Growth inhibition due to continuous cropping of

Soil Science and Plant Nutrition

ISSN: 0038-0768 (Print) 1747-0765 (Online) Journal homepage: http://www.tandfonline.com/loi/tssp20

Growth inhibition due to continuous cropping of dryland rice and other crops Wilbur Ventura & Iwao Watanabe To cite this article: Wilbur Ventura & Iwao Watanabe (1978) Growth inhibition due to continuous cropping of dryland rice and other crops, Soil Science and Plant Nutrition, 24:3, 375-389, DOI: 10.1080/00380768.1978.10433117 To link to this article: http://dx.doi.org/10.1080/00380768.1978.10433117

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Date: 06 August 2017, At: 21:45

Soil Sci. Plant Nutr., 24 (3), 375-389, 1978

GROWTH INHIBITION DUE TO CONTINUOUS CROPPING OF DRYLAND RICE AND OTHER CROPS

Downloaded by [Center for Water Res Dev Mngnt] at 21:45 06 August 2017

Wilbur VENTURA and Iwao WATANABE The International Rice Research Institute, Los Banos, Laguna, Philippines Received January 21, 1978

Rice, corn, sorghum, mungbeans, and cowpeas were continuously grown on the same land at 2- to 6-week intervals between crops to determine effects on yield. Decline of growth and yield occurred during continuous cropping. Dryland rice, mungbeans, and cowpeas were affected most, corn slightly, and sorghum was not visibly affected. Growth inhibition in dryJand rice and mungbeans occurred after one or two crops; growing the same crop in rapid succession resulted il1low grain yields. The persistence of inhibitory effects was indicated. One crop rotation or 5 months of fallow removed only a part of the harmful effects. The causal agent appears to have a specific affinity to the host crop. Root residues of the previous crop may serve as a source of the causal agent for the subsequent crop. Biological agents were involved in growth inhibition of rnungbeans and dryland rice. In mungbeans, the inhibitory effects appeared to be directly dependent on microorganisms. It appeared that microorganisms were not the primary cause of harmful effects in dryland rice. Additional Index Words: soil sickness of upland rice, alJe!opathy.

The Philippines has a climate where crops may be grown throughout the year as long as irrigation water is sufficient. But growing a crop continuously on the same piece of land may lead to some injurious soil effects. Inhibition in the growth of dryland rice when grown annually is well known in Japan (7,8). In the Philippines there is no clear evidence of harmful effects from its continuous cropping. Oryland rice farming is gaining in popularity. In increasing the cropping intensity of rice-based farming, grain legumes are grown more frequently in the rotation for additional farm income and to maintain soil nitrogen. Growing legumes more than once a year resulted in reduced grain yields (2,3). Thus farmers were reluctant to plant legumes in rice-based rotations (4). The decline in the quality of a crop when grown on the same land year after year has been called the "soil sickness problem" (17). Soil sickness, however, is a general term and does not specifically identify causal agents. Such decline of yield after continuous cropping may be produced from interwoven factors such as the build-up 375


376

W. VENTURA and I. WATANABE

of soil-borne pathogens, depletion of a certain mineral nutrient, adverse change of soil structure due to similar tillage, and the accumulation of toxic substances (allelopathy). Toxic substances may be produced by leachate from the plant canopy or by decomposition of crop residue. The accumulated microflora or microfauna under the influence of the same crop may produce toxic substances and, in return, the toxic substances may reduce the resistance against the built-up pathogens. An attempt was made to determine the yield decline due to continuous cropping among several crops. After confirming the yield decline, greenhouse and laboratory experiments were conducted to clarify the involvement of allelopathic effects and biological agents. MATERIALS AND METHODS

Cultivation of plants in the field. Oryland rice (IR2061-464-2-4 and IR747B2-6-3), mungbeans, cowpeas, sorghum, and corn were grown continuously in the same plots at International Rice Research Institute, Los Banos, Philippines, for 3 years. A continuous fallow was also maintained. Each crop had four replications and was planted into long plots 4.5 x 16 meters each. They were arranged in a randomized block design. Just before every planting, 100-60.60 kg of N·P20$-K20/ ha basis were applied intherow. All crops were grown in the row: 20 cm spacing of rows in dryland rice, 75 cm in corn and sorghum, and 50 cm in legumes. The rate of seeding was 80 kg/ha in rice, two plants/hill at 50 cm apart in the row in corn, and seedlings were thinned to IS/linear meter at 2 weeks of age in sorghum and in legumes. High levels of cultural management and crop protection were imposed-frequent irrigation, frequent weeding, and a program of pest and disease control. Pesticide applications (carbofuran, azodrin, folidol, malathion, and benlate) were more frequent than in ordinary practice. Carbofuran at 3 kg active ingredient/ha was drilled in the row before sowing seeds. No herbicidal spraying was done. At harvest the plants were cut at ground level and straw residues removed from the experimental plots. All plants were taken for yield determination except for the two border rows at each side and a 1 meter strip at each end of the plot. The period between harvest and land preparation for the next crop varied from 2 to 6 weeks. Pot experiments. Without air-drying, soil materials from the continuously cropped plots were put in glazed porcelain pots at 3.5 kg/pot. Root residues, the amount based on the quantity present in the field, were cut into small segments and, without washing and drying, added to the fallow soils. Sterile roots were obtained by mild steam sterilization at 0.3 kg/cm3 for 10 min, and chemical soil sterilization was done with 2 ml of chloropicrin (CCI 2N0 2)/pot. Pots received 100 ppm each of N, P20&, and K 20. Two separate mungbean plants or one upland rice plant was


Cropping of Dryland Rice and Other Crops

377

grown in each pot. Rhizobium sp. was inoculated in all the mungbean soils. All treatments had three replications. To determine the effect of flooding on the occurrence of soil sickness, a set of pots with soil taken from the continuous rice plot was kept flooded 10 days before transplanting and throughout the growth period; rice plants were grown simultaneously with the dryland culture. In mungbeans, soil material from the continuous cropping plot was taken 3 weeks earlier than in the other treatments and was kept either flooded or air-dried for 2 weeks. The flooded soil was then allowed to dry before sowing the seeds, and mungbeans were grown as in upland culture. Dark culture. A simple and rapid method of determining the potentiality of soil sickness, designed and suggested for upland rice by SUZUKI and KUBOTA (15), was tried for grain crops and legumes. This technique is based on the fact that normal plants grown under complete darkness die due to autolysis within 2 to 4 weeks; infected plants die much earlier. Soil materials from the continuous cropping plots were put into small pots (500 grams soil) without air-drying. For comparison, soil materials from the fallow or rotation plots were also taken and treated in the same manner. Partial soil sterilization was done by treating with 10% acetone or by steam for 1 hr at 1 atm. The seeds were surface sterilized with formalin solution. The seedlings were grown in complete darkness (30°C constant temperature) except for watering and daily observation, which took 5 min with a 20-watt lamp. Each treatment had five pots. RESULTS

Harmful effects of continuous cropping Table 1 shows that in dryland rice (IR2061-464-2-4 and IR747-B2-6-3) there was marked reduction in plant growth and yield. The decrease continued with the number of croppings. After two successive plantings of IR2061-464-2-4 the yields were very low and the time of heading and maturity were not uniform among replications. After the second cropping, seed germination and plant growth at early stages were normal, with growth becoming increasingly poorer towards heading, which may result in few and mostly empty grains. Plant growth were uniformly inhibited in the plot. The effect was especially evident in the fifth cropping of IR2061-464-2-4 and in the sixth cropping of IR747B2-6-3; only one or two replications produced many filled grains. The inhibition in plant growth was relieved a little in the sixth and seventh croppings of IR2061-4642-4, but yields remained less than 0.60 tonfha. Corn and sorghum did not show a definite pattern or sign of the inhibitory effect of continuous cropping. Differences among croppings may have reflected seasonal variation or the incidence of pests. Grain yield was best for the two crops when planted during the. dry season. The injurious effect of continuous cropping in mungbeans and cowpeas appeared


378

Table 1. Growth and yield of upland grain crops in a continuous cropping pattern, IRRI Los Banos, Philippines. 1974-77. Crop


Upland rice (IR2061·464·2-4)

Cropping No.

Month of planting

Maturity (days)

Plant ht& (cm)

Grain yield& (t/ha)

1

Jun.

1974

104

84±5

2. 36±0. 52

2

Nov. 1974

106

S6±2

1.09±O.14

3

Mar. 1975

0.50±0.32

Aug. 1975

122 b 108 b

62±5

4

S6±2

0.38±0.12

S

1976 1976

107 b 107

40±4

6

Jan. Jun.

54±2

0.29±0.17 O. S8±0. 25

7

Oct.

1976

109 0

57±1

0.43±0.15

8

Mar. 1977

135

53±1

0.37±O.08

Upland rice (IR747·B2·6-3)

1

Jun.

1974

85

75±1

1. 77±O. 56

(7th and 8th crop. IR5)

2

Sep.

1974

87

68±2

O.87±O.17

3 4

Jan. 1975 May 1975

108 b 98 b

SI±4

O.97±O.34

61±9

O. 51±O. 34

5

89b

62±5

0.95±0.12

6

Oct. 1975 Feb. 1976

l00b

42±3

O.09±0.09

7

Jun.

1976

131

68±2

0.87±0.29

8

Jan.

1977

153

47±2

O.OO±O.OOe

Jun.

1974

88

221±6

O. 46±0. 16 4

Corn (DMR·2) 2

Sep.

1974

92

198±2

1. 77±0. 074

3

Jan.

1975

90

249±7

4.S1±0.17

4

May 1975

85

286±2

3.78±0.13

5

Aug. 1975

89

244±6

2.05±0.22

6

Nov. 1975

93

211±5

3.83±0.16

7

Mar. 1976

91

250±3

2. 64±0. 15

8

Jun.

1976

92

250±3

9

Oct.

1976

93

189±3

1. 83±0. 22 1. 3S±0. 21

Jun.

1974

105

157±4

2. 69±O. 24

2

Oct.

1974

86

137±2

2. 34±0. 70

3

Jan.

1975

83

5. 24±0. 27

4

May 1975

108

159±2 174±2

5

Oct.

88

142±1

3. 45±0.16

Sorghum (Cosor 2)

1975

3.00±0.09

6

Feb. 1976

92

167±3

6. 10±0. 17

7

Jun.

1976

107

194±7c

1. 89±0. 13 •• 4

8

Oct. 1976

85

144±7c

O. 45±0. 28 •• 4

Average ± standard deviation of the average. b Average growth duration in four replications. Average of two replications. 4 Heavy insect, rat, bird or disease damage. • Plants had no filled grains. a

C



379

as early as the second crop, where there was marked reduction in grain yield (Table 2). It was characterized by poor seed germination, and those that germinated were generally stunted and pale. Several plants wilted and died. Replanting was usually not successful. When the effect was not yet severe, plants that were able to survive the first month became normal by the flowering period. However, in the seventh cropping the injurious effect was so intense that few plants survived. After a poor crop there was a tendency for the next one to be better. The growth of legumes was similar to rice. There was a general reduction of growth throughout the plot. Root diseases (Sclerotium rolfsii Sacco and Phytium sp.), which usually attack succulent seedlings, were sometimes a problem in cow peas and to a lesser extent in mungbeans. The diseases cov~red specific areas in the plot and were quite differentiated from the reduction in plant growth and yield due to continuous cropping. The incidence of root diseases in cowpeas increased with the number of croppings.

Table 2. Growth and yield of legumes in a continuous cropping pattern, IRRI, Los Bai'los, Philippines, 1974-77. Crop

Cropping No.

Mungbeans, Phaseolus aureus Roxb. (MG50·10A)

1 2 3 4 5 6 7 8 9 10 11

12 Cowpeas, Vigna Sinensis (Linn.) (EO green pod :;2)

1 2 3 4 5 6 7 8 9

Month of planting Jun. Aug. Nov. Feb. May Aug. Nov. Jan. Jun. Oct. Mar. JuI. Jun. Oct. Feb. May Aug. Nov. Mar. Jun. Oct.

Average ± standard deviation of the average. or disease damage.

IL

1974 1974 1974 1975 1975 1975 1975 1976 1976 1976 1977 1977 1974 1974 1975 1975 1975 1975 1976 1976 1976

Maturity (days) 67

Plant htlL (cm)

72 77 65 68 79 88 91 83 82 84

85±3 85±7 45±1 65±O 79±4 S7±3 33±3 36±2 4O±1 37±3 47±3 53±1

107 78 77 77 82 95 72 89 92

243±19 235±39 173±30 230±25 107±12 34±2 38±6 155±lOb 44±2b

64

Grain yield lL (t/ha) 0.47±0.11 O.11±O.O3 O.14±O.O3 1. 29±O.22 O.80±0.13 O. 66±0. 03 0.21±0.06 O.75±0.18 0.53±0.04 0.09±0.02 1. 24±O.20 O.55±O.13

b Average of two replications only.

l.08±O.18 c O. 61±O. IS c 1. 01±O. 27 1. 10±0. 17 O.46±O.16 O.60±O.07 0.05±O.02c O. 30±O. Olb,. O. 6O±O. 06 b c Heavy pest


380


Effects as determined by various cropping patterns

Growing a crop continuously on the same piece of land for 3 years gave a general picture of the injurious effects of a one-crop system. However, growth of most of the crops examined were also determined by the season of planting, which may interfere with the characterization of harmful effects due to continuous cropping. Crops, therefore, were selectively rotated using for rotation cropping one-fourth of the mother plot. Comparison in growth performance was made among plants grown at the same time. Table 3 shows that growth and yield of dryland rice, mungbeans, and cowpeas were improved when grown in rotation with other crops. This gives circumstantial evidence of harmful effects of continuous cropping in those crops. There is, however, a tendency for the inhibitory effect to be corrected at later stages of growth in mungbeans. Although plant growth in corn did not change, a higher grain yield was obtained by growing it in rotation in the cowpea pattern, which may indicate that it was also affected by the adverse effects of continuous cropping. Such effect Table 3. Growth and yield of continuous and first croppings, IRRI, Los Baiios, Philippines. Crop Rice (IR2061-464-2-4)

Previous crop treatment

3 continuous rice crops (IR2061-464-2-4) 15 mo. continuous fallow 5 continuous mungbean crops Rice (IR2061-464-2-4) 6 continuous rice crops (IR2061-464-2-4) 7 continuous cowpeas Rice (IR747.B2·6-3) 4 continuous rice crops (IR747-B2-6-3) 4 continuous sorghum Mungbeans (MGSO-lOA) 5 continuous mungbean crops 3 continuous rice crops Mungbeans (MGSO-lOA) 8 continuous mungbean crops 6 continuous sorghum Cowpeas (EG green pod 12) 5 continuous cowpea crops S continuous corn crops Corn (DMR 2) S continuous corn crops S continuous cowpea crops Sorghum (Cosor 2) 4 continuous sorghum crops 4 continuous rice crops

Plant ht (cm)

Grain yield (t/ha)

108

S6 b

0.36 b

108 108 113

82 a 82 a 45 b

1. 75 a 1. 28 a 0.67 b

105 88

88 a 62 a

3.02 a 0.95 b

88 68 68 91 91

74 a 57 b 72 a 39 b 81 a 3S b 137 a 211 a 223 a 142 a 135 b

1. 55 a 0.66 a 0.68 a 0.53 b 1. 21 a 0.60 b 1.64 a 3.63 b 4.74 a 3.45 a 3.71 a

Maturity (days)

9S

91 93 93 88 88

In a crop. means followed by a common letter in a column are not significantly different at the

~~~U

5:-%

381

Cropping of Dryland Rice and .other Crops Table 4. Effect of one rotation crop on growth and yield in the continuous cropping pattern, IRRI, Los Banos, Philippines. Crop variety Upland rice (IR747.B2·6-3)


Cowpea (EO green pod t2) .

Corn (DMR.2)

Sorghum (Cosor 2)


Maturity Plant ht (cm) (days)

Grain yield (t/ha)

5 continuous IR747

100

42a

O.OSa

One sorghum rotation after 4 continuous IR747

100

43a

O.09a

6 continuous cowpeas

72

38 b

0.05a

One com rotation, after 5 continuous cowpeas

72

61 a

0.65a

6 continuous corn

91

250 a

2.64b

One cowpea rotation after 5 continuous corn

91

24Sa

4.45a

5 continuous sorghum

92

16Sa

6.10a

One rice rotation after 4 continuous sorghum

92

170a

6.51 a

In a crop, means followed by a common letter in a column are not significantly different at the 5% level.

may be considered minimal as compared with upland rice and the legumes, and this yield reduction might be due to the other factors. Sorghum was not affected by crop rotation. To see if one rotation crop could eliminate harmful effects in a continuous cropping pattern, the rotation portion of the mother plot was again planted to the same continuous crop. Table 4 shows that one rotation crop of sorghum failed to reduce harmful effects in dryland rice. Soil sickness in cowpea, meanwhile, was partially corrected by one rotation crop of corn. In corn, one rotation crop of cowpea eliminated all the detrimental effects of continuous cropping. Sorghum did not visibly show any response to crop rotation and continuous cropping. Persistence of harmful effects in dryland rice and mungbeans

As the most susceptible crops, attention was focused on upland rice and mungbeans. The second crop of rice (Table 5) and mungbeans (Table 6) already showed significant decreases in either plant growth or yield, suggesting early occurrence of growth inhibitory effects in those crops. Keeping the field in fallow for 5 months during the dry season improved growth and yield of both rice and mungbeans in the continuous cropping pattern. But the second crop after 5 months of fallow brought the soil back to a condition as sick as before. Removal of the inhibitory effect by keeping the soil unplanted during the dry season may be considered as only a partial cure, as growth and yield were still inferior to those of the first time in the cowpea or sorghum plots.


382 Table

s.

Effect of previous cropping condition on the growth and yield of dry land rice, IRRI, Los Banos, Philippines.

Cropping period (rice variety)


Jun.-Sep. 1976 (lR2061-464-2-4)

Oct. 1976-Feb. 1977 (IR2061-464-2-4)

Plant ht (cm)

Previous crop treatment 5 continuous rice crops

Dry matter wt (g/ml)&

Grain wt (g/m2) ..

54 b

585 c

16.5 c

5 mo. fallow in the continuous rice pattern

61 b

772 b

214 b

7 continuous cowpea crops

88 a

1,265 a

446 a

6 continuous rice crops

57 c

.5.54 b

149 c

One rice crop after .5 months fallow in the continuous rice pattern

59 c

742 b

117 bc

One rice crop after 7 continuous cowpea crops 68 b 8 continuous cowpea crops 81 a

1,046 a t,20S a

308 b

Jan.-Jun. 1977 (lR5) 7th dryland rice crops

385 a

47 b

Ob

2-1/2 years of continuous fallow

63 a

127 a

One IRS crop after 5 months fallow in the continuous rice cropping pattern

44 b

ob

For each column in every period, means followed by a common letter arc not significantly different at the 5,% level by DMRT. .. Plant sample for yield determination taken from 20 x 20 cm at 4 locations in the plot. Heavy rat damage of plants in the October cropping made yield data determination from bigger sampling area not possible. Table 6. Effect of previous cropping condition on the growth and yield of mungbeans (MGSO-I0A), IRRI, Los Banos, Philippines. Cropping period Jun.-Sep. 1976

Oct. 1976-Feb. 1977

No. of plants at 30DAS (lOS/ha)

Plant ht (cm)

Dry matter wt (g/10 plants)

Grain yield (t/ha)

8 continuous mungbean crops

170. .5

39 c

100 a

0.53 b

5 mo. fallow in the continuous mungbean pattern

240.0

152 a

1. 11 a

6 continuous sorghum crops

254.0

60b 81 a

140 a

1. 21 a

37 b

75 b

0.09 c

39 b

99 b

0.16 be

One mungbean crop after 6 continuous sorghum crops

58 a

140 a

0.23 b

7 continuous sorghum crops

78 a

161 a

0.4.5 a


9 continuous mungbean crops One mungbean crop after .5 months fallow in the continuous mungbean pattern

For each column in every cropping period, means followed by a common letter are not significantly different at the S% level by DMRT.

The three rice varieties-IR2061.464.2.4, IR747·B2.6·3, and IRS-all ap"peared to be affected by growth inhibition due to continuous cropping. The IRS


383

Table 7. The effect of continuous cropping on nutrient status of dryland rice soil, IRS, IRRI 1977 dry season, Los Banos, Philippines. Previous cropping conditions 7 continuous

dry land rice crops


Soil analysis

pH (1 : 1 w/v HaO) EC(mmhos) Exchangeable cations: Na K

Mg Ca Total exchangeable bases (meq/lOO gm) Available P (ppm) Total N (%) Organic C (%) Total elements (ppm): P K

Mg Cu Mn Zn

Fe(%)

2-1/2 years of

continuous fallow

One IRS crop after 5 mo. fallow in the continuous dryiand rice plot

Surface

Subsoil

Surface

Subsoil

.Surface

Subsoil

6.1 0.31 1.2 0.8 7.5 16.2 25.8

6.1 0.43 1.4 0.6 7.5 15.5 25.0

6.0 0.28 1.1 1.0 6.2 14.5 22.9

6.0 0.44 1.2 O. 7 6.5 16.0 24.4

6.2 0.26 1.4 0.8 8.0 16.5 26.7

6.1 0.43 1.4 0.4 7.8 16.8 26.1

13.0 0.13 1. 23 534 1,354 4,177 168 2,565 126 9.14

11. 0 o. 10 0.99 651 1,089 4,085 172 2,438 125 9.03

15.0 O. 13 1. 25 645 1,536 4, 188 167 2,716 131 9.15

11. 0 0.11

13.0 0.13 1. 35 545 1,292 4,207 165 2,562 122 9.00

10.0 0.11 1.02 664 1,075 4,093 165 2,451 121 8.94

1.11

576 1,257 4,060 173 2,560 129 9.19

crop that had immediately followed IR747-B2-6-3 showed symptoms of the harmful effects, including reduced yield. Before the first crop, the pH was 6.2 and total nitrogen content 0.13% for this Maahas clay soil. These values were maintained after 6 continuous crops of dryland rice, but the pH dropped to 5.7 and the nitrogen level rose to 0.14% after 9 continuous crops of mungbeans. After the eighth crop of IR5 in the dryland rice, the nutrient status of the soil was determined (Table 7). There were no distinct differences in nutrient content among plots of continuous rice, 2-1/2 years of continuous fallow, or continuous rice with a short fallow period, indicating that these nutrient elements may not be the primary factors in the reduction of growth and yield due to continuous croppings.

Root residues as source of harmful effects Harmful effects were less in the greenhouse than in the field, suggesting that the


384

Table 8. Effect of soil sterilization, flooding, and incorporation of rice root residues on the growth and yield of upland rice (IR2061-464-2-4), IRRI greenhouse, May-Oct. 1976, Los Banos, Philippines. Source of soil medium


Continuous rice (7th crop) Fallow soil (another field)

Root residues

Soil treatment

Grain wt (g/pot)

Plant ht (cm)

Straw wt (g/pot)

66 d

19.4 be

6.9 b

Unsterilized

With

Sterilized

With

78 b

30.2 a

16.5 a

Unsterilized, flooded

With

101 a

22.2 b

15.5 a

Unsterilized

Added

70 d

15.5 c

5.6 b

U nsterilized

Not added

71 cd

21. 5 b

8.0 b

Unsterilized

Sterile roots added

73 be

21. 3 b

7.2 b

Not added

78 b

30.2 a

15.2 a

Sterilized

In a column, means followed by a common letter are not significantly different at the 5% level by DMRT. Table 9. Effect of root residues and pretreatment of soil on the growth and yield of mungbeans, IRRI greenhouse, Oct. 1976-Jan. 1977, Los Banos, Philippines. Field cropping condition 9 continuous mungbean crops

27 months continuous fallow

6 continuous upland rice crops

Soil treatment

Plant ht (cm)

Grain wt (g/pot)

None

25 c

1.2 e

2 weeks flooding

2' e 16 d

2.2 de

2 weeks air-drying None

30 ab

3.6

UnsteriJized roots added

28 b

Sterile roots added

29 b

2.9 be 4.3 ab

None

33 a

5.5 a

1.0e

be

In a column, means followed by a common letter are not significantly different at the 5% level by DMRT.

causative agents had been disturbed during collection and preparation of soil for pot culture. In dryland rice, partial soil sterilization improved plant growth, but this happened even in the fallow soil (Table 8). The unsterilized root residues when added to the other soil may have carried a part of the inhibitory agent as there was a reduction in the weight of rice straw. Mild steam sterilization of the root residues did not cause such inhibition although there was no difference between sterilized and unsterilized roots on grain yield. Converting dryland rice soil of continuous cropping into lowland improved growth and yield of the rice plants. In mungbeans, two trials of pot culture were conducted. In the first trial seeds did not germinate when unsterilized root residues of mungbeans were present, but in their absence germination was perfect and sterilization of mungbean soil or of root


385


residues improved seed germination to the 100% level. In the second trial the harmful effects were somewhat relieved as seed germination was not inhibited. Plants grown in the fallow and upland rice soils were better than those in the continuous mungbean soil (Table 9), but the addition of unsterilized root residues to the fallow soil did not clearly inhibit plant growth. Two weeks of flooding or drying of the sick soil before planting failed to reduce the harmful effects.

Determining the potentiality of harmful effects In all crops grown under complete darkness, except sorghum, partial soil sterilization slowed the rate of autolysis, indicating that microorganisms have a part in the appearance of "soil sickness." If this method is to be used in gauging the potentiality of soil sickness, mungbeans show great response. When grown under complete darkness in the other soils, the rate of mungbean autolysis was slower than from the continuous culture soil and the addition of unsterilized root residues in the fallow soil hastened the rate of autolysis (Table 10). Dryland rice responded to partial soil sterilization but the difference in seedling autolysis between sterilized and unsterilized soils was not as clear as in mungbeans (Table 11). In the second set, no clear difference was observed among continuous, fallow and mungbean soils. The addition of unsterilized root residues to the fallow soil, however, hastened the rate of seedling autolysis. The behavior of cowpeas in the dark culture was dubious. Although a response was obtained by partial sterilization of the cowpea no clear difference was observed among continuous culture, rotation, and fallow soils (Table 12). Sorghum appears to have no response to partial soil sterilization under dark culture. Table 10. Number of autolyzed mungbean plants (MGSO-I0A) in the dark culture grown in soils from the continuous cropping pattern, L0S Banos, Philippines. Field soil condition Continuous mungbeans (lOth crop) 27 months continuous fallow

Continuous rice (7th crop)

a All seedlings autolyzed.

Sterilization treatment

Days after sowing

5

7

9 10 12 14 16 19 21

23 2S 27

Unsterilized

0

3 16 26 301.

10% Acetone Steam Unsterilized

0 0 0

0 0 0

Unsterilized with root residues 10% Acetone Steam. Unsterilized

3 0 0 0

5 16 20 29 341. 0 0 0 0 0 1 11 22 32 341. 0 0 0 1 1 4 21 28 341. 0

0

0

0 12 21

10% Acetone Steam

0 0

0 0

0

0 0

0 0

0 0 0

0

0 0 0

1 1 2 5 12 28 331. 4 6 11 22 27 301. 9 23 30 331.

0 4

311.

6 18 26 33 341. 9 21 30 33 341-


386

Table 11. Number of autolyzed rice plants (IR2061-464-2-4) in the dark culture grown in soils from the continuous cropping pattern, Los Banos, Philippines. Sterilization treatment

Field soil condition


Continuous rice crops (4th crop) 1st rice crop

Continuous rice crops (7th crop) 30 months continuous fallow

Continuous mungbeans (10th crop)

a

Unsterilized 10% Acetone Steam U nsterilized 10% Acetone Steam

Days after sowing 16 18 20 22 24 26 28 30 32 34 36 0 0 0 0 0 0

3 0 0

0 1 0

U nsterilized 0 3 10% Acetone 0 0 Steam 0 0 Unsterilized 0 0 Unsterilized 6 17 (with root residues) 10% Acetone 0 0 Steam 0 0 Unsterilized 0 1 10% Acetone 0 0 Steam 0 0

Feb. 1976 dark culture 8 27 37S 22 26 36 37 37 0 5 7 14 25 25 1 14 22 31 33 35 a S 13 16 18 18 32 4 5 7 19 0 Feb. 1977 dark culture 14 16 22 25 3011. 0 3 10 12 18 23 0 3 13 17 23 27 6 19 23 24 3011. 24 28 30 a 0 7 13 20 24 0 12 19 23 27 2 10 20 24 29 0 S 14 20 21 3 8 16 17 27

3936 39-

26 29 3011. 24 28

34 37 3923 33 3911.

28 28 3011. 30 a

26 28 3011. 30 a 24 28 30 28 30

All seedlings autolyzed. Table 12. Number of autolyzed plants in the dark culture grown in soils from the continuous cropping patterns, IRRI, Los Banos, Philippines. Crop

Cowpea

Field soil condition

Continuous cowpeas Unsterilized 10% Acetone Steam lst cowpea crop Un sterilized (after S corn crops) 10% Acetone 18 months fallow

Sorghum

a

Sterilization treatment

Continuous sorghum (5th crop)

All seedlings autolyzed.

Steam Unsterilized 10% Acetone Steam U nsterilized 10% Acetone Steam

Days after sowing 10 12 14 16 18 20 22 24 26 30 32 34 4 9 11 0 0 0 0 0 0 0 11 19 0 7 8 0 0 0 0 0 2 0 0 0 0 0 0 0 0 9 0 2 6 0 1 4

12 0 0 26 8 0 11 0 0 20 17 14

14 20 26 4 0 0 0 27 31 32 8 8 8 0 0 0 18 22 29 0 0 0 0 0 0 23 27 28 21 26 28 21 24 29

38 25 19 4011. 11 11 4011. 4 7 9 3011. 3011. 3011.

32 11 3 34 9 0 38

39 4011. 36 40& 4011. 36 38 4011. 39 4011. 20 38 4011. 22 36 38


387


DISCUSSION

Crops differed in their reaction to continuous cropping. Dryland rice, mungbeans, and cowpeas, when planted in rapid sequence, declined in production. Corn was less susceptible, while sorghum was not affected. Reduction of plant growth and yield occurred in spite of the addition of sufficient fertilizer and intensive control of pests and diseases and management factors. The occurrence of soil-borne diseases, however, would not be entirely eliminated under field conditions. Frequent and heavy application of carbofuran used indirectly as a nematocide had eliminated nematode build-up in the soil. The dry summer months had adversely but uniformly affected plants in the various cropping patterns. The inhibitory effect of continuous cropping occurred rather early-the second or third crop had already shown marked decreases in growth and grain yield. Once established, it appears the harmful effects persist in the soil for quite sometime: To bring the effects to an agronomically low level of activity, it is best to avoid planting the same or related crop for at least 1 year. The agent of growth inhibition may be considered to have a specific affinity to the crops, as growth retardation will not occur in mungbeans when grown in a· sick rice soil and vice versa. But crop residual effects may be found among related crops, as a previous soybean crop caused grain yield reduction in the next mungbean crop (3).

Converting dryland rice soil with a history of growth inhibitory effects into flooded soil substantially improved the growth of plants. The same observation was made by NISHIO and KUSANO in Japan (8). In a rice-based system, the practice of keeping the soil flooded during the wet season and then planting it to the susceptible crop once during the dry season would not therefore create a problem of growth inhibitory effects. But submerging the soil for 2 weeks before planting or drying it for the same period did not reduce the harmful effects in mungbeans. It would be expected then that periodic rainfall causing flooding of the field for several days or water stress during the growth of plants does not minimize these harmful effects. Keeping the soil in fallow for 5 months or drying it for at least 30 days (3), however, beneficially reduced soil sickness. The causal agent would still be there as the succeeding crop had not yet fully recovered. Chemical sterilization with chloropicrin under greenhouse conditions improved growth and yield. The effect may not be entirely biological, as soil fumigation was found to affect the chemical properties of the soil and the release of plant nutrients (5, 13, 16). It appears, however, as an effective way of reducing the harmful effects due to continuous cropping. In legumes, there is a need to inoculate the soil with the right strain of Rhizobium after fumigation. Definitely, the inhibitory agents also stayed in the root residues, which may persist in the soil for quite sometime and may serve as a source of infection for the suc-


388


ceeding susceptible crop. Dark culture of seedlings proved to be a simple yet rapid method of determining the potentiality of "soil sickness." Results obtained from dark culture may, however, be open to several questions. Since dark culture presented conditions quite different from those of light culture or of the field it does not necessarily mean that the same harmful organisms could detrimentally affect healthy plants under light culture. Inhibitive action under dark culture should therefore be considered to be just a potential effect of soil sickness, giving only indirect and supplementary clues to finding causative agents of growth inhibition due to continuous cropping. The growth of plants under controlled conditions in the greenhouse and in· the dark culture suggested the involvement of biological agents in the decline of growth and yield of both mungbeans and dryland rice. The degree of biological involvement appears to have differed between the two crops; mungbeans were more clearly affected by microorganisms. Further study is needed to determine whether the microorganisms, possibly a fungus, caused direct damage to the root system as a soil-borne disease or caused the production of toxic substances inhibitory to mungbean growth. The situation with dryland rice is more complicated because, although the involvement of microorganisms was suggested, there appear to be other things that had primarily caused the decline of plant growth. In Japan, a fungus, Pyrenochaela sp., which has specific affinity to dryland rice, was found by NISHIO and KUSANO (7, 9, 10) to produce a growth inhibitor to several monocotyledonous crops including dryland rice. In the Philippines, harmful effects, however, appeared much earlier and visual symptoms differed, which may indicate differing activities of the biological agents. Under field conditions, it is most probable that in the monoculture soil, microbial balance tipped toward a specific organism which either infects directly or tends to produce substances that inhibit growth of the specific crop (1). It is speculated then that in dryland rice soils in the Philippines, microorganism involvement gave an indirect effect in the transformation of certain accumulated compounds into substances that are toxic as a result of continuous cropping. The close association between root exudates and microorganisms had been reported (6, lI, 12, /4). REFERENCES 1) BoRNER, H., Liberation of organic substances from higher plants and their role in soil sickness problem, Bot. Rev., 26, 393-424 (1960) 2) INTERNAnoNAL RICE RESEARCH INsnTUTE, Annual Report for 1973, Los Banos, Philippines, 1974. pp. 28-29 J) INTERNAnoNAL RICE RESEARCH INSTITUTE, Annual Report for 1974, Los Bailos, Philippines, 19705. pp.334-335 f) INTERNAnoNAL RICE RESEARCH INsnTUTE, Annual Report for 1975, Los Banos, Philippines, 1976. p.363 5) LADD, J.N., BRISBANE, P.O., BUTLER, H.A., and AMATO, M., Studies on soil fumigation. III.



389

Effects on enzyme activities, bacterial numbers and extractable ninhydrin reactive compounds, Soil BioI. Biochem., 8, 255-260 (1976) . 6) MCCALLA, T.M. and HASKINS, F.A., Phytotoxic substances from soil microorganisms and crop residues, Bact. Rev., 28, 181-207 (1964) 7) NISHIO, M. and KUSANO, S., Fungi associated with roots of continuously cropped upland rice, Soil Sci. Plant Nutr., 19,205-217 (1973) 8) NISHIO, M. and KUSANO, S., Effect of root residues on the growth of upland rice, ibid., 11, 391-395 (1975) 9) NISHIO, M. and KUSANO, S., A growth inhibitor specific to upland rice produced by Pyrenochaeta sp., ibid., 11, 227-286 (1976a) 10) NISHIO, M. and KUSANO, S., A fungal substance of selective action on plant growth produced by Pyrenochaeta sp., ibid., 12, 467-472 (1976b) 11) PATRICK, Z.A., The peach replant problem in Ontario. II. Toxic substances from microbial decomposition products of peach root residue, Can. J. Bot., 33, 461-486 (1955) 12) ROVIRA, A.D., Plant root exudates, Bot. Rev., 35, 35-57 (1969) 13) ROVIRA, A.D., Studies on soil fumigation. I. Effect on ammonium, nitrate and phosphate in soil and on the growth, nutrition and yield of wheat, Soil BioI. Biochem., 8,241-247 (1976) 14) SUBBA-RAO, M.S., BIDWELL, R.G.S., and BAILEY, D.L., Studies of the rhizosphere activity by the use of isotopically labeled carbon, Can. J. Bot., 40, 203-212 (1962) 15) SUZUKI, T. and KUBOTA, S., A simple method for determining the degree of soil sickness by dark culture of plant, J. Sci. Soil Manure, Japan, 42,126-128 (1971) (in Japanese) 16) SUZUKI, T. and WATANABE, I., Re-establishment of nitrifying microorganisms in field soil after chloropicrin fumigation, ibid., 37,579-589 (1966) (in Japanese) 17) TUCKEY, H.B., Implications of allelopathy in agricultural plant science, Bot. Rev., 35, 1-15 (1969)