Submerged Soils

Pedogenesis, Morphology and Classification of submerged soils

Pedogenesis

1. Soil forming factors and processes are playing an important role in pedogenesis

2. Alluvial and rice soils and all soils which are devoid of O2 come under the category of submerged soils

Factors influencing the pedogenesis of soils

Two conditioning factors for the distribution or submerged soils are 

                                   (1) Climate       and

                                   (2) Physiography. 

Most of the submerged soils in the world are geologic formations because the submerged soils are young soils. There will not be full development of horizons due to anaerobic condition. With reference to diagnostic horizons and epipedons, the epipedons are mostly absent and typic epipedons are not formed.

Conditions amenable for the formations of submerged soils

1. Humid condition – More rainfall, particularly during rainy season favors seasonal saturation of soils with water and seasonal flooding of low lying areas. Mostly the submerged soils in Asia are produced as a result of the condition of humid climate with heavy concentration of rainfall and abundance of low lying land such as flood plains, deltas and coastal lowlands.

2. Topography - Apart from these, submerged soils could be formed even in terraces when flooded / saturated with water as long as the land is plain. Development of submerged soils in slopes is ruled out because saturation is not possible. When the soil is heavily textured with impermeable nature, formation of submerged soils could not be averted. So, flooding the soil either artificially or by natural agencies cause the formation of submerged soils.

Soil forming factors

These include,

F = (cl, o, r, p, t) climate, organisms, relief, parent material and time.

The submerged soils will develop only in the aquic soil moisture regime in the soil profile or an aquic soil moisture regime in the sub-soil. The soils will be totally devoid of dissolved O2 because the soils are saturated either with ground water or by capillary fringe. The occurrence of these submerged soils is not restricted to the activity   of specific soil forming factor but mostly to hydrologic and pedogenic processes.  However, in the development of submerged soils, saturation is the most important factor but soil forming factors play only a marginal or low role. The hydrologic and pedogenic processes provide the excess water for saturation and sufficient biomass for activity of microorganisms and sufficient time to effect reduction and gleying. Among the different soil forming processes, the following are important.

1. Gleying

Process of soil formation leading to the development under the influence of excessive moisture of a Gley horizon in the lower part of the solum. The materials in this horizon will be of either bluish gray or olive gray in colour, more or less sticky, compact and structureless.

2.Mottling

Actually it is the development of spots of different colours which are irregularly marked due to reduced iron oxide.

Reduction and Gleying

These occur by both the processes when specifically associated with the biological activity of the anaerobic environment but many of the soil forming processes are common for aerobic and anaerobic moisture regimes. To site an example, presence of diagnostic horizons like ochric, mollic, cambic, albic, argillic, natric, salic, calcic and plinthic are found in both the moisture regimes within a location. In addition, there is greater similarly in the climate and edapic (vegetation) properties of aquic and non aquic soil moisture regimes within a geological area. Therefore, in the soil formation, the aquic soils (submerged soils) were distributed among the soil orders on the basis of diagnostic properties that persist upon drainage. There is no name in taxonomy as rice or paddy soil but it is a generic name compared to that of forest soil, grass land soil etc. Therefore, rice soils are considered as artificial hydromorphic soils according to Kanno (1962). As such rice soils are   distributed in most of the soil orders.

Orders - sub orders - great groups of rice soils

Oxisols            Aqu ox               (1) Ochr    (2) Plinth,    (3) Gibsic       (4) Umbr

Ultisols            Aquults.              (1) Trop    (2) Pale       (3) Plinth         (4) Frag                                                                                                              (5) Alb

Alfisols             Aqualfs              (1) Trop    (2) Plinth      (3) Natr        (4) Dura

Aridisols          Nil                        Nil

Inceptisols       Aquepts            (1) Trop   (2) And       (3) Hal        (4) Sulf                                                                                                              (5) Plinth

Entisols           Aquents              (1) Trop.    (2) Fluv        (3) Sulf       (4) Hydr

Vertisols          Nil                        Nil

Mollisols         Aquolls               (1) Hapl    (2) Argi       (3) Calci          (4) Dura                                                                                                              (5) Natr

Spodosols      Aquods                (1) Trop (2) Dura

Histosols         Nil                        Nil

Genesis of Genetic Horizons

The most important pedogenic processes involved in the formation of rice soilshave been well documented by Kanno (1962), Dudal (1965), Brinkman (1970), Wadaand Mitsumota (1973) and other workers. The most important processes have beeninfluenced by redox conditions, addition and removal of chemical compounds and soilparticles accompanied with change in soil physical, chemical and biological propertiesthrough irrigation, drainage or both. The various processes comprise of

1. Gleysation

2. Synthesis of mottles

3. Eluviation

4. Illuviation

5. Graysation

6. Plough sole formation

7. Illimerization / degradation

8. Cutan formation

9. Redistribution of exchangeable bases

10. Accumulation or decomposition and alteration of organic matter

All these processes will lead to the profile differentiation in rice soils

Distribution of submerged Soils

Rice soils are mainly distributed in the orders Entisols, Inceptisols and Alfisols.

• They occur mainly in humid and per humid climatic conditions.

• Wetlands are widely distributed even in dry regions. But, these soils are not distributed uniformly throughout the world.

• The submerged soils are more in tropical Asia which form 1/16 of cultivable area and 1/3 of the total Indian soils.

• We can use wetland soils and alluvial soils as synonyms because alluvial soils are brought under wetlands

• Nearly 2/3 of the world alluvial soils are in Asia and hence rice forms the staple food of most Asian countries

• After Asia, South America accounts substantial area of wetlands that is 25 million ha which are potentially utilized for rice cultivation

Profile Characters of Submerged Soils

Kanno (1962) introduced various notations to represent soil profile. Rice needs stagnant water, so puddling is done to form a dense layer which will prevent infiltration. Depending upon the deposition of particles, plough sole is a transitional zone between eluvial and illuvial zone. Occurrence of gley horizon is possible when water table is high whereas in well drained soil, parent material will be oxidized and in ploughed layer we  get gleying. In high water table area, parent material gets reduced and materials move upward due to capillary rise leading to the formation of gley horizon. According to Kanno (1978), a typical horizon sequence of well drained rice soil will be composed of,

1. Reduced eluvial horizon

2. Predominantly oxidized illuvial horizon

3. Periodically reduced illuvial horizon

4. Permanently reduced horizon

Ploughed layer : This will be represented with APg / ApG / APgG as described by Kanno (1962). This is the upper most layer where profound oxidation reduction involving the reducing effects of organic matter decomposition products takes place. When submerged, this layer may be divided into (1) a thin upper oxidized layer (1-2 mm) and (2) a lower reduced layer. The important process acted in this layer is the reduced eluviation. This is called graysation, the term coined by Mituschi (1984). A large portion of reduced Fe is oxidized to form a variety of mottles in the aerable condition.Ploughed sole Ag /AgG : This consists part of eluvial A horizon and illuvial B horizon or both. This layer is more or less compacted subsurface horizon in which the redox conditions under the leaching of reduced Fe and Mn resemble those of upper Apg (Zone of eluviation).Predominantly oxidized illuvial horizon Bg : This is observed in well drained rice soils where ground water is usually below 1m. As leaching proceeds, dark brown magniferrous mottles disappear from the Bg horizon and reddish yellow or orange yellow iron mottles remain behind.Periodically reduced illuvial horizon BgG  : This develops between AG and G horizons in poorly drained rice soils. This is atransitional horizon characterized by the predominance.Permanently reduced horizon :This horizon develops under impeded conditions where ground water level is within 1.5 m from the surface. This will appear either as bluish gray or grayish blue in colour due to the reduction of iron through the activity of microorganisms.

Physical properties of submerged soils

When the soil is flooded, we have only solid and liquid phases and devoid of gaseous phase. Even the micropores are filled with water depending up on the degree of submergence, the properties of solid and liquid phases vary.

1. Soil texture : In general, the texture of soil varies from sand to clay. But rice soils are not sandy because rice grows under submerged condition and it is difficult to submerge the sandy soil due to its highly porous nature.

2. Structure

Since rice soils are flooded and puddled, there will be disintegration of macrostructure and only the microstructure will remain. Sometimes a portion of micro structureis also destroyed under alternate wetting and drying conditions. The structure can be once again restored when the puddled soil is allowed to dry, the soil will form a massive structure.

3. Bulk density

Most of the soils do not have the ideal BD because the gaseous phase is totally eliminated due to flooding. All the pores will be filled with water. In general, the BD of rice soils will be lower than the normal soils that is 1.4 to 1.8. But there are conditions that the BD is reduced to 1.1 to 1.3, that conditions is called as Fluffy soil. In these fluffy soils the work animals and machines may sink and even ploughing may become difficult. If these soils are not treated, it will be very difficult to have cultivation in these soils. We have to operate a roller 5-6 times before flooding in order to compact the soil.

4. Soil consistency

It denotes the manifestation of physical forces of cohesion and adhesion acting   within the soil at various moisture regimes. The manifestations include the following  aspects.

1. The soil behaviour towards gravity, pressure, thrust / pull is affected due to cohesion

2. The tendency of the soil mass to adhere to foreign bodies on submergence.

3. The sensation evidenced by the feel of the fingers.

The definition implies that the soil consistence includes such soil properties as resistant to compression and shear, friability, plasticity and stickiness. These properties are manifested differently as the forces of cohesion and adhesion within the soil mass vary. The soil consistency varies with texture, OM content, amount and nature of colloidal materials, structure and moisture content. Because of submergence, soil will be wet and moist. On puddling it becomes pasty. On drying it becomes hard and it becomes so hard that malloting is required to break it. At optimum moisture soil can be rolled into balls or ribbons in between fingers by giving pressure.

5. Water permeability

1. Seepage : Defined as the lateral movement of surface water

2. Percolation : refers to vertical movement of water beyond root zone particularly to water table. Rate of seepage and percolation varies widely in rice soils depending on soil physical conditions mainly soil texture. Both seepage and percolation take place simultaneously depending on moisture status. In reality it is very different to separate seepage and percolation because of the transitional flow or water which can not be clearly classified. In general, rice soils are considered to have 2 systems. In the process of puddling, coarse particles settle first followed by finer soils particles (dense) and as a result percolation of water is restricted leading to stagnation of water which is suitable for rice cultivation. The two layer system include, 1. Ploughed layer – Permits flow of water ; 2. Second layer – restricts the movement.

6. Soil Aeration

Macropores are completely replaced with water due to flooding. This leads to anaerobic condition lending to soil reduction. Under normal condition within 24 hours of flooding, O2 content will reach zero level resulting in the accumulation of CH4, H2 S etc.

7. Soil temperature

Due to submergence, soil temperature is reduced because of the presence of water. Usually temperature is less by 5-10º compared to atmospheric temperature. Reduced temperature is due to reduction in atmospheric solar energy due to insulating effect of submergence. Variation in temperature of water is more than atmospheric , because of high sp. ht. of water ( 1.0 ) which absorbs radiation.

Effect of flooding on soil

Gaseous exchange takes place Macro pores are filled with water leading to anaerobic condition / reduction. Soil with 2:1 clay leads to volume expansion and disruption of aggregates takes place leading to reduced aggregate stability.

Transformation of Nutrients in submerged soils     ( Please refer to the latter part of this Lecture notes )

Eh values                                 Electron activity is due to

< - 200 m V                               Oxygen – nitrogen range

+200 to +100 m V                     Iron range

+100 to 0.0 m V                        Sulfate range

0.0 to – 200 m V                       Methane - hydrogen

Sposito (1989) proposed to this classification is,

Pe value                       Name of soil

> 7                                 Oxic soils

+2 and +7                      Suboxic soils

< + 2                               Anoxic

 He made this classification at pH 7. These ranges correspond roughly to redox control by oxgen – nitrogen, manganese – iron, and sulphur couples.

Berner (1981) proposed categories for redox as

Pe value                     Name                                 Electron activity

+7 to +13                     Oxic                                    Oxygen – nitrogen

+2 to +7                       Pastoxic                              Iron

- 2 to +2                       Sulfidic                               Sulfur

-6 to -2                         Methanic                            Methane - hydrogen

POISE

Poise is a useful concept in understanding potential measurements and the behaviour of mixed systems. The poise of a redox system is its resistance to changes in potential upon the addition of small amounts of oxidant or reductant.

Poise increase with the total concentration of oxidant plus reductant) and for a fixed total concentration it ismaximum when the ratio of oxidant to reductant is 1. The poer poise of natural aerated systems is due to the absence of a reversible in sufficiently high concentration.

Poise also had a bearing on he potential of mixed systems. When two or more systems are mixed, a redistribution of electrons takes place (I  enegy ban- iers absent) which brings the potentials of the systems to a commen value, the final potential determined by the system which is present in excess.

Thermodynamic sequence of soil reduction in submerged soils

After the disappearance of O2 in the submerged soil, the need for electron acceptor by facultative and “ obligate anerobes result in the reduction of several oxidised components. Ponnamperuina (1977) reported the reduction sequence in submerged soil and Eo the source of e1 at low activity are NADH, NADPH & ferrodoxin produced by anaerobic bacteria.

Soil redox and physical properties

The redox is an important electrochemical property, decides the aeration status, flocculation etc which inturn influences the growth of microorganisms and plants.

1.      Aeration status of soil

The redox potential, can be used to indicate the aeration status in soil

Soil aeration status                                      Eh (V)

Well – aeration soils                                  0.4 to 0.7 or higher

Somewhat poorly aerated soils                0.3 to 0.35 or lower

Waterlogged soils                                     -0.4 or lower

2.      Effect of redox on flocculation of clay particles

3.      Oxygen concentration and redox potential in soil aggregates

Role of redox conditions in soil genesis

1. Human formation,  2. Horizon development and  3. Soil colour

Mottling is a variegated colour pattern in the soil. In many situation it indicates that if the soil is saturated with water during some part of the year. The presence of mottling particularly low chroma mottling has been used for many years as an indirect indicator of soil saturation and is the result of the iron reduction. High chroma mottles result from the reoxidation of the reduced iron and its precipitation as iron oxide minerals. Therefore, the influence of soil variables on the soil colour should be important to study the soil saturation.

Factors affecting the redox potential of the soil

1.Organic matter

The course, rate and magnitude of the Eh decrease depend on the kind and the amount of organic matter. The presence of native or added organic matter sharpens and hastens the first minimum, nitrate abolishes it. The rapid initial decrease of Eh is apparently due to the release of reducing substances accompanying oxugen depletion before Mn (IV) and Fe (III) oxide hydrates can mobilize their buffer capacity. Katyal (1977) reported that the addition of organic matter caused  a steep fall in Eh in the early stages because of the accumulation of reducing substances as a result of O2 depletion  (ponnamperuma, 1966). The Eh values in general were lower with organic matter than without it.

2.Alternative flooding and drying

Sajwan and Lindsay (1986) reported that redox chances under alternate flooding and drying. During the drying cycle Pe + PH value increased to near 15, whereas Pe + pH value increased to near 6 during flooding cycle. These fluctuations 88 can be duet to the increased electron activity in flooded soils as O2 is depleted and is not available as an electron acceptor. During the drying cycle O2 is re-supplied again and increased the Pe + PH to near 15.0.

3. Influence of tome and soil depth

Sajwan and Lindsay (1986) reported that the chances in redox (Pe + PH ) occurred with time and depth below the soil – water interface. There was a regular and consistant pattern of redox changes. In 1cm above the soil-water interface the Pe + pH remained poised near 15.0 and did not change during the growing period. Solubilization of O2 from the atmosphere and convective movement of the flood water apparently kept the flood water oxidized near Pe + pH 15.0 throughout the growing season. Beloe the soil-water interface Pe + pH dropped rapidly and levelled off after 30d. at depths of 0.5, 2, and 6cm, the Pe + pH stabilized at approximately 5.5, 4.2 and 3.90 respectively.

4. Effect of submergence and salinity

Mahrous ct al. (1983) demonstrated the influence of the salts on the redox potential of the submerged soils. They reported that, in water – logged soils oxidation-reduction potential can become as negative as -300 mV, if the reduction processes are sufficiently intense. Reduction of soil is a consequence of anaerobic respiration by soil bacteria, the source of energy is the organic matter, and Fe + Mn can be used as electron acceptors in anoxic environments. Among the different soils studied, salinity effect is small on Eh values. In the willows clay soil, however, a salt treated sample has lower Eh values than the control. It happens since soil originally had a high EC value and the control sample was leached with distilled water to bring the EC down. This leaching process presumably contributed to the loss of some oxidized components, especially NO3 to be used as electron acceptors in the anaerobic reduction process, other soils under study, showed a slight increase in Eh values of salinized samples compared to their controls. This salinity effect is likely due to a direct influence of the ionic strength and a I depression effect on the biological activities of the soil microorganisms. The slight increase of Eh values after they reached their lowest values in some soils could be due to the increase of PH values with submergences.

5.Influence of chemicals on redox

Nhung and ponamperuma (1966) reported the effects of CaCO3, MnO2, Fe(OH)2 and prolonged flooding on the Eh changes in the growth of rice on a flooded acid sulphate soil. The much lower redox potentials of the limed soils than of the untimed ones are a manifestation of the pronounced influence that PH has on the Eh of the system Fe (OH)3 – Fe2+, which is probably the dominant system in most flooded soils. MnO2 at 1.0% markedly retarded the fall in Eh which is due to the slightly increased pH by the reduction of So4= .  similar is the case with Fe (OH) 3 at 0.5percent.

6. Effect of agricultural chemicals and wastes

Header et al. (1994) reported that effects of furden, sewage sludge and time of incubation on Eh (mV) in waterlogged soil. The application of furdan have a remarkable effect on the Eh of sewage sludge amended and non-amended soil comparing control. After 3 weeks of incubation especially at 50 and100ppm furdan, high redox potential were recorded particularly, in non-amended soils. This is due to the application of sewage sludge stimulate the reducing condition which will decrease the Eh.

7. Reducing substances

There is a close relationship between the intensity factor (redox potential) and the capacity factor (amount of reducing substances) of redox properties of soils. The lower the Eh in the soil, the higher the amount of reducing substances, the lowering of Eh is due to the reduction of oxides of iron and manganese and sulphate after the formation of organic reducing substances.

8. Effect of tillage on redox potential

Soil redox potentials are relative indication of aeration. Based on this, Oiness et al.(1989) reported the changes in redox potentials throughout the growing season ina continued tillage- continuous maize which reflected differences in tillage implement on aeration status.This Eh at the 15cm depth remained relatively high throughout the growing season. Potentials in no-till (NT) and Chisel (CH) tillage treatments were briefly at below 300 mV early in the growing season, but they had reached 500 mV by mid-july for CH and late july for NT. The man ridge- tilled (RT) treatment values were usually >550 mV throughout the season. Eh in the moldboard – plow (Mp) tillage treatment appeared to reach a maximum mean value of about 650 mV, stated to decline in late july, and reached values of about 430 mV by late sept. This is due to that, tillage with Mp, incorporate surface residues to a depth of about 25cm, undoubtedly affected aeration of the surface zones.