Which substrate is known for high ammonium toxicity when used in plants?

Composted manure is known for having higher levels of ammonium, which can lead to toxicity in plants if not managed properly. When manure is composted, the microbial activity breaks down organic matter and generates ammonia as a byproduct. This ammonia can convert into ammonium, which is readily available for plant uptake. However, excessive ammonium can lead to toxicity symptoms such as leaf burn, stunted growth, and overall plant stress if the concentration in the substrate becomes too high.

In contrast, other substrates listed, like composted garbage, sawdust, and vermiculite, generally do not have the same level of ammonium concentration or carry the same risk of toxicity when used in plant cultivation. Composted garbage can vary widely in composition, sawdust may lack nutrients necessary for plant growth, and vermiculite is primarily used for its aeration and moisture retention properties rather than nutrient content. Therefore, the risk of ammonium toxicity is specifically associated with composted manure, making it the correct choice in this context.