The soil productivity index, formula and all

What the index is

A soil productivity index is a single 0 to 100 score for how productive a soil is for crops. It exists because a full soil report is a lot to read and hard to compare. Reducing it to one number lets you rank field against field or parcel against parcel at a glance, then dig into the detail only where the score says it matters.

The honest caveat that belongs on every index: it is a model of inherent soil potential, not a promise of a yield. Weather, management, drainage upkeep, and price all move the actual result. What a good index does is tell you what the ground is capable of before any of that, which is exactly what you want before a lease, a purchase, or a program decision.

The AgriTerra formula, published

AgriTerra computes its productivity index as a weighted blend of four USDA SSURGO soil fields. Here is the whole thing, weights included.

The AgriTerra soil productivity index: inputs and weights

Input	SSURGO field	Weight
Land capability class	Non-irrigated capability class (I through VIII)	40%
Drainage class	Natural drainage class	25%
Organic matter	Organic matter (percent)	20%
Available water capacity	Available water capacity (inches of water per inch of soil)	15%

Each input is scored on its own 0 to 100 sub-scale (shown below), then the four are weighted and summed to a single 0 to 100 index. The inputs are read straight from USDA SSURGO; the weighting is AgriTerra modeled.

Each input is scored on its own sub-scale before the weights are applied:

Land capability class (40% of the score)

• Class I (few limitations): 100 of 100
• Class II (moderate limitations): 85 of 100
• Class III (severe limitations): 65 of 100
• Class IV (very severe limitations): 40 of 100
• Class V to VIII (generally unsuited to cultivation): 10 of 100

Drainage class (25% of the score)

• Well or excessively drained: 100 of 100
• Moderately well drained: 80 of 100
• Somewhat poorly drained: 50 of 100
• Poorly drained: 25 of 100
• Very poorly drained: 5 of 100

Organic matter (20% of the score)

• 4 percent or more: 100 of 100
• 3 to 4 percent: 80 of 100
• 2 to 3 percent: 60 of 100
• 1 to 2 percent: 40 of 100
• Under 1 percent: 20 of 100

Available water capacity (15% of the score)

• 0.20 or more: 100 of 100
• 0.15 to 0.20: 80 of 100
• 0.10 to 0.15: 60 of 100
• Under 0.10: 30 of 100

So the composite is: land capability class score times 0.40, plus drainage score times 0.25, plus organic matter score times 0.20, plus available water capacity score times 0.15, rounded and held to the 0 to 100 range. Nothing hidden, nothing proprietary. If you disagree with a weight, you can see exactly what to argue about, which is the point of publishing it.

From score to modeled yield

The index is not the end of the line. AgriTerra runs the score through a published yield curve to estimate row-crop potential: modeled corn yield equals 140 bushels per acre plus the index as a fraction of 100 times 100 bushels, held within a 140 to 240 bushel range. A parcel scoring 50 models to about 190 bushels; a top-scoring parcel approaches the 240 bushel ceiling. That modeled yield is labeled as modeled, never as a guarantee. See what farmland produces for how yield turns into per-acre economics.

Productivity index vs NCCPI vs CSR2

Three indices come up most often. They measure similar things in different ways, and knowing which is which saves a lot of confusion.

• NCCPI (National Commodity Crop Productivity Index, USDA NRCS): a national model that arrays soils by their inherent capacity to grow dryland commodity crops, through four submodels for corn, soybeans, small grains, and cotton. It reports on a 0.01 to 1.0 scale and ships inside SSURGO, so it exists for every mapped soil in the country.
• CSR2 (Corn Suitability Rating 2, Iowa State University): an index of inherent row-crop productivity for Iowa soils on a 5 to 100 scale, heavily used in Iowa farmland assessment and land values. It is Iowa-specific and does not exist outside Iowa.
• AgriTerra productivity index: a transparent 0 to 100 composite of four SSURGO fields, published above, computed for any parcel in the country. It is designed to be read and checked, not treated as a black box.

The practical takeaway: if you farm in Iowa, CSR2 is the local currency and worth knowing. Anywhere in the country, NCCPI gives you a USDA-native national score and the AgriTerra index gives you a published, checkable one in the same read as your drainage, water, and revenue options. They will not agree to the decimal, because they weight things differently; used together they triangulate the same underlying truth about the ground.

To see where these fields come from, start with how to use Web Soil Survey. For how every figure on this site is sourced, see how we get our numbers.