Evaluation of the effluent from different retention times as fertilizer for growing water spinach (Ipomoea aquatica)

 

San Thy and T R Preston

University of Tropical Agriculture Foundation
Chamcar Daung, PO Box 2423, Phnom Penh 3, Cambodia
santhy@utafoundation.org
regpreston@utafoundation.org

 

 

Abstract

A Randomized Complete Block design with three treatments and three replications per treatment was used to study growth rate of water spinach (Ipomoea aquatica) fertilized with 122 kg N/ha in 28 days (0.43g N/ m² /day) as effluent from biodigesters with hydraulic retention times of 10 (ERT10), 20 (ERT20) and 30 days (ERT30). There were two plantings and two consecutive harvests (28 days interval) for each planting season with the same land and level of effluent N. The first planting was towards the end of the dry season (no natural rain); the second was in the middle of the rainy season.

 

There are significant different (P= 0.001) from the first experiment and the highest yield from ERT20. But in the second experiment, there is no significant of water spinach yield (P=0.108). The first yield was higher (26.44 tonnes /ha) than second yield (16.67 tonnes/ ha) with the same level of N 122kg/ harvest because.

There was effect on soil fertility by number of planting; the soil fertility was decreased consecutive with number planting.

 

Keywords: Effluent, retention time, fertilizer, effluent, Ipomoea aquatica,

 

Introduction

The objective in recycling of anima wastes is to  improve soil fertility through return of plant nutrients and organic matter  (Rodriguez and Preston 1997). Processing the raw manure in a biodigester also reduces environmental pollution as the methane produced by anaerobic fermentation can  be collected and used as fuel (Preston and Leng 1989; Simon 1990).

 

Besides contributing to environmental preservation, biodigesters provide a very good source of fertilizer for crops, water plants and fishponds (Preston and Rodriguez 1996). Gas production for cooking reduces the need for firewood, and saves time, labour and finance for finding firewood  (NRC 1981). The integration of livestock with trees, food crops and aquaculture is seen as the most appropriate way to use the natural resources in a system that is productive and sustainable in the long term (Preston 2000; Marchaim 1992).

 

 

 

The population in Cambodia is primarily engaged in agriculture with some 80% of the population in the rural areas, growing mainly rice and other crops or vegetables. Most farmers cannot afford chemical inputs and as a result yields are low (CARDI ...).  Animal manure is a substitute for chemical fertilizer and is traditionally used by poor farmers in Cambodia. However, the supply is not enough and is not managed efficiently as rice fields are usually some distance away from the homestead where the animals are kept.

 

The biodigester programme in Vietnam (Bui Xuan An et al 1997a,b; Khang et al 2002) has promoted the introduction of low cost tubular polyethylene biodigesters, which makes it possible for small-scale farmers to convert manure into biogas and a nutrient-rich effluent. Biodigester effluent is potentially a better fertilizer than the raw manure, because the anaerobic digestion process results in conversion of organic nitrogen in the manure to ionized ammonia (NH₄⁺), which can be used directly by plant roots (Forchhammer 1994). Thus it has been found in Vietnam that  the effluent was a better fertilizer compared with raw manure for application to cassava and duckweed (Le Ha Chau 1998a,b), although there are few reports of trials to compare the two sources of plant nutrients .

 

Water spinach is used conventionally in Cambodia as a vegetable for consumption by people and animals. It has a short growth period of about one month and is resistant to most common insect pests. Kean Sophea and Preston (2001) showed that water spinach is highly sensitive to N fertilization, and responded linearly to application of biodigester effluent up to a level equivalent  to 140 kg N/ha in the 28 day growth period.

 

Thus water spinach appears to be a good indicator of the potential fertilizer value of  biodigester effluent and was therefore chosen as the medium for evaluating the effluent from biodigesters managed with different retention times as described by Santhy et al (2003).

The hypothesis to be evaluated was that retention time would increase the proportion of organic nitrogen converted to ammonia-N in the effluent and this would be reflected in a higher fertilizer value when applied to water spinach grown on soil plots.
 

Materials and methods

Experimental design

The 3 treatments were the effluents from biodigesters charged with diluted pig manure (4% DM) and with hydraulic retention times of 10 (ERT10), 20 (ERT20) and 30 days (ERT30). The test crop was water spinach. The design was a Randomized Complete Block (RCBD) with 3 replications (Table 1). The plot size was 1m x 1.5m. The applications of effluent to the water spinach were at the same level of nitrogen (122 kg N/ha), and plant growth was measured over a 28 day period.  

Planting and fertilization

Water spinach was planted from seeds (1 to 2 cm depth) at a row width of 15 cm and 1 to 2 cm spacing between seeds. 

 

The biodigester effluent was collected every day during the 10 days of data collection of period 1 in Experiment 2 (Santhy et al (2003) and stored in closed PVC containers.  The amount of effluent was planned to be the equivalent of 140 kg/ha. However, due to an error in the calculations, only the ERT20 and ERT30 treatments provided approximately the planned amount of N (Table 2). Applications of the effluent were made every second day on 10 occasions, beginning on day 1 through to day 21, using the same total volume of liquid (water and effluent) on each plot. The exact amounts of effluent and water were in the ratio 1:2. The effluent was applied at the same time for each treatment, at 5.00 pm because at this time the evaporation is lower.

 

Water irrigation

Watering was used to apply water twice a day (7:00 am in the morning and 5:00 pm in the afternoon) at the rate of 4 litres/ m²). On rainy days no water was applied.

Data collection and analyses

 

Source of the soil: Sample of soil was taken from each plot before planting and after harvest for analysis of DM, OM, pH, Nitrogen (N), after taking samples, were mixed by treatments

Effluent: samples were kept of every application to the plots of water spinach and analyzed for pH, DM, OM, Nitrogen and ammonia nitrogen (NH₃-N).

 

Water spinach: Height of the water spinach was measured every week before applying the fertilizers. At harvest after 28 days, the green biomass is weighed and analyses for DM, OM and Nitrogen. All plants in individual plots were weighed. Leaf and stem was separated and analysed to determine dry matter by microwave radiation (Undersander et al 1993), nitrogen by Foss Tecator kjeldahl apparatus (AOAC 1990) and OM by converting from ash (Combustion in Furnace) to determine OM = DM-Ash.

Statistical analyses

The data will be subjected to analysis of variance (ANOVA) by using the General Linear Model (GLM) of the MINITAB software (Release 13.31, 2002). 

Result and Discussion

 

Height growth of water spinach

 

Height grow at the first planting must be better and are faster than second planting. In first planting, treatment from effluent 20 days retention time is height in daily grow rate if compare to other two treatments and significantly (P =0.02) in between three treatments. Second planting height growth are not different from each treatment (P = 0.526).  This is because of the effect of rain-fed very often in rainy season and washed nutrient through soil erosion from the plots, so that the second planting, the growth rate is lower than the first planting (table 3, figure 3). The maximum high growth of first planting 59.0 cm of  and the second planting the maximum is 46.7 cm  from treatment, effluent 20 days retention (Figure 1 and 2).

 

Table 3: Height growth of water spinach from first and second planting, cm
First planting	Treatment, effluent retention time, days
	ERT 10	ERT 20	ERT 30	SEM	Prob.
Days
7	7.02	6.29	6.39	0.477	0.546
14	17.7	16.5	16.02	1.141	0.596
21	35.1	35.0	31.9	1.832	0.432
28	53.8	59.0	53.8	1.290	0.045
Daily height growth, cm	2.25	2.52	2.26	0.054	0.02
Second planting
7	6.49	7.09	6.53	0.415	0.551
14	18.7	20.4	19.2	1.054	0.531
21	36.3	39.5	34.2	2.130	0.285
28	43.1	46.7	42.4	2.972	0.582
Daily height growth, cm	1.82	1.96	1.75	0.129	0.526

 

 

 

 

 

Yield of water spinach

 

The high yield of water spinach was 26.44 tones fresh biomass/ha in 28 days of growth from treatment of 20 days of effluent retention time at the first planting.  On a yearly basis, assuming water is available, this is equivalent to 317.28 tones fresh biomass/ha, which with a dry matter content of 8.35% is 26.49 tones of dry matter a year, while the highest yield of the second planting is 16.7 tones/ harvest of 28 days period so it is equivalent to 200 tones/ year that is 16.5 tones/ ha/ year in DM, which is DM of 8%. There is different yield from Kean Sophea and Preston (2001) the yield is 18.6 tones fresh biomass/ha in one month of growth equivalent to 223 tones fresh biomass/ ha/ year, with a dry matter content of 8% is 18 tones of dry matter. This indicates there is a very high potential for the use of water spinach as a suitable crop to develop the use of biodigester effluent in integrated farming systems in rural Cambodia. Kean Sophea and TR. Preston (2001) researched by planting water spinach and applied effluent and urea with no fertilization and 75 kg N/ ha. 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 standard to different levels of N as effluent from pig manure. The fresh biomass yield was linearly related to level of effluent N (R²= 0.96) the yield is reaching 23.6 tonnes/ha with140 kg N/ha.

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,

There was an interaction (P=0.001) between growth by treatment and effects of the retention time of effluent fertilizer on final biomass harvesting (Figures 4 and table 4). Water spinach had different height growth on all effluent from different retention time. The effluent application, which were different amount of the total nitrogen in table 2 are 10, 20 and 30 days retention time as following 102, 144 and 129 N kg /ha.

Table 4: Yield of water spinach from different retention time of effluent biodigesters
	Effluent from different retention time, day
	ERT10	ERT20	ERT30	SEM	Prob
The first planting
per plot, kg	3.28	3.96	3.08	0.088	0.001
Tons per ha/harvest	21.8	26.4	20.5	0.588	0.001
The second planting
per plot, kg	1.83	2.5	2.2	0.184	0.108
Tons per ha/harvest	12.2	16.6	14.6	1.224	0.108

Sath Sonetra  et al (2002) studied on waste water from row rubber, cow manure and chemical fertilizer, the yield of water spinach had response with increasing level of nitrogen from 0 up to 400kg N/ ha, the yield was similar between waste water and NPK and lower yield with Cow manure( 26.84, 28.58 and 24.22 tonnes/ha) was higher than in biomass by applied effluent from biodigester but also high nitrogen application until 400kg/ha. If compared with treatment from 20 days effluent retention time in biodiester with 144 kg N /ha is similar with treatment cow manure 26.44tonnes/ ha.

The application are different levels of effluent N as the main plots (15, 30, 45 and 60 kg/ha) association of the cassava and water spinach. Yield of water spinach was positively and linearly related to the level of biodigester effluent and was higher on the complete defoliation treatment. . The yield increased from 2.1 to3.54 tonnes fresh biomass/ha as the effluent N was increased from 10 to 60 kg N/ha (Sokphoun Sopheak and T R Preston, 2002)

 

There was no interaction between substrate and level of N fertilization. The results indicated that soil revealed to be a better substrate than sand. Biomass yield, protein yield and growth response of water spinach to N fertilization was curvilinear and the best treatment (P<0.01) was a level of 50 kg N/ha. Biomass and protein yield were 2.53 and 1.70 t/ha for sand and 2.86 and 1.97 t/ha for soil (Krailas  Kiyothong, 2001)

 

Ngo Tien Dung (2001) Biomass yields and protein content of water spinach increased linearly (P<0.02) with increasing levels of the effluent and latex waste water. At the highest level (40 kg N/ha of effluent) the biomass yield of water spinach reached 2.44 tones/ha and crude protein content reached 29.6% in dry matter.  The dry matter of water spinach decreased linearly (P=0.001) as effluent level increased.

 

Meanwhile the research on Ipomoea aquatic is sowing seed directly into the bed. Plants are spaced 12 cm apart and fertilized heavily with organic materials and when rainfall is not adequate, the crop is irrigate. Harvest with the entire plant can be made 50- 60 days after planting and after the first cutting, the harvest can be made every 7 to 10 days. Annual yield is 90 tonnes from wet culture equivalent to13.56 tonnes /ha with 55 days interval harvest (Yamaguchi 1990)

All the research results above are different yield because of the fertilizers, soil property, temperature or sunlight, quality and species of seed and wet condition. 

 

 

Proportion of leaf and stem

 

The 10 plants of water spinach were selected three times of sampling from each plot until get 10 plants, the leaves and stem was separated and analysis immediately.

The interaction of stem and leaf are high different in DM (P=0.002), nitrogen (P=0.004) but not different between treatments (P>0.05).

 

 

 

 

The contents of dry matter and N in the water spinach were similar to the values reported by Kean Sophea and Preston (2001) and Sath Sonetra et al (2002). The leaves were low in DM and contained more nitrogen than the stems and petioles (Table 5) with 4.53, 4.3 and 4.28 % N dry matter from 10, 20 and 30day of effluent retention times with nitrogen application 102, 144 and 129 N kg/ha at the first planting and second planting.

The proportion of the leaf and stem in DM, and N as percentage are different, was high in the first planting. The stems of treatment of 10, 20 and 30 days (effluent retention time in biodigester) were 66.64, 65.86 and 61.53 % was not different and the second planting were 71.8, 72.6 and 60.1% were not different but the first experiment were lower than in the second experiment.

Dry matter in the stem in table 5, there was very little varying of the two plantings. Dry matter in the leaf from the first planting was higher in the second planting. Nitrogen of the stem were higher is in the first planting but nitrogen of the leaf was not different (table 5).

 

 

 

   

(** St: mean Steam, L: leaf, ERT: effluent retention time of...)

 

Soil property

 

Soil analysis was taken before and after planting, DM, OM and N, there was increased from before and after first planting in soil characteristics as in table 6, and decreased when the second planting was ready to finished in soil properties and fertilizing effluent were the same in both planting in table 2. The main effect on the second planting because in during this experiment was rainy very often most of heavy rain.   

 

                               

 

There were differences among fertilizer treatments in the soil pH or in the organic matter content after harvesting of the water spinach (Tables 6).  However, on all fertilizer treatments the soil pH was almost increased the soil pH higher when before and after growing the water spinach (table 6).  By contrast, the organic matter content increased when fertilizer was applied. There were no changes for the three effluent treatments. 

Nitrogen contain in the soil at the beginning higher and then after the first planting and second planting was lower this was logical because of plants up taking the nutrient  and it increased the organic matter (table 6).

 

5. Conclusion and Recommendation

·         Effluent from 20 days hydraulic retention time in biodigester can be used a good fertilizer for water spinach production, and improve soil productivity.

·         The high yield of the water spinach from the 20 days effluent retention in biodigester compared with the first and the second planting

·         The average yield of the first planting is higher than the second planting because of rainfall washed the nutrient.

 

References

 

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