Soil Test → Rate & Turn ppm Into a Real Recommendation

It runs your soil-test value through a published calibration — the Tri-State Fertilizer Recommendations (Ohio State, Purdue and Michigan State). The calibration defines a critical level and a maintenance range for each nutrient and method. Below the critical level the tool builds up toward it and adds crop removal; inside the maintenance range it applies crop removal only; above the range it recommends zero and lets you draw the soil bank down. The result is an actual P2O5 or K2O rate, not a generic 'add some fertilizer'.

It is the soil-test calibration logic at the heart of Tri-State and most land-grant systems. If your soil test is below the critical level, you BUILD it up over a few years while also replacing what the crop removes. Once it sits in the maintenance range you simply MAINTAIN — apply roughly what the crop removes so the test stays put. Above the maintenance range you DRAW DOWN — apply nothing and let the crop mine the surplus, saving money with no yield penalty.

For phosphorus it supports Mehlich-3 P, Bray-P1 and Olsen P; for potassium it uses the NH4OAc / Mehlich-3 K test. Mehlich-3 and Bray-P1 read similarly on most soils, while Olsen is the right test on calcareous, high-pH soils. Pick the method your lab actually used — the critical levels differ, so the wrong method gives the wrong rating and rate.

The rating bins your value relative to the critical and maintenance levels: very low (well below critical), low (below critical), optimum (in the maintenance range), high (above maintenance but moderate), and very high (well above). Very low and low soils trigger build-up; optimum triggers maintenance; high and very high trigger draw-down. The rating is the quick read; the rate is the action.

Build-up adds enough nutrient to raise the soil test from its current value up to the critical level. The Tri-State build factor — pounds of P2O5 or K2O to raise the test one ppm over the plow layer — is multiplied by the ppm gap to critical, then spread over the build period you choose (commonly three to four years) so the cost is not all in year one. That annual build-up is added to the crop-removal replacement.

Crop removal is the nutrient hauled off the field in the grain or hay, in pounds of P2O5 or K2O per bushel or ton. Even when you are building the soil test, you must replace what this season's crop takes or the build never gets ahead. In the maintenance range, removal IS the whole recommendation — you are replacing exactly what leaves so the test holds steady.

When your soil test sits above the maintenance range, there is more nutrient in the soil bank than the crop needs this year, so applying more is wasted money with no yield response. The economically correct move is to apply zero and draw the surplus down over a season or two until the test falls back into the maintenance range. The tool flags this as 'draw down'.

Yes. The tool multiplies the ppm gap to critical by the build factor and the price you enter for the oxide nutrient to show the total build-up cost per acre over your chosen build period. That figure separates the one-time investment of raising a depleted soil from the ongoing maintenance cost of replacing removal, so you can budget a build programme properly.

The calibration here is the Tri-State system, built for the corn-belt states of Ohio, Indiana and Michigan and broadly applicable across similar temperate grain regions. Critical levels and build factors are regionally calibrated, so if your state extension publishes its own soil-test calibration, prefer that. The build–maintenance–draw-down logic, however, is universal — only the numbers shift.

For phosphorus, a single pre-plant or banded application is usually fine because P is immobile. For potassium on sandy or low-CEC soils, splitting a large build rate can reduce leaching and luxury uptake. The tool gives the annual rate; how you place and time it is an agronomic decision that depends on your soil, tillage and crop.

Yes, through the removal term. A higher yield goal removes more nutrient, so the crop-removal component of the rate rises proportionally. The build-up component, by contrast, depends only on how far below critical your soil test sits, not on yield — so on a very low soil the build-up can dominate, while on an optimum soil the whole rate is removal driven by yield.