Yoshida - 1981 - Fundamentals of Rice Crop Science

162 FUNDAMENTALS OF RICE CROP SCIENCE
deficiency is unlikely but toxicity may occur. There are several reasons, however,
why manganese toxicity in lowland rice is uncommon (Tadano and Yoshida
1978):
• Rice roots have a high degree of manganese-excluding power. In solution
culture, the tissue manganese content increases only four fold with rice plants
grown at 300 ppm Mn compared with those grown at 0.1 ppm Mn. It is 109
times for barley and 700 times for radish (Tanaka et al 1975).
• Rice tolerates a high manganese content. The critical tissue content for
manganese toxicity is 700 ppm for beans, 1,200 ppm for barley, and 7,000
ppm for rice (Cheng and Quallette 1971).
• Compared with nitrate, ammonia has a retarding effect on manganese uptake
(See Table 3.18).
• A high level of iron in submerged soils may counteract an excessive manganese
uptake (Ishizuka et al 1961).
In many cases, a high manganese content in rice tissues is frequently associated
with high yields, possibly indicating that a high manganese content in the soil is
associated with various favorable soil conditions.
3.13. SULFUR
3.13.1. Occurrence of deficiency
Sulfur deficiency is a common nutritional disorder in upland crops but it rarely
occurs in lowland rice. Sulfur deficiency of lowland rice has been reported from
Mandalay, Burma (Aiyar 1945); the Lower Amazon Basin, Brazil (Wang et al
1976a, b); and South Sulawesi, Indonesia (Mamaril et al 1979). A recent trend in
the fertilizer industry to shift from ammonium sulfate to urea and from superphosphate
to nonsulfur phosphatic fertilizer may induce more widespread sulfur deficiency
in lowland rice.
3.13.2. Sulfate in soil solution
Sulfate is reduced to sulfide in flooded soils. As a consequence, the soil sulfate
concentration declines rapidly and is associated with a sulfide accumulation (Fig.
3.28). Thus, the availability of soil sulfur decreases as soil reduction proceeds
(Nearpass and Clark 1960).
The rate of sulfate reduction in submerged soils depends on soil properties.
Concentrations as high as 1,500 ppm SO 4
2-
in neutral and alkaline soils may be
reduced to zero within 6 weeks of submergence (Ponnamperuma 1972).
3.13.3. Natural supply of sulfur
Sources of natural sulfur are soil, irrigation water, and atmospheric precipitation.
a. Soil. Soil contains sulfur in organic and inorganic forms. The A value, to
assess the available sulfur in submerged soils, is highly correlated with the soil
sulfur extractable with Ca (H 2 PO 4 ) 2 or KH 2 PO 4 solution (Suzuki 1978). Hence,