Answer:
When all available water has been used up in a soil, the remaining water is typically unavailable to plants. Therefore, option 1 is the correct answer.
Plants can only absorb water that is in the soil's pore spaces and is not too tightly bound to soil particles. Once all the available water has been used up, the remaining water is held too tightly to soil particles for plants to access it. This can lead to water stress and drought conditions for plants, and can also negatively affect soil fertility and other aspects of ecosystem health.