Given an equal amount of water, which soil will make water more easily available for plants?

Question

BlackTom AI · Accepted Answer

Large soil particles (sand) create bigger pore spaces and hold water more loosely, so that water is released at lower suction and becomes easier for plant roots to take up — therefore the correct answer is a. Sand.

Relevant concepts:
- Pore size distribution: sand has large macropores; clay has many tiny micropores.
- Matric potential (soil water tension): water in small pores is held at higher tension (more strongly) than water in large pores.
- Plant-available water: the portion of soil water between field capacity (after gravity drains) and the permanent wilting point (when plants can no longer extract water).

Step-by-step reasoning:
1. When equal amounts of water are added to two soils, the water distributes into the available pore spaces. Sand’s larger pores fill and drain more quickly; clay’s tiny pores retain water more tightly due to greater surface area and stronger adhesive forces.
2. After gravitational drainage, the remaining water in sand is held at lower tension (less negative matric potential) so roots can extract it with less effort. In clay, much of the retained water is held at much higher tension and is therefore harder for roots to remove.
3. Plants extract water down to the wilting point; because clay holds more water tightly, a larger proportion of its water can be below the plant-available threshold. Even if clay can store more total water, that water is less accessible.
4. Therefore, for an equal amount of water applied, sand will make a greater fraction of that water readily available for plant uptake.

Why the other option is incorrect:
- Clay (option b) does retain more total water per volume, but its high surface area and small pores bind water strongly. Much of that retained water is held at tensions beyond what roots can extract, so it is less available to plants.

Confirming the final answer: based on pore size, matric potential, and plant-available water concepts, the correct choice is a. Sand.

Given an equal amount of water, which soil will make water more easily available for plants?单项选择题

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