Yoshida - 1981 - Fundamentals of Rice Crop Science

MINERAL NUTRITION OF RICE 155
3.21. Effect of acetic acid on the 65 Zn
absorbed by the rice variety IR8 at pH
4.5 and 7.5 (Forno et al 1975a)
In some problem soils, organic acids may accumulate in concentrations as high
as 10–30 mM. The kinetics of organic acids in submerged soils show that they
accumulate in much smaller quantities at high temperatures, and decompose more
quickly at high temperatures. Thus, whether organic acids could accumulate in
quantities large enough to retard zinc uptake depends on temperature regimes.
The inhibitory effects of bicarbonate and organic acids on zinc absorption lead
to the hypothesis that, in calcareous soils and other alkaline soils where zinc
deficiency occurs, flushes of microbial activity after submergence cause transitory
–
peaks in HCO 3 and organic acids, which, either separately or together, cause the
temporary inhibition of zinc uptake by the rice plant.
3.10.4. Critical plant zinc content
The critical level for zinc deficiency in the rice plant is about 15 ppm (Fig. 3.22).
Similar values are reported for upland crops (Thorne 1957, Carroll and Loneragan
1968).
For purposes of diagnosing zinc deficiency, the following criteria have proven
useful when combined with visible obsevations and soil analyses (Yoshida et al
1973):
Zinc content in the whole shoot Diagnosis
20 ppm Unlikely