Ollas work through a process called soil moisture tension. This means that when the soil around an olla is drier, it pulls moisture through the terracotta into the ground. This also affects when and how often you need to fill them up. 'During a rainy stretch, for example, you might not need to refill ollas at all until the ground dries out, because ollas release water only when the surrounding soil is dry,' Karen from Sprouted Garden says, 'but when the heat of summer aligns with the peak of a plant's production, you might need to refill it every couple of days.'