Subplastic
This concept is derived from field texture and is a property of soils that have clay contents by particle size analysis of 35% or more. Such soils are various forms of clay texture and plasticity and are further explained below.
Texture
When textured in the field, all clay soils are sticky when wet and have:
- a moist plastic bolus
- some resistance to shearing between thumb and forefinger,
- will ultimately form a ribbon at least 50mm long, and
- in a plastic state, can be rolled into threads without breaking and moulded into any shape.
Clay soils range from light clay (35-40% clay) to heavy clay (50% or more). As the clay contents get higher, the soil becomes more sticky, more resistant to shear, ribbons longer, become more plastic and can form thinner threads on rolling.
Plasticity and subplasticity
The degree of plasticity or subplasticity is determined at the soil moisture content used for field texturing, that is, just below the sticky point. During field texturing, the moist soil is kneaded to break up any soil aggregates into its' basic particles of clay, silt and sand. The property of subplasticity is found in soils with strong silt and sand sized microaggregates of clay that resist being broken up by kneading. Aggregating agents such as iron oxides, organic matter, calcium carbonate or gypsum are commonly responsible.
Extra kneading is needed to break up the microaggregates into clay sized particles causing the field texture to become increasingly clayey.
A Plastic clay can be field textured within 2 to 5 minutes and will remain that same texture on further kneading.
A Subplastic clay will increase in field texture 1 to 2 texture groups after 10 minutes of kneading. For example, a soil that textures within 5 minutes as clay loam but textures as light clay after 10 minutes.
A Strongly subplastic clay will increase in field texture more than 2 texture groups after 10 minutes of kneading.
For example, a soil that textures as light clay within 5 minutes textures as medium-heavy clay to heavy clay after 10 minutes of kneading.