The math behind every rate. Open. Cited. Yours to check.

Every number Zone Forge prints belongs to one of five tiers. Knowing which tier a number sits in is how you know whether to trust it, scrutinize it, or override it.

Your soil test, graded in your state's own words — or honestly left measured.

Upload a soil test and Zone Forge grades it for you — worst nutrient first, color- and word-coded, with variability hints and your fertility trend across past uploads.

In the 26 states whose extension labs publish rating words, we grade in that state's own language — Arkansas's "Very Low → Above Optimum," Delaware's Fertility Index Value, Oklahoma's "Low / Sufficient." In the other states, we show your measured value with a note on why we don't grade that lab format yet. An honest "measured" beats a wrong grade — a soil test run on a lab method our table isn't calibrated to degrades to "confirm your method" instead of mis-grading. pH and lime are calibrated for all 50 states, so that verdict shows everywhere.

How Zone Forge picks a rate.

The default Hybrid philosophy treats your soil-test value differently depending on which band it falls into. Below is the exact decision tree.

Different driver, different units, different product axis.

Liming is fundamentally different from N-P-K fertilization. The driver is the lab's reserve-acidity value (Mehlich Buffer Acidity for Penn State, Buffer pH for midwest crosswalks), the output is tons CaCO₃-equivalent per acre, and the product split is calcitic vs dolomitic — not which fertilizer maximises analysis %. We keep the code paths separate so the math never crosses.

1. Resolve target pH. Crop-specific (alfalfa 6.8, corn/soy/wheat 6.5, oats/rye/sorghum 6.0) → profile default → system default of 6.5. The provenance flows through to the UI so a farmer seeing alfalfa at 6.8 vs corn at 6.5 can see why.
2. Compute CaCO₃-equivalent demand using the right state's method. Zone Forge ships 50-state lime coverage where each state's source matches that state's land-grant extension publication. Penn State uses Mehlich Buffer / Exchangeable Acidity (the method PSU publishes today; Adams-Evans was retired). Delaware, New Jersey, South Carolina, Florida, and Alabama use Adams-Evans Coastal Plain (UDel anchor table, Auburn / Clemson / Rutgers / UF parallel citations). Georgia uses UGA's direct Ca(OH)₂ titration (LBCEq, 2025). Vermont uses reactive aluminum (the only state on this method). Illinois uses water pH × CEC class. Maryland uses texture × physiographic province → lb oxides/A. Arkansas uses pH + soil-test Calcium. Iowa, Tri-State (OH/IN/MI), and the other midwest / Plains states use buffer-pH crosswalks for SMP / Sikora / Woodruff. Western states with alkaline soils (AZ, NV, NM, UT, WY, most of CO) surface honest "lime not needed" messaging. If measured pH is already at or above target, return 0 — no lime even when reserve acidity is present.
3. Pick calcitic vs dolomitic from soil Mg. If Mg ppm < 50 or Mg base saturation < 10% ⇒ dolomitic (corrects pH and Mg together). Otherwise calcitic. No Mg data ⇒ default to calcitic — safer than loading Mg into balanced soil. Thresholds are configurable per source via the dolomitic_if JSON block, not hardcoded.
4. Convert to a physical product rate using ECCE (Effective Calcium Carbonate Equivalent — varies 60–100% across products). physical_tons = caco3_tons / (ecce / 100).

Mg correction stays uncoupled. The fertilizer engine produces a Mg fertilizer rate when soil Mg is low (defaults: dolomitic lime when liming is also required, otherwise K-Mag / langbeinite). The lime engine uses Mg only to choose calcitic vs dolomitic. Epsom salt is tagged as specialty / advanced and is excluded from default recommendations — at field scale on row crops, it isn't the practical product.

# Worked example — corn, low pH, low Mg, Tri-State source
measured_ph     = 5.8
buffer_ph       = 6.7
source          = "Tri-State 2020"
crop            = "corn"
mg_ppm          = 40

# Resolve target pH (crop > profile > system default)
target_ph       = crop_target_pH["corn"]
                = 6.5      # provenance: crop

# Lime required? 5.8 < 6.5 ⇒ yes
tons_caco3      = lookup(buffer_ph=6.7)
                = 3.0      # tons CaCO₃-equivalent / ac

# Source choice: Mg 40 < 50 ⇒ dolomitic
recommended     = "dolomitic"

# Convert to dolomitic lime product (ECCE 85)
product_rate    = 3.0 / 0.85
                = 3.53     # tons dolomitic lime / ac

Honest about our limits.

Trust runs both ways. Here's what Zone Forge intentionally does not do today.

• No MRTN curves for nitrogen. Today's nitrogen Rx is yield-driven (yield × removal × tie-up factor). MRTN integration with PSNT and prior-crop credits is on the roadmap.
• No tissue-test corrections. Soil-test only. Tissue-test integration may come later.
• No micronutrient blending. Zone Forge now blends macronutrients — assign N, P, and K to products and build one prescription (multi-layer by default, or a single blended bag) — but micronutrients aren't part of that yet: three micronutrient deficiencies still generate three separate prescriptions. Micronutrient blending is on the roadmap.
• No live fertilizer price feeds, and no margin/break-even math yet. Fertilizer cost is your input — we don't pull product prices from market APIs. We do show USDA AMS local cash bids for your crop on the field map (your nearby grain price, not futures); margin and break-even on top of that are coming.
• No low-pH lockout on phosphorus. You can sequence prescriptions yourself (lime first, then P) — Zone Forge doesn't enforce a particular order.
• No cation base-saturation correction. Albrecht-style Ca/Mg/K ratio balancing isn't in the engine. OM, CEC, and base-saturation values from your soil report show in the soil-data view as informational diagnostics, not as adjustable targets. Adding ratio-correction would require a different engine — we'd rather ship nothing than ship a half-built one.
• No proprietary "secret sauce." If a number isn't in a published table or computable from one, it's flagged as a placeholder. Always.