Agriculture Reference
In-Depth Information
Certain soils may undergo volume changes in response to variation in moisture status.
Vertisols shrink reversibly on drying‚ often presenting a surface pattern of deep polygonal
cracking that disappears on rewetting. The B horizons of certain andisols may shrink on
drying and beyond a certain level of drying this is largely irreversible (Maeda
et al.‚
1977).
2.2.4.2
Water retention by dead plant material and soil organic matter
Like clays‚ dead plant material and soil organic matter have very high water-absorbing
capacities and may hold up to several times their own weight of water. Guano for example‚
may retain 273 % water at field capacity (Vannier‚ 1970). Papendick and Campbell
(1981) report that a mulch of wheat-straw residues of 12 Mg may absorb up to 2.5
mm of rainfall thereby preventing much of the water incident on a site from reaching the
surface of the mineral soil.
The characteristic draining curve for plant material follows an exponential relation-
ship (Papendick and Campbell‚ 1981) of the form:
where is the matric potential‚ is the gravimetric water content and a and b are con-
stants. Most of this water is held at relatively low water potentials and is of limited use
to plants or micro-organisms except those‚ such as certain fungi‚ that are adapted to using
it. Myrold
et al.
(1981) showed that at -1.5 MPa‚ chopped wheat straw residues still
contained 45 % water and 26 % at -3.1 MPa while water contents at the same potentials
in a mollisol of silt loam texture were 9 % and 7 %‚ respectively. These authors also
showed that the osmotic potential of plant material may contribute appreciably to its
total water potential.
As plant material decays‚ the characteristic draining curve changes such that water is
held at a higher potential for any given level of gravimetric or volumetric moisture content.
Dix (1985) showed that the leaves and wood of oak
Quercus robur
change with degree
of decomposition such that water becomes progressively more available for the decom-
posing fungi (Figure I.26); this may have a substantial influence on the succession of
organisms decomposing organic substrates.
2.3
Hydrological regimes
The fate of the water that falls on the landscape is a critical factor in all environments.
The amount and intensity of the rainfall incident on the soil surface controls the severity
of soil erosion‚ the quantity and quality of river water‚ recharge of subterranean aquifers
and the supply of water for man's use. In the context of agriculture‚ the rainwater
incident on a site and its subsequent movement have important implications for such
cultural activities as the timing and nature of cultivation‚ the planting and harvesting of
crops and the need for irrigation or drainage.
