Approximately what fraction of the moisture from the top quarter of the root zone is available to plants?

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Multiple Choice

Approximately what fraction of the moisture from the top quarter of the root zone is available to plants?

The key idea is plant-available water in the soil—the portion of moisture roots can actually use. In the upper part of the root zone, not all stored water is readily accessible; some is held too tightly in the soil or drains away after irrigation. A commonly used rule-of-thumb is that about 40% of the moisture present in the top quarter of the root zone is accessible to plants. The remaining 60% is either too tightly bound (not easily extracted) or lost to drainage. So the best estimate among the options is 40%. Keep in mind that the exact fraction can vary with soil texture and structure.

The key idea is plant-available water in the soil—the portion of moisture roots can actually use. In the upper part of the root zone, not all stored water is readily accessible; some is held too tightly in the soil or drains away after irrigation. A commonly used rule-of-thumb is that about 40% of the moisture present in the top quarter of the root zone is accessible to plants. The remaining 60% is either too tightly bound (not easily extracted) or lost to drainage. So the best estimate among the options is 40%. Keep in mind that the exact fraction can vary with soil texture and structure.

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