Soil fertility is determined by its ability to:

Options

A. Decay organic matter
B. Hold organic matter
C. Hold water
D. Support life

Explanation

To determine how soil fertility is assessed, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Soil Fertility**: Soil fertility refers to the ability of soil to provide essential nutrients to plants. This includes not only the nutrients themselves but also the capacity of the soil to retain and supply water to the plants.

2. **Evaluating the Options**:
- **Option A: Decay of organic matter**: While the decay of organic matter contributes to soil health, it is not the sole determinant of soil fertility.
- **Option B: Amount of organic matter it holds**: The presence of organic matter is important for nutrient supply, but again, it is not the only factor.
- **Option C: Amount of water it holds**: Water retention is crucial for plant growth, but it does not directly define soil fertility.
- **Option D: Life support it provides**: This option encompasses the overall ability of the soil to support plant life, including nutrient supply, water retention, and the overall health of the ecosystem.

3. **Conclusion**: The best answer is **Option D**, as it reflects the comprehensive role of soil in supporting plant life through nutrient and water supply.

### Final Answer:
Soil fertility is determined by its ability to **support life**.

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