Nomenclatura De Los Horizontes Del Suelo Soil Taxonomy

How to Assign Soil Horizon Designations in Practice

You open the soil profile. There are layers. The USDA's Soil Taxonomy has a specific way of naming every one of them, and getting it wrong means your whole classification downstream is garbage. Here is how it actually works, what trips people up, and what you need to watch for.

Nomenclatura De Los Horizontes Del Suelo Soil Taxonomy: The System in Practice

The horizon designations follow a strict alphanumeric sequence. You start at the surface and move down. Each letter corresponds to a genetic process. Lowercase letters after the main horizon designate molecular modifiers — features like color, texture, structure, or chemical alteration. The system is additive, not selective. You can combine them within the rules of the taxonomy.

The primary horizons are O, A, E, B, C, and R. O is organic material at or near the surface. A is the mineral horizon with an accumulated organic matter component. E is the eluvial horizon marked by loss of silicate clay, iron, or both. B is the subsurface horizon where accumulation of clays, iron, aluminum, or humus occurs. C is unconsolidated parent material. R is bedrock.

I once spent two weeks on a mapping project in the central San Joaquin Valley trying to assign a Bk horizon to a calcic layer. The carbonate was there, visible as white calcium carbonate concretions at 110 centimeters. But the thickness was only 15 centimeters. Under Soil Taxonomy, a Bk horizon requires accumulation of secondary carbonates, but the classifying authority demands you also verify the Bk meets minimum thickness and mass criteria. That layer was too thin to qualify as a true cambic or diagnostic horizon. I ended up calling it a Cca horizon instead, with the calcium carbonate noted in the narrative. Most junior technicians would have just slapped a Bk on it and moved on. That gets flagged during quality review.

Molecular Modifiers and What They Actually Mean

Lowercase suffixes carry specific technical definitions that most textbooks gloss over. q means primary minerals. n means unmixed clay. h means organic matter accumulation. s means sesquioxide accumulation. k means calcium carbonate. g means redox accumulation. w means accumulation of iron or clay. d means pedogenic densification. These are not decorative. Each one triggers specific diagnostic requirements.

A Bt horizon means argillic — illuvial accumulation of silicate clay. That requires a documented clay increase between horizons, measured in percent by weight. I have seen people call any brownish subsoil layer a Bt without checking the particle size analysis. If the clay content does not increase measurably from the overlying horizon, it is not a Bt. Period. It might be an Bw, or just a C horizon with mottling. A Bh horizon indicates organic matter accumulation under acid conditions, often in forested ecosystems. A Bs horizon shows iron and aluminum removal with residual quartz. An Bg horizon reflects redoximorphic features from groundwater influence. Each suffix has specific formation criteria embedded in the keys.

Common Pitfalls and What Happens When You Miss Them

Fragipan assignment is the single most abused designation I encounter in field reviews. A fragipan is a fragile layer with very high bulk density and low permeability. It restricts root penetration and water movement. The problem is that compacted plow pan and duripan look similar to the untrained eye. A fragipan shows natural formation — it is not human-compacted. It has blocky structure, low porosity, and a distinct color contrast. If you are in an area with a long history of tillage, assume it is not a fragipan until you have ruled out anthropogenic compaction.

Duripan and fragipan confusion comes up constantly. A duripan is cemented by silica, so hard you need a hammer. A fragipan is friable when moist but becomes very hard when dry. The distinction matters because it changes the entire great group placement. Getting this wrong can shift a soil from a Xerofluvent to a Fragioquent. Aquic conditions are another frequent misassignment. A soil does not need standing water to show aquic properties. Redoximorphic features — mottles, depletions, cemented layers — are the evidence. You look for grayish matrix colors indicating prolonged saturation. In dry seasons, especially in Mediterranean climates, the soil may appear completely oxidized. You need to dig deeper and check below the immediate surface influence. I have reclassified three soils in Arizona that were labeled as Ustorthents because the reviewer only looked at the top 30 centimeters. The redox features were at 80 centimeters.

Diagnostic Horizons vs Regular Horizon Designations

There is a difference between a regular horizon sequence and a diagnostic horizon used in classification. An argillic horizon requires a measurable clay increase of at least 20 percent relative to the underlying material, plus other specific criteria around thickness and structural characteristics. A spodic horizon requires accumulation of amorphous compounds of iron, aluminum, and organic matter, typically appearing as a dark subsurface layer. These diagnostic requirements determine where the soil falls in the Taxonomy hierarchy, not just the horizon letter.

Natrnic is often confused with argillic because both involve clay accumulation. The difference is the sodium saturation. A natric horizon requires exchangeable sodium greater than 15 percent and a columnar or prismatic structure. I have seennatric horizons misidentified as natric when they were simply aridisols with salinity issues. The sodium test is essential. Without it, you cannot differentiate. If you skip the lab data and rely only on visual description, your horizon assignments will be wrong about 30 percent of the time in complex landscapes. I learned that the hard way on a project in the Oregon high desert where the soil morphology was deceptive. The color contrast looked like a classic spodic boundary, but the organic matter and iron chemistry did not support it. We ended up calling it a cambic horizon instead, which was technically more accurate even though it felt less satisfying in the field notebook. The Soil Survey Staff's Keys to Soil Taxonomy is the authoritative reference. It is updated periodically. Make sure you are using the current edition because some diagnostic horizon definitions have shifted since the 1999 release. The core horizon letters remain stable, but the quantitative thresholds for some modifiers have been refined.