Which statement best explains why plant available water is not the same as total soil water?

Question

BlackTom AI · Accepted Answer

Plant available water is the fraction of soil water that plants can actually extract — mathematically PAW = θ_FC − θ_PWP, where θ_FC is volumetric water content at field capacity and θ_PWP at the permanent wilting point. Field capacity (~ −33 kPa) is the water retained after gravitational drainage; permanent wilting point (~ −1500 kPa) is when remaining water is held so tightly that plants cannot create sufficient suction to extract it.

Step-by-step reasoning:
1. Total soil water (θ_total) includes three conceptual pools: gravitational water in large macropores, plant-available capillary water in mesopores, and unavailable hygroscopic/adsorbed water in micropores. So θ_total = θ_gravity + θ_available + θ_unavailable.
2. After rainfall or irrigation, large pores drain by gravity until the soil reaches field capacity; that drained fraction cannot be used by plants because it percolates away beyond the root zone.
3. The remaining capillary water between field capacity and wilting point is available to roots because root water potential can overcome the adhesive forces holding this water.
4. Water at potentials drier than the wilting point is held too tightly on particle surfaces (adsorption) and is effectively inaccessible to roots.

Why the correct answer is correct:
- "Some soil water drains away and some is held too tightly for plants to access" succinctly describes the two reasons limiting plant-available water: gravitational loss and strong adsorption/unavailability.

Why the other options are incorrect:
- "Water in clay soils is always fully available" is false: clay holds more total water but a larger fraction is held at very low potentials and is not available.
- "Soil water is only controlled by rainfall amount" is false: soil texture, structure, and pore-size distribution control retention and availability as well as rainfall.
- "Plants can use all water held in soil pores" is false: large-pore water drains and very tightly held water in micropores is inaccessible.

Conclusion: therefore the best explanation is the provided option: Some soil water drains away and some is held too tightly for plants to access.

Which statement best explains why plant available water is not the same as total soil water?单项选择题

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