Page 100 - Enzymes in Tropical Soils

88 Enzymes In Tropical Soils

The soil water content is also affected by soil porosity. The soil water
content may increase with porosity when micropores dominate. As discussed
previously, the soil micropores may hold water molecules more tightly than do the
soil macropores. Therefore, the soil water content may decrease with the increase
in soil porosity when the soil macropores dominate. In this case, most of the soil
water may drain as the gravitation water.
Not all water in soils is available for absorption. The available soil water lies
between the soil field water capacity (FC) and the soil permanent wilting point
(PWP)as shown in Fig. 6.3 and Fig. 6.4. FC is the water hold by soil matrix at water
potential of 10 – 35 kPa while PWP is the water hold by soil matrix at water
potential of about 1,520 kPa (Singer and Munns, 1987). The value of available
water is different for different soils dependent the soil texture, structure, total
porosity, and pore distribution.

AVAILABLE

WATER

SATURATED FC PWP DRY
AIR

Fig.6.3. Soil water content and availabilities (Salam, 2012).

The soil water is dynamic. It can move to any directions. Water may move
downward as a result of gravitational force and move to any direction due to
capillary force, which is the combination of adhesion and cohesion forces. The soil
water may move downward through soil macropores. However, at the same time,
the soil particle may attract water with adhesion adn cohesion forces. When the
gravitational force is weaker than the adhesion and cohesion forces, the downward

Abdul Kadir Salam - 2014