Although humus is important to soil, it is relatively short-lived and must be replaced constantly by the action of microorganisms on undecomposed material. Smaller particles have a higher water-holding capacity because of the large surface area per unit of volume. Water is retained by surface tension. Surface tension is displayed in the beads of water that form on the outside of a cold glass. Until the beads gain enough weight to overcome the force holding them to the glass, they seemingly will defy gravity.
Surface tension causes an interesting phenomenon in soils.
Water will not move out of a small diameter void into a large diameter void until the force of gravity overcomes the surface tension. Therefore, soil with a mixture of large and small particles may have a higher water-holding capacity than a soil made up only of small particles. Chemical Properties Cation Exchange Capacity Cation exchange capacity, or CEC, is the ability of a soil to retain positively charged ions. Since potassium, ammonium, calcium, magnesium, zinc, iron, copper, and manganese exist in the soil solution as cations, the soil's ability to hold onto these is important for the health of a plant. CEC is determined almost entirely by the amount and type of clay and organic matter in the soil.