Effect of Soil Amender (Biochar or Charcoal) and Biodigester Effluent on Growth of Water Spinach

Sisomphone Southavong, T R Preston* and Ngo Van Man**

Champasack University

Champasack province, Lao PDR

spdeuk@yahoo.com

^*Finca Ecológica, TOSOLY, UTA (Colombia)
AA #48, Socorro, Santander, Colombia

**Nong Lam University, Ho Chi Minh city, Vietnam

Abstract

A biotest was carried out at the research centre of Champasack University, Lao PDR to determine the effect of biochar, charcoal and biodigester effluent on growth of water spinach. The fifteen treatments in a completely randomized 3*5 factorial arrangement with 3 replications were: soil amender (biochar or charcoal or none) at 40 tonnes/ha and level of effluent (0, 25, 50, 75 or 100 kg N/ha) applied to samples of soil held in fifteen litre capacity plastic baskets. Sixty seeds of water spinach were planted in each basket. After germination, some seedlings were removed to balance the number in each basket (40 seedlings) for the rest of the experiment. The plants were irrigated every morning and evening. Measurements were made of height, number of leaves, and weight of above-ground biomass after 35 days and again (re-grow) after a further 35 days.

Both soil amenders (biochar and charcoal) gave similar improvements in water holding capacity of between 27 and 38%. Soil pH was increased from 4.7 to 6.6 due to addition of biochar.  Biochar increased foliage yield of the water spinach in both the first and second harvests, but there was no apparent effect on foliage growth from application of charcoal.  In the first harvest, there were curvilinear responses to biodigester effluent for biochar and charcoal amenders, with the peak occurring at between 50 and 75 kg N/ha. For the un-amended soil the response was linear with the highest yield at 100 kg N/ha. In the second harvest, the response to effluent for the biochar amender was again curvilinear with the peak at 50-75 kg N/ha; by contrast the response to effluent with the charcoal amender was linear with maximum yield requiring 100 kg N/ha. On the un-amended soil there was no relationship between effluent level and biomass yield.

Key words: biotest, TLUP gasifier stove, rice husk, soil pH, water holding capacity

Introduction

The world faces with major perturbations, a financial crisis precipitated by simultaneous and interrelated/interactive events including Peak Oil (the end of inexpensive energy), other global resource depletion and climate change all of which are undermining world food economy. There is an urgent need to respond to these challenges in order to produce and deliver food to maintain the present world population, let alone the increased population predicted by 2030 of 8-10 billion people (Leng 2009).

Water spinach (Ipomoea aquatica) is a vegetable that is consumed by people and animals; it has a short growth period, is resistant to common insect pests and can be cultivated either in dry or flooded soils. Moreover, it has been found that water spinach has a high potential to convert nitrogen from biodigester effluent into edible biomass with high protein content (Kean Sophea and Preston 2001). Hongthong Phinmasan et al (2004) reported that water spinach as the only source of feed for growing rabbits appears to support acceptable growth rates of close to 20 g/day with a DM feed conversion of 2.7. This simple feeding system may be attractive for small-holder farmers in the tropics, due to the possibility to raise rabbits with a local resource (water spinach) that is easy to grow and needs no processing.

The fertility of soils is important in agriculture particularly in making decisions on planting of crops. Terra Preta ("black earth") was discovered by Dutch soil scientist Wim Sombroek in the 1950's, when he discovered pockets of rich, fertile soil in the Amazon rainforest (otherwise known for its poor, thin soils). Carbon dating has shown them to date back between 1,800 and 2,300 years (Glaser et al 2002).

Biochar is a charcoal produced from biomass, by a process known as pyrolysis. Pyrolysis means heating in the absence of oxygen, which prevents complete burning of the organic biomass (which happens in open fires). It is a stable solid and rich in carbon content, and thus can be used to lock carbon in the soil (Lehman 2007).

The application of biochar (charcoal or biomass-derived black carbon (C)) to soil is proposed as a novel approach to establish a significant, long-term, sink for atmospheric carbon dioxide in terrestrial ecosystems. Apart from positive effects in both reducing emissions and increasing the sequestration of greenhouse gases, the production of biochar and its application to soil will deliver immediate benefits through improved soil fertility and increased crop production (Lehman  et al 2006). Moreover, some researchers claim that biochar may be an immediate solution to reducing the global impact of farming (and in reducing the impact from all agricultural waste). It has been shown that biochar has multiple uses, when added to soil it can significantly improve soil fertility and also act as a sink for carbon (Lehmann 2007). In this way, the carbon is removed from the atmosphere in a process called sequestration (Zwietenoe 2006; Davies 2007).

The increase in crop yield with biochar application has been reported elsewhere for crops such as cowpea (Yamato et al 2006), soybean (Tagoe et al 2008), maize (Yamato et al 2006; Rodríguez et al 2009), and upland rice (Asai et al 2009). Haefele (2007) and Haefele et al (2008) discussed the possibility of biochar applications for rice-based cropping systems. Reichenauer et al (2009) applied biochar in tsunami-affected paddy fields in Sri Lanka, and the experimental results showed that the application of 2 tonnes rice-husk-biochar per ha increased the grain yield from less than 4 tones per ha for the control treatment to more than 5 tones per ha for the biochar treatment. Boun Suy Tan (unpublished data) has indicated clearly that applying biochar (from a gsifier stove) to the soil at 40 T/ha in combination with compost could triple the yield of rice from 1,252 to 3,756 tonnes/ha.

Besides that, biochar can act as a soil conditioner enhancing plant growth by retaining nutrients and by providing other services such as improving soil physical and biological properties (Glaser et al 2002; Lehmann and Glaser 2003; Lehmann and Rondon 2005).

The pH of biochar produced by gasification of bagasse and rice husks is 9.5 (Kong Saroeun and Preston 2008) and biochar produced from rice husk by gasifier stove is 9.8 (Southavong and Preston 2011). As these soil conditioners have high pH value, they should be used in the low pH soil (acid soil) because they can increase the pH of the soil and thus increase the yield of acid sensitive crops (Lickacz 2002; FFTC 2008).

Charcoal is a black substance that resembles coal and generally made from wood that has been burnt, or charred, while being deprived of oxygen so that what's left is an impure carbon residue. Charcoal helps to clean the soil of pollutants, it also acts as a soil conditioner. It is used as a top dressing for gardens, bowling greens and lawns. Charcoal also acts as a substitute for lime in soil additives because of the potash content, and it can be a little cheaper than lime (http://www.buyactivatedcharcoal.com/natural_fertilizer). In addition, Ogawa (1987) reported that charcoal applied to the soil could stimulate the activity of soil microorganisms and promote the formation of root nodules and vesicular-arbuscular mycorrhizae in soybean roots.

Effluent is one kind of nutrient-rich fertilizer which is the liquid waste from low cost biodigesters that farmers can set up for multipurpose utilization in their family. When applied to vegetables and plants, it can lead to increases in biomass yield and a higher content of crude protein.  Examples of these effects were observed in Chinese cabbage (San Thy and Pheng Buntha 2005), water spinach (Kean Sophea and Preston 2001; Ho Bunyeth and Preston 2004; Nguyen Van Hiep and Preston 2006), mulberry (Phiny et al 2009), cassava (Le Ha Chau 1998), maize (Rodríguez et al 2009; Sokchea and Preston 2011) and rice biomass (Southavong and Preston 2011).

Objectives

·         Evaluate the effect of biochar and charcoal as soil amenders in combination with effluent from biodigester charged with pig manure on growth of water spinach (biotest).

·         To test the long-term effect of biochar application to soil in improving soil and crop production.

Materials and Methods

Location, duration and climate of the study area

The experiment was conducted at the at the research centre of Champasack University, about 13 km far from Pakse City, Champasack province, southern Laos. The trial was covered the period of March to May 2011. The climate in this area is tropical monsoon with a rainy season between May and October and a dry season from November to April. The mean air temperature is 28.2°C. Average annual rainfall is 2,000mm/year.

Experimental design

The experiment was arranged in a completely randomized design (CRD) as a 5*3 factorial with 3 replications (Table 2 and Photo 1).

The factors were:

·         Level of biodigester effluent: 0, 25, 50, 75 or 100 kg N/ha

·         Soil amender: biochar, charcoal at 4kg/m² or 40tones/ha or none

Materials

The biochar (Photo 2) was produced locally by burning rice husks in a top lid updraft (TLUD) gasifier stove (Olivier 2010) (Photo 4). Charcoal was bought locally from the farmer nearby the University campus. The effluent were taken from biodigester of a “plug-flow” tubular polyethylene with UV filter (5 m³ liquid volumes) (Photo 5) charged daily with washing (1.5 m³) from pig pen holding on average 21 pigs of 50 kg mean live weight. Water spinach seeds were bought locally from the market.

Procedure and data collection

The 15 kg of acid soil with or without soil amender (biochar and charcoal) were put into the plastic baskets (size 35*48cm) according to the experimental layout in table 2. Water spinach seeds (dry-land species) were soaked in water over night before planting for better germination. Sixty seeds of water spinach were planted in baskets. After germination, some seedlings were removed to balance the number in each basket (40 seedlings) for the rest of the experiment. The distance between rows was 8cm and 2-3 cm between seeds. The baskets are set up with the net so that the excess water can drain away easily (Photo 6). The water was applied uniformly to all baskets every morning and evening. In raining day additional water was not applied. Colour of plant, germination and growth of plants were observed every day.

The heights of the plants, number of leaves were measured every 7 days over a total period of 35 days. At the end of the trial, green biomass (leaf + stem) was harvested and weighed and again (re-grow) after a further 35 days were measured; then samples of foliages were analysed for dry matter (DM) content. Samples of soil were analysed at the beginning and end of the trial for pH, OM, water holding capacity, N. Biochar and charcoal were analysed for DM, pH and ash content.

Fertilizing

The fertilizer (biodigester effluent) was applied at the beginning and every 7 days interval for 35 days (total of 5 times) during the growing period. The quantities were calculated according to the N content of the effluent based on the treatments over the 35 days period (20% in each time). For the re-grow period, there was no additional of effluent applied.

Chemical analysis

The DM content of the water spinach and soil samples was determined using the micro-wave radiation method of Undersander et al (1993). Organic matter (OM) and N of soil and effluent were determined by AOAC (1990) methods. The pH of soil was determined using digital pH meter.

Statistical analysis

The data were analyzed according to the General Linear Model option in the ANOVA programme of the Minitab (2000) software.  Sources of variation were effluent, soil amender, interaction effluent*soil amender and error. Tukey test in the Minitab software were used to separate mean values that differ when the F-test is significant at (P<0.05).

Results and discussion

Chemical composition of experimental materials

The pH content of biochar was higher than from charcoal (Table 3) and the result are similar to those reported by Southavong and Preston (2011). The OM content was higher for charcoal than from biochar (Table 3). The N content of the effluent was considerably much lower compared to those reported by Rodríguez et al (2009); Southavong and Preston (2011); Sokchea and Preston (2011) the reason for this may have been the newly installed of the biodigester and feed sources of pigs.

Water holding capacity

Both soil amenders (biochar and charcoal) gave similar improvements in water holding capacity of between 27.4 and 38.7% (Table 4). The value was considerably lower than was reported for biochar obtained from an updraft gasifier in Colombia charged with sugar cane bagasse and biochar derived from TLUD gasifier stove (Southavong and Preston 2009) in their study they compared two types of biochar and 5 different levels range from 0 to 8% and the results showed that the value was between 37.9 to 59.6%. The difference can probably be explained by the difference of the soil properties, it was much higher of WHC in the soil in their experiment (37.9%) compared to the present experiment (27.4%). Similar finding was reported by Sokchea and Preston (2011) the WHC was increased from 43 to 62% when biochar was added to acid soil.

Effect of biochar and effluent on water spinach biomass yield

Biochar increased foliage yield of the water spinach in both the first and second harvests, but there was no apparent effect on foliage growth from application of charcoal.  In the first harvest (Figure 3; Table 5), there were curvilinear responses to biodigester effluent for biochar and charcoal amenders, with the peak occurring at between 50 and 75 kg N/ha. For the un-amended soil the response was linear with the highest yield at 100 kg N/ha. In the second harvest (Figure 4; Table 5), the response to effluent for the biochar amender was again curvilinear with the peak at 50-75 kg N/ha; the biochar showed the long term effect in improving the biomass yield of WS in agreement with Sombroek et al (2003); Glaser et al (2002); Lehmann and Glaser (2003); Lehmann and Rondon (2005) reported that when biochar is applied to soil and it helps to retain the nutrients in soil and remain available to plants thus increase the plant growth and yield; by contrast the response to effluent with the charcoal amender was linear with maximum yield requiring 100 kg N/ha. On the un-amended soil there was no relationship between effluent level and biomass yield.

Effect of soil amender on soil pH

The pH of the soil was significantly increased when biochar was applied and there were effect on soil pH due to level of effluent (Table 6). In the research reported by Rondon et al  (2007) the biochar was made by pyrolysis of eucalyptus logs and contained only 0.3% of ash. Their data showed an increase in soil pH from 5.0 to 5.4 after applying 40g biochar per 1 kg of soil, much less than the increase from 4.7 to 6.6 in our experiment.

Conclusions and recommendations

·         Biochar increased foliage yield of water spinach in both first and second harvests but there was no apparent effect on foliage growth from application of charcoal.

·         Soil pH was increased from 4.7 to 6.6 due to addition of biochar.

·         Both soil amenders (biochar and charcoal) gave similar improvements in water holding capacity of between 27 and 38%.

Acknowledgement

The authors would like to express their sincere thanks to Sida MEKARN program funded by sida SAREC project for financial support, special thanks to Dr. Phetsamay Vyraphet for its useful
advises and Champasack University for providing the study site to carry out this trial.

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