Plantand Soil 163: 131-139, 1994. @ 1994KluwerAcademicPublishers.Printedin the Netherlands.

Differential acidity tolerance of tropical legumes grown for green manure in acid sulfate soils S u n t h o r n P o o l p i p a t a n a I a n d N. V. H u e 2 1Department of Soil Science, King Mongkut's Institute of Technology, Bangkok 10520, Thailand and 2Department of Agronomy and Soil Science, University of Hawaii, Honolulu, HI 96822, USA Received20 September1993.Acceptedin revisedform25 March1994.

Key words: acid sulfate soils, acidity tolerance, Ca deficiency, Ca/A1 ratio, A1 toxicity, critical A1 concentration, tropical green manures

Abstract The growth of four tropical legumes ( Cajanus cajan, Sesbania aculeata, S. rostrata, and S. speciosa) used as green manures in the tropics was studied in a glasshouse experiment. Two acid sulfate soils (Typic Sulfaquept, Bang Pakong Series; and Sulfic Tropaquept, Rangsit Series) were adjusted to four pH levels: 3.8 or 4.0 (original soil pH), 4.5, 5.5, and 6.5 (amended with lime). Dry weight was determined 49 days after sowing. Concentrations of N, P, K, Ca, Mg, Fe, Mn, and A1 were also determined in aerial plant parts at harvest. The legumes responded differently to soil acidity and liming, but not to soil type. Cajanus cajan had the highest biomass production, followed by S. aculeata, S. rostrata and S. speciosa, in this order. The N concentration closely paralleled biomass production, suggesting that the growth of symbiotic rhizobia and nodulation were perhaps more susceptible to soil acidity than were the host plants. Liming to pH 5.5-6.0 was recommended for the legumes' growth based on the quadratic relationships between dry-matter yield and soil pH. In the unlimed soils, the Ca concentration in C. cajan and S. aculeata (0.32%) was twice as high as that in the two low-yielding legumes (0.15%). Furthermore, plant Ca increased exponentially (or quadratically in case of S. speciosa) as lime additions increased. It was estimated that for adequate growth, the Ca requirement in the shoot dry matter was approximately: C. cajan 1.2% Ca, S. aculeata 0.8%, S. rostrata 0.6%, and S. speciosa 0.4%. In contrast with Ca, the concentration of Fe, and to a lesser extent Mn, was significantly lower in C. cajan and S. aculeata than in S. rostrata and S. speciosa. The ratio of Ca to A1 in plant tops was used to characterize plant tolerance to soil acidity, and to quantify the critical AI concentration in the plants. It appears that _> 90% maximum growth was attained only when Ca/AI was _> 150 for C. cajan and S. speciosa, _> 200 for S. rostrata, and > 300 for S. aculeata. Cajanus cajan tolerated up to 80 mg A1 kg- 1 in the shoot dry matter, whereas significant growth reduction occurred in the Sesbania species at levels > 30 mg A1 kg- I.

Introduction Use of fast growing legumes, such as Cajanus cajan, and Leucaena and Sesbania species, as green manures has long been an important cultural practice of subsistence farmers of the tropics (Evans and Rotar, 1987; Yost and Evans, 1987). Yet, little scientific information is available on the edaphic adaptability of these multipurpose plants. Data from India indicate that these green manure plants can grow well in calcareous or

sodic soils, but their performance in acid soils has not been vigorously tested (Evans et al., 1983; Mappaona and Yoshida, 1993). As for crop tolerance to soil acidity, C. cajan and S. rostrata were the only tropical legumes for which data were available (Joshua et al., 1989; Nakano et al., 1992). C. cajan reportedly grew better than S. rostrata in acid soils (Nakano et al., 1992). Perhaps because of this tolerance, C. cajan has been recommended as a promising crop for the Northeast Thailand (Khon Kaen province), where most

132 soils are sandy and acid (Wallis et al., 1988). Recently, Thai farmers have been actively growing legumes for green manure in annual crop rotation on an increasingly large area of clay-textured acid sulfate soils of the Bangkok Plain of Thailand. These soils are strongly acidic, particularly when aerated because of the oxidation of pyrite and jarosite minerals (Parkpian et at., 1991). Given these circumstances, characterization of green manure plants that are tolerant of soil acidity is urgently needed. Thus, the objective of this study was to evaluate the acidity tolerance of four common green manure legumes when grown on acid sulfate soils.

Materials and methods

Plant selection from afield survey Initially, a field survey of the Bangkok Plain was conducted to determine those local tropical legumes with a potential for use as green manure crops, including species of Cajanus, Crotalaria, Desmodium, Leucaena, Vigna, and Sesbania. Each location was visited several times in order to question farmers on growth behaviour, biomass production and ability to survive under adverse conditions. After evaluation, the four most promising species [Cajanus cajan, Sesbania aculeata (also known as S. bispinosa), S. rostrata, and S. speciosa] for green manure were selected for the glasshouse study.

Soil sampling and analysis Acid sulfate soils were taken from two locations where paddy rice (Oryza sativa) and tropical fruits were grown. They were the Bang Pakong (Bg) and Rangsit very-acid phase (Ra) series. The Bg soil was characterized as a pre-oxidized potential acid sulfate soil (Typic Sulfaquept) and the Ra soil was an acid sulfate soil (Sulfic Tropaquept). Soil samples for the experiment were collected from the Ap horizon and the jarositic layer. The samples were air-dried, ground, sieved to pass a 2-mm screen and stored for chemical analysis (Table 1). Soil pH and electrical conductivity (EC) of 1:1 soil to water suspensions were measured after 1 h of intermittent shaking. Cation-exchange capacity (CEC) was determined by 1 M NHaOAc, pH 7.0 (Chapman, 1965). Extractable P was determined by the Bray II method (Bray and Kurtz, 1945). Calcium and Mg from the NHa0Ac extract were determined by atomic absorp-

Table 1. Selectedphysical and chemical properties of the two acid sulfatesoils used in the greenhouseexperiment. Soil properties pH (1:1; soil: water) EC (1:1; dS/m) organic carbon, % Extractable P (mg/kg)a CEC (cmolc/kg)b Exchangeable K (cmolc/kg) ExchangeableCa (cmolc/kg) ExchangeableMg (cmolc/kg) ExchangeableNa (cmolc/kg) Exchangeableacidity (cmolc/kg) Extractable AI (cmolc/kg) AI saturation, % of ECEC DTPA-ExtractableFe (mg/kg) DTPA-ExtractableMn (mg/kg) Water soluble SO42- = (mg S/kg) Particle size distribution Silt (%) Clay (%)

Soil series Bang Pakong Rangsit 3.8 0.7 2.4 8.4 21.4 0.1 2.2 6.7 2.1 14.3 7.5 29.5 1440.9 36.8 502.4

4.0 0.2 2.2 9.0 23.8 0.2 1.0 1.8 0.1 8.1 4.3 38.4 711.1 22.6 134.4

40.8 56.8

35.6 61.7

aBray P-2 method. b1 M NH4 OAc, pH 7.0 method.

tion spectrophotometry, and K and Na by flame photometry. Aluminum was extracted by 1 M KC1 and determined colorimetrically (Barnhisel and Bertsch, 1982). Water-soluble sulfate-S was extracted by shaking 10 g soil with 50 mL water for 30 min, followed by 5 min centrifugation at 2000 g (relative centrifugal force). The supernatant was filtered and sulfate measured turbidimetrically, using a spectrophotometer at 420 nm (Freeney, 1986). Soil texture was determined by the pipette method of Gee and Bauder (1982).

Pot experiment The treatments consisted of a factorial combination of the four legume species and four soil pH levels: 3.8 or 4.0 (unamended), 4.5, 5.5 and 6.5, replicated three times in a randomized complete block design. The soil pH levels of 4.5, 5.5 and 6.5 were established by adding 10, 22 and 38 cmol ( O H - ) kg -1 as Ca(OH)2 to the Bg soil, and 5.5, 14 and 22 cmol ( O H - ) kg - t to the Ra soil. To ensure that plant growth was not limited by nutrient deficiencies, nutrients were applied in solution as follows (mg kg-1 soil): Na2MoO4 H20, 0.67; H3BO3, 0.83; CuSO 4 5H20, 5; ZnSO47H20,

133 10; MnSO4H20, 15; and KH2PO4, 176. Nitrogen was only applied at the beginning of the experiment to initiate growth at a rate of 24 mg N kg-l as NH4NO3. The P rate was selected based on the results of Jugsujinda et al. (1978) to produce 90% maximum growth to minimize the ameliorating effects that higher rates of P could have on A1 toxicity. The nutrients were mixed with the soils which were then incubated for 14 d at field water holding capacity before planting. Five seeds of each legume, inoculated with appropriate rhizobial strains, were planted in 15-cm diameter pots filled with 2 kg of soil, and thinned to two plants per pot after emergence. The pots were watered daily with deionized water, initially to 80% of field capacity, and later to field capacity. The experiment was carried out in a glasshouse at King Mongkut's Institute of Technology, Bangkok, Thailand, during SeptemberNovember 1991. Day temperatures ranged from 30 to 35°C and night temperatures from 18 to 220 C. At 49 d after sowing, plants were removed from the pots and aerial biomass (dry weight of tops) was obtained from oven dried samples (70-80°C for 48 h). A subsample from each replicate was digested in a 3:1 nitric: perchloric acid mixture, and analyzed for K, Ca, Mg, A1, Fe, and Mn by atomic absorption spectrophotometry and P by the molybdate/ascorbic colorimetric method. Plant N was measured by the micro-Kjeldahl method. Analysis of variance was used to test the effect of soil pH, soil type, and plant species on drymatter weight and nutrient uptake of the legumes. The relationship between relative dry-matter yield and Ca/A! ratio of plant tissues was established by nonlinear regression analysis, using PLOTIT ® software (Haslett, M1, USA) to derive critical A1 concentrations.

Results and discussion

Differential growth as measured by dry-matter yield and N accumulation The dry-matter weights of the legumes as affected by soil pH are listed in Table 2. Dry weight was affected significantly (p < 0.01) by plant species, soil pH and species x pH interactions, but not by soil series (Table 2). Soil series also had no significant effect on plant composition (statistical analysis not shown). For this reason, data from the two soils were combined for regression analysis (Table 3). Among the four legumes, S. speciosa and S. rostrata had the low-

Table 2. Dry-matter yield of the four green manure legumes grown on two acid sulfate soils from Thailand, and the associated analysis of variance Dry weight, g/pot C. cajan S. aculeata S. rostrata

Soil pH

S. speciosa

Bang Pakong soil (Typic Sulfaquept) 3.8

1.64

1.56

1.27

0.93

4.5 5.5 6.5

3.48 5.04 5.55

2.61 3.96 4.61

2.40 3.29 4.50

1.95 2.36 2.64

Rangsit soil (Sulfic Tropaquept) 4.0

1.97

1.86

1.70

1.17

4.5 5.5

5.16 6.76

2.77 4.55

2.31 4.17

2.32 3.54

6.5

6.58

4.76

4.18

3.00

Variance Block Treatment Soil series (S)

df

Analysis of variance (ANOVA) MS F value Significance

2

0.36

31 1

7.57 9.2

1.05 22 27

**a NS

Soil pH

3

44.7

Legume (L)

3

21.8

S × pH S x L

3 3

0.9 1.8

2.65 5.3

9

1.6

4.7

,,*

9 62

0.4 0.34

1.2

NS

pH × L SxpHxL Error

131

NS a

64

,,,, ,,,, NS NS

aNS: non-significant at the 0.05 level; **: significant at the 0.01 level.

est yields, whereas C. cajan and S. aculeata grew much better throughout the pH range tested (Table 2). Since high biomass is an important factor in selecting legumes for green manuring, C. cajan and S. aculeata appear to be much better than S. speciosa and S. rostrata as green manures. Within species, growth improved considerably as pH increased (Table 2). Quadratic relationships between dry-matter yield and soil pH (Table 3) suggested that 95% of the maximum yield would be attained when soil pH is 5.5 for C. cajan, 5.9 for S. aculeata, 6.3 for S. rostrata, and 5.3 for S. speciosa. Along with biomass, high total N accumulation (mostly from N fixation) is another desirable characteristic of legumes to be used as green manure. This N criterion clearly indicates that C. cajan and S. aculeata are better green manures than S. speciosa and S. rostrata (Fig. 1A). In fact, S. speciosa grew slow-

134 250

Table 3. Regression equations of dry-matter weight (Y, in g/pot) against soil pH (X) and plant N concentration (N, in %). There were 24 observations for each legume Regression e q u a t i o n

Determination coefficient r2

0.72** 0.59**

E))

150

\ Z

C ~)

100 1

C

0 0

z

Sesbania aculeata Y = -12.8 + 5.2 X - 0.39 X2 Y = -0.59 + 1.22 N

200

50-

0.83** 0.47**

I

3.5

Sesbania rostrata Y = -9.7 + 3.9 X - 0.27 X2 Y = -1.69 + 1.72 N

4.0-

0

3.0-

0.78** 0.46**

C ~) 0 C 0 0

2.0-

**Significant at the 0.01 level.

3.9

I

I

I

4.5

5.5

6.5

4.5

5.5

6.5

(B)

0.83** 0.52**

Sesbania speciosa Y= -14.8 + 6.1 X- 0.52 X2 Y = 0.21 + 0.92 N

T

E

Cajanus cajan Y =-30.1 + 12.2 X- 1.0 X2 Y= -2.15 + 2.07 N

0 0-

(A)

Z

1.0 3.5

ly, produced the least biomass and had the lowest N concentration among the four legumes studied (Fig. 1B). Thus, this Sesbania species, in spite of its photoperiod insensitivity (i.e, a potentially short-season, fast-growing legume), does not appear to be suited as a green manure crop when grown in acid sulfate soils. Furthermore, strong linear relationships between biomass and plant N concentration (Table 3) indicate that N nutrition partly controlled dry matter production, suggesting that either the symbiotic rhizobia and the host legume had similar responses to soil acidity and liming or the rhizobia were more susceptible to soil acidity. The latter possibility is more likely, based on the facts that (i) both S. speciosa and S. rostrata, the two low-yielding legumes, always contained < 3.0% N (Fig. 1B), a level considered inadequate for good growth, and (ii) the appropriate rhizobia for S. speciosa often fail under adverse environmental conditions as reported by Evans and Rotar (1987) and Dr. P. Prabuddham (personal communication) while the stem-nodulated rhizobia for S. rostrata might not have enough time to be fully active. Alva et al. (1986a) and Suthipradit (1989) reported that A1 toxicity had a greater effect on rhizobial nodulation than on host plant growth, including that of soybean (Glycine max),

3.9

Soil pH

Fig. 1. TotalN content (A) and plant N concentration (B) of the four green manurelegumes grown in acid sulfate soils at differentpH levels. CC: Cajanus cajan, SA: Sesbania aculeata, SR: S. rostrata; SS: S. speciosa. Vertical bars are standard errors. Unlimed pH is designated as 3.9 which is the average of 3.8 and 4.0, the actual pHs of the two unamended soils.

cowpea (Vigna unguiculata), and green gram (Vigna

radiata). Chemical composition of the legumes Phosphorus Increasing soil pH from 3.9 to 6.5 steadily increased P concentration in the plants probably because of more vigorous growth at higher pH and because soil P becomes more available for plant uptake at pH range of 5.5-6.5. In C. cajan, plant P increased from 0.19% in the unamended soils to 0.30% at pH 5.5 and to 0.36% at pH 6.5 (Table 4). Given the plateauing of the dry-matter yield between pH 5.5 and 6.5 (Table 3) it appears that a plant P concentration o f 0.30% would be adequate for Cajanus cajan growth. In fact, this P level agrees well with the adequate range of 0 . 3 0 -

135

Table 4. Nutrientcompositiona of fourtropicallegumesusedas greenmanuresas affected by differentpH in two acid sulfate soils Soil pH

Plant top composition Ca Mg (%)

P

K

3.9 4.5 5.5 6.5 LSD.05

0.19 0.21 0.30 0.36 0.08

1.87 2.03 2.25 2.38 0.60

0.32 0.77 1.35 2.13 0.37

3.9 4.5 5.5 6.5 LSD.05

0.16 0.24 0.30 0.31 0.05

1.76 2.42 1.89 2.36 0.43

0.31 1.03 1.18 1.25 0.46

3.9 4.5 5.5 6.5 LSD.05

0.17 0.20 0.29 0.30 0.06

1.90 1.99 1.94 1.97 0.50

0.15 0.51 0.81 0.88 0.33

3.9 4.5 5.5 6.5 LSD.05

0.16 0.21 0.27 0.29 0.07

1.64 1.51 1.66 1.46 0.32

0.14 0.44 0.88 0.47 0.21

A1

Mn (rag kg- a)

Fe

175 125 82 75 32

146 126 93 88 35

236 151 143 81 32

147 121 76 57 29

173 1 48 133 71 28

409 322 228 130 73

161 124 76 44 30

207 173 151 144 34

513 464 371 158 84

141 88 41 37 24

272 243 223 162 49

616 490 244 165 72

Cajanus cajan 0.47 0.92 1.22 1.47 0.37

Sesbania aculeata 0.55 0.64 0.98 1.58 0.32

Sesbania rostrata 0.44 0.72 1.04 1.03 0.26

Sesbania speciosa 0.38 0.56 0.66 0.61 0.21

aAverageof data from the two soils.

0.35% P tabulated by Reuter and Robinson (1986) for this legume species sampled 60 d after sowing. In the Sesbania species, P increased steadily from 0.16% at pH 3.9 to about 0.30% at pH 5.5 then practicaUy leveled off at higher pH (Table 4). Thus, 0.30% P also seems to be the adequate level for Sesbania species growth. Evans and Rotar (1987) also reported 0.30% P as a "normal" concentration in many Sesbania species, and Singh et al. (1992) listed 0.32% as the average P concentration (with a range of 0.21-0.40%) in S. aculeata. In general, the four legume species behaved similarly in terms of P nutrition in response to soil acidity.

Potassium Although herbage K apparently increased from 1.87% to 2.38% in C. cajan (the increases were not statistically significant, however), and fluctuated between 1.8 and 2.4% in S. aculeata, it remained virtually constant at 1.9% in S. rostrata and 1.6% in S. speciosa, as lime rates increased (Table 4). Thus, in our experiment, soil acidity apparently had little effect on K nutrition of the legumes, perhaps because K was supplied adequately by all the treatments. [Adequate concentration of K was reportedly > 1.7% in C. cajan (Reuter and Robinson, 1986) and approximately 1.5% in many Sesbania species (Singh et al., 1992).]

136 ~5 Q. L_

6.0

10.0 •

"5

8.06.0-

LID

4.0~

E

2.0~ 0.0~

E -• ~-/ e e • • C a j a n u s c a l a n " y-- 6.75.(1.0 - e-1'31X),r = = 0.73

o'.o

1.10 '

210

I~

2.0-

a Y = 4.36'(1.0 - e-1'96X}, r z = 0.42

0.0

0.0

3.0

018

1.'2

1.L6

210

Plant Ca, %

Plant Ca, % "5 Q.

014

6.0-

4.0-

• :

:

"5 ,t-i

•

\

3.0-

4.02.0-

>~

¢'~

>~

Y = 4.20"(1.0 - e-2"8OXl, r' = 0.60

0.00'.0

014

018

1.'2

1.'6

Plant Ca, %

¢'~ 210

1.0-

Sesbania speeiosa Y = 3 . 0 4 " ( 1 . 0 - e - 3 ' 8 5 X l , r2 = 0 . 6 5

0.0

o'o

o's

lo

1'5

Plant Ca, %

Fig. 2. Relationshipbetweendry-matteryieldof the fourgreen manurelegumesand the Ca concentrationin plant tops.

Calcium Adding Ca(OH)2 to raise the soil pH also raised plant Ca significantly (Table 4). In C. cajan, plant Ca increased linearly from 0.32% in the unamended treatment to 2.13% in the highest lime treatment. By assuming that Ca was the most limiting factor to the legume growth in the acid soils, an estimate of the critical Ca concentration was obtained by plotting the dry-matter yield vs. plant Ca (Fig. 2). Figure 2 shows that 1.75% and 1.20% Ca would be required by C. cajan to maintain growth at 90 and 80% of the maximum, respectively. The lower value agrees well with the adequate range of 0.8-1.2% Ca reported by Reuter and Robinson (1986), and is probably closer to the "true" Ca requirement because under the AI toxicity stress (as in our case) Ca requirement has been shown to be higher than in the absence of such a stress (Alva et at., 1986b). On the other hand, Ca levels of 0.32% and 0.77% in plants of the unamended soils (pH 3.9) and the lowest lime treatment (pH 4.5) were clearly deficient, which were partially responsible for the observed poor growth. Using the same approach, adequate Ca levels were identified as 0.82% for S. aculeata, 0.57% for S. rostrata, and 0.42% for S. speciosa (Fig. 2). Evans and Rotar (1987) also listed 0.8-1.1% Ca as the normal range for good Sesbania growth. It is worth noting that the two high-yielding legumes (C. cajan and S. aculeata) were able to obtain and accumulate more Ca at each pH level than the

other two low-yielding species (Table 4). For example, in the unamended soils, C. cajan and S. aculeata contained about 0.32% Ca as compared to 0.15% Ca in S. rostrata and S. speciosa. Perhaps, the ability to efficiently absorb Ca from Ca-poor sources has made C. cajan and S. aculeata well adapted to acid soils of Thailand as our survey indicated.

Magnesium In the unlimed treatment, C. cajan contained 0.47% Mg, S. aculeata 0.55%, S. rostrata 0.44%, and S. speciosa 0.38%. Despite these rather high initial concentrations, plant Mg did increase two to three fold as soil pH increased (Table 4). Given the fact that the soils were inherently high in Mg (Table 1) and that adequate Mg level was 0.3% in both C. cajan (Reuter and Robinson, 1986) and S. sesban (Singh et al., 1992), this Mg increase was due mainly to a more vigorous growth of the legumes at higher pH and at better Ca supplies. Since Mg was not a growth limiting factor, its uptake pattern among the four legume species was not clearly different, except that Mg in the two high-yielding species seems to increase steadily with liming while Mg in the two low-yielding species seems to level off at 1.0% (S. rostrata) and 0.6% (S. speciosa). Iron and manganese The four legumes responded differently to the potentially excessive Fe in the acid sulfate soils. C. cajan

137 accumulated only 236 mg Fe kg- 1 in the unlimed soils and maintained its internal Fe between 151 and 81 mg kg-I as lime quantities were added to raise the soil pH to 4.5 and 6.5 (Table 4). According to Reuter and Robinson (1986), 150-190 mg Fe kg -1 is considered adequate for C. cajan growth; a deficiency would only occur with < 60 mg Fe kg -1. Thus, it seems logical to speculate that C. cajan has an ability to regulate Fe uptake and to prevent Fe from becoming detrimental to its metabolism when grown on acid sulfate soils. By contrast, S. rostrata and S. speciosa accumulated 513 and 616 mg Fe kg -l, respectively, when grown on the unlimed soils (Table 4). Whether these high Fe levels were a result or a cause of poor growth (low dry matter yield) could not be resolved in this experiment because the data were taken only at harvest (49 d after sowing). However, it is clear that the two high-yielding legumes (C. cajan and S. aculeata) always had lower Fe concentration than the other two low-yielding legumes, especially at the two lowest soil pHs of 3.9 and 4.5. Plant Mn shows a response pattern similar to plant Fe: lowest in the high-yielding C. cajan and S. aculeata and highest in the low-yielding S. rostrata and S. speciosa (Table 4). This observation agrees well with that made by Nakano et al. (1992) in explaining a higher biomass production of C. cajan as compared to S. rostrata and Crotalariajuncea, when grown in an acid red soil of southern Japan. However, the differences in Mn concentrations in the four legumes were smaller than those of Fe; and absolute Mn concentrations were all below 275 mg kg- 1. Since _> 300 mg Mn kg- 1 has been considered detrimental to the growth of C. cajan (Reuter and Robinson, 1986), it is unlikely that Mn was a cause of poor growth of any legume in this experiment. Thus, the greater accumulation of Mn by the low-yielding legumes than their high-yielding counterparts might have been due to their genetic differences because even at pH 6.5 where dry-matter yields were relatively high, S. speciosa and S. rostrata still contained 162 and 144 mg Mn kg -1, respectively, nearly twice the Mn concentration in C. cajan (88 mg kg -1) and S. aculeata (71 mg kg- 1). Aluminum Given the high exchangeable A1 in these acid sulfate soils (Table 1), AI phytotoxicity was likely when no lime was added. In fact, in the unlimed soils, all the four legumes contained from 140 to 175 mg A1 kg -1, which subsequently declined exponentially as lime rates increased, a typical response of a living

organism to most toxicants (Table 4). At pH 6.5, plant A1 was 37 mg kg-1 in S. speciosa and 75 mg kg-l in C. cajan. Regarding A1 toxicity threshold, we found no published data for the legumes used in this study. However, in a published abstract, Licudine and Hue (1992) suggested that A1 levels _> 40 mg kg -t and _> 85 mg kg-l would reduce dry matter yield of 42-dold S. cochinchinensis by 10% and 50%, respectively. Similarly, Helyar (1979) related A1 toxicity to concentrations exceeding 40 mg A1 kg-1 in soybeans at 43 d after planting. It is worth noting that these reported critical AI levels were in the lower limit of A1 concentrations found in our experimental plants even at pH 6.5. In our experiment, however, defining the critical A1 concentration is not simple because Ca was also affecting growth. It has been well accepted that Ca strongly interacts with A1 in terms of ameliorating A1 phytotoxicity (Alva et al., 1986b). To deal with this Ca/A1 interaction, we plotted the relative yield of each legume as a function of its Ca/A1 ratio (Fig. 3). Such plots show that approximately 80% variations in the dry matter yield could be attributed to CaJAI ratios in the plants. Also, relative yields would attain > 90% of the maximum if Ca/A1 equals 150 for C. cajan and S. speciosa, 200 for S. rostrata, and 300 for S. aculeata (Fig. 3). Based on Ca levels of 1.2% for C. cajan, 0.82% for S. aculeata, 0.57% for S. rostrata, and 0.42% for S. speciosa, that were considered minimum requirements for adequate growth of these respective species (Fig. 2), we estimated that critical A1 concentration, above which significant yield reductions would be expected, was 80 mg kg- l for C. cajan, and approximately 30 mg kgfor the three Sesbania species. Thus, C. cajan was the most A1 tolerant while the three Sesbania species had similar Al-toxicity but different Ca-deficiency tolerances.

Summary and conclusions Increasing use of fast growing legumes as green manures for improving soil productivity and crop production necessitates an evaluation of their adaptability to acid soils, especially to acid sulfate soils which occupy a major portion of Thailand's coastal areas. Based on green manuring criteria of high biomass production and high N content, C. cajan and S. aculeata were better suited to the acid soils than S. rostrata and S. speciosa. Since dry-matter increases were most pronounced in the pH range of 4.5-5.5, liming to pH 5.5

138 lOO

~_~

J

lOO-

r

÷

80

o

25

8o~~~~ ~e e ~ ~

60

Sesbaniarostrata

80

40 ~

:=~

20 ~ ~

"0 rc

t

0

e ; i m ' ~ ~ S e s b a n i a aculeata

40 ¥ = -71.0 * e -0"01× + 97.5 R2 : 0.85 0

100

200

300

Y = -83 7 * e -00055X + 104 R== 0.86

20O-

4 0

o

500

2;0

4;0

100 ~ i

•

'F, • ::t~

"

•

80 -

40

~ m/'~

•

•

6O-

t ~-

Sesbania speciosa

O~

40-

9. * e -0'017X + 81.3

r

20 t? O]

R :0.79 ,

r

100

,

I

200

'

F

300

8;0

looo

'

i

400

e

100

80 60~

600

Ca/AI ratio

Ca/AI ratio

]

500

200

?

Cajanus cajan Y = -67.9 * e -0"015X + 74.8 R= = 0,80

,

i

200

Ca/AI ratio

,

i

400

.

-

600

i

800

,

0

10 0

Ca/AI ratio

Fig. 3. Relationship between relative biomass of the four green manure legumes and the Ca/AI ratio in plant tops.

was recommended for the growth of these legumes after taking a cost/benefit analysis into consideration. (In many developing countries where green manures are most needed, lime may be expensive or not readily available because of poor infrastructure. Thus, a small yield increase after pH 5.5 might not cover the lime cost.) The legumes responded differently to stresses imposed by soil acidity: C. cajan and S. aculeata were able to absorb much more Ca but much less Fe and Mn than the other two legumes. Calcium concentrations required for adequate growth were estimated to be 1.2% for C. cajan, 0.8% for S. aculeata, 0.6% for S. rostrata and 0.4% for S. speciosa. Although the four legumes had similar P and K uptake patterns, C. cajan could tolerate nearly three times the levels of A1 as the Sesbania species: critical A1 concentrations (for 10% dry-matter reduction) were 80 mg kg -1 for the former and 30 mg kg- 1 for the latter.

Acknowledgements This research was supported in part by grant No. 8.098 to the junior author, from the Program in Science and Technology Cooperation, Office of The Science Advisor, U.S. Agency for International Development.

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