Maclean et al. - 2002 - Rice almanac source book for the most important e

Table 1. Typical profile of a flooded rice soil.
Horizon
Ofw
Apox
Apg
Apx
B
Description
A layer of standing water that becomes
the habitat of bacteria,
phytoplankton, macrophytes (submerged
and floating weeds),
zooplankton, and aquatic invertebrates
and vertebrates. The chemical status
of the floodwater depends on the
water source, soil, nature and biomass
of aquatic fauna and flora, cultural
practices, and rice growth. The pH of
the standing water is determined by
the alkalinity of the water source, soil
pH, algal activity, and fertilization.
Because of the growth of algae and
aquatic weeds, the pH and O 2
content
undergo marked diurnal fluctuations.
During daytime, the pH may increase
to 11 and the standing water becomes
oversaturated with O 2
because of photosynthesis
of the aquatic biomass.
Standing water stabilizes the soil
water regime, moderates the soil temperature
regime, prevents soil erosion,
and enhances C and N supply.
The floodwater-soil interface that
receives sufficient O 2
from the floodwater
to maintain a pE + pH value
+
above the range below which NH 4
is
the most stable form of N. The
thickness of the layer may range from
several millimeters to several
centimeters, depending on
pedoturbation by soil fauna and the
percolation rate of water.
The reduced puddled layer is characterized
by the absence of free O 2
in
the soil solution and a pE + pH value
below the range at which Fe(III) is
reduced.
A layer that has increased bulk density,
high mechanical strength, and
low permeability. It is frequently
referred to as a plow or traffic pan.
The characteristics of the B horizon
depend highly on water regime. In
epiaquic moisture regimes, the horizon
generally remains oxidized, and
mottling occurs along cracks and in
wide pores. In aquic moisture regimes,
the whole horizon, or at least
the interior of soil peds, remains
reduced during most years.
Mn. Plowing and puddling often result in the
development of a dense layer below the cultivated
topsoil.
Three types of water saturation occur in rice
soils: (1) endosaturation, in which the entire soil
is saturated with water; (2) episaturation, in
which upper soil layers are saturated but underlain
by unsaturated subsoil layers; and (3) anthric
saturation, a variant of episaturation with
controlled flooding and puddled surface soil.
The properties of a typical soil profile of a
flooded rice soil during the middle of a growing
season are shown in Table 1.
Many rice-growing countries have developed
classification systems that distinguish
natural wetland soils from rice soils. The only
soil classification systems applicable worldwide
are found in the legend of the FAO-UNESCO
Soil Map of the World and the U.S. Department
of Agriculture Soil Taxonomy.
In the FAO-UNESCO Soil Map of the
World, Gleysols, Fluvisols, Planosols, Plinthosols,
and Histosols make up most of the wetland
soils. Gleyic subunits of Arenosols, Andosols,
Cambisols, Solonetz, Solonchaks, Chernozems,
Pharozems, Greyzems, Luvisols, Podsoluvisols,
Podsols, Lixisols, Acrisols, and Alisols are also
mostly wetland soils. Although Vertisols, Nitosols,
and Ferralsols have no gleyic subunits,
these soils may be artificially flooded and sown
to rice.
Soil taxonomy does not recognize wetland
soils, but classifies soils with aquic conditions at
the suborder level and soils with hydromorphism
at the subgroup level. Most hydromorphic soils,
which have an aquic moisture regime, are
equivalent to wetland soils in soil taxonomy.
Suborders with aquic conditions are found in the
orders of Spodosols, Andosols, Oxisols,
Vertisols, Ultisols, Mollisols, Alfisols,
Inceptisols, and Entisols. Most Histosols are
wetland soils per se. Practically all Aridisols are
upland soils. Soils within the aquic subgroups
showing hydromorphism generally are not
wetland soils because signs of wetness are found
only in subsoil horizons.
Research is now being conducted to grow
rice on dry, but irrigated land. See “aerobic rice”
on page 32.
The rice plant and its ecology 15