What defines a soil fumigant?

A soil fumigant is specifically defined as a pesticide that transforms into a gas or vapor when applied to the soil. This gaseous state allows the fumigant to permeate through the soil, effectively targeting a range of pests, including nematodes, pathogens, and weeds.

The correct answer highlights that a nematicide can volatilize as a gas, which is a key characteristic of soil fumigants. When these substances are applied, they need to be able to move through the soil to reach pest populations effectively, and achieving this through vaporization is vital for their efficacy.

Other options do not fit the definition of soil fumigants. Solid pesticides, liquid herbicides, and natural pesticides do not exhibit the same properties necessary for fumigation. Solid pesticides do not vaporize, liquid herbicides typically target plant growth rather than soil pests, and natural pesticides might not possess the specific characteristics, such as volatility, that soil fumigants require. Thus, the focus on the gaseous nature of a soil fumigant, specifically in the context of nematicides, makes this answer the most accurate.