Understanding the schema is the difference between struggling with SSURGO and mastering it. This is your permanent reference — join chain, key tables, live column lookup, and the most common mistakes.
Every SSURGO query follows this spine. Four tables. Three joins. One pattern to rule them all.
Pre-aggregated map unit attributes — 150+ soil properties already summarized. AWS, KSAT, flooding frequency, farmland class. Use when you don't need component-level detail.
Soil interpretation ratings — NCCPI productivity, Fragile Soil Index, drainage suitability, building site ratings. Join on cokey, filter by mrulename and ruledepth=0.
Root-limiting layers: fragipan, lithic bedrock, paralithic bedrock, duripan. Use to find depth-to-restriction for root zone depth calculations.
Texture class and modifier for each horizon. Join on chkey, filter rvindicator='Yes' for representative value. Returns sandy loam, silty clay loam, etc.
Rock fragment type, shape, hardness, and volume % for each horizon. Essential for SOC calculations (fragment correction) and construction suitability.
MLRA overlap — which Major Land Resource Areas a map unit falls in. Used for regional soil analysis and NCCPI normalization within MLRAs.
Search any table name to see its columns, data types, and descriptions — fetched live from SDA MetadataColumn.
Without AND c.majcompflag = 'Yes', you get every component including minor inclusions that cover only 5–10% of a map unit. Your results will have 3–10× more rows than expected and weighted averages will be skewed.
AND c.majcompflag = 'Yes' when you want dominant soil data.A common mistake is joining legend to mapunit using ON l.areasymbol = mu.areasymbol — but mapunit has no areasymbol column. The correct join is always ON mu.lkey = l.lkey.
INNER JOIN mapunit mu ON mu.lkey = l.lkeyHorizon AWC (awc_r) can be NULL for organic surface layers, bedrock, and water features. A raw multiplication will return NULL for the entire horizon sum.
ISNULL(ch.awc_r, 0) in AWS calculations to treat NULL as zero thickness contribution.The Soil Intelligence companion site has a D3 force-directed ER diagram of all 134 SSURGO tables, sortable column browser with all 12 metadata fields, and a live domain value decoder.