Which of the following is NOT considered a soft deposit?

Calculus is considered a hard deposit, which differentiates it from soft deposits. Soft deposits primarily include substances that can be easily removed or disrupted from the tooth surfaces.

Food debris refers to remnants of food that can stick to the teeth but can be easily brushed away. The dental pellicle is a thin film that forms on the teeth, composed mainly of proteins from saliva, which can be wiped off but is not hard and should not be confused with any type of calculus. Materia alba is a loose, soft deposit composed of dead cells, bacteria, and food particles that can easily be removed through regular oral hygiene practices.

In summary, calculus is mineralized plaque that forms hard deposits as it calcifies on the teeth, requiring professional intervention to remove, while all the other options are considered soft deposits that can be managed through routine oral care.