea; over a 10 ha zone that is about 2,174 kg of urea. DAP supplies P2O5 at 46 percent and MOP supplies K2O at 60 percent, and the tool uses whichever carrier you pick per nutrient.

They are nutrient-removal coefficients — kilograms of N, P2O5 and K2O removed per tonne of produce — from IPNI and ICAR removal tables, multiplied by your target yield. Wheat removes about 25 kg N, 11 kg P2O5 and 20 kg K2O per tonne; the reference table on this page lists every crop's default yield and removal figures the tool uses.

These are the three soil-test fertilizer philosophies. Build-up applies more than the crop removes to raise a low soil reserve toward the critical level. Maintenance replaces exactly what the crop removes to hold the soil test steady. Draw-down applies less than removal to mine a surplus on a high-testing zone, saving fertilizer while the reserve falls back to the optimum.

It depends on how variable your field is. If most zones already test high, VRA can cut total product and cost substantially by drawing those zones down, while still building the low zones. The tool reports the kilograms of product and the cost saved versus the flat maintenance plan, plus the saving as a percentage, so you can see the payback for your specific zones.

Enter your realistic target yield for the crop, because removal scales directly with it — a higher target removes more nutrients and so raises the maintenance baseline. If you leave it at the crop default, the tool uses the typical target yield from the removal table. Match it to your field's history for the most accurate prescription.

For nitrogen, urea (46-0-0) or UAN (32-0-0); for phosphorus, DAP (18-46-0) or TSP (0-46-0); for potassium, MOP/potash (0-0-60) or SOP (0-0-50). The carrier's analysis fraction sets how much product each nutrient rate needs, and the tool also reports the implied whole-field blend grade from your totals.

Yes — the calculator starts with four example zones, each with its own area and N, P and K ratings, and you can edit every value live. Add a zone for each distinct fertility area your soil-test map shows; the totals, blend grade and flat-rate comparison update immediately as you change any zone.

On a genuinely high or very-high testing zone, no — the soil reserve already exceeds the crop's needs, so applying below removal mines that surplus without yield loss, which is the point of draw-down. The risk is mis-rating a zone, so base the ratings on a recent soil test, and keep at least a starter rate (the Very High 0.15 multiplier) for early vigour.

It is a planning calculator that gives you the per-zone rates, product and cost so you can build and justify a prescription. It does not export a shapefile or controller file directly; take the rates it produces into your VRA software or spreader controller. The figures are agronomic; calibrate your spreader to deliver them accurately in the field.