Smith - 2003 - Rice origin, history, technology, and production

Soil Fertilization and Mineral Nutrition in U.S. Mechanized Rice Culture 375
significant loss mechanism, because the rice soils in the United States do not become
sufficiently reduced for H 2S formation.
Sulfur Nutrition, Fertilization Practices, and Diagnosis of Deficiency
In the United States, rice normally does not require S fertilizer, due to the low permeability
o f the soils, coupled with adequate amounts o f S naturally provided from
mineralization from soil organic matter and extraneous S sources, such as irrigation
water and precipitation. Sulfur deficiencies normally occur after establishing the perm
anent flood during active tillering and again during late reproductive growth when
the plant is producing biom ass rapidly and the panicle is developing. Sulfur-deficiency
symptoms during vegetative growth include chlorosis o f the entire rice plant, starting
with the younger leaves, reduced tillering, and stunted growth. Although chlorosis
may start with the younger leaves, this distinction may not be observed, as S-deficient
rice can become uniform ly chlorotic quite rapidly. W hen S deficiency occurs during
the rice reproductive growth stage or about 2 to 3 weeks before heading, deficiency
symptoms are observed in the top two or three leaves. Leaves o f deficient rice plants
have alternating vertical dark and/or yellow (chlorotic) streaks that began at the tip
o f the leaf and extend toward the leaf base. The bottom two to three leaves almost
always appear norm al. During flag leaf exsertion, the flag leaf may show less severe
symptoms than the other top two to three leaves. But once fully exserted, the flag leaf
eventually develops alternating dark and yellow strealcs. M aturity o f the rice will be
delayed if S deficiency is not corrected in a timely manner.
Sulfur-deficiency symptoms during the rice vegetative growth stage appear similar
to those o f N. The difference is N-deficiency symptoms begin as a yellowing o f
the older leaves. Since visual symptoms are difficult to distinguish from N deficiency,
plant tissue analysis is often required for positive identification. M inim um concentrations
o f S in the rice plant for optim um growth during active tillering and panicle
initiation are 0.17 and 0.15% , respectively (Bell and Kovar, 2000). Rice crop uptake
and removal o f S are approximately 26 and 9 kg S/ha, respectively, based on an average
yield of 9000 kg/ha and total aboveground biomass (grain and straw) o f 20,000 kg/ha.
Thus the rice plant does not require a large amount o f S for optim um growth.
Sulfur fertilizer usually is applied in the plant-available SO4 form . Ammonium
sulfate [(NH4)2S 0 4 ; 24% S] is most often used to alleviate or prevent a S deficiency
in rice. Experience has shown that for m ost S-deficient soils a 112-kg/ha application
o f (NH4)2S 0 4 , which supplies 27 kg S0 4 -S/ha, is sufficient for optim um rice growth
and production. Although this appears to be a marginal rate, along with the S supplied
in irrigation water and precipitation, this rate is ordinarily more than adequate. The
best tim e to apply (NH4)2S 04 is early in die season with the N fertilizer at preflood
application time, when the rice plant is beginning to tiller (De Datta, 1981). On sandy
soils with high permeability, an additional application at midseason is often required,
especially on fields with a history o f S deficiency occurring during late reproductive
growth. Am m onium sulfate usually is recommended on sandy soils, because they may
suffer from both S and N loss due to leaching. Precision-graded soils suffer from
many nutrient abnormalities in addition to S, because o f organic matter removal
and the exposure o f subsoils deficient in some elements and toxic in others. Poultry
litter and at times gypsum (CaS0 4 , 18% S) are applied to these soils for reclamation.