A fertility strategy: below the critical level, apply extra to raise the soil over a few years, then apply crop-removal rates to hold it there.
Buildup-and-maintain is one of the two main philosophies for P and K. When a soil tests below its critical level, you apply more than the crop removes ('buildup') to raise the soil-test value over several years; once you're at the target, you switch to applying roughly what the crop removes ('maintenance') to hold it.
The alternative is sufficiency, which applies only what this year's crop needs and lets the soil-test value ride. Different states lean different ways, and the buildup factors (how much it takes to move the test a point) are calibrated locally — another reason the source matters.
Related: Mehlich-3 · Cation exchange capacity (CEC)