Soil Productivity Index Explained — Why the Difference Between 119 and 132 Costs $2,000 Per Acre

The Number That Drives Illinois Farmland Value

If you own farmland in Illinois and you do not know your ground's Productivity Index score, you are navigating the most important financial decision in that asset without the most important data point. The PI — developed by the University of Illinois — is the single most consistent predictor of per-acre farmland value in the state. Understanding it is not optional for serious landowners or buyers.

What the PI Scale Measures

The Productivity Index is a numeric rating assigned to each soil type in Illinois based on that soil's capacity to produce corn under average management conditions. The scale runs from roughly 50 on the state's least productive soils — poorly drained, heavy clay, or thin topsoil profiles — up to 147 on the deepest, best-structured dark loam soils found in the most productive farming counties in the state. Most farmland in central Illinois falls somewhere between PI 100 and PI 140.

The score is assigned by soil type, not by individual parcel or management history. If your ground is mapped as Drummer silty clay loam — one of the most productive soil series in Illinois — every acre of that soil type carries the same PI regardless of whether it has been farmed by an excellent operator for 40 years or neglected for a decade. The PI reflects the soil's inherent capacity, not what any particular tenant has done with it.

This matters for valuation because it creates an objective anchor that is not subject to interpretation. Two buyers looking at the same farm can disagree about the quality of the drainage tile, the condition of the farmstead, or the favorability of the existing lease — but they cannot reasonably disagree about the PI score of the soil types mapped on the ground. The USDA has mapped every square mile of Illinois, and that data is publicly accessible.

The Dollar-Per-PI-Point Math

At current central Illinois market pricing, the relationship between PI and per-acre value runs at approximately $100 to $150 per PI point. That range reflects the fact that different counties trade at different underlying multiples — McLean County farmland trades at a higher absolute price than Menard County farmland at equivalent PI — but the per-point increment within a county is relatively consistent.

The practical implication: the difference between PI 119 and PI 132 — thirteen points — translates to approximately $1,300 to $1,950 per acre at current pricing. On a 160-acre farm, that gap is $208,000 to $312,000. On a 320-acre parcel, the difference between owning PI 119 ground and PI 132 ground in the same county approaches $600,000 in total asset value. This is not a rounding error. PI is the core variable that needs to be understood before any valuation conversation begins.

How to Look Up Your PI — The USDA Web Soil Survey

The USDA Natural Resources Conservation Service maintains the Web Soil Survey at websoilsurvey.sc.egov.usda.gov. This is the public interface for the national soil mapping database and it allows anyone to pull soil data for any parcel in Illinois. The process is not complicated: navigate to the tool, use the area of interest selector to outline your parcel, and request the Soil Data Explorer report. Under soil properties and qualities, you can access the Illinois-specific Productivity Index ratings by soil type.

The Web Soil Survey does not give you a single PI number for your farm — it gives you the PI for each mapped soil type that appears on your ground, along with the acreage of each type. To calculate a weighted average PI for your farm, multiply the acreage of each soil type by its PI score, sum the results, and divide by total farm acres. That weighted average is what buyers, appraisers, and assessors use as the baseline PI for your property.

Two caveats: the Web Soil Survey mapping is based on aerial interpretation and field sampling done decades ago in some cases. Soil mapping is not perfect. Boundaries between soil types on the ground may not match exactly what the survey shows. And drainage improvements — particularly tile drainage installed after the original soil survey was conducted — can materially improve the functional productivity of soils that are rated lower on the PI scale due to drainage limitations. An accurately tiled field that was mapped at PI 112 due to wetness issues may perform more like PI 120 ground in practice.

The Board of Review Connection

In Illinois, farmland is assessed for property tax purposes based on its agricultural economic value, which is tied to soil productivity. The assessment formula uses the PI score of the soil types on your ground to determine the expected crop revenue and derive the assessed value. If your ground is incorrectly mapped — if the USDA soil survey places a lower-PI soil type on acreage that actually contains higher-PI soil — your assessed value may understate your ground's taxable productivity, or in some cases overstate it.

The Board of Review process allows landowners to challenge assessments they believe are incorrect. In Menard County, where I serve on the Board, soil classification challenges come up regularly. The successful challenges are built on documentation — actual soil sampling, yield data over multiple years, and in some cases independent soil scientist review. When the documentation supports a reclassification, the Board has authority to adjust the assessment accordingly. It is a legitimate process and worth pursuing when the underlying mapping error is documented and material.

Using PI in a Transaction

Whether you are buying, selling, or just trying to understand what your ground is worth, the PI conversation should happen early. For buyers: request the Web Soil Survey data before you make an offer. For sellers: know your weighted average PI before you set an asking price or go to auction. For landowners evaluating an unsolicited offer: understand that the buyer knows the PI — and if they are not mentioning it, there is usually a reason.

The PI is not the only variable in farmland value. Drainage infrastructure, access, lease structure, and local market dynamics all matter. But PI is the anchor. Everything else is an adjustment above or below that anchor.

Jared Williams is the Managing Broker and owner of Archer Realty & Auction LLC. He serves on the Menard County Board of Review and specializes in agricultural land sales across central Illinois, with direct experience in soil productivity analysis and farmland valuation. Start the conversation at archerrealty.net.