R layer

Underlying consolidated bedrock. See also horizon (soil).

rainfall interception

See precipitation interception.

rawhumus

See mor.

reaction (soil)

The degree of acidity or alkalinity of a soil, usually expressed as a pH value. Descriptive terms commonly associated with certain ranges in pH are: - extremely acid, <4.5 - very strongly acid, 4.5-5.0 - strongly acid, 5.1-5.5 - moderately acid, 5.6-6.0 - slightly acid, 6.1-6.5 - neutral, 6.6-7.3 - slightly alkaline, 7.4-7.8 - moderately alkaline, 7.9-8.4 - strongly alkaline, 8.5-9.0 - very strongly alkaline, >9.0

regolith

The unconsolidated mantle of weathered rock and soil material overlying solid rock.

Regosol

The only great group in the Regosolic order. The soils in the group have insufficient horizon development to meet the requirements of the other orders.

Regosolic

An order of soils having no horizon development or development of the A and B horizons insufficient to meet the requirements of the other orders. See also: Regosolic Soil Classification.

relief

Elevations or inequalities of a land surface, considered collectively. Land having no unevenness or differences of elevation is called level; gentle relief is called undulating, strong relief, rolling, and very strong relief, hilly. See also microrelief.

rendzina

A group of soils having brown to black surface horizons, which have developed on parent material that contains more than 40% calcium carbonate equivalent. Not used in Canadian taxonomy.

residual material

Unconsolidated and partly weathered mineral materials formed by the disintegration of consolidated rock in place.

residual shrinkage

The decrease in volume after the proportionality between water loss and volume change ceases.

residual soil

Soil formed from, or resting on, consolidated rock of the same kind as that from which it was formed and in the same location.

retentivity profile (soil)

A graph showing the retaining capacity of a soil as a function of depth. The retaining capacity may be for water, for water at any given tension, for cations, or for any other substances held by soils.

reversion

Changing of essential plant nutrient elements from soluble to less soluble forms by interaction with or reactions in the soil. Reversion is usually restricted to the conversion of monocalcium phosphate to the less soluble dicalcium phosphate.

reworked

Descriptive of material modified after its preliminary deposition, commonly by water or wind.

rhizobia

Small heterotrophic bacteria of the genus Rhizobium capable of forming symbiotic noduies on the roots of leguminous plants. In the nodules the bacteria fix atmospheric nitrogen that is used by the plants. The bacteria receive their energy from the plants.

rhizoid

In the lower plants, one of the unicellular or multicellular rootlike filaments that serve for attachment and absorption.

rhizoplane

The external surface of roots and of the soil particles and debris adhering to them.

rhizosphere

The soil surrounding and directly influenced by plant roots.

rill

A narrow, very shallow, intermittent water course having steep sides. It presents no obstacle to tilling.

rill erosion

An erosion process in which many small channels a few centimetres deep are formed; it occurs mainly on recently cultivated soils. See also rill.

rill erosion

See erosion (2.).

riverwash

Barren, usually coarse-textured, alluvial soil in and along waterways, exposed at low water levels and subject to shifting during flood periods. A miscellaneous land type.

roche moutonnée

A rounded hummock (boss) of rock smoothed and striated by glacial action and often showing evidence of plucking on the lee side.

rock drumlin

An elongated hill having a veneer of glacial drift over a rock core.

rockland

An area of which usually 25% to 90% is occupied by rock outcrops and most of the remainder by shallow soils. A miscellaneous land type.

rough broken land

An area having steep slopes and many intermittent drainage channels, but usually covered with vegetation. See also miscellaneous land type.

runoff

The portion of the total precipitation on an area that flows away through stream channels. Surface runoff does not enter the soil. Groundwater runoff or seepage flow from groundwater enters the soil before reaching the stream.