irps 5 physicochemical properties of submerged soils in relation to fertility

8/4/2019 IRPS 5 Physicochemical Properties of Submerged Soils in Relation to Fertility

1/34


2/34

IRPSNo.5, February 1977

PHYSICOCHEMICAL PROPERTIES OF SUBMrRGED SOILSIN RELATION TO FERTILITY

ABSTRACTThe pH, Eh or pE, specific conductance, ionic strength, ion exchange, sorptionand desorption, chemical kinetics, and mineral equilibria profoundly influencethe fertility of submerged soils by controlling the availability of plantnutrients and regulating their uptake by rice roots. A pH of about 6.6, an Ehfrom 0.01 to 0.12 V or a pE from 0.2 to 2.0 at pH 7.0, a specific conductanceof about 2 mmho/cm at 25~'and a temperature from 30 to 35C (all in the soilsolution) appear to favor nutrient uptake by rice. Under these conditions theavailability of nitrogen, phosphorus, potassium, calcium, magnesium, iron,manganese, and silicon is high, the supply of copper, zinc, and molybdenum isadequate, and injurious concentrations of aluminum, manganese, iron, carbondioxide and organic acids are absent. In normal tropical soils these conditionscan be achieved by incorporating organic matter and keeping the soil submergedfor 2 to 4 weeks before planting.

1by F. N. Ponnamperuma, principal soil chemist, International Rice ResearchInstitute, Los Banos, Philippines. Paper presented at the Seminar on thefertility of paddy soils, Taipei, Taiwan, 26-31 July 1976. Submitted to theIRRI Research Paper Series Committee, 5 January 1977.


3/34


4/34

IRPS No.5, February 1977

PHYSICOCHEMICAL PROPERTIES OF SUBMERGED SOILSIN RELATION TO FERTILITY

Soil fertility is the status of a soil with respect to its ability to supplynutrients essential to plant growth. The ability to supply nutrients dependson the presence in the soil of adequate nutrients in forms the plant can absorb;the soils ability to deliver nutrients by mass flow and diffusion to the rootsurface, which according to Parish (1971), is high in soils with a high watercontent; the presence of a favorable ionic composition, and the absence in thesolution of substances that interfere with movement of nutrients into the root.These factors are strongly influenced by the physicochemical properties of thesoils. In submerged soils the important physicochemical properties or processesthat control fertility are the negative logarithm of hydrogen ion concentration(pH),redox potential (Eh), specific conductance, ionic strength, ion exchange,sorption and desorption, chemical kinetics, and mineral equilibria.

pH AND FERTILITYThe pH values of acid soils increase on submergence whereas the opposite occursin calcareous and sodic soils (Fig. 1). For most soils the fairly stable pHattained after several weeks of submergence is between 6.5 and 7.0.

The pH of a submerged soil exerts a marked influence on the capacity of the soilto supply nutrients through

1. direct effects on nutrient absorption,2. direct effects on the concentrations of nutrients or

toxic substances in the soil solution,3. indirect effects on chemical equilibria, and sorption

a nd d es or pt io n,4. influence on microbial processes connected with the

release or loss of plant nutrients and the generationof toxic substances.


5/34


pH ,-------.----------------------------

Soil'Io , 'Iextu re pi[ O.M.% F'(' ~.~ Mn C %28 day 4. L.'l 4.70 0.0835 clay 3.4 (l.t) L.GO 0.0 I40 clay 3.8 7.2- 0.08 0.0057 clay loam 8.7 2.2 0.(,3 0.07'l4 clay (1.7 2'(, o.s 0.09L)


6/34

IRPS No.5,February 19775

undergo reversible oxidation-reduction seasonally, the activity of Fe2+ in thesoil solution (the concentration will be higher) is given by the equationpH - 1/2 pFe2+ = 5.4. The iron(II) hydroxide potential (pH - 1/2 pFe2+) appearsto be a characteristic for a given soil and remains constant during long periodsof submergence (IRRI, 1968 ; Ponnamperuma, 1972). If its value is determined for2+a soil, it can be used to calculate the activity of Fe . The only additionalparameter required for the calculation of concentration is the specificconductance from which the ionic strength is derived (Ponnamperuma et al., 1966b).

The concentrations of Fe2+ in the solutions of four soils, derived from theiriron(II) hydroxide potentials, are in Table 1. The peak values for more than100 wetland rice soils ranged from 6,600 ppm at pH 5.67 for an acid sulfatesoil to 0.07 ppm at pH 7.25 for a calcareous soil low in organic matter.

Table 1. Influence of soil properties on the Fe(II) hydroxidepotential pFe(OH)2 and the concentration of water-solubleFe 2+ at pH 6.5 and ionic strength of 0.03 mol/liter.

Soil O.M. Fe Mn pFe(OH)2 Fe2+no. pH (%) (%) (%) (ppm)21 4.6 4.1 2.8 0.02 5.41 6297 5.9 3.3 1.7 0.33 5.09 19527 6.6 2.0 1.6 0.31 4.90 6426 7.6 1.5 0.3 0.06 4.8 3 43

The practical implications of the pH - Fe2+ relationship for the fertility ofsubmerged soils are that at high pH values rice suffers from iron deficiency,and that at low pH values rice is affected by iron toxicity (Ponnamperuma,1955; Tanaka and Yoshida, 1970) and by phosphorus and potassium deficiencies2+induced by excess Fe (Yamada, 1959).

High concentrations of aluminum and manganese are common causes of cropfailure in acid aerobic soils. Except in certain acid sulfate soils,aluminum toxicity disappears within a few weeks of soil submergence because


7/34


of the increase in pH. Nhung and Ponnamperuma (1966) observed aluminumconcentrations similar to those derived from the equation pH - 1/2 pAlM = 2.2(Raupach, 1963). In the pH range 3.5 to 5.0 for a submerged acid sulfate soilthose values are

pH Al (ppm)3.0 7003.5 704.0 74.5 0.75.0 0.07

The toxicity of carbon dioxide and organic acids lS more pronounced at lowpH than at high pH because the concentrations of the undissociated species(H2C03 and RCOOH) increase as pH decreases, and because H2C03 and RCOOH aremore toxic to rice than HC03- and RCOO-.

The H2C03 concentration in reduced soils can be calculated from the followingequations:Ultisols (Ponnamperuma et al., 1969) : pH 6.1 0.58 log PC02 (1)Calcareous soils (Ponnamperuma, 1967): pH 6.1 0.67 log PC02 (2)Sodic soils (Ponnamperuma, 1967): pH 7.8 + log (alkalinity)

- log PC02 (3)

Effects on chemical equilibria and sorption and desorptionMineral phosphorus is present in soils mainly as aluminum and iron phosphatesat low pH, and as calcium phosphates at high pH, and is found sorbed on thesurfaces of clay and oxides of aluminum and iron (Russell, 1973). An increasein pH values favors the solubility of aluminum and iron phosphates anddesorption of phosphorus, and a decrease in pH favors the solubility ofcalcium phosphates (Stumm and Morgan, 1970). Thus the solubility ofphosphorus in soils tends to be maximum in the pH range from 6 to 7 (Fig. 2,3).

The increase in pH of acid soils and the decrease in pH of calcareous andsodic soils increase the availability of phosphorus in submerged soils.This is a benefit of flooding rice soils (Ponnamperuma, 1965).


8/34


0 oc0 / c -/ v - Q

.o~OJ ':o~

2 0 1 >0~~o::" 03 1 >

40'"n, 5NIa.6

7

pHFig. 2. Solubility of phosphate compounds

in soils at 250 C and 0.005 M Caconcentrations (Lindsay and Moreno,1960).

100zQf-: : : >c oa ::f-en0w(!)


9/34


2+Increasing the pH of a submerged soil decreases the concentration of Mg ,2+ 2+Ca ,and Mn (Table 2), apparently through carbonate equilibria involvingthese lons. These equilibria can be described by the equation

pH + 1/2 log M2+ + 1/2 log PC02 K (4)

where M is magnesium, calcium, or manganese, and K2+ 2+Ca ,and 4.06 for Mn (Ponnamperuma, 1967).

6.60 for Mg2+, 4.92 for

Table 2. Influence of pH on the concentration of some cationsin the solution of Maahas clay, 6 weeks after submergence.

Initial Solution pH Concn (EEm)soil pH at 6 weeks Fe Mn Ca Mg

6.4 6.59 61 23 369 1918.4a 7.63 2.2 1.8 39 35

a f .A ter amendment wlth NaHC03.

There is evidence that calcium deficiency is a factor limiting the growth ofrice on submerged sodic soils (IRRI, 1973).

When soils are submerged, the concentrations of water-soluble zinc and copperdecrease and the concentration of water-soluble molybdenum increases (Table 3),partly because pH increases when acid soils are flooded. The solubility ofthe three elements is highly dependent upon pH, as the following equationsfor some aerobic soils show:

(zn2+) 106 (H+) 2 (Lindsay and Norvell, 1969) (5)(Cu2+) 103.2 (H+)2 (Lindsay and Norvell, 1969) (6)

2- 10-20.5 CH+)2 (Lavy and Barber, 1964) (7)(Mo04 )


10/34

IRPS Noo5, February 19779

Table 3. Kinetics of water-soluble boron, copper, molybdenum,and zinc in three submerged soils.

Concn (ppm)Element 1 wk 2 wk 3 wk 4 wk 6 wk

Luisiana clay (pH: 4.8; O.M. : 2.8%)B 0.55 0.49 0.19 0.46 0.30Cu 0.11 0.11 0.03 0.04 0.02Mo 0.18 0.06 0.28 0.15 0.24Zn 0.30 0.09 0.05 0.08 0.03

Maahas clay (pH: 6.6; O.M. : 2.8%)B 1.80 1.01 1.15 1.15 1.18Cu 0.06 0.05 0.03 0.03 0.02Mo 0.04 0.09 0.08 0.17 0.12Zn 0.18 0.08 0.04 0.06 0.03

Keelung silt loam (pH: 7.7; O.M. : 6.9%)B 0.48 0.55 0.55 0.68 0.52Cu 0.05 0.05 0.05 0.04 0.03Mo 0.09 0.10 0.34 0.17 0.27Zn 0.14 0.08 0.06 0.04 0.03

2+ 2+ 2-The Zn ,Cu ,and Mo04

are also present, with B(OH)Z, sorbed on hydrousoxides and clays (Jenne, 1968; Ellis and Knezek, 1972). Changes in pH valuesmay cause sorption or desorption depending on the isoelectric points of theadsorbing substances.

2+The pH value can also affect Zn2-the activity of S .d C 2+ lob 0 h hOffan u equl l rla t roug lts e ect on

For a given total sulfide concentration, a high pH will2- 2+ 2+increase the activity of S and favor the removal of Cu and Zn as

insoluble sulfides. There is some evidence of the presence of Zlnc sulfidein flooded soils (IRRI, 1973).

A high pH in the surface water of flooded soils may lead to substantial lossesof broadcast nitrogen fertilizer by volatilization.

Effects on microbial processesThe decomposition of organic matter in submerged soils is done chiefly by anaerobic


11/34


bacteria. They function best at a pH near 7, at which the release of bothnitrogen and phosphorus from soil organic matter should be maximum. It isdifficult to study the effect of pH on microbial processes in submerged soilsbecause pH values spontaneously change, but the increase in rate ofammonification that followed liming of acid sulfate soils of Thailand seemsto confirm the adverse effect of low pH on ammonification. Denitrification,also, is slow below pH 5.5 (Alexander, 1961), which may account for thepersistence of nitrate for several weeks in acid submerged soils( Po nn am pe ru ma , 1 95 5) .

Sulfate reduction occurs best at low redox potentials and in the pH rangefrom 5.5 to 8 .5 (Starkey and Wight, 1946). Thus the increase in pH of acid2-soils on submergence is conducive to sulfur deficiency on soils low In S04 .

Implications for riceThe pH of the solution of a reduced soil is an important factor determiningthe fertility of rice soils. Concentrations of iron, aluminum, and PC0 2 canbe calculated from the pH value using the equations below

Ultisols: pH 1/2 F 2+ 5.4 (8 )eAcid sulfate soils: pH 1/2 pAlM 2.2 (9)Ultiso1s: pH + 0.58 log PC0 2 6.1 (10)Calcar eous soils: pH + 0.67 log PCO 6.1 (11)2

The pH values of reduced soils are higher than those of the correspondingsoil solutions because of the loss of carbon dioxide during measurement(Ponnamperuma et al., 1966a).

In terms of soil fertility, the optimum pH (measured in the solution of thesubmerged soil) for rice is about 6.6. At that pH value the microbial releaseof nitrogen and phosphorus from soil organic matter and the availability ofphosphorus are high; the supplies of copper, zinc, and molybdenum areadequate; and the concentrations of substances that interfere with nutrientuptake, such as aluminum, manganese, iron, carbon dioxide, and organic acids,are below the toxic level. However, the rates of denitrification and SO~-reduction are higher at pH 6.6 than at lower pH values.

Soil and water management practices should, therefore, aim at attaining a pH


12/34


of 6.6 at planting time and at maintaining that value at least until panicleinitiation. In soils containing more than 1% active iron oxide, and lncalcareous and sodic soils, pH 6.6 is achieved by the incorporation of organicmatter in the submerged soil at least 2 weeks before planting.

REDOX POTENTIAL (Eh) AND SOIL FERTILITYThe Eh is perhaps the most important physicochemical property controlling thechemical and biochemical characteristics of submerged soils. Eh, or the pE(negative logarithm to the base 10 of the electron activity, equal toEh/0.05~,measures the intensity of oxidation or reduction. High andpositive Eh or pE values indicate oxidation conditions; low or negative valuesind icate re duct ion condi tion s.

Within a few hours after submergence, a soil's dissolved and gaseous oxygen isconsumed by aerobic microorganisms. Then the facultative anaerobes, followedby the obligate anaerobes, use soil components and dissimilation products oforganic matter as electron acceptors in their respiration, reducing the soilln the sequence predicted by thermodynamics (Table 4) and lowering its Eh orpE (Fig. 4).iron (III) to

Thus, NO] is reduced to N2, manganese(IV) to manganese(II),2- +iron (II), S04 to H2S, CO2 to CH4, N2 to NH3, and H to H2.Because manganese (II) and iron (II) compounds are more soluble than their

oxidized counterparts, a submerged soil is charged with water-soluble Mn2+2+and Fe . These ions and HCO;, formed during the decomposition of organicmatter, increase the specific conductance of the soil and have marked influenceon ionic equilibria in submerged soils.

Redox potential affects1. the nitrogen status of the soil;2. the availability of phosphorus and silicon;

2+ 2-Mn and S04 directlyd 2-. di Ian Mo04 ln lrect y; and those of

.. f 2+

t he c on ~e nt ra tl on s 0 Fe ,2+ 2+ 2+ 2+Ca , Mg , Cu , Zn ,4. the generation of organic acids and hydrogen sulfide.

In the quantitative study of soil reduction and its implications it is moreconvenient to use pE than Eh.


13/34


Eh,mv~ - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - ~

300

Luislana clay (pH ".6, O.M.2.9-t .)~ Kalayaan clay loam (pH 5.4, O.M. 16.0-t.)0---0 Luna clay (pH 7.3, O.M. 12/.)........., Son Pablo clay loam (pH 6.7, O.M.2.6-t.)v---q Cotabata silty clay loam (pH 8.7 IO.M. 2.2/.)

400

200

100

o

4 6 8 10 12 1 4Weeks submerqed

Fig. 4. Kinetics of solution Eh in five sub:m.erged soils.

Th e n it~o ge n st atu sThe pE determines the stable form of nitrogen in a soil. The equation forthe N03 N system

pE 21.06 - 115 pNO; + 1110 pN2 - 6/ 5 pH (12)shows that in aerobic soils at pH 7.0 (P02 = 0.2), the activity of N03 is104.9. That means that N03 is stable in aerobic soils. The same equationshows that NO) is extremely unstable in submerged soils epE = -1 to 3).Thus within a few days after submergence the bulk of original or added N03is lost as N2 by denitrification (Ponnamperuma, 1972). For 280 Philippinerice soils the loss ranged from 10 to 78 kg N/ha per season with a mean of26, representing an annual loss (recalculated) of 52,000 tons of nitrogen(IRRI, 1973). The loss of nitrogen by denitrification in submerged soils


14/34


can be minimized by keeping the soil continuously flooded (Castro and Lantin,1 97 6) .

A submerged soil is not uniformly at a low pE. The surface water, and a fewmillimeters of the soil in contact with it, can have a high Eh (Mitsui, 1960)or pE. Ammonium from fertilizers broadcast on the surface, or NH4+ diffusingupward to the zone in which pE is high, is rapidly converted to N03 bynitrifying bacteria. The N03 then moves by mass flow and diffusion into theanaerobic zone and is denitrified. Thus N03 fertilizers are not advisablefor submerged soils, and nitrogen efficiency is reduced by alternate wettingand drying of the soil (Patrick and Mahapatra, 1968 ). However, inunfertilized soils the losses of nitrogen are low and midseason drying mayeven benefit rice CIRRI, 1973). The efficiency of nitrogen use can beincreased by placing NH4+ in the rice root zone (De Datta et al., 1974).

In aerobic soils the end product of the mineralization of organic nitrogen+lS NO;; in submerged soils it is NH4 The equationpE +14.91 - 1/8 pNO; + 1/8 pNH4 - 5/4 pH (13)

shows that at low potential of submerged soils (pE = -1 to 3) the+concentration of NO] that can be in equilibrium with NH4 is infinitesimal.Thus NH4+ accumulates in submerged soils. Concentrations of nitrogen as NH4+as high as 300 ppm are reported for submerged soils within 20 days of flooding( Po nna mpe rum a, 1 965 ).

The presence of nitrogen as NH4+ benefits soil fertility because bothleaching and denitrification losses are less than with nitrogen as N03This is one reason why submerged soils are more fertile than upland soils( Po nn am pe ru ma , 1 97 5) .

The low pE of submerged soils also helps maintain nitrogen fertility by+encouraging nitrogen fixation. The reduction of N2 to NH4 needs a highelectron activity as shown by

pE +4.64 - 1/6 pN2 + 1/3 pNH4 - 4/3 pH (14)


15/34


+At pH 7.0 and a pN2 of 0.78 atm, a detectable amount of NH4 will be presentonly at pE = -2.35. The source of electrons at low activity are NADH, NADPH,and ferredoxin produced by anaerobic bacteria (Ponnamperuma, 1972) (Table 4).

Table 4. Thermodynamic sequence of soil reduction.

Ferredoxin (ox) + e Ferredoxin (red)

Eo7a(V) pEo 7b

0.8 14 13.800.741 12.660.401 6.80

-0.158 -2.67-0.185 -3.13-0.214 -3.63-0.244 -4.14-0.278 -4.69-0.317 -5.29-0.329 -5.58-0.413 -7.00-0.431 -7.31

System+02 + 4 H + 4 e = 2 H20

2 NO; + 12 H+ + 2 e N2 + 8 H20Mn02 + 4 H+ + 2 e Mn2+ + 2 H 20CH3COCOOH + 2 H+ + 2 e = CH3CHOHCOOH

+ 2+Fe(OH)3 + 3 H + e Fe + 3 H202- +S04 + 10 H + 8 e = H2S + 4 H20+C02 + 8 H + 8 e + CH4 + 2 H20+ +N2 + 8 H + 6 e = 2 NH4+ +NADP + 2 H + 2 e = NADPH

NAD+ + 2 H+ + 2 e = NADH2 H+ + 2 e = H2

corrected to pH 7.0.b pEo corrected to pH 7.0.

Aerobic, nitrogen-fixing bacteria can thrive In the high-pE surface soillayer and the rhizosphere of rice, sustained by fermentation productsdiffusing from the low-pE soil matrix. The anaerobic bulk of the soil wouldbe an excellent medium for such anaerobic, nitrogen-fixing bacteria asClostridium. Fixatior.. is higher in the presence of rice plants than in theirabsence -- 55 vs. 32 kg/ha (IRRI, 1971).

Availability of phosphorus and siliconThe availability to rice of original and added phosphorus increases on soilsubmergence (Ponnamperuma, 1965; Patrick and Mahapatra, 1968 ). In acid soilsthe increase in phosphorus availability is associated with a decrease in Eh


16/34


or an increase in iron(II) (Ponnamperuma, 1955; Patrick and Mahapatra, 1968),suggesting a role for iron(III)-bonded phosphates. The increase in phosphorusavailability may be due to direct reduction of iron(III) phosphates or tothe increase in pH accompanying soil reduction.

When lake muds undergo reduction, large amounts of soluble silica are releasedinto the hypolimnium (Hutchinson, 1957). The concentration of silicon in thesolution of submerged soils increases slightly after flooding and thendec reas es gr adua lly ( Ponn ampe ruma , 1965). The increase in concentrationof silicon by soil reduction may be due to the release of silicon sorbed onhydrous oxides of iron(III), following reduction of iron(III) to iron(II).

The increase in availability of phosphorus and silicon is regarded as oneof the benefits of flooding rice soils (Ponnamperuma, 1955, 1965, 1975).

2+ 2+Concentrations of water-soluble Fe and MnWhen a soil is flooded, iron(III) and manganese(IV) oxides are reduced and2+ 2+large amounts of Fe and Mn are brought into solution. The theoreticale qu at io ns ( Po nn am pe ru ma , 1972) relating the activities of Fe2+ and Mn2+ topE and pH before the peaks of water-soluble Fe2+ and Mn2+ are

pEpE

17.87 + pFe2+ - 3 pH20.85 + 1/2 pMn2+ - 2 pH

(15)(16)

f 2+. 15 2+T he a ct iv it ie s 0 Fe In most soils conform to equation but the Mnactlvltles are much lower than those predicted by equation 16. After thepeaks of water-soluble Fe2+ and Mn2+, their activities obey the equations

pEpE

23.27 + 3/2 pFe2+ - 4 pH18.57 + 3/2 Pe02 - pH

(17)(18)

2+ 2+ .The concentrations of Fe and Mn In reduced soils ranged from 0.07 to6,600 ppm and from 1 to 100 p pm , r es pe ct iv el y.


17/34


r"}_+-The increase in concentration of water-soluble Fe" is an important benefitof flooding rice soils. It not only eliminates iron deficiency in all soilsbut also depresses manganese toxicity in acid soils (IRRI, 1966, 1973;Ponnamperuma et aI., 1955; Tanaka and Yoshida, 1970).

2+Equation 15 shows that to have a concentration of 1 ppm Fe in the soilsolution, at an ionic strength of about 0.03 mmol!liter, the pE must be 2or the Eh, 0.12 V.

2+ 2+The large amounts of Fe and Mn brought into solution at low pE values,along with HCO~, increase the specific conductance of the soil solution and2+ 2+ +displace large amounts of Ca ,Mg ,and K by cation exchange reactions(Ponnamperuma, 1965, 1972).

The increase in pH caused by the decrease in Eh or pE depresses the solubilityof copper and zinc wh i.I.e increasing that of molybdenum.

Reduction of sulfate 2-Within 4 to 6 weeks after submergence the concentration of water-soluble S04in normal, nearly neutral soils becomes extremely low. In acid soils the2-decrease is s l.ower CIRRI, 1966). The d i.sapp ea r anc e of S04 is due to its2-reduction to hydrogen sulfide with pE 5.12 - 1/8 pS04 + 1/8 pH2S - 5/4 pH.The hydrogen sulfide formed reacts with lron compounds, giving FeSnH20.This reaction reduces the concentration of hydrogen sulfide in normalsubmerged soils to less th~n lO-6M.

The reduction of SO~-to hydrogen sulfide has practical implications for thefertility of submerged soils.

1. In soils low in SO~-, sulfur deficiency may occur, especiallyif urea is the source of fertilizer nitrogen.

2. In soils low in active iron, such as white sandy soila, peatsoils, and some acid sulfate soils, hydrogen sulfide may hindernutrient uptake by rice roots or poison the pl:Glt.

3. Hydrogen sulfide may lower the availability of copper and zinc.

FermentationWhen organlc metaoollces 01 oaccerlB ct~C d~ ~~ec~~'J~ accepLors, d W1G2


18/34


variety of organlc substances is produced (Ponnamperuma, 1972). Of these,the fatty acids have been shown to be toxic to rice (Takijima, 1963). Othertoxic substances produced during the decomposition of organic matter at lowpotentials are thiols, organic sulfides, hydrogen sulfide, and ethylene.

Implications for soil fertilityThe decrease in Eh brought about by soil submergence and its secondarychemical and physicochemical effects have positive and negative effects onthe fertility of flooded rice soils. The benefits are lncrease in thesupply of nitrogen, phosphorus, potassium, iron, manganese and molybdenum,and suppreSSlon of manganese and aluminum toxicities. The disadvantagesare losses of nitrogen by denitrification; decrease ln availability of sulfate,copper, and zinc; and production of substances that interfere with nutrientuptake or that poison the plant directly.

Available data suggest that the optimum level of reduction may be in therange of Eh 10 to 120 mV or pE 0.2 to 2.0 at pR 7.0 of the soil solution.

Soil potentials are of little diagnostic value in rice culture (Ponnamperuma,1965) .

SPECIFIC CONDUCTANCEWhen a soil is submerged, the specific conductance increases in the earlyphase of submergence, reaches a maXlmum, and then decreases to a fairlystable value, which varies with the soil (Fig. 5). The increase inconductance during the first few weeks after flooding is due to

2+ 2+1. release of Fe and Mn , following the reduction of insoluble

2.iron(lII) and manganese(IV) hydrous oxides;accumulation of NH4+, RCO;, and RCOO ;

3.7+ 2+

displacement of cations from soil colloids by Fe- and Mn .Thus there is a close similarity between the kinetics of specific conductance

2+ 2+ .and those of Fe and Mn and other catlons (Ponnamperuma, 1972).

In normal submerged soils the peak specific conductances are of the order of2-4 mmho/cm at 25"C. But submerged sandy soils that are rich in organic


19/34


Specif ic conductance (mmho/cm ot 25C)

8 Luiliane cloy (pH 4.6 IO.M.2.9%)... ... ... Kalayoon cloy loom (pH 5.4 IO.M.16.0%)0--0 Lune cloy (pH 7.3 IO.M. 1.2%). ... .... .., SonF'oblo clay loom (pH 6.7 IO.M. 2.6%)fI---I9 Cotabota Iilty clay loom (pHS.7 j O.M. 2.2/0)

Weeks subrnerqed

Fig. 5. Kinetics of specific conductance of the solutions of fivesubm.erged soils.

matter may attain values exceeding 4 mmho/em, which lS the harmful limit forrice (Ponnamperuma, 1972).

Apart from its direct effect on the rlce plant, the accumulation of ionsincreases the ionic strength and alters the ionic composition of the soilsolution. That affects ionic equilibria involving plant nutrients and theuptake of ions by rice roots.

The chemical kinetics of Maahas clay (pH 6.6; O.M. 2%) amended with sodiumchloride to give initial specific conductances of 5, 10, 20, and 25 mmho/cmrevealed that as the salt content increased, the concentration of cationsincreased (Fig. 6). The increase in concentration of cations brought about


20/34


200 '.'" . . . . . .~\ . . ... . ..,\ . . .,\-,_'...... t. '- . . . . . . . . . _ _ ._._._._.- - - - - - - - -150

1200

-,. '..~. .~ . . _ _ .--', ., . . . . ., '., . . . . . . . . ." .... _ ..... _ .'?I_ ..... ,~ . . . . ....",.- . - . . . . . . _ " . .- - - - ------ . . . . . . . _ -00400

2 4 6 1 2We ek s s ubm er ge d W e ek s s ubm erg ed

Fig. 6. Effect of salt on the concentration of cations in the solutionof submerged Ma aha s clay.

by the addition of sodium chloride may be attributed to displacement of+cations from soil colloids by Na , decrease in pH caused by the presence ofextra electrolyte, and effect of increased ionic strength on the solubilityof soil minerals.

T~ Although the concentration of ~ increases with increase in salt concentratlon,.+ a r +a.the activity ratio of K , (K/ CaMg), decreases (IRRI, 1973). If the

potassium-activity ratio is a measure of K+ availability, K+ availabilitydecreases when the salt content increases in spite of an increase ln theconcen trat.i.onof po tassium in the soil solution.


21/34


In addition to the direct injurious effect of excess salt, a high specificconductance of a submerged soil may imply the presence of injurious amountsof Fe 2+ and ionic antagonisms that hinder the uptake of ions. Thus irontoxicity is aggravated by adding sodium chloride. On the other hand, amoderate amount of sodium chloride may benefit rice by increasing theconcentration of ions that are in short supply.

CHEMICAL KINETICS AND SOIL FERTILITYFlooding a soil sets in motion a series of chemical and biochemical changesthat profoundly affect the availability and loss of nutrients, and thegeneration of substances that can interfere with nutrient uptake or thatpoison the plant directly.

Availability of nutrientsSoil properties, duration of submergence, and temperature strongly influencethe concentrations of

1.2.3.4.S.6.

water-soluble N03; +exchangeable and water-soluble NH4 ;2+ 2+total and water-soluble Fe and Mn ;

+ 2+ 2+ 2-water-soluble K ,Ca ,Mg ,S04;water-soluble copper, zinc, and molybdenum;carbon dioxide and organic acids.

Nitrate is present in soils at the time of flooding and throughout submergencein the soil surface and the supernatant water. Nitrate is readily absorbedby rice and nitrate disappearance represents the loss of a valuable nutrient.Figures 7 and 8 show that the rate of disappearance of NO} varied markedlywith the soil and temperature. Denitrification was fastest in Pila clayloam, moderately fast in the three acid soils, and slowest in Keelung silt loamdespite its high pH and high content of organic matter. In all soilsdenitrification was hardly detectable at So C and increased rapidly withincrease in temperature.

The mineralization of organic nitrogen In submerged soils stops at the NH4++stage. Velocity of NH4 release is, therefore, a good index of the capacity


22/34


23/34


of a soil to meet the nitrogen demands of a rice crop (Kawaguchi and Kyuma,1969). The kinetics of NH4+ release is important in rice nutrition becausenitrogen holds the key to productivity of rice, because more than half thenitrogen taken up by a rlce crop comes from the soil, and because grain yieldis highly sensitive to an excess or a deficiency of nitrogen at criticalphases of growth.

Soils vary widely in their capacity to produce NH4+ (Fig. 9). Soils rich in+organic matter rapidly release NH4 and attain concentrations exceeding 300+. ..ppm NH4 -N ln the sOll wlthin 2 weeks after submergence (at 25-35C). Soils+low in organic matter may release as little as 40 ppm NH4 -N even after 50days of submergence. The soil solution may contain 5 to 110 ppm nitrogendepending on soil texture and organic matter content (IRRI, undated).

Ammonia production follows an asymptotic course,release can be described by the equation

+and the kinetics of NH4

log (A - y) = log A - ct (19)+where A is the mean maximum NH -N in ppm of the dry soil, y lS the4concentration after t days, and c is a parameter depending on the soil. For31 mineral tropical soils, A was highly correlated with the organic mattercontent of the soil (IRRI, 1966) and may be taken as a measure of theavailability of nitrogen.

NH4 -N (ondrysoil basis)400r---------------------------------------~

200

/Silf loom pH 5.7; o.rn .

300

,.. a _._ _ _................, 0Cloy loom pH7.5; O.m. 1.1 10

o 10 20 30 40 50 60 70Days subrnerqed

Fig" 9. Kinetics of ammonification in sixsubmerged soils at 25-350 C.


24/34


Temperature has a marked influence on the rate of NH4+ release. Figure 10shows that the rate increased progressively from 15 to 45C in all soilsexcept Luisiana clay, in which the rate at 45C was less than that at 35C.

NH3-N (ppm)

400 Casiguran clay loam

100

300

Luisiana clay

200

Keelung silt loam

6 7Incubation (weeks)

Fig. 10. Influence of tem.perature onam.m.onification in four sub-m.erged soils.


25/34


The concentrations of both water- and acid-soluble phosphorus increase onflooding, reach a peak value, and decrease (Fig. 11, 12). The peak valueswere lowest for acid clay soils and highest for a calcareous sandy loam (IRRI,1968 ; Ponnamperuma, 1972). The kinetics of water-soluble phosphorusextractable in a pH 2.7 acetate buff_er indicates that the increases are smalland short-lived in acid clay soils, and that available phosphorus afterflooding is correlated with available phosphorus before flooding (IRRI, 1968).

To xi c s ub st an ce sCarbon dioxide, organic acids, and hydrogen sulfide (above certain limits)are known to be toxic to rice (Ponnamperuma, 1965).

One to three tons of carbon dioxide are produced in the plowed layer of 1 haof soil during the first few weeks after submergence (IRRI, undated). Beingchemically reactive, carbon dioxide forms carbonic acid, bicarbonate, andinsoluble carbonates. The partial pressure of C02 (PC02) is a good measure ofcarbon dioxide accumulation and can be calculated from pH HC03 concentration,and specific conductance (Ponnamperuma et al., 1966a), or from pH and totalcarbon dioxide determined by gas chromatography (IRRI, 1968).

The PC02 In a soil increases on flooding, reaches a peak of 0.2 to 0.8 atm 1to 3 weeks later, and declines to a fairly stable value of 0.05 tc 0.2 atm(IRRI, undated). Acid soils high in organic matter but low in iron andmanganese show a rapid increase in PC02 to about 0.5 atm within 1 to 2 weeksof flooding, followed by a slow decline in PC02 to about 0.3 atm. Neutralsoils low in organic matter give PC02 values that are less than 0.2 atm.

Carbon dioxide injury may occur in rice soils low in active iron and highin organic matter, especially at low temperature (Cho and Ponnamperuma, 1971).

When a soil is submerged the concentration of volatile organic acids increases,reaches a peak value of 10 to 40 mmol/liter in 1 to 2 weeks, and then declinesto less than 1 mmol/liter a few weeks later. Organic acids persist longer incold soils than in warm soils (Yamane and Sato, 1967; Cho and Ponnamperuma,1971). At pH values below 6.0 those acids are toxic to rice (Tanaka andNavasero, 1967).


26/34

E0.0.o,

25IRPS No.5, February 1977

27

162292 1

28

1 r - - - t - - - 1 ~ - - ~ - - - - - - ~ - - - - - - - - ~ - - - - - - ~ 1 42 4

Fig. 11. Kinetics of P soluble in pH 2. 7 acetate buffer in eightsubrne r ged soils.

Weeks submerged

P(ppm)

SoilNo. Texture pH O.M.% Fe %1 sandy loam 7.6 2.3 0.18

14 clay 4.6 2.8 2.1325 sandy loam 4.8 4.4 0.1826 clay loam 7.6 1.5 0.3027 clay 6. 6 2. 0 1.604.0

3.0

Week s s ubme r gedFig. 12. Kinetics of water-soluble P in five subrnerged soils.


27/34


Temperature has a marked effect on the kinetics of PC02' organic acids, and2+ .water-soluble Fe . Temperatures hlgher than 30C hasten and sharpen thepeaks of PC02' organic acids, and Fe2+, and low temperatures retard andbroaden the peaks (Fig. 13, 14, 15). Low-temperature injury to rice due tothe soil chemical regime is more severe on acid soils than on neutral soils(Cho and Ponnamperuma, 1971). It is associated with high concentrations ofcarbon dioxide, organic acids and Fe2+.

The concentration of water-soluble hydrogen sulfide increases on flooding,reaches a peak 1 to 3 weeks later, and then decreases to a low value.Harmful concentrations of hydrogen sulfide may be present in acid sulfatesoils, and in acid soils low in iron, during the first few weeks afterflooding but not later as generally supposed (IRRI, 1973).

ImpZ ications for riceChemical kinetics of submerged soils has the following implications forthe fertility of rice soils

1. Soils high in available nitrogen need no nitrogert fertilizer.2. Soils medium in available nitrogen need only a topdressing of

nitrogen at panicle initiation.3. Soils low in available nitrogen need both basal application and

topdressing.4. Soils whose temperature is below 25C need more basal nitrogen

fertilizer than those whose temperature is 30 to 45C.5. NH4+ toxicity is possible on warm sandy soils high in organic

matter.6. Acid clays need phosphorus fertilizer in spite of the increase ln

availability of phosphorus brought about by flooding.7. To prevent the toxic effects of excess Fe2+, carbon dioxide,

organic acids, and hydrogen sulfide, soils should be keptsubmerged for at least 2 weeks immediately before planting intropical lowlands and longer in cooler areas.

IONIC STRENGTH, ION EXCHANGE, SORPTION, AND DESORPTIONSubmerging a soil brings large amounts of ions into solution through soil


28/34

~.-----------------------~

27

~guran sandy loam


500

~400c,++

Fig. 13.

Luisiona cloy

10 12 14 0

,,,,i-.~---- . . ----4 6 8 10Weeks submerged

12 14 0

Moohos cloy

~ 15~31""'24 C9---'" 20 C

30C38~ 23 C

-~--- . . . . . . - -4 10 12 14

Influence of temperature on the kinetics of FeZ+ in three soils.~ r - - - - - - - - - - - - - - - - - - - - - - - - - ~

KeeUlg sil t loan

~


29/34


reduction, NH4+ production, and the solvent action of carbon dioxide oncarbonates. These ions increase the specific conductance and the ionic

2strength (I = 1/2 Im.z. ) of the soil solution.1 . - 1 . -The increase in ionic strength decreases the activity coefficients of allions according to the Debye-Huckel equation, log y = Az2I1/2/1 + aBI1/2,and thus at a given pH, Eh, or Pca ,increases the concentrations of all ions2in equilibrium with their solid phases. That affects ionic equilibria andperhaps also ion uptake by the rice plant; but little information isavailable on the influence of ionic strength on nutrient uptake by rice.

In aerobic soils, ion exchange plays an important role in the replenishmentof the soil solution with anions and cations absorbed by plant roots. Itplays an important role in increasing the fertility of acid soils by liming,and of saline and sodic soils by leaching. In submerged soils the largeamounts of Fe2+ and Mn2+ brought into solutions displace cations from the

I . + + 2+ 2+ .clay comp ex, lncreaslng the concentrations of NH4 ' K , Ca , and Mg lnthe soil solutions. The slight increase in concentration of water-soluble+K brought about by cation exchange may not be significant in rice nutrition

Ca2+ and Mg2+ut loss from the soil of the displaced from the soil colloidsinto the solution may lead to soil acidification by "ferrolysis" (Brinkman,1 97 0) .The elements known to be sorbed by clay, organic matter, or hydrous oxidesof iron, manganese, and aluminum are: nitrogen, phosphorus, silicon, boron,cobalt, copper, zinc, and molybdenum (Mortland and Wolcott, 1965; Jenne,1968 ; Russell, 1973). Sorption may be due to electrostatic attraction,covalent bonding, or isomorphous replacement in the crystal lattice. Becauseof the changes in surface properties brought about by changes in Eh or pH,lons held by electrostatic attraction or covalent bonding may be releasedinto the soil solution when a soil is flooded.

Thus the concentrations of water-soluble phosphorus, silicon, and molybdenumincrease when a soil is submerged. Cobalt, copper, and zinc are also probablyreleased into the soil solution but later fixed, apparently as insolublesulfides.


30/34


MINERAL EQUILIBRIA AND SOIL FERTILITYMineral equilibria play an important part in controlling the availability ofphosphorus, potassium, calcium, magnesium, iron, manganese, zinc, and copper,and the toxicity of carbon dioxide and hydrogen sulfide in submerged soils.These equilibria include the equations+ 2+Fe (OH) 3 + 3 H + e;: Fe + 3 H20+ 2+Mn02 + 4 H + 2 e~Mn + 2 H20+ 2+Fe3 (Oll) + 8 H + 2 e= 3 F e + 8 H20+3 Fe(OH)3 + H + e Fe3(OH)8 + H20+CaS04 + 8 H + 8 e + H20 = CaC03 + H2S2+ 2-FeS - : Fe + S

ZnS::: Zn 2+ + S2-CuS ~ Cu2+ + S2-MC03 + H20 + C02=M2+ + 2 HC03where M = Mg, Ca, or Mn

(20)(21)(22)(23)

+ 4 H2O (24)(25)(26)(27)(28)

The Eh, pH, ionic strength, and PC02 levels, as shown earlier, exert a stronginfluence on fertility through these equilibria.


31/34

IRPSNo.5, February 197730

LITERATURE CITEDAlexander, M. 1961. Introduction to soil microbiology. John Wiley and Sons,New York. 472 p.Arnon, D. I., and C. M. Johnson. 1942. Influence of hydrogen ion concentrationon the growth of higher plants under controlled conditions. Plant Physiol.17:525-539.Brady, N. C. 1974. The nature and properties of soils, 8 th ed. Macmillan, NewYork. 639 p.Brinkman, R. 1970. Ferrolysis, a hydromorphic soil forming process. Geoderma3:199-206.Castro, R. U., and R. S. Lantin. 1976. Influence of water management on theN supply of rice soils. Philipp. J. Crop Sci. 1(1): 56-59.

Cho, D. Y., and F. N. Ponnamperuma. 1971. Influence of soil temperature on thechemical kinetics of flooded soils and the growth of rice. Soil Sci.1 12: 18 4-194 .De Datta, S. K., E. A. Saladaga, W. N. Obcemea, and T. Yoshida. 1974.Increasing efficiency of fertilizer nitrogen in flooded tropical rice.Pages 265-268 in Proceedings FAI-FAO Seminar on Optimising AgriculturalProduction Under Limited Availability of Fertilizers, New Delhi.Ellis, E. G., and B. D. Knezek. 1972. Absorption reactions of micro-nutrientsin soils. Pages 59-78 in J. J. Mortvedt, P. M. Giordano, and W. L.Lindsay, ed. Micronutrients in agriculture. Soil Sci. Soc. Am., Madison,Wisconsin, USA.Hutchinson, E. G. 1957. A treatise on limnology. Vol. 1. John Wiley and Sons,New York.International Rice Research Institute. (Undated). Annual report 1964. LosBanos, Philippines. 335 p.International Rice Research Institute. 1966. Annual report 1965. Los Banos,Philippines. 357 p.International Rice Research Institute. 1968 . Annual report 1967. Los Banos,Philippines. 308 p.International Rice Research Institute. 1971. Annual report 1970. Los Banos,Philippines. 265 p.International Rice Research Institute. 1973. Annual report 1972. Los Banos,Philippines. 246 p.Jenne, E. A. 1968 . Controls on Mn, Fe, Co, Ni, Cu, and Zn concentrations insoils and water: the significant role of hydrous Mn and Fe oxides.Pages 337-396 in R. A. Baker, ed. Trace inorganics in water. Adv. inChem. Sere 73. Am. Chem. Soc., Washington, D.C.


32/34

IRPS No.5, February 1977 ,32

Ponnamperuma, F. N., R. Bradfield, and M. Peech. 1955. Physiological diseases ofrice attributable to iron toxicity. Nature 175:265.Ponnamperuma, F. N., R. U. Castro, and C. M. Valencia. 1969. Experimental studyof the influence of the partial pressure of carbon dioxide on the pH valuesof aqueous carbonate systems. Soil Sci. Soc. Am., Proc. 33:239-241.Ponnamperuma, F. N., E. Martinez, and T. Loy. 1966a. Influence of redox potentialand the partial pressure of carbon dioxide on the pH values and suspensioneffect of flooded soils. Soil Sci. 101:421-431.Ponnamperuma, F. N., E. M. Tianco, and T. A. Loy. 1966b. Ionic strengths of thesolutions of flooded soils and other natural aqueous solutions from specificconductance. Soil Sci. 102:408 -413.Raupach, M. 1963. Solubility of simple aluminum compounds expected ln soils.III. Aluminum ions in soil solutions and aluminum phosphate in soils. Aust.J. Soil Res. 1(1):46-54.Rorison, I. H. 1972. The effect of extreme soil acidity on the nutrient uptakeand physiology of plants. Pages 223-254 in H. Dost, ed. Acid sulfate soils.Proceedings of the International Symposium, August 13-20, 1972. Wageningen.Vol. 1.Russell, E. W. 1973. Soil conditions and plant growth, 10th ed. Longman, London.Starkey, R. L., and K. M. Wight. 1946. Anaerobic corrosion of iron in soils. NewBrunswick, New Jersey, USA. 108 p.Stumm, W., and J. J. Morgan. 1970. Aquatic chemistry. Wiley Interscience. NewYork. 58 3 p.Takijima, Y. 1963. Studies and behavior of the growth inhibiting substances inpaddy soils with special references to the occurrence of root damage in peatypaddy fields. Bull. Natl. Inst. Agric. Sci. Jpn. Ser. B, 13:117-252.Tanaka, A., and S. Navasero. 1967. Carbon dioxide and organic acids in relationto growth of rice. Soil Sci. Plant Nutr. 13:25-30.Tanaka, A., and S. Yoshida. 1970. Nutritional disorders of rice in Asia. Int.Rice Res. Inst., Tech. Bull. 10. 51 p.Yamada, N. 1959. Some aspects of the physiology of bronzing. IRC Newsl.

8(3) : 11-15.Yamane, 1., and K. Sato. 1967. Effect of temperature on the decomposition oforganic substances in flooded soils. Soil Sci. Plant Nutr. 13:94-100.


33/34


34/34

O the r pa pe rs in th is s e rie sNo.1 Recent studies on rice tungro disease at IRRINo.2 Specific soil chemical characteristics for rice production in AsiaNo.3 Biological nitrogen fixation in paddy field studied by in situacetylene-reduction assaysNo.4 Transmission of rice tungro virus at various temperatures:A transitory virus-vector interaction

The International Rice Research InstitutePO. B ox 9 33 . M anila , P hilip pin esStamp

irps 5 physicochemical properties of submerged soils in relation to fertility

Documents