The plasticity diagram of Casagrande is traversed by the so-called A line, which separates the mineral clay soils from the cohesive soils with organic admixtures, i.e. the organogenic and medium plastic silts.

"The German classification standard DIN 18196, which defines and designates soil types according to their particle size distribution and, for cohesive and organogenic soils, refers to the position to the A line, also refers to DIN 4022 Part 1 for a uniform design and description of soil types and rock. The A line is a significant line in the plasticity diagram that classifies clays of a different plasticity and clays/silts with organic admixtures, in the illustration of the liquid limit w_l compared to the plasticity index I_p" (http://www.geodz.com/deu/d/Klassifizierung_von_Boden_und_Fels)

A low plasticity index I_p (small space between the liquid limit and the plasticity limit) indicates that the soil significantly changes its consistency with slight changes of its water content. According to the plasticity diagram, clays have a higher plasticity index than silts, although they have the same liquid limit. Thus, with regard to their consistency, silts are more prone to changes in the water content than clays. Clays can absorb more water without significant changes in their consistency.

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