Which of the following substances is not allowed in agricultural practices?

Arsenic is indeed not allowed in agricultural practices due to its toxic nature and its classification as a toxic heavy metal. The use of arsenic can lead to soil and water contamination, posing serious health risks to both humans and wildlife. This makes it an unsuitable substance in agriculture, especially as sustainability and safety become increasingly important to farming practices.

Lime sulfur, glyphosate, and organic compost, on the other hand, have specific regulations and contexts in which they can be used. Lime sulfur is permitted in some organic systems for controlling pests and diseases. Glyphosate, while controversial and subject to restrictions in certain areas, is still widely used in conventional agriculture and is not outright banned across the board. Organic compost is generally encouraged in agricultural practices for enhancing soil health and fertility. Thus, these substances can be utilized under certain guidelines, distinguishing them from arsenic, which is prohibited due to its inherent risks.