Livestock Research for Rural Development 13 (6) 2001 | Citation of this paper |
Two experiments were carried out from 22 July to
21 October 2001 in the University of Tropical Agriculture farm on the campus of the Royal
University of Agriculture, Phnom Penh. The main objective was to
measure the effect of different sources of fertilizer and the response in yield of water
spinach (Ipomoea aquatica, var. reptans) to
increasing levels of effluent from a biodigester charged with pig manure. In both studies the first fertilizer application was made
one week after planting and the rest at weekly interval for 3 weeks. The total period from
planting to harvest was 4 weeks.
The first experiment was conducted to evaluate the
effect of different fertilizing practices on water spinach (Ipomoea aquatica, var. reptans) yield. The crop was located on a sandy, poor soil derived from alluvial
deposits (pH 5.45, N 0.13%). A completely randomized block design with four treatments was
employed: no fertilization (control), 75 kg N/ha as urea, 75 kg total N/ha as biodigester
effluent and 75 kg ammonia-N/ha as biodigester effluent. There was no difference in fresh
biomass yield of water spinach between the two treatments with biodigester effluent (17.6
and 18.6 tonnes/ha, for total-N and ammonia-N, respectively), which were higher than the
control (5.6 tonnes/ha) and tended to be higher than when the N source was from urea (15.5
tonnes/ha).
In
the second experiment the yield of water spinach was used as response criterion to
different levels of N (0, 20, 40, 60, 80, 100, 120 and 140 kg N/ha) as effluent from a
biodigester charged with pig manure. The fresh biomass yield was linearly related with the
level of effluent N (Y = 7.12 + 0.118X, R2=
0.96) reaching 23.6 tonnes/ha with140 kg N/ha. The yield response in the second harvest
(re-growth), when the same levels of effluent N were applied, was much less reaching a
maximum of 16 tonnes/ha of fresh biomass with 140 kg N/ha, and with a more variable
response (Y = 7.03 + 0.0502X, R2= 0.64).
It was concluded that: on the basis of total N content, biodigester effluent had a similar value as urea for fertilization of water spinach; and the yield response to effluent was linear over the range of 0 to 140 kg N/ha. In the first growth period, with 140 kg N/ha of effluent, 54% of the applied N was converted to N in the water spinach.
In Cambodia about 50% of all
children aged 0-5 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
(UNDP 1997). Micronutrient deficiencies in the diet result in improper physical and mental
development in children as well as adults, which is responsible for lower productivity and
inferior quality of life. Leafy vegetables can contribute significant amounts of vitamins
and minerals, and are especially excellent sources of protein, carotene (vitamin A), iron
and ascorbic acid (vitamin C). The vegetable consumption in Cambodia is reported to be one
of the lowest in the world, being only 35 kg per person per annum (DOA 1994). This is
equivalent to only 30% of the recommended daily intake of vegetables (300g/day according
to WHO). Therefore, a home garden for growing of vegetables is one way to
improve the quality of life of the Cambodian people by increasing nutritional status and
generating income as well.
Cambodias economy is
largely based on agriculture. Approximately 80% of the population live in rural areas and
grow rice as their staple crop. Vegetable production is of secondary importance and is
mainly in lowland areas, especially in the Mekong delta region where the soil is very
fertile. The farm size in Cambodia is generally small
especially concerning the vegetable garden and the soil is
very poor. So the intensive cultivation and improved productivity per unit area is a key
factor for rural economic and social development.
The vegetable production in
Cambodia is mainly dependent on external inputs in the form of seeds, pesticides and chemical fertilizers. Most
farmers cannot afford the chemical inputs and this leads to very low yields. Vegetables
require many nutrient elements for good growth and production but N, P and K are the three
elements of most concern. Leafy vegetables are especially heavy users of nitrogen.
Animal manure is a potential
replacement for chemical fertilizer and is traditionally used by poor farmers in Cambodia.
However, it is not properly managed so that the efficiency of utilization of the manure is
very low. 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 a
nutrient rich effluent. Le Ha Chau (1998a,b) showed that cassava and duckweed yielded more
biomass of a higher protein content when effluent rather than manure was used as the
fertilizer.
Water spinach is used traditionally in Cambodia as
a vegetable for consumption by people and animals. It
has a short growth period and is resistant to common insect pests. However, there appears
to be no information in the literature on its response to fertilizer especially fertilizer
of organic origin as is produced by the anaerobic digestion of livestock manure.
The following experiments were carried out to
evaluate the potential of water spinach to utilize the effluent from biodigesters charged
with pig manure. The first experiment aimed to compare biodigester effluent with urea,
which is the fertilizer most commonly used by vegetable farmers in Cambodia. The second experiment was designed to derive the
response curve of biomass yield to increasing levels of biodigester effluent as the only
fertilizer.
The experiment was done in the Ecological farm of the University of Tropical Agriculture situated in the campus of the Royal University of Agriculture, Chamcar Daung, Dangkor District, about 10 km from Phnom Penh. The experimental area was located on sandy soil derived from alluvial deposits. The pH was 5.45 and the soil had 0.13% of nitrogen (UTA 2001, unpublished data).
The treatments were:
C: control (no fertilizer)
N: Effluent from a biodigester charged with pig manure at rate of 75 kg N/ha
NH3: Effluent from a biodigester charged with pig manure applied at 75 kg ammonia-N/ha
Urea: Urea applied at 75 kg N/ha
There were 8 replicates of each treatment arranged within two blocks in a completely randomized block design. The plot size was 1.5*1.00 m with a space of 50 cm between plots in each block, which were 10 m apart.
The soil was cultivated by hoe, two times and a raised bed
prepared, which was 15 cm high.
Seeds of
dry-land water spinach were planted on 22 July 2001, in rows across the bed, at spacing between seeds of 1-2 cm
and at 2-3 cm depth. The distance between rows was 20cm. The distance between plots and
rows was 50 cm.
The fertilizers were applied 3 times in the growing
period. Increasing quantities equivalent to 10, 40 and 50% of the total allowance were
applied on days 7, 14 and 21, respectively (Table 1). For each application fresh
quantities of effluent were brought from a biodigester charged with washings (manure,
urine and water) from pigs fed broken rice and water spinach (Ipomoea aquatica).
The effluent was analysed for total nitrogen (N) and ammonia-N (NH3-N). The
quantities of effluent were calculated on the basis of the total amount of N (75 kg/ha)
and the proportion of the total (10, 40 or 50%) to be applied.
Table 1: Quantities of effluent and urea applied per plot
of 1.5 m2 during the experiment |
||||
Days |
% of
total |
Effluent (N) |
Effluent (NH3-N) |
Urea, g |
7 |
10 |
4.89 |
5.02 |
2.45 |
14 |
40 |
13.6 |
11.4 |
9.78 |
21 |
50 |
16.1 |
26.8 |
12.2 |
Total |
100 |
34.6 |
43.2 |
24.5 |
A watering can was used to apply water twice a day
(morning and afternoon) at the rate of 3 to 4 litres/m²). On rainy days no water was
applied.
Plant height was measured and number of leaves counted
weekly before applying the fertilizers. The water spinach was harvested 28 days after
planting. All plants in individual plots were weighed. Leaf and stem were separated and
analysed immediately after harvest to determine dry matter by microwave radiation
(Undersander et al 1993) and nitrogen (Foss Tecator kjeldahl apparatus; AOAC 1990).
Samples of soil were taken from each plot before planting and after harvest for analysis
of dry matter, nitrogen, ash, organic matter and pH.
Before planting and after harvesting the water spinach,
two samples of soil (2 kg) were taken from the 0 20 cm layer in each plot and put
into plastic bags. Three seeds of maize were planted in each plastic bag. The bags were
watered every day. One week after planting the number of maize plants was reduced to one
per bag for the growing test. After 30 days the height to the growing point was recorded
and the biomass harvested and weighed fresh.
The data were analysed by ANOVA using the General Linear
Model (GML) software of Minitab (Version Release 12.21). The variables were fertilizer
treatments, blocks and error.
The experiment was carried out in the same location and
with the same plots used for Experiment 1.
The design was a production function with 8 levels of fertilization with biodigester effluent, equivalent to N levels of 0, 20, 40, 60, 80, 100, 120, 140 kg/ha. Two harvests were made at intervals of 28 days and the fertilizer treatments were repeated in each of the two consecutive growth periods There were 4 replicates of each treatment. The plots used for each replicate were those used previously for each of the four fertilizer treatments applied in Experiment 1.
The plots were cultivated by hoe two times before re-making the raised beds. There was an interval of 7 days from the time of the harvest in Experiment 1 and planting the seed in Experiment 2, which was done on 25 August 2001.
The procedures were similar to those described in Experiment 1, except that no analyses were done on the soil.
Two harvests were made at intervals of 28 days.
The original hypothesis was that the water spinach would
respond more to the ammonia nitrogen present in the effluent rather than to the total
nitrogen a proportion of which would be in organic form and thus less available to a
rapidly growing plant such as water spinach. Unfortunately
there appeared to be considerable variation in the proportion of ammonia in the effluent
applied on each occasion, with values ranging from 0.49 to 0.99 (Table 2).
Table 2: Composition of the effluent and urea applied on
the three occasions during the growth of the water spinach in the first growth period |
|||||
Days |
Effluent, |
Effluent, |
Effluent,
mg NH3-N/litre |
Effluent, |
Urea, |
7 |
230 |
224 |
184 |
0.802 |
46 |
14 |
330 |
395 |
325 |
0.986 |
46 |
21 |
350 |
210 |
173 |
0.494 |
46 |
The digested waste (effluent)
is a high quality fertilizer. The digestion process converts a proportion of the nitrogen
in the organic materials to ionic ammonia (NH4+), a form that becomes more stable when ploughed into the
soil. The ammonia ion is readily fixed in the soil so that it can be absorbed
by plants. However, there is little
information on the proportion of the nitrogen that is converted to the ionized form. It is
not known if the very high value recorded in the effluent applied on day 14 was a true
value or the result of analytical error. Recent analyses of total nitrogen and ammonia
nitrogen in the manure fed into the biodigester and the corresponding effluent indicated
proportions of ammonia nitrogen of 0.11 and 0.06, respectively in fresh manure from cattle
and pigs increasing to 0.62 and 0.67 in the effluent (Pok Samkol and Chan Noeng 2001,
unpublished data). As the application of the effluent in the N and NH3-N treatment was based on the reported analyses (Table 2), the amounts
of effluent applied on day 21 were much greater for the NH3-N treatment (26.8 litres) than
for the N treatment (16.1 litres) with corresponding total amounts applied during the
experiment being 43.2 and 34.6 litres (Table 1).
There were no differences among fertilizer treatments in
the soil pH (P=0.28) or in the organic matter content (P=0.43) after harvesting of the
water spinach (Tables 3 and 4 and Figures 1 and 2). However,
on all fertilizer treatments the soil pH was almost one unit higher after growing the
water spinach (P=0.001). By contrast, the
organic matter content decreased when no fertilizer was applied and increased on the urea
treatment. There were no changes for the two effluent treatments.
Table 3: Least
square means for pH of the soil before and at the end of the first growth period for the
control, N , NH3 and urea fertilizer treatments. |
||||||
|
Control |
N |
NH3 |
Urea |
Mean |
SEM/Prob |
Before |
4.64 |
4.73 |
4.80 |
4.49 |
4.67 |
0.039/0.001 |
After |
5.52 |
5.41 |
5.42 |
5.40 |
5.44 |
|
Mean |
5.08 |
5.07 |
5.11 |
4.94 |
|
|
SEM/Prob |
0.055/0.17 |
|
|
|
Table 4: Least
square means for organic matter of the soil before and at the end of the experiment for
the control, N , NH3 and urea fertilizer treatments |
||||||
|
Control |
N |
NH3 |
Urea |
Mean |
SEM/Prob |
Before |
2.76 |
1.97 |
2.15 |
1.92 |
2.20 |
0.184/0.852 |
After |
2.23 |
2.04 |
2.19 |
2.54 |
2.25 |
|
Mean |
2.49 |
2.01 |
2.17 |
2.23 |
|
|
SEM/Prob |
0.26/0.62 |
|
|
Figure 1.
Effect of different sources of fertilizer on soil pH before and after growing of water
spinach |
|
There was reasonable uniformity among treatment
plots in overall soil fertility before starting the experiment, as measured by the maize
test (P=0.311) but a tendency for it to be less on the urea treatment (Tables 4 and 5;
Figure 3). On all treatments there was an
apparent decrease (P=0.091) in soil fertility after growing the water spinach, however, as
the maize growth is subject to the prevailing climatic conditions during the test, caution
is needed in interpreting differences between tests done at different times. The use of sand as a reference medium, as
demonstrated by Chamnanwit Promkot (2001), would have given more validity to the
interpretation of differences between tests at different times.
Figure 3. Effect of different sources of
fertilizer on growth rates of maize
(soil biotest) before and after planting water spinach
Table 5: Least
square means for the biotest (weight of maize plant, g)
of the soil before and at the end of the experiment |
||||||
|
C |
N |
NH3 |
Urea |
Mean |
SEM/Prob |
Before |
16.7 |
13.4 |
20.1 |
18.3 |
17.1 |
1.59/0.311 |
After |
8.91 |
10.6 |
11.9 |
7.83 |
9.83 |
|
Mean |
12.8 |
12.0 |
16.0 |
13.0 |
|
|
SEM/Prob |
1.27/0.001 |
|
|
There were no differences between the two levels
of effluent (based on total N and ammonia-N) in the effects on the height and green
biomass yield of the water spinach. Both these treatments tended to support higher yields
than urea alone, which in turn was superior to the control (Table 6; Figures 4 and 5).
Table
6: Least square
means for height and yield of water spinach in plots of 1.5m² according to fertilizer
treatment |
|||||
|
Control |
N |
NH3 |
Urea |
SEM/Prob. |
Height, cm |
19.7c |
34.5a |
34.8a |
30.7b |
0.71/0.001 |
Biomass yield, g fresh weight per plot |
|||||
Green
material |
6.58c |
17.6ab |
18.6a |
15.5b |
1.23/0.001 |
Roots |
1.50b |
2.24a |
2.16a |
2.12a |
0.14/0.001 |
abc Means within rows without letter in common are different
(P< 0.05) |
Figure 4. Effect of different sources of fertilizer on height of water spinach after 28 days growth |
Figure 5. Effect of different sources of
fertilizer on green biomass yield of water spinach at 28 days |
The plots receiving the ammonia-N treatment received 25%
more effluent than those where the effluent was
applied according to the total N content, yet the yields differed by only 6%. This implies
that the total N content is as good a guide to the fertiliser value of biodigester
effluent as is the content of N in the form of NH4+ and that there was little benefit from the extra 25% of N
on the NH3 treatment. There is no
obvious explanation for the latter result, other
than that the uptake of N was less when the highest quantities
were applied on day 21 of the growth period. That effluent should be superior to urea is
not surprising as the effluent supplies a variety of other nutrients, including
phosphorus, potassium and other minor elements all of which are needed for plant growth.
In an experiment in southern Minnesota, comparing pig manure and nitrate-N fertilizer,
maize yields were greater for the pig manure than for the commercial fertilizer at six of
the seven sites (Randall 1998). Cushman et al (1999) compared commercial nitrate fertilizer and swine effluent for
production of
vegetable crops in Mississippi and showed that all treatments receiving swine effluent always yielded equal to or greater than
treatments that received commercial fertilizers. These results
also showed that differences in the predominating nitrogen form, that is nitrate nitrogen
(NO3) from commercial fertilizer sources or ammonia-N from
the effluent source, did not affect yield or quality of the vegetables (Cushman et al
1999).
The yield on the best treatment was equivalent to
18.6 tonnes fresh biomass/ha in one month of growth.
On a yearly basis, assuming irrigation is available, this is equivalent to 223
tonnes fresh biomass/ha, which with a dry matter content
of 8% is 18 tonnes of dry matter. This indicates there is a very high potential for the
use of water spinach as an appropriate crop to exploit the use of biodigester effluent in
integrated farming systems.
There was an interaction (P=0.001) between growth
periods and effects of the level of effluent in
the first harvest (Figures 6 and 7). The water spinach had a very similar height on all
effluent levels following the first application of effluent, which was 20% of the total
amount. At the second measurement on day 14, differences due to effluent level became
apparent, and these were magnified considerably by day 21 (Table 7).
Figure 6. Effect of effluent level on height of
water spinach |
Figure 7. Effect of level of effluent on number of
leaves of water spinach. |
There were no differences among levels of effluent
in the effects on the number of leaves (Table 8), but there were big differences between
periods (P=0.001).
Table 7: Least square means for height (cm) of water
spinach in plots of 1.5m² according to level of effluent N |
|||||||||
Growth |
Level of effluent, kg
N/ha |
SEM/Prob |
|||||||
0 |
20 |
40 |
60 |
80 |
100 |
120 |
140 |
||
14 days |
8.07 |
8.01 |
6.70 |
7.74 |
8.52 |
9.02 |
8.40 |
11.1 |
0.37/0.001 |
21 days |
15.3 |
18.15 |
16.4 |
18.9 |
21.3 |
23.5 |
22.8 |
26.4 |
0.66/0.001 |
28 days |
23.9 |
31.0 |
30.24 |
37.3 |
39.3 |
45.4 |
42.6 |
48.8 |
1.18/0.001 |
Table 8: Least square means for numbers of leaves of water
spinach in plots of 1.5m² according to level of effluent |
|||||||||
Periods |
Level of effluent kg N/ha |
SEM/Prob |
|||||||
0 |
20 |
40 |
60 |
80 |
100 |
120 |
140 |
||
14 days |
5.90 |
5.95 |
5.50 |
5.65 |
5.97 |
5.87 |
5.95 |
6.17 |
0.14/0.001 |
21 days |
8.95 |
9.27 |
8.47 |
8.82 |
9.42 |
9.42 |
9.05 |
9.80 |
0.19/0.001 |
28 days |
11.3 |
12.0 |
11.5 |
12.0 |
12.4 |
12.9 |
12.1 |
12.8 |
0.22/0.001 |
Figure 8. Effect of
level of effluent on dry matter and crude protein content
of whole water spinach (stem and leaves).
There were tendencies for the dry matter content to decrease slightly (Y = 9.16 0.0497X±0.0026) and for crude protein to increase (Y = 15.6 + 0.0492X±0.016) as the effluent level was increased (Figure 8).
Figure 9. Effect of
effluent on green biomass yield of water spinach (first harvest after planting 28 days)
The response of biomass yield to effluent level
was linear over the range of 0 to 140 kg N/ha (Figure 9; Table 9).
Table 9: Least square means for fresh biomass yield of
water spinach according to level of effluent (first harvest) |
|||||||||
|
Level of effluent kg N/ha |
SEM/Prob |
|||||||
0 |
20 |
40 |
60 |
80 |
100 |
120 |
140 |
||
Biomass, tonnes/ha |
6.66 |
10.5 |
10.6 |
14.1 |
17.4 |
20.7 |
20.0 |
23.6 |
1.44/0.001 |
The fact that the yield response was linear up to the maximum level applied (140 kg N/ha) implies that there might have been responses to even higher levels of application of effluent N. Although climatic conditions and soils are very different, it is interesting to compare the levels of N used in the present study with the recommendations for leafy vegetables, typically recommended in the UK (MAFF 1994) (eg: 300 kg N/ha for sprouts, 250 kg N/ha for cauliflowers and 200 kg N/ha for lettuce).
Biomass yields were lower for the second harvest
(Figure 10; Table 10) compared with the first harvest (Figure 9; Table 9) and the response
was less consistent, with apparently lowest yields for the 40 and 60 kg N/ha levels. The
variation may have been caused by the exceptionally heavy rain which fell during the
re-growth phase. The yield of fresh biomass with 140 kg N/ha was 16 tonnes/ha compared
with 24 tonnes/ha in the first harvest.
Table 10: Least square means for fresh biomass yield of
water spinach according to level of effluent (second harvest) |
|||||||||
|
Level of effluent, kg N/ha |
SEM/Prob |
|||||||
0 |
20 |
40 |
60 |
80 |
100 |
120 |
140 |
||
Fresh biomass, tonnes/ha |
9.25 |
9.16 |
7.41 |
7.00 |
11.0 |
11.0 |
13.2 |
16.3 |
1.92/0.04 |
Figure 10. Effect of
different levels of effluent Non biomass yield
of water spinach (second harvest)
The dry matter content of the leaf tended to be higher than in the stem but there was no apparent effect of the level of effluent (Figure 11).
Figure 11. Effect of different levels of effluent N on the
dry matter content
of the leaves and stem of water spinach (second harvest)
The protein content of the leaf of water spinach was higher than in the stem but, as in the case of the dry matter content, there was no relationship with the level of effluent N that was applied (Figure 12).
Figure 12. Effect of
different levels of effluent N on the crude protein content
of the stems and leaves of water spinach
The efficiency with which nutrients are recycled
is an important indicator of the sustainability of the farming system. For the first
harvest, the transfer of nitrogen into the foliage of the water spinach was 54% of the
quantity of nitrogen applied in the effluent (Table 11).
The comparable figure for cassava managed as a forage crop and fertilized with
effluent from a biodigester charged with pig manure was 67%.
Table 11. Recovery of nitrogen in the foliage of
water spinach and cassava when fertilized with biodigester effluent |
||||||
Crops |
N applied, |
Biomass yield, |
DM, |
N in DM, |
N in biomass,
kg/ha |
Recovery of N |
Water spinach |
140 |
24000 |
9 |
3.50 |
75.6 |
54.0 |
Cassava# |
100 |
8700 |
22 |
3.52 |
67.4 |
67.4 |
# Le Ha Chau 1998a |
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