Introduction to Soil Chemistry

D
C
water 75
B
Figure 4.3. A wide-diameter capillary tube (A) draws colored a shorter distance than does a
narrow-diameter capillary (B). Colored water cannot move from the narrow-diameter capillary
to the wide-diameter capillary (C). Colored water can move from a larger diameter capillary to
a smaller capillary (D).
large pores will be drawn into smaller pores, but water in small pores cannot
move into large pores unless energy is exerted.Thus, water will not move from
a sandy horizon—smaller pores—into underlying gravel layer—larger pores—
unless the water exerts enough pressure to move from the small pores into the
larger pores in the gravel. This relationship between pores and water movement
is illustrated in Figure 4.3 using capillaries.
Some pores do not drain because they are simply too small and where the
interior is large but the entrances and exits are so small that they prevent
drainage as shown in Figure 4.4. Both of these can be called restricted pores.
Restricted pores can cause problems in any extraction and analysis procedure
because they prevent the complete removal of solution from the soil.
Exchange with extracting solutions is limited by the slow process of diffusion
and can result in the component of interest occurring at low levels even in the
last of multiple extractions [5].
A