Yoshida - 1981 - Fundamentals of Rice Crop Science

NUTRITIONAL DISORDERS 189
ance for nutritional disorders in various crops (Wright and Ferrari 1976). A
similar attempt in rice is under way (Ponnamperuma 1976a, 1977).
To overcome a nutrient deficiency by varietal improvement depends on the
assumption that a soil contains a certain amount of the nutrient to be used. One
variety may be better able to use a soil nutrient than another. In fact, the
contribution of varietal tolerance to rice grain yield was estimated at 0.5–0.8 t/ha
for phosphorus deficiency, 0.5–1.5 t/ha for zinc deficiency, and 0.2–0.7 t/ha for
iron deficiency at yield levels of 2.5–4.0 t/ha (Mahadevappa et a1 1980). When the
plant’s requirement is large relative to the soil nutrient reserve, however, its
differential ability may not contribute much. In theory, once a soil nutrient is
depleted, varietal tolerance is no longer effective. This consideration may apply to
deficiencies of most nutrients except iron. Iron is a micronutrient for higher plants
but it is a macroconstituent of soil. Under such conditions, varietal improvement
of the plant’s ability to use soil iron is a logical approach to the cure of iron
deficiency in plants.
One more point worth considering is to compare gains and losses for the varietal
improvement as a means to overcome a nutrient deficiency in the soil. A long-term
phosphate experiment indicates that grain yield decreases with time unless the crop
receives phosphate fertilizer (Fig. 4.1). When no phosphate is applied, the total
soil phosphorus content in both the surface soil and in a furrow slice decreases as a
result of the crop’s removal of phosphorus. The total soil carbon and nitrogen
contents also decrease. Studies on nitrogen fixation by azolla (Watanabe et al
1977) and blue-green algae (Mitsui 1960) show that phosphate application is
necessary to increase nitrogen fixation. If an efficient phosphate user (rice
variety) is grown without phosphate fertilizer, the nitrogen fixation by azolla and
4.1. Effect of phosphate
application on rice yield
in the long-term fertilizer
experiment, Shiga, Japan.
1930–1957 (Shiga Pref.
Agric. Exp. Stn. 1961).