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VI. Conditions for Plant Growth

VI. Conditions for Plant Growth

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strongly reduced. Rorison (1973) has listed the factors detrimental to

plant growth under strong acid conditions. These include:

1 . Direct effect due to injury by H+ions

2. Direct effect due to low pH

( a ) Impaired absorption of calcium and nitrogen

(b) Increased solubility, and thus toxicity, of iron, aluminum and


(c) Decreased availability of phosphorus, caused by aluminumphosphorus interactions, and of molybdenum

3. Low base status, leading to deficiency of calcium, magnesium, and


4. Abnormal biotic factors, such as impairment of the nitrogen cycle

and mycorrhizal activity, and increased virulence of pathogens.

Most of these have been studied on acid soils, but a few on very acid soils.

In addition, the presence of aluminum salts could lead to salt injury.

Under strong reducing conditions the following factors may contribute

to poor plant growth: ( 1 ) production of hydrogen sulfide; ( 2 ) mobilization of ferrous and manganous ions; (3) production of organic acids.

In temperate areas cropping of acid sulfate soils takes place after the

soils have been drained, so that in these regions we are concerned only

with the effects of acidity. In the tropics, such soils may be drained for

annual or perennial crops or temporarily waterlogged for rice, and in these

regions we are concerned with toxic factors under both oxidizing and reducing conditions.



I. Hydrogen Ion and p H

At the pH levels of acid sulfate soils some Htmay be present, and

whereas root injury as a direct result of Htions below pH 4 has been

suggested by work in solution culture, Rorison points out that plants may

tolerate relatively large concentrations of Htions, so long as the concentrations of other cations are large, and the concentration of toxic polyvalent

cations is small. Thus it is likely that the detrimental effects of aluminum

and manganese in the soil solution far outweigh those of H+ ions (Adams

and Pearson, 1967).

Low pH inhibits the conversion of ammonium to nitrate, so that NH,'

ions accumulate. Some plants take up both ammonium arid nitrate ions,

but calcicole species may give poor or no growth when supplied with

at this



at pH 4.2, whereas calcifuge plants flourish with NH,-N

pH (Rorison, 1973). Since conditions more acid than these are common



in surface soils of acid sulfate areas, the behavior of tropical species, particularly in their reaction to NH,-N and NOs-N, is obviously of interest.

2. Aluminum

Aluminum is normally the major exchangeable cation in very acid soils.

Little Fe3+appears to exist in exchangeable form, but both aluminum and

iron exist in fairly accessible hydroxy forms as coatings and on edge sites.

Little Fe3+can exist in solution at pH > 3.5 so that ferric iron is unlikely

to be a cause of toxicity, except in very acid conditions, such as might

occur in the subsoil where active pyrite oxidation is taking place. Aluminum, on the other hand, is appreciably soluble above this pH. The oft

quoted work of Magistad (1925) gives the solubility of aluminum as 0.3

ppm at pH 4.50 and 76.4 ppm at pH 3.11. This refers to the A13+ion,

but other aluminum ions, e.g., Al(OH)2+,may have a less steep increase

in solubility with increasing acidity. Tanaka and Navasero (1966a) found

that the concentration of aluminum in culture solution was less than 1

ppm when the pH > 5.5, regardless of the amount added.

Tolerance to aluminum differs greatly between and within species, and,

at least for a number of temperate crop plants, this tolerance has been

thoroughly studied; the uptake of aluminum has been reviewed by Jackson

(1967). Foy and Brown (1964) showed that oats yielded 75% of the

maximum yield with 6 ppm of aluminum in solution, whereas the yield

of mustard was only 7%. Reid et al. (1969) showed that within-species

tolerance is genetically controlled, and could be detected in greenhouse

trials. Of interest in this respect is the work of Chadwick and Salt (1969),

who examined the tolerance of Agrostis tenuis, a frequent colonizer of colliery tip spoil. By sampling areas with a known history of colonization,

they showed that tolerance to aluminum toxicity could probably evolve

in about ten generations.

Except for rice, tropical crops do not seem to have had the same attention as regards aluminum tolerance. Rubber, oil palms, and coconuts grow

well in soils of pH 4 or above. Bananas and cassava also grow well at

this value, and pineapples at below pH 4.0. Adams and Lund (1966) demonstrated the detrimental effect of aluminum to cotton, which appears particularly sensitive to aluminum toxicity and/or calcium deficiency, and

Turner and Bull (1967) described aluminum toxicity symptoms in oil


Aluminum toxicity to rice plants was first reported by Miyake in 1916,

who showed that 1.2 ppm of aluminum in solution was toxic. Cate and

Sukhai (1964) summarized the literature on toxic levels for rice, and gave

values varying from the 1.2 ppm of Miyake to 270 ppm given by Hart

(1959). Tanaka and Navasero (1966a), using water culture, found that



the critical concentration in the culture solution was about 25 ppm for

plants with adequate contents of other nutrients, particularly phosphorus.

However, their work illustrates the difficulties of using aluminum in culture

solution, for even when 200 ppm was added to the solution, only 49 ppm

of soluble aluminum was found on analyzing the culture solution; even

at pH 3.5, they found only 175 ppm in solution, when 500 ppm had been

added. Thus it would appear that split root techniques or mist spray cabinets are essential to keep large amounts of aluminum in solution. In Section 111, C we recorded groundwater samples from Thailand with 2.12

mmoles of aluminum per liter, and such levels would obviously be extremely toxic to rice. Tanaka and Navasero (1966b) leached small samples of acid sulfate soils from Vietnam and Malaya, with initial p H s of

about 3.5, and found that the first leachate had a pH of 3.7 and contained

70 ppm of aluminum; rice plants grown in this leachate died, but normal

growth was obtained in the fifth successive leachate, which had a pH of

3.8 and contained 21 ppm of aluminum.

The toxicity of aluminum to rice plants is not in doubt, but the amount

in the soil solution after the acid soils have been waterlogged for some

time, is in doubt. Thus Tanaka and Navasero (1966b) reported a pot experiment in which initial soil leachate contents of 35 ppm dropped to less

than 1 ppm with 3 weeks flooding or 30 days of incubation. These experiments showed an increase in pH of less than half a unit, i.e., from about

3.5 to 3.8, a critical range for aluminum, but they offer no other explanation for the large decrease of aluminum in solution. They record a very

large increase in ferrous iron in the leachates after flooding or after incubation. Patrick and Wyatt (1968) have suggested that the reaction





+ OH-

is responsible for the increase in p H on waterlogging, but the existence

of Fe(OH), under these conditions seems improbable, though it is often

postulated. The formation of ammonia from decomposing plant material

is probably the major factor in raising the pH of a waterlogged soil. It

is hydroxyl that is responsible for neutralizing aluminum, rather than ferrous iron, as Cate and Sukhai (1964) suggested.

Increases in pH, many of them greater than those reported by Tanaka

and Navisero, have often been recorded. Ponnamperuma et al. (1973)

reported rises from about pH 3.5 to 6.1 in 12 weeks’ flooding of acid sulfate soils from Vietnam, much of the increase taking place in the first 2

weeks. Nhung and Ponnamperuma (1966) reported an increase from pH

3.6 to pH 4.7 in 16 weeks’ flooding, whereas Tomlinson (1957) reported

an increase from 2.7 to 6.2 when mangrove soils in Sierra Leone were

3 00



flooded with rainwater. Kanapathy (1973) gave an increase from pH 2.6

to 6.3 in 60 days’ waterlogging. Beye (1973) found increases that varied

with the soil treatment. In some polders the pH increased from 3.8 to

5.7; in others to less than 4.0. Soils in pots might be expected to behave

differently from those in fields, especially if soluble salts were leached in

the field.

In conclusion it would appear that, whereas aluminum toxicity is certainly a very important factor in dryland agriculture, in rice culture, in

which the land is flooded before and during the crop-although admittedly

this is not always possible on these soils-aluminum toxicity is unlikely

except in the early stages of submergence.

3. Iron

Unless the pH falls below about 3.5, ferric iron is not soluble. Thus

few examples of ferric iron toxicity would be expected, although Martin

and Evans (1964) have reported that sugar cane can accumulate toxic

amounts, giving reddish or rust-colored leaf symptoms. However, the concentration of ferrous iron increases greatly under reducing conditions.

Nhung and Ponnamperuma (1966) report that the concentration of Fez+

reached 800 ppm after 6 weeks’ submergence. Hart (1959) extracted 500

ppm of Fez+with 0.2 M KC1 from mangrove soil at 65% moisture and

a pH of 6.5, and 900 ppm at a pH of 5.5 and 50% moisture. In drier

conditions (40% moisture) less than 100 ppm was extracted. Tanaka and

Navasero (1966b) also reported very large increases in the iron contents

of leachates in flooded conditions, with or without incubation, values of

800 to 1700 ppm being obtained. Ferrous iron concentrations of 5000

ppm, 2 weeks after submergence, were reported by Ponnamperuma et al.

(1973), but values of 500-1000 ppm are more common; however, some

soils apparently give very little. Toxic concentrations of ferrous iron are

given as about 500 ppm by Nhung and Ponnamperuma. Tanaka and

Navasero (1966b) concluded that iron was the main cause of poor growth

of rice on acid sulfate soils. However, not all acid sulfate soils give large

amounts of iron on reduction; this can be attributed to small iron contents,

or the absence of “easily reducible” iron oxides, an ill-defined term. Iron

oxides are plentiful in many nonacid sulfate rice soils, yet no toxic symptoms develop. Takijima ( 1965) suggested that under certain conditions,

e.g., in peaty soils, rice roots lose their ability to form a protective zone

around them.

Iron in the rice plant gives some indications of the conditions in the

soil, for Tanaka and Navasero (1966b) showed that contents may reach

6000 ppm in plants severely affected by iron toxicity, whereas normal

plants contain about 200 ppm. In their liming experiment, rice roots had



1600 ppm in unlimed pots and 230 ppm in soils receiving 5 g of calcium

carbonate per kilogram of soil. Tanaka et al. (1966) reported that rice

leaves containing more than 300 ppm of iron exhibit iron toxicity symptoms. In culture solution, 100 ppm of iron and a pH of 3.7 may give toxicity symptoms, but normally more than 500 ppm are needed; young plants

are more susceptible to iron toxicity.

Rorison (1973) reviews information on the plant mechanisms of tolerance to toxicity; in waterlogged soils these have been related to the evolution of oxygen from roots, so precipitating ferric oxide and lessening the

transpiration rate. Rice plants obviously do cause precipitation of iron

oxide round their roots, yet under normal conditions they absorb sufficient

iron to maintain healthy growth, even though the crop has a large need

for this nutrient.

4 . Manganese

Little is known about the toxicity of manganese in acid sulfate soils for

most attention has been given to aluminum toxicity in dryland soils and

iron toxicity in reduced soils. However, soils rich in manganese and with

a low pH give manganese toxicity, particularly on alternate wetting and

drying. The temporary acidity brought about by using acid hydrolysis fertilizers, e.g., monocalcium phosphate; can also give manganese toxicity

(Agricultural Research Council, 1967). Perhaps few acid sulfate soils contain any appreciable quantities of manganese, having lost much of it during

the soil formation process. Manganese is readily mobilized in rice soils,

and accumulations of manganese dioxide often occur in a horizon below

that of iron accumulation. Tanaka and Navasero ( 1 9 6 6 ~ )stated that manganese toxicity is rare in rice, and they found (1966b) that the manganese

content of rice plants was low, irrespective of the amount in the soil solution. Lockard (1959) obtained no symptoms of toxicity in plants receiving

2 ppm manganese, but did so with 8 ppm in culture solution. He also

found a strong varietal difference in the uptake of manganese; at 8 ppm

of manganese in the culture solution one variety had 200 ppm in the

leaves, another over 4000 ppm.

5 . Salt Eflects

There are large increases in soluble salts when pyritic soils are dried

and oxidized; Ponnamperuma et al. ( 1973 ) give specific conductance

levels (mmho/cm) exceeding 10 in some acid sulfate soils. Very frequently

reports on conductivity do not define whether soil suspensions, solutions,

or saturation extracts were used, so that comparisons are impossible. It

would appear, however, that serious salt injury is possible in many soils.

Not only are salts formed on oxidation, but considerable amounts of neu-




tral salts may be left by periodic flooding with sea water (Section I ) . Such

salts are washed out by the early rains and by subsequent flooding by fresh

water during the wet season.

6 . Hydrogen Sulfide

Hydrogen sulfide formed in anaerobic soils is not always completely immobilized, and plant growth suffers where this occurs. Vamos (1964) reported that damage by hydrogen sulfide in some paddy fields in Hungary

is usually preceded by decreases in the temperature and atmospheric pressure; in one instance, enough hydrogen sulfide was released from a pond

to kill unfledged birds in the neighborhood. Postgate (1960) described

a spectacular evolution of hydrogen sulfide off the coast of South West

Africa in the winter of 1950, when poisoned fish were strewn to a depth

of several feet on the shore of Swakopmund. Morton (1965) recorded

examples in Florida where dredging mangrove swamps released so much

hydrogen sulfide from buried mangrove debris that buildings were discolored and, on one occasion, workmen were killed.

Rice is the crop principally concerned in hydrogen sulfide toxicity, although Dommergues et al. (1969) describe similar effects on lucerne and

broad beans on saline soils in Tunisia; Ford and Calvert (1969) reported

hydrogen sulfide toxicity to citrus in Florida, 0.5 ppm hydrogen sulfide

in the groundwater causing death of citrus roots. Ford (1973) demonsstrated a time-H,S concentration relationship with root damage.

The physiological disease of rice associated with hydrogen sulfide is

known in Japan as akiochi (autumn decline). The respiratory activity of

the roots is impaired; affected plants are deficient in silica and bases, and

at later stages of growth, in N and K as well (Okajima and Takagi, 1953,

1955, 1956a,b; Baba et al., 1953; Baba and Harada, 1954; Mitsui et al.,

1954; Yamada and Ota, 1958; Vamos, 1967; Park and Tanaka, 1968).

Plants affected by akiochi are susceptible to Helminthosporium leaf spot,

and infection with this fungus is taken as an indicator of akiochi (Baba

and Harada, 1954; Mitsui, 1956; Tanaka and Yoshida, 1970). The iron

content of rice affected by akiochi tends to be greater than in healthy

plants; Tanaka et al. (1968) found that sulfide decreased the root respiration of rice plants grown in water culture, even in the presence of excess

Fez+,and increased the iron content of the shoots. It seems that the oxidizing power of the roots is diminished by the effect of hydrogen sulfide,

and the roots lose their ability to absorb nutrients and the plants become

susceptible to Helminthosporium and other diseases. The plants seem to

become more sensitive to hydrogen sulfide when vigorous growth is

checked by overcast weather, presumably because their oxidizing power

is decreased (VBmos, 1964). Vimos (1958b) found that the application



of nitrate fertilizers inhibited sulfide formation. Ponnamperuma et al.

( 1965) corrected “suffocation disease” of rice by applying manganese

dioxide to the soil before it was flooded.

Brusone, another disease of rice, is caused by Piricularia oryza, and

seems also to be associated with damage caused by hydrogen sulfide

(VBrnos, 1958a, 1959a,b; Zsoldos, 1962).

Soils on which akiochi occurs are characterized by being iron deficient,

and the persistence of free hydrogen sulfide is ascribed to the lack of sufficient iron to react with all the sulfide formed in the waterlogged soil. It

seems that hydrogen sulfide toxicity is not an intrinsic property of these

soils, in that they do not contain excessive amounts of sulfate, but that

it arises from the use of artificial fertilizers that contain sulfate (Baba and

Harada, 1954); nowadays akiochi seems to be no longer a serious problem

in Japan (Tanaka and Yoshida, 1970).

Sulfide toxicity in akiochi soils can be prevented by incorporating subsoil, etc., to increase the iron content (Shiori and Tanada, 1954), but

Yamane and Sat0 (1961) found that adding hydrated ferric oxide to a

muck paddy soil caused little decrease in the amount of free hydrogen

sulfide that formed in the flooded soil. Allam (1971) has also reported

that, in Louisiana rice soils, the Fez+concentration had no appreciable

effect on hydrogen sulfide accumulation, and suggested that it was sorbed

on the clay fraction. Pitts (1971) working with the same soils postulated

that a flexibacterium, that is capable of destroying hydrogen sulfide, exists

in rice soils. Rodriguez-Kabana et al. (1965) examined the influence of

hydrogen sulfide on nematodes in rice soils and recorded considerable control. They found 0.1 to 1 ppm hydrogen sulfide in the soil-water phase

5-7 days after flooding with as much as 30 ppm in some soils after 100

days’ flooding. Although these levels killed the nematodes, the authors report no damage to the rice.

When discussing the immobilization of sulfide in flooded soils, Japanese

writers consistently refer to “active ferric oxides,” but we have found no

definition of this quantity.’ However, it is implicit in the use of this term

that not all the soil iron is capable of reacting with sulfide. It is to be

expected that flooding a soil would increase the reactivity of the iron it

contains, and it is significant that mud from river beds, etc., is more effective

than hill soil as an amendment to degraded paddy soils (Baba and Harada,

Motomura and Yokoi (1969) distinguished active and inactive ferrous iron, the

former being extracted by 0.2% aq. AICI, followed by 1 N NaOAc, pH 3; inactive

ferrous iron was calculated as the difference between total Fez+ extracted by

HF/HzS04 and the active fraction. It is unlikely that this distinction has any real

significance, as Pruden end Bloomfield (1969) showed that both AICI, and HF

give spurious values for ferrous iron in soils.



1954). Bloomfield (1969) found that a slight excess of a laboratory-prepared hydrated ferric oxide, relative to the total sulfate, was sufficient

to immobilize all the sulfide formed in microbiological sulfate-reduction

experiments. However, when soil containing a severalfold excess of iron

was substituted for the artificial ferric oxide, under the same conditions a

considerable proportion of the sulfide was not trapped. Less free hydrogen

sulfide was obtained with a periodically waterlogged soil than with a soil

from a well-drained site, although the HC1- and dithionite-soluble iron contents of the two soils were almost identical. The greater reactivity of iron

in the periodically flooded soil was illustrated by the amounts of iron dissolved when the two soils were incubated anaerobically with plant matter,

without added sulfate-about 3 times as much iron was dissolved from

the poorly drained as from the well-drained soil. The proportion of free

hydrogen sulfide decreased asymptotically as the proportion of soil in the

incubation mixture was increased, so that in this respect the relationship

between active and total iron was not linear (C. Bloomfield, unpublished).

Further, different proportions of free hydrogen sulfide were obtained with

different forms of plant matter, although without added sulfate, weight for

weight the two forms of plant material dissolved the same amounts of iron

in anaerobic incubation experiments. It thus seems that biochemical factors

also influence the extent to which sulfide combines with iron in anaerobic


Sulfate-reducing organisms do not operate in very acid conditions (Section 11, B) so hydrogen sulfide toxicity is unlikely in acid sulfate soils,

except where the acidity decreases after prolonged waterlogging. Some

toxicity in dryland crops is therefore possible when the acid sulfate horizons are waterlogged to counteract acidity, but none has been reported.

The more vigorously the rice plant is growing the better it can cope with

hydrogen sulfide in the soil solution, by the detoxification' mechanisms of

the roots, possibly oxidation, and/or by proliferation of its root system

to compensate for root destruction. The latter has been reported from

Malaya (Annual Report, 1957) where injection of hydrogen sulfide saturated solution into the soil doubled the weight of the roots, root proliferation being greatest near the surface.

7. Organic Acids

Hollis and Rodriguez-Kabana ( 1967) showed that acetic, propionic,

and n-butyric acids, the latter is very small amounts, accumulated in Louisiana rice soils. Acetic acid was dominant, with a concentration of


me per liter in the soil solution. The undissociated molecule is

regarded as the toxic factor, and as dissociation is pH dependent, an increase in pH from 4.5 to 5.5 decreases the concentration of this 10-fold.



Tanaka and Navasero (1967) also showed that acetic and butyric acids

had a deleterious effect on growth in a culture solution of pH 4.0, but

had no effect at pH 6 or 8. They concluded that as the pH of most rice

soils rises above 6, the concentrations of organic acids are normally not

great enough to cause damage.



I . Phosphorus

Rorison ( 1973 ) states that aluminum inhibits the uptake of phosphorus

and seriously limits the growth of susceptible species, but Jackson ( 1 967)

draws attention to aluminum accumulator plants, like tea, that contain normal amounts of phosphorus in the aboveground portions, even when they

take up large amounts of aluminum. Precipitation of aluminum phosphate

may occur outside the root and also in the intercellular spaces of the cortex. The large amounts of exchangeable aluminum in acid sulfate soils,

under dryland conditions, could cause severe phosphate deficiency in susceptible crops, but few investigations on phosphate behavior have been

reported, though many reports show the need for phosphate by rice on acid

sulfate soils. Tanaka and Navasero (1966a) found that acid sulfate soils

from Vietnam and Malaya had little available phosphorus, plants in pot

culture having very small amounts. Heavy dressings of phosphate eliminated iron toxicity symptoms and gave normal growth of the plants.

Moorman (1961) found that 800 kg of rock phosphate per hectare had

no residual effect in the second year on rice soils in Vietnam. Hesse

(1963) and Watts (1969) reported large retention of phosphorus by fresh

mangrove muds, and Watts also reported very low levels of phosphorus

in the water of fishponds in acid sulfate soils in Malaya. Adding phosphate

to the water greatly increased the yield of fish. Conditions for residual

effects probably vary, for there are reports from Senegal of residual responses after 5 years from phosphate application (IRAT, 1971 ).

It is likely that phosphate deficiency is very widespread in acid sulfate

soils and that good responses will be obtained when other limiting factors

are removed. It is uncertain, however, whether the deficiency can be attributed to acidity per se, or to overall shortage of phosphates. Variscite and

strengite are the most likely stable end products of phosphate reactions

in acid soils. In the crystalline form both are of low solubility except at

very acid p H s ; Bache (1963) found that, in pure systems, the pH of the

equilibrating solution had to be below 3.1 for the phosphate concentration

in the solution to be controlled by the solubility product of variscite. However, the initial precipitation products are possibly amorphous compounds



adsorbed on the hydroxy iron and aluminum materials, and the release

of phosphate from these would not be controlled by pH.

Under the reducing conditions of rice soils, iron phosphate compounds

become available ta the plant (Patrick and Mahapatra, 1968). Thus the

reactions of phosphate in acid sulfate soils may depend on the timing of

application. If applied when the soils are dry and are at their maximum

acidity, with large amounts of aluminum in solution, aluminum phosphate

may be precipitated. Although this is likely to be in an amorphous form,

it would crystallize on aging and hence be of limited solubility as soon

as the pH increased with waterlogging. Phosphate added after the soils

are flooded and reduced would react with the ferrous iron to form ferrous

phosphate, which would remain available to the rice plant.

2 . Copper Deficiency

Copper deficiency in pineapples has been reported by Moorman

(1961). This may be associated with the rather higher organic contents

of the soil, as pineapples show copper deficiency on nonacidic peat soils.

Grant (1973) reports copper contents of 10-100 ppm, mainly in the form

of chalcopyrite, in muds in Hong Kong. In Nigeria, copper sulfate gave

a 40% increase in rice yields when applied to mangrove muds (Annual

Report, 1962).

3. Cations

Bases are removed as sulfates and replaced on the exchange complex

by aluminum, and to some extent magnesium, during the formation of acid

sulfate soils. Acid sulfate soils are therefore likely to be deficient in calcium

and potassium, but exchangeable magnesium may be quite high; however,

Turner and Bull (1967) state that oil palms of acid sulfate soils frequently

show symptoms of severe magnesium deficiency.

Reports on the amounts of bases show large differences. Sombatpanit

(1970) quotes 3.5-5.0 me/100 g exchangeable calcium and 3.0-3.2

me/100 g exchangeable magnesium in the top 35 cm of an acid sulfate

soil in Thailand. Amounts of this order are also given by Nhung and

Ponnamperuma (1966) for a soil from Vietnam. The figures for the nonacid sulfate soil from the same area are 5.5-10 me/100 g calcium and

5.3-6.1 magnesium. Nonacid sulfate soils derived from marine alluvium

in Malaya also have exchangeable calcium contents of 5-10 me/100 g

(Coulter, 1972), but the well-weathered soils on nonalluvial materials have

often less than 1 me/100 g exchangeable calcium. As many tropical crops

have relatively small needs for calcium, these values for the acid sulfate

soils are not unduly small. Pham et al. (1961) give 3.0 me/100 g ex-



changeable calcium in the 0-35 cm horizon, 2.9 in the “cat” clay horizon,

and 3.6 in the reduced horizon; corresponding figures for magnesium are

4.1, 5.7, and 9.0; similar values are given by Vieillefon (1969) for acid

sulfate soils from Senegal. In Sarawak, on the other hand, Andriesse et

al. (1973) show that soils from lobster mounds, which had become oxidized and leached, had a maximum of 1.6 me/100 g exchangeble calcium

with none in some horizons. Although the hinterland rocks from which

these soils are derived are relatively poor in bases, recent marine deposits

in the same area had 5 to 6 me/100 g exchangeable calcium, and it would

therefore seem that under high rainfall and virtually continuous leaching

very small amounts of exchangeable bases will be retained. Where there

is a prolonged and intense dry season, as in Thailand and Vietnam, there

is less leaching of bases.

In temperate countries, e.g., the Netherlands, exchangeable calcium and

magnesium contents are quite large, the values at the surface reflecting the

addition of lime; even in the deep subsoil of drained profiles 3-5 me/100 g

exchangeable calcium has been reported.

Potassium may be deficient in some acid sulfate soils but, as in the case

of calcium and magnesium, no detailed investigations have been reported.

Clays of marine origin often contain much potassium, and Sombanpanit

(1970) found 1.2-1.7% total potassium in soils from Thailand.




Little is known of the biological conditions in acid sulfate soils, other

than those reported for the sulfur cycles. Much research has been done

on the general effects of acidity on microorganisms, and many studies have

reported on the effects of acidity on Rhizobia. Tropical species are apparently much more tolerant of acidity than temperate species, a reflection

of their tolerance of aluminum, though they are susceptible to large

amounts of manganese. Cover crop legumes grow on acid sulfate soils with

a pH of about 4.0 (Bloomfield et al., 1968) and, given adequate phosphate, legumes such as Centrosema and Pueraria flourish at low pH.

Nitrogen fixation by nonsymbiotic microorganisms has been reported

as occurring around the roots of rice plants (IRRI, 1971). The amounts

of nitrogen fixed are relatively small, but if fixation ceases under very acid

conditions, then the supply, already low in many rice soils, will be


Mycorrhiza contribute substantially to the uptake of phosphorus in

phosphate-poor soils; although they are extremely tolerant of acidity, they

are likely to be affected at the pH values of acid sulfate soils.

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VI. Conditions for Plant Growth

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