What typically characterizes soil residual herbicides?

Soil residual herbicides are characterized by their ability to persist in the soil, having both short-term and long-term effects on weed management. These herbicides work by forming a residue in the soil that can inhibit the growth of weed seeds and seedlings over an extended period. This persistence allows them to provide ongoing weed control and reduce the frequency of applications needed.

The effectiveness of these herbicides can vary based on several factors, including soil type, moisture, temperature, and microbial activity, which can all influence how long the herbicide remains effective in the soil. Thus, the ability of soil residual herbicides to have both short-term effects on immediate weed populations and long-lasting effects that help prevent future outbreaks makes option C the accurate characterization.

In contrast, the other options describe attributes that don't align with the nature of soil residual herbicides. For instance, some herbicides may be absorbed quickly by plants, but this is not a defining characteristic of soil residual types. Likewise, the assertion that they require constant applications goes against their purpose, as they are designed to remain effective over time. Lastly, if they do not persist in the soil, they would not provide the intended long-term weed control, further affirming why option C is the most suitable answer.