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Why can excess borates act as herbicides?

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A labeled diagram of the periodic table entry for boron shows its atomic number 5, symbol B, electron configuration, atomic weight range, and classification as a solid, weakly acidic, other nonmetal with a rhombohedral crystal structure at room temperature. — boron Properties of boron.

While boron is an essential micronutrient required for the normal growth and functioning of plant apical meristems, it acts as an unselective herbicide when present in excess quantities in the soil.1

The mechanism by which excess minerals, including borates, damage plants involves two primary pathways:

Direct Toxicity: High concentrations of these substances can directly cause stunting, deformities, and dieback in plant tissues.2
Nutrient Interference: Excess minerals can interfere with the plant's ability to absorb and utilize other essential nutrients, which may lead to secondary deficiency symptoms.2

Because these effects are broad and impact the fundamental physiological processes of plant growth, borates are classified as unselective herbicides when applied at high levels.1