In urea manufacture and handling, biuret content is kept below about 1%. What is the main agronomic reason for limiting biuret to such a low level?

Introduction / Context:
Biuret forms by the condensation of urea molecules at elevated temperatures or long residence times. While chemically related to urea, biuret has undesirable biological effects on certain crops, especially when urea is foliar-applied or used in sensitive stages of growth.

Given Data / Assumptions:

• Biuret content targeted < 1% in many fertiliser specifications.
• Exposure of leaves/seedlings to higher biuret can cause phytotoxicity.
• Manufacture and granulation conditions influence biuret formation.

Concept / Approach:
Biuret interferes with plant metabolism and can cause leaf tip burn and reduced growth. Therefore, process conditions and product cooling are managed to keep biuret low, protecting agronomic performance and enabling safe foliar use in some crops.

Step-by-Step Solution:

Verification / Alternative check:
Fertiliser standards and supplier datasheets often cite biuret limits (e.g., ≤ 1% for general use; even lower for foliar-grade urea), reflecting agronomic evidence.

Why Other Options Are Wrong:

• Corrosive or explosive: not the primary concerns for biuret in fertilisers.
• Helpful in decomposition: incorrect; biuret does not enhance useful decomposition in soils.
• Granule hardness: not the main driver for the limit.

Common Pitfalls:
Assuming biuret limits relate to storage stability; the dominant reason is crop safety.

Final Answer:
toxic and harmful to some crops