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| Photo 1: Planting the stem cuttings | Photo 2: Fertilising the water spinach after 14 days |
Citation of this paper |
A split-plot design was used to study growth of water spinach (Ipomoea aquatica) as affected by the type of planting material (seeds or stems) and by biodigester effluent (0, 50, 100, 150 and 200 kg N/ha) used as fertilizer.
Fresh biomass yields after 24 days were higher (15 tonnes/ha) when water spinach was established from seed than from stem cuttings (9.18 tonnes/ha). Biomass yield of water spinach increased linearly with effluent N level when planted from stem cuttings and logarithmically when planted from seed. The N content of water spinach leaves increased from 3.08 to 5.56% in DM (19.3 to 34.8% crude protein) by application of 200 kg N/ha as biodigester effluent. Stems were much lower in N (1.2 to 2.0% in DM) and this index tended to decrease with increasing application of effluent N. Water extractable N tended to be higher in leaves than in stems but the converse was true for the water extractable DM. Water extractable DM in leaves and stems increased with level of effluent N up to 100 kg/ha but then declined. Effluent N level had no effect on water extractable N.
It is recommended to establish water spinach from seed rather than from stem cuttings.
Key words: Biodigester, effluent, fertilizer, Ipomoea aquatica, nitrogen, water spinach
Cambodia is one of many countries in Asia South Pacific bordered by Thailand, Lao, Vietnam and the gulf of Thailand. The country has a sub-tropical climate with monsoon rains. The population is concentrated in the central basin, where rice, the mainstay of the diet, is grown. Rice production is a key factor in household food security in the rural areas which are occupied and farmed by approximately 80% of the population.
Unfortunately, many disasters particularly the long time of civil war have caused many problems in Cambodia, resulting in poverty and hunger. The land has became less fertile and many parts are still affected by land mines. Land use patterns are also distorted in many areas. Loss of biodiversity, land degradation and deforestation are identified as major problems. Many households are still not self-sufficient in rice. About 50% of all children aged under 5 years are either stunted or underweight, which appears to be due to long term chronic under-nutrition rather than wasting from short-term, severe food shortages (Heifer-Cambodia 2002). Malnutrition is broadly recognized as a main problem for both human well-being and also animal production in the world, especially developing countries like Cambodia. Lack of information on technical systems relating to sustainable agriculture makes the farmers with a small land area feel less confident, or they pay less attention to production, resulting in an increase in landless families or loss of land among the poor farmer families.
Using water spinach (Ipomoea aquatica) as vegetable for family consumption and animal feeding is very common in Asian countries, as it takes a short time to grow and is highly resistant to common insect pests, and is rich in protein. However, most farmers cultivate water spinach by using urea as fertilizer. The introduction of low-cost biodigesters in Southeast Asia (Bui Xuan An et al 1997) has made it possible for small-scale farmers to convert manure into biogas and nutrient rich effluent. Kean Sophea and Preston (2001) showed that water spinach responded dramatically to fertilization with biodigester effluent. This concept is based on processing the raw manure in a biodigester to produce gas for cooking and at the same time reducing environmental pollution (Preston and Leng 1989). An advantage of water spinach is that it grows in soil or water and can be planted from vegetative cuttings or from seed. However, there is no information about the productivity of water spinach when established from seed or stems.
To study growth of water spinach planted as seed or stems and with fertilizer recycled in the form of biodigester effluent.
Two main treatments were compared in a split-plot arrangement with two replications (Table 1):
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Table 1. Layout of the experiment (S is seed; P stem cuttings) |
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Level of effluent N, kg/ha |
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|
Block |
Replicate |
200 |
0 |
100 |
150 |
50 |
|
1
|
1 |
S |
P |
S |
P |
S |
|
1 |
P |
S |
P |
S |
P |
|
|
2 |
2 |
P |
P |
S |
S |
S |
|
2 |
S |
S |
P |
P |
P |
|
PVC baskets, lined with polyethylene (capacity about 50 litres, area 0.16 m²) were filled with soil to a depth of 20 cm. The soil contained 0.097% of N and had a pH of 7.46.
The dry-land water spinach species was chosen for both seed and stem planting, which was done on August 3, 2003. The seeds were kept in water at ambient temperature for 12 hours before planting in rows across the basket at a spacing of 3-4 cm and at 1-2 cm depth. The distance between rows was 10cm, and the overall seeding density was 62.5g/m2. The stems for planting were each 5 cm long and were taken from a section starting 4-5 cm from the main root. They were planted at spacing of 5cm x 5cm, equivalent to an average of 30 plants per basket (equal to 200 plants/m²) (Photos 1 and 2).
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| Photo 1: Planting the stem cuttings | Photo 2: Fertilising the water spinach after 14 days |
The biodigester effluent was applied every 4 days for a total of 6 times during the growing period. It contained 410 mg N/litre, thus the total amounts used were 0, 1.95, 3.9, 5.85 and 7.8 kg in each basket over the 24 day period, divided in 6 equal applications. The effluent was from a biodigester charged with cow manure managed with a 20 day retention time and a charging rate of 4 kg solids per 1 m³ of liquid volume (Bui Phan Thu Hang 2003).
The plants were irrigated twice a day (morning and afternoon) at the rate of 3 to 4 litres/m²). On rainy days no additional water was applied.
Plant height was measured every 4 days before applying the effluent. The measurement was done on a random number of plants (10 plants/basket). The water spinach was harvested at 24 days after planting. All plants were separated into stem and leaf, and after weighing the two components, samples were analysed immediately to determine dry matter by microwave radiation (Undersander et al 1993), N (AOAC 1990) and water extractable DM and N (Ly and Preston 1997).
The data were analysed by ANOVA using the General Linear
Model (GML) software of Minitab (Version Release 13.2). Sources of
variation were: planting materials (S, P), effluent N level, the
interaction of plant material*level N, blocks and error.
The height of the water spinach increased linearly with level of effluent up to 150 kg N/ha (Table 2 and Figure 1) with no further increase for 200 kg N/ha. The water spinach planted from seed had faster growth in the first 4 days (P=0.023) but there was no difference in the rate of growth in height between the two plant materials (Table 3). There were differences between the two blocks in rate of growth in height (P=0.008) with the highest value for block 2, situated close to a wall which perhaps shielded the plants from excessive wind and rain, which occurred at intervals during the experiment.
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Table 2. Least square means for height (cm) due to effluent level effect |
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|
N level |
4days |
8days |
12days |
16days |
20days |
24days |
Increase, cm/day |
|
0 |
4.94 |
8.39 |
12.8 |
14.8 |
18.0 |
20.6 |
0.78 |
|
50 |
4.03 |
7.66 |
14.4 |
19.2 |
25.3 |
28.5 |
1.29 |
|
100 |
4.74 |
8.73 |
14.9 |
21.8 |
29.9 |
35.4 |
1.60 |
|
150 |
4.58 |
8.03 |
15.5 |
23.8 |
32.5 |
38.4 |
1.79 |
|
200 |
4.46 |
8.34 |
16.7 |
24.8 |
33.7 |
39.4 |
1.85 |
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SEM |
0.278 |
0.494 |
0.68 |
0.86 |
1.02 |
1.02 |
0.054 |
|
Prob |
0.270 |
0.634 |
0.032 |
0.001 |
0.001 |
0.001 |
0.001 |
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Table 3. Least square means for height due to planting material effect |
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|
4days |
8days |
12days |
16days |
20days |
24days |
Increase, cm/day |
|
Stem |
4.21 |
8.00 |
13.2 |
19.6 |
26.9 |
28.8 |
1.45 |
|
Seed |
4.89 |
8.46 |
16.5 |
22.1 |
28.8 |
32.7 |
1.47 |
|
SEM |
0.176 |
0.313 |
0.428 |
0.547 |
0.645 |
0.647 |
0.034 |
|
Prob |
0.023 |
0.325 |
0.000 |
0.009 |
0.069 |
0.567 |
0.72 |
The growth in height of the water spinach was similar to the findings of Ngo Tien Dung (2001) (42 cm with 40 kg N/ha over 28 days) and San Thy and Preston (2001) (47 cm with 140 kg N/ha in 28 days), taking account of the shorter growth period (24 days) in the present experiment.
Figure 1. Mean values for height of water spinach after 24 days according to effluent N levels
and source of planting material
The hypothesis that biomass yield would be higher by planting from seed was verified by the results (Table 4). Response to effluent N level was curvilinear for planting from seed and linear for planting with stem cuttings (Figure 2). Maximum yield for seed planting (15,000 kg/ha) was lower than was reported in Cambodia (23,600 kg/ha with 140 kg N from effluent) by Kean Sophea and Preston (2001). Differences in soil and climate could be the reason for the differences. There appears to be no information in the literature on yield responses to fertilizer with stem cuttings as plant material.
The proportion of leaf in the fresh biomass was higher for stem planting, presumably because the plants were less mature, but there was no consistent trend for effect of effluent N level (Table 3).
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Table 4. Least square means for fresh biomass yield and proportion of leaf material according to effluent N level and source of planting material |
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|
Stem |
Seed |
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|
Treat |
Yield, kg/ha |
% leaf |
Yield, kg/ha |
% leaf |
|
0 |
2000 |
51.2 |
4219 |
38.8 |
|
50 |
4531 |
45.7 |
7656 |
39.6 |
|
100 |
6094 |
43.2 |
11563 |
36.6 |
|
150 |
7187 |
42.4 |
13500 |
36.8 |
|
200 |
9219 |
43.7 |
15000 |
39.9 |
|
Mean |
5820 |
45.3 |
10380 |
38.3 |
|
Yield: SEM/Prob
plant material effect 403/0.001 |
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Figure 2: Response in biomass yield to effluent N level
with two sources of plant material
The DM content of the leaves and stems decreased as effluent fertilizer level was increased; and leaves had higher DM content than stems (Table 5; Figure 3). Nitrogen (and therefore crude protein) in leaves increased with effluent N level as far as 150 kg N/ha, but then leveled off (Figure 4). Crude protein in the stems was much lower than in the leaves, and appeared to decrease with increasing effluent N level, but the trend was not uniform (Figure 4).
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Table 5. Least square means for DM and N in stems and leaves of water spinach according to level of effluent N in kg/ha |
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|
|
0 |
50 |
100 |
150 |
200 |
SEM |
Prob. |
|
Dry matter content, % |
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|
Leaf |
10.2 |
10.3 |
9.42 |
9.20 |
9.37 |
0.194 |
0.007 |
|
Stem |
8.09 |
7.30 |
6.43 |
6.13 |
6.33 |
0.194 |
0.001 |
|
N content in DM, % |
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|
Leaf |
3.08 |
3.74 |
4.33 |
5.66 |
5.56 |
0.156 |
0.001 |
|
Stem |
2.01 |
1.10 |
1.26 |
1.40 |
1.19 |
0.191 |
0.121 |
Figure 3: DM content of leaves and stems of water spinach according to level of effluent N
Figure 4: Mean values for content of crude protein in leaf and stem of water spinach according
to effluent N level
There was no effect of planting materials, and no interaction between planting materials and N level of effluent on the water extractable DM and N, therefore the data in Table 6 and Figure 5 are from the average of the sources of the planting material.
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Table 6. Least square means for water extractable DM and N according to N levels of effluent |
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|
Water extractable DM |
Water extractable N |
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|
Level N |
Leaves |
Stems |
Leaves |
Stems |
|
0 |
24.6 |
34.8 |
60.8 |
52.9 |
|
50 |
36.8 |
43.3 |
65.1 |
64.4 |
|
100 |
46.3 |
50.3 |
52.6 |
51.3 |
|
150 |
41.9 |
53.4 |
55.4 |
62.2 |
|
200 |
39.6 |
44.6 |
61.4 |
56.3 |
There was an indication that the water-extractable DM was higher in the stems than in the leaves (Figure 5) but there were no differences between leaves and stems for water-extractable N (Figure 6). Water-extractable DM increased in both leaves and stems as effluent N level was increased up 100 kg N/ha, but thereafter there was no further response, and in fact a decrease with 200 kg N/ha. Values for both indices were relatively high and this is in agreement with in vitro measurements of the nutritive value of water spinach (Ly and Preston 2001).
Figure 5. Water extractable DM in leaves and stems according to level of effluent N
Figure 6. Water extractable N in leaves and stems according to level of effluent N
The recovery of the N applied in the effluent, calculated as [(N in plant biomass/Total N applied)*100], decreased with increasing levels of application of N in the effluent (Figure 7), and was higher in the water spinach planted from seed. Higher values than these were reported by Kean Sophea and Preston (2001), but they planted the water spinach on raised beds of soil, and obtained higher yields, probably because of better utilization of soil nutrients.
Figure 7. Recovery of N by water spinach fertilized with increasing levels of effluent N
This experiment was carried out as part of the MSc (2003-05) course, supported by the MEKARN project financed by sida-SAREC. The authors expresses their gratitude to all the personnel of the Agriculture and Natural Resources faculty of An Giang University, for assistance with the experiment. Grateful thanks are given to colleagues (Dr J Ly, Mr Chhay Ty and Mr San Thy) for advice and supervision during the conduct of the present study.
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Received 5 June 2004; Accepted 10 August 2004