Question 1: How do you conceptualize "plant available water". Can plants access all of the water in the soil at given point in time? If you want to use the technical terms, consider saturation, field capacity, and wilting point.
To get you thinking: What happens after it rains and the soil becomes saturated? Can the soil hold all of that water, or is some lost...?
Also: Is there any water held (against gravity) by the soil that plants can't take up?
Question 2: In a few sentences, why is testing and managing soil pH (usually in the range of 5.5-7.0) important for plant nutrition?