Ancient works on Vastu stress on the importance of quality of soil as one of the fundamental factors while selecting land for construction. According to Vastu the soil is divided into Brahmin, Kshatriya, Vaishya and Shudra categories on the basis of colour, taste and smell of the soil. Soil which is white, reddish or yellow, has a sweet smell and is fertile, is considered very auspicious for residential and commercial purposes. The soil which is black in color, has bad odor and tastes bitter is considered inauspicious.
Vastu advocates construction on land with a high soil density. Modern engineering also prefers land with a high soil density and undertake soil testing for assessing the strength and suitability of soil. It is beyond doubt that our ancestors were aware of every aspect of building construction. They were aware of not only the modern principles of architecture but also the subtle cosmic energies that radiate in and around a human being. Vastu advocates examination of soil to check the strength of the soil as it has to bear the full weight of the structure and withstand forces of nature like Air, Wind and Earth-quakes etc.
One of the tests is to dig a pit in the center of the plot and to fill it up again with the dug up soil, If after filling up the pit, there is a mound of soil on the place, the site is considered fit for construction. In case the pit is full and no soil is left. It is considered medium. In case, the soil is not enough to fill the pit, it is considered unfit for construction. Filling up the dug-up pit with water and observing the fall in the water and observing the fall in the water level also tested the strength of the soil. The soil condition can also be tested by other simple experiments like ploughing the soil, which indicate the soil composition in the upper crust of the site. The presence of animal wastes like bones and hair and other inauspicious objects indicate unsuitable soil conditions. Soil condition is also considered bad if the presence of vermin and termites is noticed as these made the soil hollow. If it is essential to construct on such a land, it is recommended to dig up and remove all the soil a depth of 5½ to 6 feet and refill with good quality soil and stones. As per Rishis like Garg Rishi, house should be constructed on a site which gives good vibrations and is pleasing to the eye.