What does "firm" soil require in terms of pressure for crushing?

The requirement for "firm" soil in terms of pressure for crushing is an indication of its compaction and structural integrity. Firm soil can support a moderate amount of pressure, reflecting its ability to maintain its shape and support loads without excessive deformation.

The correct answer, which is two fingers' pressure, indicates a moderate amount of force necessary to assess the compactness and strength of the soil. This level of pressure suggests that the soil is neither too loose nor too compact, indicating a balanced state that ensures stability and the ability to support structures.

In practical terms, when conducting a field test or assessing soil, using two fingers to apply pressure allows for an adequate evaluation of the soil's quality. It gives an idea of how well the soil can maintain its structural integrity under typical load conditions.

Other options, such as no pressure or using one finger's pressure, indicate that the soil is either too weak or inconsistent, while three fingers' pressure would suggest it might be overly compacted, which could affect drainage and biological activity within the soil. Thus, two fingers' pressure is the appropriate indicator of firm soil stability.