Vergara - 1976 - Physiological and morphological adaptability of ri

NITROGEN RESPONSE or RICE IN TROPICAL commosis
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three factors. In most cases. grain yield increases at 6O kg N/ha were significant
(Fig. l5). At 120 kg N/ha. C-22 in the June l7 seeding lodged early and gave
poor nitrogen response. Similarly. IR5 and Ml—48 also lodged and thus had
low grain yields in the June 2, June l7, and June 27 (first to third) seedings.
Solar radiation values during the ripening periods were higher for the two
later seedings than for the two earlier ones. Moisture supply was uniform for
the later plantings as indicated by even rainfall distribution and low soil moisture
tensions (l-‘ig. l5). Therefore. the relatively‘ low yields in the last two
seedings were not due to poor environmental conditions, but to susceptibility to
diseases such as blast in Ml—48 and IR442—2—58, and sheath blight in IRS and
M1—48.
The results at the two sites demonstrate that early seeding is desirable with
late-maturing lines. like IRS. which can recover from dry spells. For the earlyto
medium-maturing lines, moisture distribution from rain is more critical than
for the late-maturing line once the crop is fully established. Furthermore.
varietal resistance to diseases such as blast and sheath blight are highly important
for high nitrogen response under fann conditions.
At the Bicol station, IRS gave the highest nitrogen response in the first two
seedings (June 20 and July 5). In the third seeding (July 24). nitrogen response
was not significant. Moisture supply from rain was unifonn in Bieol. As a result,
the differences in nitrogen response at various dates of seeding were not significant.
Soil moisture tension never exceeded 2O cb. Solar radiation totals during
the reproductive and ripening stages were not different from one date of seeding
to the other. 13.9 kcal/sq cm for July 24 and 14.4 kcal for July 5 (Fig. l6).
At the La (iranja station in the Visayas nitrogen response was significant for
Ml—48 and 111442-2-58 in the first seeding, IRS in the second, and C-22 and
IR442—2—58 in the third (Fig. 17).
The rainfall at La Granja was low for l week in mid-July and for 2 weeks in
early to mid-August, causing the soil moisture tension to rise to 45 and 65
cb for the two dates, respeetiittelyt These two periods of high moisture tension
corresponded to the reproductive and ripening periods of the crop, causing
yield reduction and low nitrogen response (Fig. l7). Soil at the La Granja
station is very coarse (Table l), and soil moisture tension rises quickly despite
uniform rainfall.
Except for IRRI in the Philippines. Peru is the only other place where experiments
have been conducted to study environmental factors affecting nitrogen
response in upland nee.
In Pent, upland rice is grown on both flat and sloping land in the Amazon
basin area. Experiments were conducted during September I968 and February
I970 in the Yurimaguas area because of its soil and climatic conditions
representative of the Selva Baja, and because it is the largest and most rapidly
expanding upland rice-producing area in the country (Sanchez and Nurefia.
I972). The monthly rainfall averages during this period were compared with
the 20-year average (Sanchez and Nurefia. 1972). There was no unusual de-