Effect of level of urea fertilizer on biomass production of water spinach (Ipomoea aquatica) grown in soil and in water

Ly Thi Luyen and T R Preston*

Goat and Rabbit Research Centre, Bavi, Vietnam
manhluyen@yahoo.com
UTA-TOSOLY, AA#48, Socorro, Colombia
regpreston@utafoundation.org

Abstract

An experiment was carried out at Angiang University campus from 2^nd August 2003 to 23^th August 2003. The main objective was to determine the response in biomass yield of water spinach (Ipomoea aquatica) growing in soil or water and fertilized with five levels of urea (0, 14, 28, 56 and 84 kg N/ha). The design was a split plot with soil or water as the split plot and the five levels of urea as the main plots. The water spinach was grown in baskets of 35 x 45 cm and 20 cm depth. In the soil plots the pH was 7.46 and the N content 0.97%. In all urea treatments the first application was 5 days after planting and thereafter at 4 day intervals. The total period from planting to harvest was 3 weeks. The water spinach was established from seed planted at the rate of 60 g/m². For the water treatment the seed was planted in a 5cm layer of soil in the bottom of the basket. Water was added 5 days after planting when the seedlings were about 4cm in height.

Growth in plant height and in fresh  biomass yield increased in curvilinear fashion according to level of urea N with the maximum yield at 30 kg N/ha. Rate of growth in height was slower (1.31 vs 1.53 cm/day) but fresh biomass yield after 3 weeks was higher (10.4 vs 8.4 tonnes/ha) in water spinach grown in the soil compared with the water. Dry matter and N content was higher in leaves than in stems with no difference between soil and water as the growth medium. There was a linear increase in N content of leaves from 3 to 5.5% in DM as the level of urea N was increased.  Of the total N present in water spinach, 65 to 70% was in the leaves.

It is concluded that water spinach is a vegetable with a high potential to convert efficiently the nitrogen in urea into edible biomass with a high nitrogen content.

Key word: Biomass, Ipomoea aquatica, soil, urea, water,  water spinach

Introduction

According to Van Soest et al (1997) leafy vegetables can contribute with significant amounts of vitamins and minerals, and are especially excellent sources of protein, carotene (vitamin A), iron and ascorbic acid (vitamin C). Nguyen Nhut Xuan Dung (1996) stated that the ash content of fresh vegetables is an important source of trace minerals. Therefore a "home garden" for growing of vegetables is one way to improve the quality of life of farmers by increasing nutritional status and generating income as well.

Vietnam's 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 second in importance and is mainly in lowland areas, especially in the Hong delta, Thai Binh region and other regions near the rivers, where the soils are very fertile. On the other hand, the farm size in Vietnam is generally small especially concerning the vegetable garden and soil is very poor in the sloping land and mountain areas. Therefore, the intensive cultivation and improved productivity per unit area is a key factor for rural economic and social development

The vegetable production in Vietnam is highly 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 three elements of most concern. Leafy vegetables are especially heavy users of nitrogen.

Water spinach (Ipomoea aquatica ) has a very high biomass yield. It is used traditionally in tropical regions for consumption by people and animals. Using water spinach as a protein source for BaXuyen and Large White sows has given good results in feed intake and digestibility (Le Thi Men et al 1999).  Water spinach has a short growth period and is resistant to common insect pests. It can grow both in soil and in water and it is very easy to grow by farmers. The traditional practice is to use urea as fertilizer but there is no information on the optimum level to use.

The following report describes an experiment where water spinach was grown in soil and in water in order to compare growth characteristics and biomass yield of the water spinach as influenced by graded levels of urea as fertilizer

Materials and methods

Location

The experiment was done on the An Giang University Campus and lasted three weeks.

Treatments and design

Two factors were compared in a split plot design. The main plots were 5 levels of N from urea  (0, 14, 28, 56 and 84 kg N/ha), The split plots were water and soil as planting medium. There were 2 replications, arranged as blocks, of each of the treatment combinations (Table 1).

Land and water preparation

Twenty baskets (0.1m deep, 0.35m wide, 0.45m long) lined with polyethylene (capacity about 50 litres) were used (Photo 1). A layer of soil (5 cm) was placed in the baskets in the water treatment to facilitate germination. Water was added to a depth of 20 cm. In the soil treatment the depth of soil was 20 cm. Holes were made in the polyethylene in this treatment so that excess water could drain from the basket.  In both soil and water treatments the urea was applied at 4-day intervals for 3 weeks.  The soil was obtained close to the experimental area.  The pH was 7.46 and the N content  0.097%.

Photo 1: Water spinach growing in the baskets (14 days after planting)

Planting

Seeds of water spinach were  put in water overnight. They were planted on 2^nd August 2003, in rows across the surface of soil in the basket at 2-3 cm depth at the rate of 60 g/m² (10g/basket). The distance between rows was 5 to 7 cm. For the water treatments,  the water was added to the baskets 5 days after planting the seeds.

Fertilization

The urea was applied as a 1% (w/v) solution in 4 equal amounts, the first time after 5 days and again at 4 day intervals during the 20 day growing period (Table 2).

Irrigation

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.

Measurements

Plant height was measured every 4 days. Total biomass was harvested at 21 days and separated into leaves and stems and analysed immediately to determine DM content. DM was determined by micro-wave radiation (Undersander et al 1993) and N according to AOAC (1990).

Results and discussion

The rate of increase in height was greater when the water spinach was grown in water than on soil (Table 3 and Figure 1). Conversely, the yield of biomass was greater when the plants were grown on soil. There was a positive curvilinear response in biomass yield with the optimum at 30 kg N/ha (Figure 2).  The maximum yield of biomass (12.4 tonnes/ha) was lower than the 24 tonnes/ha  reported by Kean Sophea and Preston (2001). These authors also showed that the response in yield was linear up to 140 kg N/ha. However, these authors used biodigester effluent and the growth period was 28 days.  Biodigester effluent provides a balanced array of plant nutrients and this was probably the reason for the higher overall yield and the difference in the response curve to applied N.

Figure 1: Effect of urea N level on growth rate in height of water spinach during 3 week growing period	Figure 2: Effect of urea N level on fresh biomass yield of water spinach at 3 weeks

On a DM basis leaves accounted for slightly over 50% of the plant biomass and this measure was not affected by the growth medium or the level of fertilizer (Table 4). The N content of leaves and stems was similar on both growth media and increased linearly as urea N levels increased (Figure 3). Leaves had almost three times the concentration of  N as stems at low fertilizer N levels and twice as much at higher levels (Figure 4).  At least 28 kg N/ha  needed to be applied in order that the  N exported in the biomass was less than the N in the urea input (Figure 5).

Figure 3: Effect of urea N level and growth media on content of N in the leaves
of water spinach after 3 weeks of growth

Figure 4: Effect of urea N level on the N content of leaves and stems of
water spinach after 3 weeks of growth.

Figure 5: N balance for growing water spinach in soil or water with different levels of urea fertilizer

Conclusions

• The biomass yield  was higher when water spinach was grown in soil rather than in water when fertilized only with urea

• The m•aximum biomass yield was obtained with 30 kg urea N/ha

• •DM and N concentrations were higher in leaves than in stems and increased linearly with increased level of urea-N fertilizer

• •From 65 to 70% of the total N was present in the leaves

• Water spinach is a vegetable with a high potential to convert efficiently the nitrogen in urea into edible biomass with a high nitrogen content.

Acknowledgements

We are grateful to SIDA-SAREC for financing this research through the MEKARN project. We would like to thank  Dr Vo Tong Xuan, Dr Vo Tong Anh and the staff of the Faculty of Agriculture and Natural Resources for providing the facilities. Dr J Ly, MSc San Thy and MSc Chhay Ty are acknowledged for valuable advice and assistance.

References

AOAC  1990  Official Methods of Analysis. Association of Official Analytical Chemists. 15^th edition (K Helrick editor) Arlington pp 1230 

Nguyen Nhut Xuan Dung 1996 Identification and evaluation of indigenous plants for livestock and humans in the Mekong Delta region in Vietnam. MscThesis. Swedish University of Agricultural Sciences, Uppsala

Kean Sophea and Preston T R 2001 Comparison of biodigester effluent and urea as fertilizer for water spinach vegetable. Livestock Research for Rural Development. (13) 6: http://www.lrrd.org/lrrd13/6/kean136.htm

Le Thi Men, Ogle B and Vo Van Son 1999 Evaluation of water spinach as a protein source for Baxuyen and Large White sows and fattening crossbred pigs.  MSc thesis, Swedish University of Agricultural Sciences, Uppsala

Undersander D, Mertens D R and Thiex N 1993 Forage analysis procedures. National Forage Testing Association. Omaha pp:154 

Van Soest P J, Robortson J B and Lewis B A 1991 Methods for dietary fiber, neutral detergent fiber and non starch polysaccharides in relation to animal nutrition.  Journal of Dairy Science 73:2583-3593