TNRCC Instructions
On-Site Sewerage Facilities (OSSF)
Soil Texture Analysis for OSSF Site Evaluation 

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Soil Texture Classes  Texture is the relative amount of the different sizes of mineral particles in a soil. All soils are composed of sand (0.5 - 2.0 mm in size); silt, which includes intermediate sized particles that cannot be seen with the naked eye, but feels like flour when pressed between the fingers (0.002 - 0.05 mm); and clay, which is extremely small in size and gives cohesion to a soil (less than 0.002 mm). The texture of the different soil horisons of soils may be classified into five general groups which will be used for determining the application rate in Table I.  

Soil Class Ia  Sandy texture soils which contail more than 30% gravel.  

Soil Class Ib  Sandy soils which contain less than or equal to 30% gravel. It includes sand and loamy sand.  

    • Sand: Sand has a gritty feel, does not stain the fingers, and does not form a ribbon or ball when wet or moist.
    • Loamy sand: Loamy sand has a gritty feel, stains the fingers (due to silt and clay components), forms a weak ball, and cannot be handled without breaking.

Soil Class II  Course loamy soils which includes sandy loam and loam textures.  
    • Sandy loam: Sandy loam has a gritty feel and forms a ball that can be picked up with the fingers and handled with care without breaking.
    • Loam: Loam may have a slightly gritty feel but does not show a fingerprint and forms only short ribbons from 1/4 to 1/2 inch in length. Loam will form a ball that can be handled without breaking.

Soil Class III  Fine loamy textured soils which include silt, slit loam, silty clay loam, clay loam, sandy clay loam, and sandy clay textures.  
    • Silt: Silt has a floury feel when moist and sticky when wet, but will not ribbon and forms a ball that will tolerate some handling.
    • Silt Loam: Silt loam has a floury feel when moist and will show a fingerprint but will not ribbon and forms only a weak ball.
    • Sandy clay loam: Sandy clay loam has a gritty feel, but contains enough clay to form a firm ball and may ribbon from 0.75 to 1.0 inch in length.
    • Clay Loam: Clay loam is sticky when moist. Clay loam forms a thin ribbon from 1.0 to 2.0 inches in length and produces a slight sheen when rubbed with the thumbnail. Clay loam produces a non-distinct fingerprint.
    • Sandy clay: Sandy clay will ribbon more than 2.0 inches in length and will feel very gritty.

Soil Class IV  Fine textured soils which generally contail more than 40% clay-sized particles. Class IV includes silty clay and clay textures!  
    • Silty clay: Silty clay will ribbon more than 2.0 inches in length and feels very smooth.
    • Clay: Clay will ribbon more than 2.0 inches and will feel neither gritty or smooth.

Presence of Gravel  If the soil has greater than 30% gravel by volume, it is not considered a suitable soil material for a standard drainfield.  


National Resource Conservation Service (NRCS) Soil Surveys for many Texas Counties can be obtained through the local NRCS office. The Soil Survey maps provide soil evaluations for all mapped areas and are useful for describing the general area, but the maps are not precise enough to "replace" a site-specific field test.  

Texas Natural Resource Conservation Commission (TNRCC) 
PO Box 13087 Austin, Texas  78711-3087  (512) 239-0010

TNRCC Permit Instructions  

All contents copyright (C) 1997-1998,  D. Fehler  All rights reserved.