What are capillary tensions in soil?

Capillary tensions in soil refer to the phenomenon where water is drawn into the small pores between soil particles due to the adhesive forces between water molecules and the soil particles themselves. This process is critical in understanding how water moves through soil and how it affects agricultural practices, drainage, and soil stability.

When water enters the soil, it does not merely fill up the spaces; rather, it is held in place by these attractive forces. The surface tension of the water creates a "meniscus" effect at the air-water-soil interface, which plays a significant role in the retention and movement of moisture in the soil matrix. This phenomenon is essential for plant growth since roots access water held in these capillary spaces.

The other options relate to different concepts in soil mechanics and hydrology. Forces caused by soil compaction pertain to how soil density is affected when external pressure is applied, while pressure caused by soil layers deals with the weight and stress distribution due to overlying materials. Forces exerted by soil on structures involve the bearing capacity of soil and how soils transmit loads to foundations. These concepts, while important in geotechnical engineering, do not directly define capillary tension as influenced by molecular attraction between water and soil.