Soil Productivity Index
Definition
A soil productivity index is a numerical rating of a soil's inherent capacity to produce crops, assigned to each soil map unit from soil survey data such as soil type, texture, water-holding capacity, slope, erosion and depth. Several states publish their own: Iowa's Corn Suitability Rating 2 (CSR2), scaled from 100 for the most productive soils down to 5; Illinois' productivity index, which AcreTrader's Illinois offerings cite; and Minnesota's crop productivity index. A farm's figure is usually an area-weighted average of the ratings of the soils on its tillable acres. Buyers and appraisers use these indexes to compare farms and to express prices per index point, because better soils support higher yields and rents. The indexes are regional and measure inherent soil potential, not the yields a particular farm achieves, which also depend on drainage, management, weather and, in the West, water.
Why it matters
Soil quality is the most durable driver of a row-crop farm's rent and value, and productivity indexes are how the market prices it. Two farms in the same county can differ by thousands of dollars an acre largely because of their soils. For investors in fractional farmland, the index is a quick check on whether a purchase price is in line with comparable sales, but it can also be misused: an index quoted for the whole farm may include acres that are not farmed, a high index may sit on land that floods without drainage, and an index from one state means nothing in another. Pairing the index with yield history, tillable acres and drainage records gives a fuller picture.
Common misconceptions
- •A soil index is not a yield. It rates inherent potential; management, drainage and weather decide yields.
- •Indexes are not comparable across states. CSR2, Illinois' index and Minnesota's index use different scales.
- •A whole-farm index can include non-farmed acres. Weight it across tillable acres.
- •A high index does not mean good drainage. Many high-rated Corn Belt soils need tile to reach their potential.
- •Price per point does not adjust for everything. Location, buildings, water and field layout still matter.
Technical details
Computing a farm's weighted index
List each soil map unit on the tillable acres with its acres and rating.
Example: 80 acres at 88, 50 acres at 76 and 20 acres at 60 (150 tillable acres).
Weighted index = (80 x 88 + 50 x 76 + 20 x 60) / 150 = (7,040 + 3,800 + 1,200) / 150 = 80.3.
Price per point: at $12,000 a tillable acre, $12,000 / 80.3 = about $149 per point.
Use the same index version (for CSR2, the ISU or NRCS calculation) for every farm compared.
The state systems investors meet
Iowa, CSR2: 5 to 100, computed per map unit from soil survey data; see Corn Suitability Rating 2.
Illinois, productivity index: cited in AcreTrader's Illinois offerings (Buffalo Hart Farm 131.1); a different scale from CSR2.
Minnesota, crop productivity index: used for Minnesota farms such as the Red River Valley.
Elsewhere: USDA soil survey data and local land capability classes, with yields and water often more important than an index.
Using the index with other evidence
Yield history: multi-year actual yields for the farm, ideally verified.
Tillable acres: index weighted over farmed acres, with non-farmed acres valued separately.
Drainage: tile maps, installation dates and outlet condition.
Rent: current cash rent against local rents for similar index values.
Comparable sales: price per acre and per point for recent nearby sales.
How investors apply the term
Investors usually apply this term by tying the definition back to the controlling documents and then asking which cash flows, rights, restrictions, or valuation assumptions it changes. A useful review looks at the term alongside fees, leverage, liquidity, reporting frequency, and manager discretion rather than treating it as a standalone concept.
The practical diligence test is simple: identify who benefits if the assumption is wrong, who has control when conditions change, and what evidence the investor receives after closing. Terms that cannot be reconciled to documents, cash movements, and monitoring data should be treated as marketing shorthand until proven otherwise.
Frequently asked questions
- How do investors measure the productivity of farmland soil?
- With the state's soil productivity index, weighted across the farm's tillable acres, alongside the farm's yield history. AcreTrader's Buffalo Hart Farm offering, for example, reports a soil productivity index of 131.1 and a thirteen-year history averaging 228 bushels of corn and 69 of soybeans an acre. The index describes the soil's potential; the yield history shows what this farm and its operators achieved.
- Can soil indexes from different states be compared?
- No. Each state's index uses its own scale and method: Iowa's CSR2 runs from 5 to 100, while Illinois' productivity index uses a different scale and AcreTrader's Illinois offerings cite values above 100. Compare farms only within the same index, and use yields and rents to compare across states.
- What is price per productivity point?
- The sale price per acre divided by the farm's index, a common way to compare land sales within a state. AltStreet's AcreTrader review notes that AcreTrader's comparisons rank offerings per tillable acre and per soil productivity point using third-party sales data. It normalizes for soil quality but not for drainage, location, buildings or water.
- What do soil indexes leave out?
- Drainage condition (a poorly drained soil with good tile may outyield a better-rated soil without it), management, weather and climate trends, irrigation and water rights, field shape and size, and access. They are a starting point for soil quality, not a forecast of yields or returns.