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VIII. Effect of Green Manuring on Availability of Plant Nutrients Other Than Nitrogen

VIII. Effect of Green Manuring on Availability of Plant Nutrients Other Than Nitrogen

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1984). Green manuring also helps in increased utilization of fertilizer P by

the crops (Venkatarao and Govindarajan, 1960). Krishna Rao el al. (1961)

and Subbiah and Mannikar (1964) have shown that green manure taps

subsoil P and makes it available to the shallow-rooted crops. Upon decomposition of green manure, organically bound P is mineralized and becomes

available to crops. P mineralization is closely related to the analogous

transformation of N (Thompson et al., 1954).Phosphorus release would be

most rapid under soil and climatic conditions favoring ammonification

(Alexander, 1977). Phosphorus content of the added organic matter is

perhaps the most important factor in regulating the release of P (Fuller et

af., 1956; Singh and Jones, 1976).

In waterlogged soils, green manure increases availability of P through

the mechanisms of reduction, chelation, and favorable changes in soil pH

(Hundal et al., 1987). Changes in soil pH due to green manuring can

influence solubility of P (P. K. Singh er af., 1981). The effect of green

manuring on available P content in the soil has been reported to be greater

in acidic and sodic soils than on calcareous soils (Table VII). Ranjan and

Kothandaraman (1986) reported increased availability of P from rock

phosphate applied to rice with green manuring. The decomposition products of green manures have significant chelation capacity, lowering the

activity of polyvalent cations such as Ca, Fe, and A1 that form insoluble

salts with P and thus liberating phosphates from the basic phosphates of

these elements at very low pH values (Agboola, 1974). Hundal et al. (1988)

reported that green manure incorporation significantly reduced P sorption

capacity of waterlogged soils. Anaerobic decomposition of green manure

reduced the bonding energy and P sorption maxima. This effect was

ascribed to release of P during mineralization of green manure, and accumulation of organic acids-complexed metal cations, thereby inducing solubilization of native soil P or reduced fixation of added inorganic P through

the acidifying and chelation mechanisms.

Table VII

Effect of Incorporation of Sesbania Green Manure (GM) on Olsen P (mg/kg) in the Soils at

Different Days after Flooding

P.K. Singh

a / . (1981)

Soil pH 5.8


Y . Singh

e t a / . (1988)

Soil p H 8.5

Swarup (1987)

Soil pH 10.3


20 days

40 days

14 days

28 days

30 days

60 days

- GM













+ GM



The leguminous green manure plants have a strong ability to absorb the

rather inaccessible K in the soil (Gu and Wen, 1981). Many workers have

reported increased availability of K in soils due to green manuring (Kute

and Mann, 1969; Debnath and Hajra, 1972; Katyal, 1977; Tiwari et al.,

1980; Nagarajah et al., 1989; Swarup, 1987). Increase in soil solution

concentration of K, Ca, and Mg in flooded soils was directly related to the

concentration of water-soluble Fe2+and Mn2+(Katyal, 1977). A significant

increase in water-soluble Ca and Mg with application of green manures to

the flooded soils has been observed by Katyal (1977) and Khind et af.

(1987a). The peaks of Ca and Mg concentration were observed after 8 days

of application of green manure.

Like K, increase in soil solution concentration of Ca and Mg in the green

manure-amended waterlogged soils is due to release of Fe2+ and Mn2+

under highly reduced conditions. In the green manure-amended soils,

active biodegradation coupled with an extensive C02 production can lead

to the dissolution of Ca and Mg carbonates. In waterlogged soils, addition

of readily decomposable organic matter in the form of green manure can

markedly influence the chemical equilibrium of sulphur (Pan, 1985).


Micronutrient contents of leguminous green manure crops do not spectacularly differ from those of the nonleguminous cereal crops. Nevertheless, the changes in the oxidation-reduction regimes, particularly in the

submerged soils, and increased chelation capacity brought about by addition of organic matter as green manure dictate the transformations of

micronutrients. A number of workers have studied the influence of green

manures on the kinetics of soil solution Fe2+ and Mn2+ in submerged

calcareous and noncalcareous soils (Thind and Chahal, 1983), black, red,

and lateritic soils (Katyal, 1977), sodic soils (Sadana and Bajwa, 1985),

neutral soils (Khind et al., 1987a),and acidic soils (Nagarajah et al., 1989).

In all the soils both Fe2+ and Mn2+ in the soil solution increased by

submergence over a period of 10 to 12 weeks, but a peak concentration of

Fe2+ (about 2 weeks after submergence) and Mn2+ ( I to 2 weeks after

submergence) was conspicuous when the soils were green manured. The

extent of increase of total Fe2+ and Mn2+ as a result of green manuring

varied from very low (<2%) in sodic soils (Sadana and Bajwa, 1985) to as

high as 30-fold in an acid lateritic soil (Katyal, 1977). Soil pH had a

remarkable effect on the amount of peak concentrations of Fe2+and Mn2+

in the solution of green manure-treated submerged soils (Fig. 4). Organic

matter added in the form of green manure also functioned as a chelating



1 .o


->E - 0.5














b b









FIG.4. Relationships between peak concentration of (a) pFe2+ and (b) pMnZf, and soil

pH in several soils amended with green manure. (Data from: 0,

Katyal, 1977; 0 ,Thind and

Chahal, 1983; X , Sadana and Bajwa, 1985; A,Nagarajah et ul., 1989.)

agent for water-soluble Fe2+and Mn2+.Work of Bao e f al. (1978) revealed

that at the beginning of submergence, the rate of formation of Fe2+ as a

result of reduction in the presence of green manure lags behind that of

organic reducing substances. Later on, most of the Fe2+in the soil solution

was found to be in the chelated form. After about a week or so, there

appears a peak of chelated Fe2+ when, due to intensive decomposition of

green manure, amount of chelating agent is at maximum.

The role of green manuring in increasing the availability of Fe and Mn

has been studied in terms of different availability indices and increased

supply to crop plants. In a calcareous sandy loam soil incubated with S .

aculeata (2 g/kg soil), DTPA-extractable Fe and Mn increased by severalfold over the initial values during a 4-8-week period (B. Singh, unpublished data). Gopala Rao (1956), Meelu and Rekhi (1981), and Swamp

(1987) obtained similar results with Gliricidia maculata, mung bean straw,

and Sesbania aculeara, respectively. From a field experiment on a coarsetextured soil, Takkar and Nayyar (1986) found that Fe deficiency in the

wetland rice was more effectively corrected by Sesbania green manure

than by soil-applied ferrous sulphate. Similarly, from a pot experiment

with a red loam soil, the success of green manure plus submergence in

mobilization of soil iron as a result of intense reduction and subsequent

retention in available form at a sufficiently high level during the growth of

rice nurseries has been demonstrated by Sharma and Katyal (1982) and

Maskina et al. (1985).

Several workers (Katyal, 1977; Iu e f a l . ,1981; Thind and Chahal, 1986;

Khind et al., 1987a)have reported that water-soluble or DTPA-extractable

Zn declined with the duration of flooding whether or not the soil was green

manured. As liberation of water-soluble Fe" and Mn2+was enhanced by



green manuring, it helped in the further lowering of soil solution Zn concentration in most of the investigations. Only in a sodic soil having pH

10.2, could green manure increase the availability of applied Zn (Swamp,

1987), possibly due to a significant reduction in soil pH as a result of green






On a laterite soil (pH 6.4), application of green leaf manure resulted in a

decrease in soil pH by 1.1-2.3 units (Sahu, 1965). Application of green

manure along with gypsum to sodic soils accelerates the reduction in pH

due to increase in Pco, and production of organic acids (Sadana and Bajwa,

1985). The pH of the calcareous paddy soils is controlled by the chemical

equilibria of the CaC03-C02 system. Application of Sesbania green manure to such soils lowers pH by as much as 0.5 unit (Katyal, 1977).

Addition of green manure resulted in a decline in pH to 7.70 and 5.87

compared with 8.03 and 7.61 under the no-green manure treatment at 2

weeks after incubation of calcareous and noncalcareous soils, respectively

(Thind and Chahal, 1986).

In a laboratory incubation study, P. K. Singh er al. (1981) reported an

increase in pH of an alluvial soil (pH 5.8) by 0.8 units after 40 days of

incubation with green manure. Khind et al. (1987a) also observed a conspicuous increase in soil pH after incorporating S. aculeata (Fig. 5). Soil

temperature influences changes in soil pH through its effect on the rate of

decomposition of green manure (Cho and Ponnamperuma, 1971). Shortterm variations in pH do not necessarily reflect the overall long-term

changes but may have a marked effect on the plant growth.

Addition of green manure to waterlogged soils leads to a reduced redox

potential (Ed. In a laboratory study, Yu (1985) observed that the diffusion

current (a voltametric measure of reducing substances) for the red soil

without the addition of green manures was only 0.39 pA/cm2 at 700 mV.

The current in the green manure-amended soil increased to 2.94 pA/cm2,

due apparently to the increase in the amount of reducing substances

produced by decomposition of green manure. Several studies have shown

that green manures caused dramatic reduction in Eh of waterlogged soils

differing in pH, organic matter, and easily reducible Fe and Mn contents

(Katyal, 1977;Thind and Chahal, 1983; Sadana and Bajwa, 1985; Khind et

al., 1987a). The magnitude of depression was more pronounced during the


0 1 2 3



5 10 20 30 40 50 I

FIG.5. Effect of green manure incorporation on changes in (a) Ehand (b) pH of a Crowley

silt loam soil after submergence.

early stage of submergence (Fig. 5). More reduction has been reported in

noncalcareous than in calcareous or sodic soils. In an acid soil with a low

organic matter content, green manuring reduced Eh within 1 to 2 days of

flooding from +200 mV to -200 mV (Yu, 1985). Katyal (1977) observed

greater depression in Eh of the black clay soil, which contained smaller

amounts of reducible Fe and Mn than laterite or red soils.

Katyal(l977) and Sadana and Bajwa (1985)observed higher and sharper

peaks of PCO, in the soil amended with green manure. High PCO, may

increase the water-soluble Fe to toxic amount or HC03- may directly

poison the rice plant. COz produced during decomposition of green manure can directly influence the photosynthesis process of rice plants

(Shivashankar and Vlassak, 1978). The increase in electrical conductivity

(EC) of the soil after submergence is related to increase in the amounts of

NH4+, Na+, K + , Fez+, Ca2+,and Mg2+ ions and to decomposition of

organic matter of the soil. Katyal (1977) and Sadana and Bajwa (1986)

observed that addition of organic matter into soil caused a sharp increase

in EC within 14 days of flooding. Sudden initial increase in EC may cause

the death of rice plants. Thind and Chahal (1986) observed a marked

increase in EC of the flooded soils amended with the green manure. The

increase was more in the noncalcareous than in the calcareous soils, and

was explained by greater release of Fez+and Mn2+ions in the former soil.

During decomposition of a green manure several organic compounds

accumulate in waterlogged soils. Ishikawa (1988) showed that organic

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