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 understanding how specific nutrients affect plant growth and development.

2. **Identifying the Technique**: The primary technique used to determine the role of individual elements in plants is through a method called hydroponics.

3. **Explaining Hydroponics**: Hydroponics is a method of growing plants without soil, where the roots are immersed in a nutrient solution. This solution contains all the essential mineral nutrients that plants need to grow.

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