Soil Type
Different soil types need different Opening Thresholds and Sensor Placements.
Sandy soil can store less total water than clay soil, but the water it stores is usually easier for plants to use. Clay-heavy soil can store more water, but it holds on to water more strongly, making it harder for plants to absorb it. This means OpenValve may still measure high moisture in clay-heavy soil, while the plant already has a hard time accessing that water. For this reason, clay-heavy soils typically need a higher Opening Threshold than sandy soils for the same plants.
Water also moves differently through different soils. Depending on the soil type and how compacted it is, water may spread more sideways, move mostly downward, or travel unevenly through cracks and easier paths. The sensor should be placed where the water reliably travels during irrigation. If the water mostly moves straight down through the soil, the sensor should line up with the point where water enters the soil. If the water spreads more sideways, the sensor does not have to be directly aligned with the water entry point. What matters is that water reaches the sensor in a repeatable way.
For a better understanding of these concepts, I recommend watching this video.