Yoshida - 1981 - Fundamentals of Rice Crop Science

MINERAL NUTRITION OF RICE 145
young panicles before heading (see Chapter 1). In practice, nitrogen topdressing is
applied at panicle initiation, when the young panicles grow about 1–2 mm long,
which is about 23–25 days before heading. The absorbed nitrogen at this time is
efficiently used to increase spikelet number and, hence, panicle size. For this
reason, topdressing at panicle initiation is called Ho-goe in Japanese, implying
panicle fertilizer.
In farmer’s fields, rice leaves usually turn yellow at around panicle initiation,
indicating a nitrogen deficiency . Under such conditions, nitrogen topdressing is
highly recommended to increase grain yield.
Another effect of nitrogen topdressing on yield could be through active photosynthesis
during ripening. Nitrogen absorbed at panicle initiation should help keep
leaves green after heading and thereby contribute to active photosynthesis for grain
production.
The timing of topdressing also affects the lodging resistance of a crop (Singh and
Takahashi 1962). Topdressing at 20 days before heading not only gives the
maximum panicle weight but also increases lodging resistance by affecting the
length and diameter of internodes, dry matter accumulation in the basal portions,
and the breaking strength of shoots.
The capacity of soil to hold applied nitrogen is an important consideration in
determining the efficiency of basal versus split applications of nitrogen fertilizer.
Recovery percentages of applied nitrogen differ among soils when the fertilizer
was placed deeply to minimize nitrogen loss through denitrification (Fig. 3.14).
Higher recovery percentages correlate with increases in grain yield attributable to
nitrogen applications. Apparently, the applied ammonia is lost through leaching,
3.14. Relationship between yield increase by
nitrogen application over no added nitrogen
and recovery percentage of the applied nitrogen
(Takahashi 1959c).