Illinois Soil Productivity Index: Farmland Assessment Values

Illinois assesses farmland using a soil Productivity Index rather than market value: every soil type in the state carries a PI score from 47 to 147, and that score drives a state formula that turns expected net farm income into a certified per-acre assessed value. For the 2026 assessment year, those certified values run from about $436 per acre on the lowest-rated soils to roughly $1,066 per acre on the most productive ground.¹ Your local tax rate is then applied to that assessment, not to what the land would fetch on the open market.

What the Productivity Index Measures

The PI is a numerical score assigned to each of roughly 800 soil types mapped across Illinois. The scale runs from 47 for the least productive soils to 147 for the richest ground, and the rating reflects how much corn, soybeans, and wheat a soil can produce under skilled management.² The deep prairie soils of central Illinois sit at the top of the scale; thinner or clay-heavy soils in the southern hills and river bluffs land near the bottom.

Soil scientists build the score from physical characteristics of the ground itself. Slope governs water retention and erosion risk. Texture and organic matter determine nutrient availability. Drainage decides whether roots can develop. A flat, well-drained field with rich organic content scores far higher than a steep, poorly drained hillside in the same county.

The scores are calibrated to yields achieved by the top 16 percent of Illinois farmers during the 1990s, roughly one standard deviation above the statewide average. That “optimum management” benchmark assumes the operator uses the best available practices and technology.² Two University of Illinois publications catalog the data: Bulletin 810 covers average-management yields, and Bulletin 811 contains the optimum-management yields and PI ratings that feed the assessment formula.

How the PI Becomes a Dollar Assessment

The Illinois Department of Revenue certifies a per-acre equalized assessed value (EAV) for every PI number by May 1 each year. The calculation uses five-year rolling averages so a single bad crop year or price spike does not whipsaw assessments.³ The formula moves through four steps:

Gross income per acre, estimated from crop yields tied to the PI, the actual crop mix grown on soils with that PI, and the average prices farmers received.
Non-land production costs, supplied by the University of Illinois College of Agriculture, covering seed, fertilizer, equipment, labor, and similar inputs.
Net return to land, which is gross income minus production costs.
Agricultural economic value, calculated by dividing the net return by a five-year average of the Federal Land Bank farmland mortgage interest rate. This capitalization step converts annual income into a total land value.

The EAV that lands on your tax bill is one-third of the agricultural economic value, matching the state’s standard 33-1/3 percent assessment level.³ Local taxing districts then multiply your total EAV by their combined rate to produce the final bill. The PI never sets your tax directly; it sets the assessed value the rate is applied to.

The 10 Percent Annual Cap

No matter what the formula produces, the certified EAV for any PI cannot rise or fall by more than 10 percent from the previous year’s value for the median cropped soil.³ If the formula pushes values up 20 percent in a year when corn prices surge, only 10 percent of that increase shows up. The remainder carries forward into future years. The cap works in both directions, so a steep drop in farm income does not crater assessments overnight either.

2026 Certified Values by PI

A few benchmarks from the 2026 certified values table show how steeply the per-acre figure climbs with soil quality:¹

PI 90: $458.05 per acre
PI 100: $526.06 per acre
PI 110: $614.44 per acre
PI 120: $730.76 per acre
PI 130: $1,066.45 per acre

The full table is published each year on the Illinois Department of Revenue’s farmland assessment page.⁴ The jump between PI 120 and PI 130 is especially sharp, which is how the system puts a heavier share of the tax load on the most productive ground.

Assessment by Land Use, Not Just Soil

Not every acre on a farm is row-cropped, and the law assesses different land uses at different fractions of the cropland value for the same PI. Four categories are defined by statute:⁵

Cropland is assessed at the full EAV for its PI, with adjustments for slope, drainage, flooding, ponding, and field size.
Permanent pasture is assessed at one-third of the cropland EAV for the same PI.
Other farmland, including woodlots and land in timber, is assessed at one-sixth of the cropland EAV.
Wasteland is assessed at its contributory value to the overall parcel, which is typically minimal.

These fractions matter in dollars. A 20-acre woodlot with a PI of 110 would be assessed at about $102 per acre in 2026, one-sixth of $614.44, while the adjacent cropped field at the same PI carries the full $614.44.¹ If your assessor has permanent pasture or timber ground recorded as cropland, you could be paying several times what the law calls for.

Certain conservation uses carry their own treatment. Land in CRP or CREP planted to grass is assessed as cropland; once trees planted on CRP ground mature past the point of easy return to cropping, it shifts to the one-sixth “other farmland” rate.⁶ Qualifying vegetative filter strips are assessed at one-sixth of the cropland EAV.⁷ Wooded acreage managed under a forestry plan accepted by the Illinois Department of Natural Resources is also treated as “other farmland” at one-sixth, and unimproved tracts of five or more contiguous acres under an approved Conservation Stewardship plan are valued at just 5 percent of fair cash value starting January 1 of the year after the plan is submitted.

What Land Qualifies for the Farmland Assessment

The PI-based assessment only applies to land that meets the statutory definition of a farm. The property must have been used as a farm for the two years immediately preceding the assessment year, and it cannot be primarily residential with farming as an incidental use.⁸ A garden behind a house does not qualify. A parcel actively in row crops or supporting livestock does, once it clears the two-year threshold. New ground converted to farmland for the first time has to wait out that period before receiving the agricultural assessment, though land folded into an existing farming operation may not face the same delay.⁹

Checking and Correcting Your Soil Records

Because the PI-based assessment rests entirely on which soils the assessor has recorded under your fields, verifying that record is the single most useful thing an owner can do. The USDA Natural Resources Conservation Service runs a free online tool, the Web Soil Survey, that overlays soil maps onto individual parcels.¹⁰ You enter an address or zoom to your property and the tool returns a color-coded map with each soil unit’s alphanumeric code. Those codes correspond to the PI ratings published in Bulletin 811.

Pull your property record card from the County Supervisor of Assessments. It lists every soil type and the acreage of each on your parcel. Compare that against the Web Soil Survey and against what the ground actually looks like today. Field boundaries shift after drainage work. Erosion changes topsoil profiles. Acreage figures can simply be wrong.

If the records do not match reality, file a complaint with the county Board of Review. The deadline is 30 calendar days after the assessment list is published, so watch for that notice in your local newspaper or on the county website.¹¹ Bring evidence: a private soil test from a certified lab, updated NRCS soil maps, photographs showing erosion or drainage changes, or a written report from an agronomist. If the Board of Review agrees with the claim, the records are corrected and the assessment adjusts in the following tax cycle. The burden to initiate that correction falls on the owner. Assessors do not revisit old mapping on their own.

1
Illinois Department of Revenue. Certified Values for Assessment Year 2026
2
University of Illinois College of Agricultural, Consumer and Environmental Sciences. Bulletin 811 – Optimum Crop Productivity Ratings for Illinois Soils
3
Illinois General Assembly. Illinois Code 35 ILCS 200/10-115 – Department Guidelines and Valuations for Farmland
4
Illinois Department of Revenue. Farmland Assessments
5
Illinois General Assembly. Illinois Code 35 ILCS 200/10-125 – Assessment Level by Type of Farmland
6
Illinois Department of Revenue. Publication 135 – Preferential Assessments for Wooded Acreage
7
Justia Law. Illinois Code 35 ILCS 200/10-152 – Vegetative Filter Strips
8
FindLaw. Illinois Code 35 ILCS 200/10-110
9
Illinois Department of Revenue. Publication 122 – Instructions for Farmland Assessments
10
USDA Natural Resources Conservation Service. Web Soil Survey
11
FindLaw. Illinois Code 35 ILCS 200/16-55