The soil holds on to water, which adheres to fine soil particles and fills small pores. During irrigation or rainfall, water enters the soil, filling most of the pores. Gravity slowly pulls it from the soil until it can drain no more, leaving a certain amount in the soil. This can be observed in the kitchen by soaking a sponge in water, then lifting it from the water and letting it drain until it stops dripping. The water still in the sponge is held in the smaller pores.
Water drains more easily from large pores, with air entering the vacated space. Good garden soil has a mix of large and small pores, so that the soil holds both water and air.
Deep soils drain better than shallow soils because capillary action draws the water downward. This can be observed with the sponge by lifting it from the water in a flat position. When it stops dripping, turn the sponge on edge with the long side parallel to the floor. It will begin dripping again. When it stops, turn it once more so the short side is parallel to the floor. It will once again begin dripping. By turning the sponge, you made it “deeper,” allowing capillary forces to pull harder. If you feel the sponge, you will see that it is noticeably wetter at the bottom than at the top, where the capillary pull is greatest.
Shallow soils, such as those in containers, stay saturated at their base longer, which is why potting soils need to have large pores to prevent roots from dying. Deeper pots drain more thoroughly than shallow ones.
Soil water is a solution of many dissolved minerals, which are absorbed by plant roots along with the water. Insoluble minerals are not available to plants—only those dissolved in soil water can be used. The solubility of some minerals depends on how acid or alkaline the soil water is, with the most minerals being available in slightly acid soil.