What should be kept constant in an investigation of water drainage in soil?

In an investigation of water drainage in soil, the following factors should be kept constant:

1. Soil type: The type of soil can greatly affect water drainage, so it is important to use the same type of soil throughout the investigation.

2. Soil moisture content: The initial moisture content of the soil should be consistent to ensure accurate and comparable results.

3. Slope of the soil: The slope of the soil can impact water drainage, so it should be kept constant during the investigation.

4. Amount of water applied: The amount of water applied to the soil should be consistent in each trial to ensure accurate comparisons.

5. Temperature: Temperature can affect the rate of water drainage, so it should be kept constant throughout the investigation.

By keeping these factors constant, researchers can ensure that any differences in water drainage are due to the specific variables being tested and not external factors.