Density, regional profiles, and soil water

Interpreting density with texture

Bulk density is dry soil mass divided by total volume, including pores. A sandy soil can have a higher density than clay without the same degree of root restriction, so fixed thresholds cannot be transferred across textures. Organic matter and mineral composition also affect particle density.

With an assumed mineral particle density of 2.65 g/cm³ and measured bulk density of 1.325 g/cm³, estimated porosity is $1-1.325/2.65=0.50$, or 50%. This calculates pore fraction, not the proportion filled with water. After a storm, most pores may contain water; after drainage, larger pores contain air. Excess water limits oxygen even when the soil's total porosity appears adequate.

Regional soil variation and actual site investigation

Oregon's maritime valleys, coastal dunes, volcanic uplands, and dry interior contain widely differing soils. The Jory series is a well-drained upland soil; it should not be used as a generic example of an impermeable lowland clay. Some valley soils have restrictive layers and seasonal perched water, while coarse volcanic or sandy soils can require careful water management because storage is limited. A regional name does not establish the actual parcel's drainage, pH, or rooting depth.

Inspect a soil profile rather than only the surface. Note topsoil depth, abrupt textural boundaries, mottling, roots, construction fill, buried debris, and water entry. A dark surface layer over dense compacted fill can disguise a poor root environment. A soil survey is a useful starting hypothesis, but grading and imported material can substantially alter urban sites. Confirm with field observations, infiltration testing appropriate to the design, and laboratory tests for chemical decisions.

Distinguish soil texture from a restrictive horizon. Clay can transmit water through stable aggregates and cracks, while compaction, a cemented horizon, or an abrupt layer can restrict drainage. Document the depth and continuity of a restriction before selecting a drain or ripping treatment. A drain with no lower approved outlet cannot correct every high-water-table problem.

Soil-Water Relationships and Infiltration

Soil matric potential can be expressed as a negative pressure relative to free water; suction or tension magnitude is positive. Water status is commonly expressed in bars or kilopascals (1 bar ≈ 100 kPa ≈ 14.5 psi):

1. Saturation (0 bar tension): All pore spaces (both micro and macro) are filled with water. Gravitational water drains downward under the pull of gravity.
2. Field Capacity (FC, commonly estimated near -0.1 to -0.33 bar matric potential): The moisture level remaining in soil after gravitational drainage has ceased, typically 24 to 48 hours following heavy rain or deep irrigation. The remaining water occupies pores and films with differing retention; field capacity is an operational condition, not one exact pressure for all soils.
3. Permanent Wilting Point (PWP, conventionally about -15 bars matric potential): The point at which soil moisture tension is so high that plant roots can no longer extract water. Plants wilt and cannot recover turgidity, even if placed in a saturated atmosphere overnight.
4. Available Water Capacity (AWC): The difference between water contents at field capacity and permanent wilting point, expressed on the same mass or volume basis. Do not subtract the pressure or tension values. Multiply volumetric available-water fraction by root-zone depth to obtain water depth:

Soil Ribbon Test: Field Textural Determination

Contractors on site can rapidly estimate soil texture using the USDA Field Ribbon and Feel Test:

1. Sample Preparation: Take approximately 25 grams (golf-ball size) of soil. Discard gravel, roots, and organic debris. Add water dropwise while kneading until the soil reaches the consistency of workable putty (moist but not sticky).
2. Ball Formation: Squeeze the soil into a tight ball. If it crumbles and cannot sustain a ball under gentle handling, it is classified as Sand.
3. Extruding the Ribbon: Place the ball between your thumb and the side of your forefinger. Gently press and squeeze the soil upward, pushing it over your forefinger until it extends out and breaks under its own weight.
   • No ribbon (< 0.5 in): Coarse sand or loamy sand.
   • Short ribbon (< 1.0 in): Sandy loam, silt loam, or loam.
   • Medium ribbon (1.0 to 2.0 in): Sandy clay loam, silty clay loam, or clay loam.
   • Long ribbon (> 2.0 in): Sandy clay, silty clay, or heavy clay.
4. Tactile Feel Test: Excessively wet a small pinch of the sample in the palm of your hand and rub vigorously with your index finger:
   • If the mixture feels gritty and makes a raspy sound, sand dominates.
   • If the mixture feels slick, smooth, and floury with no grittiness, silt dominates.
   • If neither grit nor slick smoothness dominates, the soil is in the loam balance range.