Download Oxidized and reduced zone in flooded soil.

Chapter 9. Soil fertility management in rice cultivation systems
Rice is the main food crop and valuable commodity of trade produced in tropics.
This crop is also cultivated in subtropics and even in some areas with temperate
climate. The wide diversity of climatic and soil conditions, cultivation practices,
and available water supply for rice production explain large variations in rice
cultivation systems. These systems may be grouped into at least five principal
ones. Although, there are also some regional rice cropping systems names of
which are connected with the season or methods of seeding and transplanting.
Each cultivation system has been further developed and performed during long
history of rice cultivation. The scientific experiments carried out during last 5-6
decades in different conditions allowed to understand the main transformations
having place in soils under rice crop and formulate recommendations on how to
improve soil fertility and determine ways of exploiting those changes to increase
productivity of rice production.
Among different systems of rice cultivation, the main ones are:
- Irrigated Lowland or Paddy system
- Rainfed lowland (paddy) system
- Upland dry land system
- Deep-water (floating) system
- Direct-seeded, irrigated system
A paddy field is a flooded parcel of arable land used for growing rice. Rice can
also be grown in dry fields, but from ancient times paddy fields agriculture became
the dominant for of growing rice. Paddy fields are a typical feature of rice growing
countries in many tropical and subtropical regions.
Irrigated lowland rice cultivation accounts for about 65 percent of the total
area in tropics, and only about third of this area relies on water supply from
different fresh water reservoirs (rivers, natural lakes or water basins). In rainforest
zones of Latin America and Africa and in ustic monsoon climate of Southeast
Asia, the process of rice cultivation begins after the advent of the first heavy rains
marking the end of the dry season. Small paddy fields of ¼ or one hectare in size
are surrounded with dukes to
trap as much rainfall as possible.
After the soil has been saturated
with water for several days,
these fields are plowed by
animal or small tractor-drown
implements achieving almost
complete incorporation of stubble and weeds into the soil.
Figure 34. Water buffalos are common to plow paddy soils.
When a field is covered with dense weed cover, some farmers bring a herd of
buffalos to a paddy field and force them to run across the field for some time until
all weeds are submerged into
the soil. At present new types of
tractor-powered rotary harrows
are used for puddling operation
on semi-flooded fields.
Figure 35. Two-wheel tractors are used
for puddling the soil in Thailand.
At the same time on a small, carefully tended lot, the seedbeds are prepared and
looked after by a family member. Two types of seedbeds are common: the
conventional one. In which soaked, pre-germinated seeds are sown directly on the
soil at the rate of 1 to 2 tons seed/ha, and the so called `dado` system, in which
seeds are prevented from contact with the soil by a layer of banana leaves or a
plastic sheet. The seed rate is five times higher. After developing mechanic rice
planters, Japanese farmers are using plastic trays 40 by 60 cm in size which
facilitate further haulage and charging of a rice planter. In both cases water level is
carefully controlled. Conventional seedbeds are pulled for transplanting at 30to 45
days after seeding, while `dapog` seedbeds have to be transplanted between 10 to
15 days after seeding because by that time nutrient reserves in the seeds begin to be
exhausted. In case of use of rice planters, seedlings are irrigated 2 to 4 times by a
week solution of a complex fertilizer 10:5:5 (2-4% solution) depending on a
nursery period which may be of 15 to 25 day long.
While the seedlings are growing in seedbeds, the main fields are puddle by
several harrowing operations conducted at progressively lower soil moisture
contents until the topsoil is converted into a uniform mud. Basal fertilizer
applications are broadcast and mixed into puddle soil during the last puddling
operation.
Before rice planting, the last two soil and water-leveling operations are made by
a tractor rotary harrow or wooden trunk pulled by buffalos. Then, after the soil is
settled in two or three days, rice
seedlings are transplanted in
`hills` or groups of three to six
seedlings at spacing ranging from
15 x 20 to 20 x 30 cm. After a few
days without flooding, the water
level is raised to 5 or 10 cm above
the soil surface and kept there
until 2 or 3 weeks before harvest.
Figure 36. Leveling of the flooded
paddy soil.
(TNAU Agricultral portal. Agriculture. // agritech. Tnau.)
One or two manual or chemical weed control operations are conducted during
the first month after transplanting. The first nitrogen top-dressing usually is made
immediately after transplanting on the flooded soil; and second one is made at the
panicle initiation stage.
Rice is harvested between 100and 150 days after seeding, usually by hand. The
management of the residues is highly variable. A second or even third crop may be
planted during the year if irrigation water is available during the dry season. About
half of the irrigated lowland area in Southeast Asia is double-cropped. These areas
account for 25 percent of the rice production of tropical Asia. Most of the new
short-statured rice varieties are grown according to this system.
Rain-fed Lowland System of rice production is similar to the one just described,
except for the lack of water control. It is the principal rice growing system in the
tropics and accounts for almost 45 percent of the total area. Rainfall dependence
means that the timing of all operations may be subject to tremendous variations, as
rainfall variability during the rainy season is usually large. The land preparation for
rice cultivation usually starts 2 months before the rainy season begins.
During this time the soil may be plowed on or two times with subsequent two
disk harrowing operations. The main purpose of the plowing and disking is to kill
weeds, to bury all plant residues, and to facilitate water penetration and
accumulation up to the level of complete water saturation. When the soil is
saturated, the further operations to prepare soil ready for rice transplanting are
similar to those described above. After transplanting the crop may suffer from
water stress at several stages of
growth, depending on the rainfall pattern. Only
one crop can be grown per year in rain-fed ustic regimes. A similar situation is
also found in certain irrigated areas of Latin America and Middle East, where the
irrigation system are very deficient and wares stress often occurs. The soil moisture
regime, unlike that for paddy rice, can best be described as intermittent flooding.
Upland rice cultivation is referred to a system in which rice is grown like any
other crop, without wetland preparation, transplanting, and dikes around the field.
This system is characterized by conventional land preparation, direct seeding in
dry soil, and complete dependence on rainfall for moisture. Upland rice is grown in
areas and seasons that have average monthly rainfall at least 150 mm. Most of the
soils used are clayey; some of them are poorly drained, others are well drained.
Upland rice is the predominant form of rice culture in the tropical America and
Africa.
Upland rice is cultivated under wide range of management intensities. It is
grown in intercropping shifting cultivation systems in Amazon Basin, much in
Africa, and in the hill areas of Southeast Asia. Almost universally, upland rice
yields are generally lower than the lowland rice yields. Water stress and small
scale subsistence farming are probably the main reasons why upland rice yields are
lower.
Deep-water rice cultivation is a unique system of rice production which has
been developed in some floodplains of the big rivers in Asia. At the advent of the
rainy season seeding is done by broadcasting rice on dry tilled land. Before the
start of heavy rains rice plants manage to develop roots and leaves, so, when deep
water comes and fields become flooded, plant are able to elongate as water rises.
The depth of water may range from 50 cm to 4 m. It is clear that only varieties
adapted to these conditions may be used. Harvesting is usually done by hand with
canoes.
Direct-seeded irrigated system has been developed in the United States of
America, Australia and Russia, and practiced in a few of the best-developed rice
areas of the tropics, primarily in Latin America. There are different modifications
in this system of large scale rice production, but the main elements of it are dry
land preparation, direct seeding, intermittent flooding (controlled irrigation), and
direct or two-stage combine harvesting. This system is practiced on large fields
that allow using big agricultural machinery. Among possible deviations in this
system may be direct seed drilling or seed broadcasting, broadcasting of
pregerminated seeds on standing water from airplanes, overhead irrigation or
flooding. All these deviations are usually dictated by local weather conditions and
available machinery.
Dry land preparation, direct seeding and light overhead watering to stimulate
germination used in this system take into account all physical and chemical
transformation having place in the soil in the period from the start of field
operations to the tillering growth stage. All these transformation will be discussed
later on. Water control during all stages of plant growth is very precise; a shallow
flood is maintained from 20 days after seeding to one month before harvest.
Nitrogen fertilizer plus required herbicides and insecticides are applied by airplane.
Rice is harvested with combines. Yields are high, and cost of production is also
high. This system requires solid initial investment capital and large turnover
financial resources.
Soil fertility management in rice cultivation systems
Rice is the only major food crop capable of growing in flooded soils, because of
its ability to oxidize its rhizosphere. Permanent flooding and intermitted flooding,
and puddling, as well, bring about a series of physical, chemical and biological
changes which provide a completely different set of soil-plant relationships from
those observed under other crops. The understanding of the nature of those
transformations should help to exploit favorable ones and suppress or avoid others
leading to unproductive losses of plant nutrients.
Physical properties of all kind of soils used for rice production drastically
change when their status is transformed from dry to flooded one. The air in the soil
pores is compressed by the advancing water until small air explosions occur,
causing the breakdown of larger aggregates or clods into smaller ones. Clay
minerals then begin to swell due to the transformation of the cementing agents. In
flooded soil hydration of reducible iron and manganese oxides, silicates, and
organic matter takes place causing destruction of the soil aggregates and increase
of swelling. In soils containing fixed ammonium between layer silicate lattices,
ammonium is released into soil solution producing dispersion of colloidal
substances. Independently microbial transformation of organic matter takes place
releasing CO2 and additional amount of ammonium into the soil solution.
Just after flooding, percolation water losses increase to a certain extent, but in 5
to 10 days percolation decreases to a negligible level. Most soils used for rice
cultivation have very low permeability because of high clay or silt contents, high
sodium saturation, high water tables, or presence of impermeable layers in the
subsoil. In such soils changes in permeability upon flooding may be less marked.
Intensive mechanical treatment of a flooded soils is called `puddling`, and the
main purpose of this treatment is breaking down of soil aggregates into a uniform
mud, accomplished by applying mechanical force to the soil at high moisture
contents. In most cropping systems puddling is an unintentional effect of tillage at
the wrong moisture content and usually results in severe yield decreases or delays
in planting. In lowland rice systems puddling is an important soil management
practice, conducted with great care for the purpose of destroying the topsoil
structure.
The process of puddling is accomplished by a series of tillage operations, which
starts at soil moisture contents above saturation and end at moisture content closer
to field capacity. Removal of the air from the flooded soil and transformation of
cementing agents in anaerobic conditions results in destruction of soil aggregates.
Additional mechanical force applied by a low at a man`s foot facilitates further
destruction of soil aggregates, elimination of noncapillary pore space, increase of
soil capillary porosity and moisture retention, decrease of percolation losses.
Puddling is a stick with two ends. The positive end: water accumulation,
prevention of water losses, better water supply for growing rice, incorporation of
plant residues into the soil, which serve as an additional source of nutrients and
decrease reduction of the soil. The negative end: fast reduction of the soil, loss of
nitrates, increase of mobility of toxic iron, manganese and sulfides.
Chemical properties of the flooded soil are strongly determined by the oxygen
contents in the soil itself and in the water above the soil. When the soil is flooded,
its oxygen content decreases to zero in less than one day. Aerobic microorganisms
quickly consume all oxygen in the soil and become dormant or die. Anaerobes or
facultative anaerobes multiply rapidly and take over the organic matter
decomposition process, using instead of free oxygen, oxidized soil components as
electron accepters. The biological changes are accompanied by a very
characteristic succession of chemical transformations of organic and mineral
substances. These products are reduced in the following thermodynamic sequence:
nitrates, manganic compounds, ferric compounds, intermediate products of organic
matter decomposition, sulfates, and sulfites.
Following the disappearance of molecular oxygen, nitrate is used as a substrate
for denitrifiers. Manganic oxides are solubilized as a result of reduction to
manganous ions, likewise orange yellow to reddish colored iron oxides are
reduced to soluble ferrous ions, decolorizing the soil. Many fermentation reactions
based on various organic substrates proceed along with these mineral
transformations, producing carbon dioxide, ammoniacal nitrogen, low molecular
weight organic acids, and so forth. As the soil becomes even more reductive,
sulfate reducers, which are strict anaerobes, produce sulfides; and methanobacteria,
also strict anaerobes, produce methane. Data presented in Table 6 summarize the
redox transformation reactions that occur after a soil is submerged.
Table 6. Principal reduction reactions occurring in Flooded Soil
in a Roughly Thermodynamic Sequence
Stage Redox,
Nature of the change
Eh,mv
Substance
0
800
O2
O2 deplition
1
430
NO3
Denitrified to N2
2
410
Mn (3,5) Reduced to Mn 2+ with higher solubility
3
130
Fe3+
Reduced to Fe2+ with higher solubility
4
- 180
OM
Fermented to produce organic acids, aldehydes,
alcohols, etc.
25
- 490
SO4
Reduced to S26
- >500(?) CO2
Reduced to CH4
pH
Converges to pH 6.5 – 7.5
Source: Simplified from Ponnamperuma, 1972.
All these biochemical changes occur vigorously for the first month after
submergence, when readily decomposable organic matter, the energy source for
microorganisms, is abundantly available. Past this stage, there will be a period
when the supply of oxygen by diffusion, though extremely slow, exceeds its
consumption at the soil/water interface. As all the oxygen is trapped by such
reduced substances as ferric and manganic ions at the interface, a thin oxidized,
orange colored layer (normally a few millimeters thick) is differentiated from the
underlying bulk of the strongly reduced, bluish-gray plow layer. The great environmental difference between the oxidized and the reduced layers exerts a profound
influence on nitrogen transformation in the later stages of paddy soil management.
Rice is known to suffer some physiological disorders under strongly reduced
conditions. At early growth stage rice is very susceptible to exchangeable
aluminum content in the soil solution; young roots suffer from aluminum
phosphate precipitation in roots because root channels are not developed yet; the
dying young roots has to be substituted by new ones, what results in delaying plant
development. At the later stage of growth, when plants are well developed, their
root system is able to supply oxygen down along the root channels preventing the
precipitation of aluminum phosphates.
The best known is a root rot, caused by the high hydrogen sulfide content
evolved in soils that are poor in readily reducible iron oxides. These soils are often
derived from pale colored, sandy, granitic sediments. They are poor, not only in
iron oxides, but also in some other plant nutrients such as Mg, K and SiO2. It is
now known that root rot is due to hydrogen sulfide and soluble iron forming ferrosulfides which also precipitates in roots causing their death. This phenomenon
takes place at the later stage of the soil reduction and it is frequently found on
degraded paddy soils, as characterized above.
The intensity of the reduction process depends on the amount of easily
decomposable organic matter which is a nutrient substrate of microorganisms. The
higher the soil organic matter content, the greater is the intensity of reduction. But
what is more important is actual amount of organic material available for
microorganisms as a nutrient substrate. As long as there is organic matter in the
soil the reduction process will be kept near the equilibrium level, preventing
reduction of sulfates and sulfites into sulfides.
Soil reduction per se is not detrimental to the rice plant, except possibly at
potentials greater than -300 mV, where sulfides may be produced at toxic levels.
The indirect consequences of the soil reduction, however, are of fundamental
importance to rice culture. Understanding the nature of all transformation having
place in the soil from the time of flooding to the harvesting time will certainly help
to take advantages of some of them and to avoid a harmful effect of others.
1. Oxidized and reduced zone in flooded soil. There are several different
horizons in a flooded soil distinguished by the unique chemical transformations
taking place in each of them. This is illustrated in Fig. 35. A superficial layer,
ranging from 1 mm to 1 cm in depth, remains oxidized because it is in equilibrium
with oxygen dissolved in the water. This layer is easily identified in flooded rice
fields because it maintains the aerobic soil color, whereas the reduced layer below
changes to the grayish-bluish color typical of reduced iron compound. The rest of
the plowed layer is reduced except for the rhizosphere of active rice roots, which is
oxidized because of the exudation of oxidized compound by the roots. This can be
visually recognized by the presence of yellowish red root coatings, caused by the
precipitation of ferric compounds on part of the root surfaces.
The subsoil may be reduced or oxidized, depending on its organic matter content
and the presence or absence of a ground water table. Most subsoils of flooded soils
may be slightly reduced, but in some cases the organic matter content is too low
for the microorganisms to cause soil reduction. Puddling of the soil pursues
translocation of clay particles to the top of the subsoil to decrease downward
movement of the water, subsequently reducing influence of the ground water.
2. Change in pH. Regardless of the original pH values, most soils reach pHs
of 6.5 to 7.2 within one month after flooding and remain at that level until dried.
Acid soils, which have pH value as low as 3.5, change pH up to 5.5, whereas
alkaline soil with pH 8.1 decrease this level to 7. These changes are function of
the Fe2+ ion concentration and partial pressure of CO2. Transformation of Fe(OH)3
and similar compounds into Fe(OH)2 or even Fe3(OH)8 results in release of OHions. Other reductions mentioned in Table 6 also produce OH- ions and decrease
H+ ions.
The pH values of alkaline soils decrease to about 7 because of increase in partial
pressure of CO2, which results in a net release of H+ ions.
In alkaline soils the effect of iron reduction is less important because they
contain little iron, whereas in acid soils the increase in the partial pressure of CO2
is less important than the reduction of iron compounds. In neutral soils the pH
changes are little because these two factors tend to balance each other.
This remarkable consequence of flooding results in a self-liming effect and
results in an optimum pH range for the availability of most nutrients. Aluminum
toxicity is quickly eliminated in acid soils when they are flooded, because
exchangeable aluminum is precipitated at a pH of 5.5.
Consequently, liming is of little value in flooded rice production. It should be
noted, however, this pH increase requires about 2 weeks of flooding. Rice may die
from aluminum toxicity if transplanted at the start of flooding in some sulfate soils.
Delaying transplantation for 2 or 3 weeks after flooding may eliminate this danger
without lime application.
Figure 37. Nitogen transformation in the flooded soil.
This phenomenon has been taken into account in the direct rice seeding system
in which the soil is treated by several operations: plowing, disking, chiseling, and
rolling. These operations provide good aeration of the soil, which results in
oxidations of iron, manganese, and aluminum compounds. Then, after seeding, the
soil is flooded (less than 5 cm) to stimulate germination. Then, seedlings are
allowed to develop 2 to 4 leaves depending on weed developments. And only then,
rice fields are flooded with water layer of 5 to 10 cm, again depending on weed
infestation. Such treatment of the soil and regulation of water supply allows for
young seedlings to develop its roots system and avoid aluminum toxicity.
1. Nitrogen transformation. After flooding, the soil oxygen is consumed by
microorganisms in one or two days. The next source of oxygen is nitrates. Amount
of which in dry upland rice cultivation and in direct seeding system may be in
order of 20 to 300 kg per hectare. Virtually all nitrates present in the soil are
denitrified within a month and lost to the atmosphere. Nitrates do not preserve in
flooded soils that is why mineral fertilizers containing nitrate for of nitrogen
should not be used ever.
Unlike nitrates, the ammonium ion is in the reduced state and therefore is stable
in anaerobic conditions. The mineralization of soil organic nitrogen stops at
ammonification state. Consequently NH4+ ions accumulate in flooded soils either
as exchangeable and soluble NH4+ or in soil solution. The quantity of ammonium
released into the soil depends on organic matter content, and especially of fresh
organic materials.
A high concentration of exchangeable iron (Fe2+) may displace considerable
quantity of ammonium ions from exchange sites into the soil solution. And there
are only three ways of the ammonium depletion in flooded soil, which are plant
uptake, leaching losses of NH4+ and water running from the field.
The presence of a thin oxidized layer over the reduced topsoil has profound
implication in nitrogen movement. Nitrification takes place in the oxidized soil,
either from organic matter mineralization or the addition of ammonium fertilizer.
Nitrate ions may move down into the reduced layer either by diffusion or by
downward water movement. In the reduced layer, denitrification quickly occurs
and N2 gas produced escapes to the atmosphere.
This mechanism is shown in Fig. 36. The practical implication is that
ammonium sources should be mixed into the reduced layer in order to avoid this
problem, and not be broadcast on the soil surface.
2. Organic matter decomposition. Anaerobic decomposition of organic
substrates in flooded soils proceeds at slow rate and low energy level and requires
less nitrogen. Consequently, soil nitrogen mineralization can proceed at higher C:N
ratios in flooded than in aerobic soils. This allows plowing down bigger amounts
of straw, which has a high C:N ratio. In spite of the slower rate of organic carbon
mineralization, the rate of organic nitrogen mineralization is often higher in
flooded than in aerobic soil conditions. Even so, greater amounts of mineral
nitrogen may be required for organic material to be completely mineralized.
The end products of organic matter mineralization are also different in flooded
soils. According to Ponnamperuma (1972), in a normal, well-drained soil the main
end products are CO2, NO3-, SO42- and resistant humified materials, in flooded soils
they are CO2, NH4+, methane, amines, mercaptans, H2S, and partially humified
residues. The pathways are similar in both conditions until the formation of
pyruvic acid. In flooded soils these and other intermediate products are further
reduced to several alcohols and organic acids, which eventually are reduced to CH4
or CO2 by strict anaerobes.
During the first few weeks of flooding, large amounts of CO2, ranging from 1
to 3 tons/ha, are produced. These peaks followed by a decline in CO2
concentration, caused by escape, leaching, and precipitation as insoluble
carbonates. The partial pressure of CO2 can reach toxic levels, particularly in soils
high in organic matter and low in iron.
3. Manganese and Iron transformations. In flooded soils reduction processes
are following in a strict sequence. After nitrates are reduced, manganese
compounds suffer reduction during next 4 to 5 weeks. Concentration of Mn 2+
increases in the soil solution reaching its peak at 5 or 6 weeks after flooding.
Then, a gradual decrease of Mn2+ occurs due to precipitation of Mn2+ as MnCO3.
Soils high in reducible manganese undergo the most pronounced changes in
spite of original pH or organic matter levels. Acid soils high in manganese and
organic matter develop peak concentration of 90 ppm Mn2+ in the soil solution,
followed by a decline and stabilization at about 10 ppm. Alkaline soils or soils low
in manganese seldom contain more than 10 ppm in soil solution.
At the end of the manganese reduction, transformation of Fe3+ compounds to
more soluble Fe2+ compounds occur. A peak in the concentration of exchangeable
Fe2+ or soil solution Fe2+ occurs normally within the first month after flooding and
is followed by gradual decline. The magnitude and intensity of these peaks vary
substantially with soil properties. The most pronounced peaks occur in acid soils
high in organic matter. These are usually Oxisols and Ultisols with large quantities
of reducible iron. Base saturated soils show small increase in Fe2+ because of their
lower content of reducible iron.
Iron reduction is considered the most important reaction occurring in flooded
soils because it raises the pH, increases the availability of phosphorus, and
displaces cations from exchange sites. The increase in Fe2+ concentration is
usually beneficial to rice in alkaline soils, reaching 20 ppm in soil solution, a level
that is usually sufficient to eliminate iron deficiency. On the other hand, increase in
Fe2+ in acid Oxisols and Ultisols may reach levels of about 350 ppm Fe2+, which
can cause iron toxicity to rice.
It is clear that all negative effects of the manganese and iron transformations
having place in flooded soils may be overcome by increasing organic matter
content in the soil and by delaying transplanting until after the peaks of reduction
have occurred.
Neither iron nor manganese can be considered a micronutrient in flooded rice
culture. The rice plant assimilates large quantities of these elements, averaging 12
or so kg Fe or Mn/ha, a slightly lower amount than the uptake of phosphorus.
Displacement of Ca2+, Mg2+, K+, and Na+ into the soil solution by Fe2+ and Mn2+
ions is another favorable effect of the reduction transformations in flooded soils.
The Fe2+ and Mn2+ ions in the soil solution may move toward oxidized upper layer
of the soil profile, forming Fe3+ and Mn4+ hydrous compounds precipitating in the
form of stains, nodules, or concretions.
4. Phosphorus availability. In acid soils in a humid climate, phosphorus is
present mainly in the form of iron phosphate (Fe-P) and aluminum phosphate
(Al-P). Neither of these is readily soluble. There are, of course, organic forms of
phosphorus that may be released during the process of organic matter
decomposition. However, in contrast to nitrogen, the quantity of organic
phosphorus compounds is normally very low, compared to the mineral forms of
phosphorus may be present in the soils.
In the process of anaerobiosis in flooded paddy soils, iron phosphate tends to be
reduced, with a release of some of the phosphorus in available forms. Moreover,
reduction of iron oxides releases some of the occluded phosphorus into the soil.
According to Ponnamperuma [31] and others, there is at least six different
mechanisms involved which raise the availability of phosphorus in paddy soils.
 The reduction of ferric phosphates into more soluble ferrous phosphates;
 The availability of reductant-soluble phosphorus compounds, caused by
dissolving previously oxidized layers surrounding the phosphate particles;
 The hydrolysis of some iron- and aluminum-bonded phosphates in acid
soils, which results in a release of some fixed phosphorus at the higher soil
pH;
 The increased mineralization of organic phosphorus compounds in acid
soils, caused by increasing the pH close to neutrality level;
 The increase of solubility of apatite in calcareous soils when the pH
decreases close to neutrality level;
 The greater diffusion of H2PO4- ions in a larger volume of the soil solution.
The extent of the manifestation of these mechanisms in different flooded soils is
not clear yet. What is clear is that in most soils there is a marked increase of the
available phosphate content beyond the 0.2 to 0.45 ppm P2O5 level, which is
considered adequate for rice growth. The magnitude of this phenomenon varies
with soil properties, and actual content of the readily available phosphates in
flooded soils should be measured by chemical analysis.
5. Sulfur transformations. Even at very intense soil reduction decomposition
of organic matter in the flooded soil is able to prevent early reduction of sulfate
ions (SO42-) to SO32- and to S2-. As long as reducible organic matter is present in
the soil transformations of sulfate ions do not take place. After soluble organic
compounds are reduced, bacteria of genus Desulfovibrio start transformation of
sulfates. The magnitude of sulfate reduction depends on soil properties. Acid soils
first show an increase of sulfate ion in the soil solution because of the release of
sorbed SO42- as pH increases. Then, a slow decrease of SO42- and SO32- may follow
depending on the intensity of soil reduction. The concentration of sulfates in the
soil solution decreases with the formation of S2- and subsequent precipitation in
form of FeS. In soils very low in iron, H2S is formed, resulting in direct toxicity to
rice plants.
6. Intermittent flooding. All the above described reactions apply to constantly
flooded soils. In practice, however, about 80 percent of rice production is based on
intermittent flooding system of rice production. Rainfed lowland fields may be
flooded throughout crop growth or be alternately flooded or dried, depending on
rainfall distribution. Many upland rice fields are temporarily flooded or
waterlogged during periods of heavy rainfall. Deep-water soil environment starts
as an aerobic system and becomes anaerobic at later stages of rice growth. Even in
constantly flooded systems the land preparation process normally includes
alternate flooding and drying at the beginning. The concepts described in the
preceding section, therefore, require modification when they applied to a particular
water regime.
The most important modifications should be related to nitrogen transformation
and dynamic under alternate wetting and drying, not only because of their
importance to fertilization practices but also because nitrification and
denitrification processes occur at relatively high redox potentials.
Under constant flooding nitrates are quickly lost through denitrification and
ammonium ions accumulate. Under intermittent flooding the following cycle
develops. Right after flooding, nitrates quickly disappear and NH 4+ contents
increase. When the soil dries, a portion of the NH4+ ions is nitrified into NO3-. In
the next flooding stage these NO3- ions are lost by nitrification or leaching.
Alternate flooding and drying cycles result in tremendous nitrogen losses, which
may be greater of the nitrogen crop removal.
The influence of intermittent flooding on phosphorus behavior and other nutrient
transformations during crop growth are not pronounces as on nitrogen
transformation, and there is no any remedy to control transformation of other
nutrients during short periods between wet and dry status of the soil.
Nutrient management in rice cultivation systems
Large nitrogen losses caused by alternate flooding and drying of soils on rice
fields require special attention to be given to water management and to a
fertilization program. Rice response to a fertilization program primarily depends on
nonsoil factors. The principal ones are plant type, solar radiation, water
management, growth duration, and, lastly, soil properties.
Nitrogen management. Rice is very sensitive to nitrogen applications, except
on recently cleared land or in circumstances where other factors severely limit its
growth. In principal tropical rice-producing countries – India, Thailand, the
Philippines, Vietnam, and Brazil - the optimum responses are obtained at rates of
45 t0 60 kg N/ha, with yields in the order of 2 to 3 tons/ha with local varieties. The
introduction of a new rice plant type by the International Rice Research Institute
has completely changed nitrogen management practices.
The traditional plant types as a rule have vigorous growth, tall stature, low
tillering capacity, weak stems and leaves, and low grain:straw ratio. These varieties
respond to nitrogen by increasing their height, which causes lodging and
subsequent significant yield losses. Tall-statured varieties response to nitrogen
application related to both panicle number per unite area and number of filled
grains per panicle. Whereas new IRRI varieties as a rule have short stature, high
tillering capacity, erect stems, and leaves, high grain:straw ratio, and resistance to
lodging. Among the three yield components, the number of panicles per unit area is
more closely related to yield increases in short-statures varieties. These varieties
respond to nitrogen application in the range of 60 to 80 kg N /ha by increase in
tillering.
The differences in nitrogen responses of the two plant types grown in rainy and
dry seasons are associated mainly with differences in solar radiation. The greater
solar radiation during dry season provides more photosynthetic energy and allows
larger nitrogen responses and yields in both plant types. The rainy season with its
high degree of cloudiness provides less solar radiation and consequently lower
yield response on nitrogen applications.
The responses to applied nitrogen are often lower with intermittent flooding as
there is always a great part of nitrogen lost trough denitrification. Under bad water
management local varieties show better yielding capacity then short0statured
varieties.
Sources of nitrogen. There are paddy areas where rice has been cultivated for
hundreds of years without receiving any fertilizer, but where yields are sustained at
1.5 to 2 tons/ha. It is estimated that about 20 kg of N is required to harvest 1 mt of
paddy. Thus, it is difficult to explain how rice yields can be sustained for so long
without any application of nitrogen [19, 20].
The greater part of nitrogen in paddy soils exists in soil organic matter. This
tends to be conserved more in paddy soils than in upland soils, because of the
anaerobic conditions. Microbial decomposition of the organic matter gradually
releases ammoniacal nitrogen (NH4+-N). As NH4+-N is stable under anaerobic
conditions, it is retained as a cation on negatively charged soil mineral and organic
particles, until the time when rice roots take it up. Thus, the leaching of
ammoniacal nitrogen from paddy fields into the environment is not significant.
There is, however, one condition under which NH4+-N becomes unstable. As
stated earlier, after a month or so from the start of flooding, a thin oxidized layer is
differentiated from the reduced plow layer at the soil surface. When NH4+-N comes
to this oxidized layer, it is readily transformed into nitrate NO3-, by nitrifying
bacteria. As an anion, NO3--N is not retained by soil particles, and is readily
washed with percolating water into the underlying reduced plow layer, and here, it
undergoes denitrification and nitrogen is lost to the atmosphere. As a way of
minimizing this loss of nitrogen, a deep placement technique for ammonium
fertilizers has been suggested and is now widely practiced.
Besides soil organic matter, there is another important source of N, i.e.
biological nitrogen fixation. In paddy soils there are many microbes that are
capable of fixing atmospheric nitrogen, such as blue-green algae, Clostridia,
photosynthetic bacteria, and many of the heterotrophic bacteria in the rice
rhizosphere. Estimates of the amount of biologically fixed nitrogen per crop of rice
vary quite widely, but 30 to 40 kg/ha would be a reasonable figure. This amount of
nitrogen is two or three times higher than the amount of nitrogen fixed in ordinary
upland soils under non-leguminous crops. Interestingly enough, this amount of
fixed nitrogen can explain the average yields of paddy rice obtained in unfertilized
fields in southeast Asia (1.5 to 2 mt/ha) on the basis of 20 kg of N for 1 ton of rice.
As explained above, paddy soils are equipped with an excellent nitrogen cycling
mechanism, with an input through biological nitrogen fixation and an output
through denitrification, as shown in Fig. 36. This appears to set the basis for
sustainability of rice cultivation as an efficient food production system.
Comparisons between nitrogen sources have shown that ammoniacal fertilizers
are those which can be recommended for application in all type of rice cultivation
systems. Even split application of urea does not show a reliable advantage in
comparison with ammonium sulfate when it was applied superficially, but the last
fertilizer is a rare on the markets. Nowadays, urea became the most preferable
fertilizer on rice plantations due to its higher N content and a more accessible good
on the market. Under intermittent flooding, sulfur-coated urea incorporated before
transplanting is superior to conventional sources applied in the same manner and,
in some instances, to split applications of regular urea.
The nitrogen is usually applied in either of two ways: incorporated in the soil
before seeding or transplanting, or broadcast at different stages of growth. The
need to incorporate ammoniacal sources into the reduced layer in systems with
constant flooding is more efficient one but technically it is difficult to implement.
Timing of nitrogen applications. Because of the rapid changes that nitrogen
undergoes in rice soils during short periods, the timing of nitrogen applications is
an extremely critical management factor. Nitrogen uptake proceeds throughout the
growth cycle of the rice plant, but its nitrogen content during two physiological
stages as critical: at the beginning of tillering and the panicle initiation stage. An
adequate supply of available nitrogen during the beginning of tillering results in
more tillers, which are closely correlated with yield in short-statured varieties.
However, excessive supplies of nitrogen after the maximum tillering stage and
before panicle initiation may result in a large proportion of unproductive tillers and
premature lodging of tall varieties. The nitrogen available between panicle
initiation and flowering is closely correlated with the number of fertile grains per
panicle. Excessive nitrogen after flowering may extend growth duration and
increase susceptibility to certain diseases. The purpose of timing nitrogen
applications is to synchronize the plant`s requirements with the availability of this
element in the soil throughout the growing season.
Under constant flooding, a basal nitrogen application entirely incorporated
before seeding or transplanting is normally sufficient for soils with low percolation
rates and for varieties resistant to lodging. In flooded soils with high percolation
rates, however, splitting the nitrogen application in two is more efficient, provided
that the second half is applied at the panicle initiation stage. For varieties
susceptible to lodging, applications at panicle initiation are advisable since they
tend to reduce initial excessive growth. The broadcast applications of urea at the
tillering or panicle initiation stages are more efficient in the presence of a thin layer
of standing water, in spite of that fact that some losses are unavoidable.
In cases of intermittent water management, when alternate flooding and drying
occur, nitrogen application should be performed after transplanting and before
panicle initiation and only in standing water. In upland rice, splitting nitrogen
applications into two parts is definitively superior to a single application,
especially during the reproductive phase. Under alternate oxidation-reduction
conditions, nitrogen losses increase. Nitrogen fertilizer recovery at harvest time
fluctuates between 20 to 30 percent. The efficiency of nitrogen use may be
increased substantially by selection of sources, placement, and timing practices
most adequate for local situations.
Phosphorus management. The most soil tests for phosphorus content fails to
predict response of rice plant to additions of a phosphate fertilizer into the flooded
soil. There are several reasons for this poor response. None of the common soil
extractants can detect reductant-soluble phosphorus, which may become available
upon flooding. Many of the common extractants (Bray, Truog, dilute double acid,
etc.) do not detect much of iron-phosphorus compounds. It would seem logical to
assess phosphorus availability in the flooded soil by determinations of the
phosphorus content directly in plant tissues. Although, it may be too late for any
correction of phosphorus nutrition of a actual crop, but results may be helpful in
programming of phosphorus applications for the next crop.
Transformations of iron and aluminum compounds in the soil having place after
flooding and organic matter decomposition affect phosphorus availability in such
magnitude that no phosphorus applications are necessary for lowland rice, whereas
aerobic crops growing on the same soil require some additions of phosphorus for
high yields. Very often, in soils testing less than 2.5 ppm of extractable P2O5, rice
does not respond to phosphorus applications. At early growth stage rice plants
may show signs of the phosphorus deficit, but later on these signs disappear, and if
fertilized, crop do not responds by yield increase.
So, the increased availability of phosphorus under flooding is soil-dependent.
And phosphorus deficiency in Oxisols, Ultisols and Andepts is a common case.
Under upland conditions, rice usually requires lower rates of phosphorus to
produce maximum yields then the other crops such as corn. Apart from the
increase in the availability of the soil phosphorus compounds, low rice response on
phosphorus application may be explained by greater physiological activity of rice
roots in comparison with other crops. It has been noted that even among the rice
varieties there are some which perform better on low available phosphorus in soils
then others.
All superphosphates are excellent phosphorus source for rice. Mono-ammonium
and di-ammonium superphosphates are also good. These fertilizers are preferable
to apply on transplanted rice. In soils with low aerobic pH values, rock phosphates
of high citrate solubility have proved to be as efficient as superphosphates. In spite
of lower phosphorus content, rock phosphates with high citrate solubility are
preferable for use because of lower cost in comparison with triple superphosphates.
The most practical method of phosphate application to rice soils is to broadcast
and incorporate it in the puddle layer before transplanting. In upland rice areas,
phosphorus is applied in bands close to the planting furrow, but it requires a lot of
manpower, what is possible on small private lots of land.
Management of other nutrients
The extremely wide range of soils and climatic conditions in which rice grows
present wide diversity of nutritional problems: nutritional disorders, ranging from
toxicities or deficiencies to complex nutrient interactions and susceptibility to
diseases. There is no a single solution to cure all these problems. In each location
similar symptoms of a certain problem may be caused by different reasons. That is
why; only accumulation of information on rice plant nutrition in the area may help
to find the right answer.
Zinc deficiency is probably the most widespread micronutrient disorder in rice
growth. It occurs in some places of India, Pakistan, the Philippines, and Colombia
under lowland conditions. In lowland rice areas, zinc deficiency is associated with
calcareous soils and is accentuated by prolonged flooding. Zinc deficiency
symptoms are more pronounced at early growth stages, and sometimes the plant
completely recovers at later growth stages. This effect is largely attributed to high
bicarbonate concentrations during the peaks of soil reduction, which result in an
immobilization of zinc in the roots.
Not all calcareous soils, however, are zinc deficient, what can be detected by soil
and plant tissue analysis. The critical level is 1.5 ppm Zn in the soil, and it is
associated with levels of 14 ppm Zn in the plant tissues. It is conceivable that the
increased availability of phosphorus with flooding can decrease the availability of
zinc. In acid soils, zinc deficiencies are widespread in upland rice, in spite of the
high zinc solubility.
Zinc deficiency can be corrected by applications of 5 to 15 kg Zn/ha as zinc
sulfate of oxide incorporated into the soil before seeding or transplanting. Other
alternative includes dipping the seedlings in 1 percent zinc oxide suspension before
transplanting, and mixing zinc oxide with presoaked rice seeds before direct
seeding.
Iron deficiency is also widespread in calcareous and alkali soils under both
upland and lowland conditions and also found in some flooded soils with organic
content too low to produce significant soil reduction. Iron chlorosis symptoms
may appear at early growth stages and disappear afterwards when increase in Fe2+
has occurred as a result of flooding. Symptoms of the iron chlorosis may be
eliminated by applying ferric or ferrous sulfate to the standing water.
More frequently iron toxicity is encountered on rice plantations then iron
deficiencies. Highly weatherd Oxisols and Ultisols, when flooded, produce
increased concentrations of ferrous ions in the soil solution, which may reach toxic
level of 300 ppm Fe or more in the rice plants. `Bronzing disease` of rice plants on
such soils is a clear symptom of the iron toxicity. The only practical solution for
iron toxicity is draining of the soil for a short time to reoxidize the ferrous iron.
Alluvial fertilization. The water used for flooding rice fields provokes big
transformation of mineral and organic compounds in the soil, at the same time
water may greatly contribute in nutrient supply to growing rice plants. At least
1000 to 1500 mm of water is used to irrigate paddy fields during one rice cropping
season. Nutrients dissolved in water, particularly basic cations such as calcium,
magnesium and potassium, as well as silica, are supplied to rice in the water. If we
assume that 1000 mm of water is used for one crop of rice, 1 mg kg-1 or 1 ppm of a
substance dissolved in water amounts to 10 kg/ha. According to the mean water
quality of most rivers,
supply of 1000 mm of water brings to a paddy field 88
kg/ha of Ca, 19 kg/ha of Mg, 12 kg/ha of K, and 190 kg/ha of SiO2. Usually more
than 1000 mm of water is used for irrigation, so the amount of nutrients supplied to
rice is larger. But the question is, to what extent can the supply of nutrients from
irrigation water satisfy the needs of rice? The yield of rice may be as big as 6.2 t/ha
of unhusked (paddy) rice. The nutrients contained in this quantity of rice exceed
the amount of K and SiO2 in the water by 3 to 5 times. Of course, soil can supply
the greater part of the rest of the rice requirements, but some K and SiO2 fertilizers
should be applied as a supplement. There is, however, no need to apply Ca and
Mg, as their supply through irrigation water far exceeds the amount required by the
rice.
The situation changes as water quality and rice yield change. In Thailand, for
example, the average level of Ca, Mg, and K in river water is roughly twice as high
as that found in Japan, whereas the mean yield of rice is about one-third. Thus, if
no fertilizers are being applied, the nutrient supply via irrigation water is much
more favorable for Thai rice.
However, the two most essential nutrients, N and P, are not supplied in any
significant quantity by irrigation water. Normally, less than 10% of the rice
requirement for these essential nutrients is supplied through irrigation water, unless
the water is severely eutrophied or polluted. Thus, there must be some other
mechanism to supply N and P to rice, if a satisfactory yield level is to be
maintained without fertilizer applications. The paddy soil-rice system has efficient
nutrient replenishing mechanisms. Basic cations and silica are supplied by
irrigation water, while biological fixation works as an inexhaustible source of N in
the system, and phosphorus availability is enhanced under anaerobic conditions.
This, along with almost perfect resistance to soil erosion, gives the paddy rice
system high productivity as well as high sustainability.
To cope with an increasing world population, the paddy rice system has to be
intensified by developing the infrastructure for irrigation and drainage, and
adopting high-input technologies. A tentative conclusion is that the intensification
of paddy rice cultivation is not likely to have as serious an adverse impact on the
environment as would be expected from the intensification of upland food
production systems.