Biodigester effluent as fertilizer for water spinach established
from seed or from cuttings

Procedure

Land preparation

 PVC baskets, lined with polyethylene (capacity about 50 litre, 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.

Planting the water spinach

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).

Fertilizing

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).  

Irrigation

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.

Measurements

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).

Statistical analysis

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.
 

Results and discussion

Water spinach growth

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.

 

Table 3.  Least square means for height due to planting material effect
	4days	8days	12days	16days	20days	24days	Increase, cm/day
Stem	4.21	8.00	13.2	19.6	26.9	28.8	1.452
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

Biomass yield

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), but higher than was found from application.  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).

Figure 2: Response in biomass yield to effluent N level with two sources of plant material

Dry matter and Nitrogen content, N recovery of the water spinach

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).

 

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

Water extractable DM and N

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.

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

N recovery

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

 

Conclusions

• Fresh biomass yields were higher (15 tonnes/ha) when water spinach was established from seed than from stem cuttings (9.18 tonnes/ha).
• Biomass yields 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.

Acknowledgements

This experiment was part of the MSc (2003-05), supported by the MEKARN project financed by sida-SAREC. The author expresses his 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  Dr T R Preston, and his colleagues (Dr J Ly, Mr Chhay Ty and Mr San Thy) for advice and supervision during the conduct of the present study.

References

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