Vergara - 1976 - Physiological and morphological adaptability of ri

TENIPERATTIRE AND truss-nest. Kinetics or FLOODED sorts 251
Ctilibqulflfl sway locrr. LUISlEIW zlcy LID-Y‘ 9115- I'm
1. Influence of temperature on the kinetics of pH in three soils.
9.5%; free Fe: 0.1%); it was least in a calcareous clay loam (pH: 7.6‘. organic
matter: 1.5%; free Fe: 0.3%). The acid sulfate soil attained a pH of4.l (the pH
above which aluminum toxicity disappears) within a week of flooding at 25°C:
at 15°C it took 10 weeks. A later study of the influence of temperature regimes
similar to those obtaining in rice fields in four typical locations (Cho and Ponnamperuma.
1971) confirmed the marked retardation of pH increase in Luisiana
clay and in another acid soil Casiguran sandy loam (pl-l 1 4.9‘. O.M.: 5.3%; free
Fe: 0.35%). by low temperature (Fig. l). It also showed that in Libon silt loam
(pl-l; 5.9". organic matter: 4.0%; Tree Fe: 0.1%) the temperature effect was not
appreciable.
The slow increase in p11 may be one factor retarding the growth Offillfi on cold
acid soils.
Specific conductance
Temperature affects the kinetics of specific conductance through its effects on the
production of water-soluble ions such as Fe“, Mn”. NPLfl HCO} and the
removal of Fe='. Mn“ as precipitates of Fe,O, -nH2O and MnCO, Thus the
changes in specific conductance roughly parallel those of iron and manganese.
In a study of eight soils at 15°. 25°, 35°. and 45°C. peak values were highest
in most soils at 45°C (IRRI. 1967). Some of these values exceeded 4 minho/cm.
the level above which rice suffers from salt inj ur_v.
CHEMICAL KINETIC S
Carbon dioxide
Carbon dioxide plays an important role in ionic equilibria in flooded soils. Its
partial pressure (Ptfog determines the pI-I. Eh, and the solubility of iron, manganese.
and calcium in flooded soils (lRRL 1966). If PCO, is too large, the concentration
of Fe" may reach toxic levels and in addition CO, may directly‘
poison the plant; if too low. the pH increases and this lowers the concentrations
of Fe='_ so the plant may suffer from iron injury.