By which technique the role of elements are defined?

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question**: The question asks about the technique used to define the role of elements in plants. This involves identifying how different nutrients affect plant growth and development.

2. **Identifying Key Concepts**: The role of elements in plants can be categorized into macroelements (like nitrogen, phosphorus, potassium) and microelements (like iron, manganese, zinc). Understanding their specific roles is crucial for plant nutrition.

3. **Exploring Techniques**: One of the primary techniques used to study the role of these elements is through controlled growth conditions. This allows researchers to isolate the effects of specific nutrients on plant health.

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