What method is used to test internal drainage in soil?

The percolation test is a widely recognized method used to evaluate internal drainage in soil. This test measures how quickly water moves through the soil layers, which is crucial for understanding the soil's drainage capacity and its suitability for various landscaping and agricultural applications.

During the test, a hole is dug in the soil, and water is added to the hole to saturate the surrounding soil. After the water has been allowed to drain for a specified period, the rate at which the water level drops is measured. This rate indicates how well the soil can absorb and drain water, providing valuable information about its drainage properties.

In contrast, the other methods listed do not specifically focus on measuring the drainage capacity. The soil drainage test is a more general term that might encompass a variety of assessments but isn't a standardized test like the percolation test. The water retention test measures how much water soil can hold, which is related but does not directly assess how quickly that water drains away. The pneumatic test examines soil compaction and aeration but does not provide direct insights into water movement and drainage characteristics. Thus, the percolation test stands out as the appropriate and standard method for testing internal drainage in soils.