2007 - Texas A&M AgriLIFE Research Center at Beaumont - Texas ...

sources of nitrogen, under flooded conditions the nitrate
form is unstable and is lost from the soil by leaching and
denitrification (a microbial process that converts nitrate to
nitrogen gas).
However, ammonium nitrogen (urea and ammonium
sulfate) is stable when below the flooded soil surface away
from air and can be used by the rice plant. Ammonium on
the soil surface or in floodwater gradually changes to nitrate
and is lost by denitrification.
Ammonium sulfate and urea sources of ammonium are
about equally efficient for rice and much more efficient than
nitrate nitrogen.
If the soils are drained for several days, urea and ammonium
sulfate can be converted to the nitrate nitrogen form.
Upon flooding the soil, the nitrate nitrogen is lost primarily
through denitrification. Therefore, to conserve and maintain
nitrogen efficiency, nitrogen fertilizer should be incorporated
or flushed into the soil with irrigation water and the soil
should remain water saturated or as moist as possible.
Another way to increase nitrogen efficiency is to use
banded fluid fertilizer. Recent research has shown that
applying fluid fertilizer in a band 2 to 3 inches below the
soil surface can improve N uptake in rice compared to dry
broadcast fertilizer.
Concerns about banding fluid fertilizer include the skill
required to apply the fertilizer uniformly over the field, the
initial cost of application equipment and the time required
to fill fertilizer tanks. To reduce application costs, attach the
fluid applicator knives to the seed drill, which allows 75 to
100 percent of the total N plus P and K to be applied while
planting.
In addition, establishing a flood at the four-leaf growth
stage rather than at the six-leaf or later stage maximizes the
efficiency of banded fluid fertilizer.
Phosphorus
Flooding soils (saturating them with water) increases
the availability of phosphorus. Flooding releases native
soil phosphorus and increases phosphorus mobility. It also
results in a soil pH change toward neutral, which converts
unavailable phosphorus to the more available form.
Phosphorus fertilizer usually increases yields on clay
soils testing below 10 ppm phosphorus and on sandy soils
testing less than 15 ppm phosphorus.
Potassium
Unlike phosphorus, potassium is not greatly activated by
flooding but is more available upon flooding. Most Texas
rice soils do not require additional potassium.
If potassium fertilizer is needed, it is on the very coarse
(sandy) soil types testing less than 50 ppm potassium.
Micronutrients
Soil flooding increases the availability of many micronutrients.
Generally, iron, manganese, boron and molybdenum
become more available under flooded soil conditions,
but zinc usually becomes less available. Although iron and
zinc deficiency may occur at any location in the rice belt,
the area most likely to be affected, historically, is west of a
line from Bay City to Wharton to East Bernard.
Environmental conditions that contribute to deficiencies
of iron and/or zinc include:
• Alkaline soils with a pH above 7.2
• History of chlorotic (yellow) seedlings
• Excessively high rates of native phosphorus
Symptoms of iron and zinc deficiencies in rice seed-lings
include:
• Entire leaves become chlorotic, then start dying after 3
to 7 days (iron).
• Midribs of the younger leaves, especially the base, become
chlorotic within 2 to 4 days after flooding (zinc).
• Chlorosis is usually more severe where the flood is
deepest and water is coldest (zinc).
• Leaves lose sturdiness and float on the floodwater
(zinc).
• Brown, bronze and eventually black blotches and
streaks appear in lower leaves followed by stunted
growth (zinc).
• Rice plants start to recover soon after the field is
drained (zinc).
In these situations, apply 10 pounds of zinc sulfate and/or
100 pounds of iron sulfate per acre at the seedling stage. If
other proven sources are used, select rates according to the
zinc and iron content and availability. Soil applications are
more effective than are foliar sprays.
Soil and plant additives
Soil additives, foliar-applied nutrient growth stimulators
and yield enhancers have not increased rice yields in research
tests or demonstrations conducted throughout the rice belt.
General fertilizer recommendations
Although soil testing is highly recommended to determine
fertilizer needs, the following general recommendations
can be used in the absence of a soil test for the first
crop, assuming semidwarf varieties planted the first week of
April.
• 170-40-0* on fine (heavy) soils
• 150-50-20 on coarse (light) soils
(*Units of nitrogen, phosphorus and potassium, respec-tively,
with 1 ⁄3 of nitrogen and all phosphorus and potas-sium
applied preplant, or by the three-leaf growth stage, 1 ⁄3 of
nitrogen on dry soil just before flood and remaining nitrogen
at panicle differentiation [PD]).
Nitrogen rates
Using these generalized recommendations, you may
need to adjust nitrogen rates, depending on location, planting
date, variety grown, water management and soil conditions.
See location and variety adjustment in Table 9.
Do not delay nitrogen topdressing when plants become
nitrogen-deficient, as the yield potential of the semidwarf
plant types drops each day they exhibit nitrogen deficiency
(yellowing).
Make further adjustments in nitrogen recognizing that
early-planted rice grows slowly in cool temperatures and
may require five to 15 more units of nitrogen than does late-
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