Which of the following is NOT a physical factor affecting herbicide selectivity?

Herbicide selectivity refers to the ability of a herbicide to effectively control specific weeds while causing minimal harm to desirable plants. Various physical factors influence this selectivity, and understanding these factors is crucial for effective pest management.

The chemical structure of the herbicide is not considered a physical factor; rather, it falls under the category of chemical properties, which greatly determine how a herbicide interacts with plants at a molecular level. The structure affects the herbicide's behavior in terms of its mode of action, absorption, and systemic movement within the target plant.

In contrast, factors such as herbicide concentration, environmental factors (like temperature, humidity, and soil type), and application techniques (including methods of dispersion and timing) all involve physical aspects that can influence how the herbicide is used and how it interacts with the target organisms. Adjusting these physical factors can enhance selectivity, making them crucial components of effective weed management strategies.