What best describes nodules and concretions found in soil?

Nodules and concretions are best described as firm bodies of cemented materials. These geological features are formed through the accumulation of minerals that have precipitated from the soil water, leading to a dense and cohesive mass. Typically, they develop around a core or center in the soil, often influencing the physical and chemical properties of the surrounding soil matrix. Their firmness and cemented nature distinguish them from other forms of soil components, reflecting a more stable mineralogical composition.

In contrast, nodules and concretions are not soft or easily removable due to their firm structure. They also do not fit the description of lightweight aggregates as they are generally more substantial than typical soft aggregates and have a higher density due to their mineral content. Lastly, they are not classified as portable organic structures since they primarily consist of inorganic materials that have crystallized rather than organic matter. This context underlines why the correct option focuses on the firmness and cementation of these features in the soil profile.