Citation of this paper

Biochar as amendment to soil growing cassava for foliage

Outhomphone Senviset and T R Preston¹

Savannakhet University, Lao PDR
outhomphone8@hotmail.com
¹ Centro para la Investigación en Sistemas Sostenibles de Producción Agropecuaria (CIPAV), Carrera 25 No 6-62 Cali, Colombia

Abstract

The design of this experiment was a 2*2*5 factorial with 3 replications in a split-plot arrangement. The main plots were biochar levels of 0, 0.5, 1, 1.5 and 2 kg/m² of soil; the split plots were cassava variety (sweet or bitter) and partial peeling (or no peeling) of the cassava stems prior to planting.

The rate of germination was much higher for stems from bitter compared with sweet variety of cassava, and was higher when the stems were partially peeled, but was not affected by soil amendment with biochar. Yields of all components of the above-ground biomass were much higher for the bitter than the sweet cassava variety. There were no effects on foliage yield due to level of biochar. Soil pH, water-holding capacity, content of organic matter and of nitrogen were all enhanced with linear trends as the level of biochar added to the soil was increased.

Key words: foliage, germination, planting method, variety

Introduction

Cassava is a staple food crop in Lao PDR. However, it has now become the second most important crop in Lao PDR, as a result of the development of industrial extraction of starch from the cassava roots. It has also recently been shown that the byproduct pulp from cassava starch extraction, as well as the whole cassava root, can be the basis of intensive fattening systems of local cattle (Phanthavong et al 2016a,b; Sengsouly and Preston 2016). The cassava foliage can also be used as the basal diet for intensive feeding of goats (Sina et al 2017).

These developments have encouraged the introduction of improved cassava varieties with higher yield potential. Most of the new varieties are considered to be “bitter” varieties because of the higher concentrations of cyanogenic glucosides. The sweet varieties are those used for human consumption.

However, there appears to be no information on the relative responses of both “bitter” and “sweet” varieties to new agronomic practices such as the use of biochar as soil amender (Lehman and Joseph 2011; Preston 2015).

The objective of the experiment to be reported in this paper was to study the response of bitter and sweet varieties of cassava to application of increasing levels of biochar. Cassava was established by planting short lengths of stems directly in the soil. Possible advantages from limited peeling of the stems to encourage germination were also examined in view of the successful use of this technique for hastening the germination and survival of stem cuttings of the mulberry tree (Morus alba) (Moreno et al 2005; Bouaravong et al 2017).

Materials and Methods

Location

The experiment was conducted on Nongpheu campus, Savannakhet University, Lao PDR. The trial covered a period of 150 days from 12 June to 8 November 2016.

Experimental design

The design was a 2*2*5 factorial with 3 replications in a split-plot arrangement. The main plots were biochar levels of 0, 0.5, 1, 1.5 and 2 kg/m²; the split plots were cassava variety (sweet or bitter) and partial peeling (or no peeling) of the cassava stems prior to planting.

The total area was 240 m² (6*40 m). The plots were 1.5*1.5m; the distance between each replication and plots was 0.5 m; the distance between rows in each plot was 0.5 m with 0.5 m between individual stems.

Procedure

The cassava stems were from cassava plants 8-12 months old. They were cut into lengths of 20 cm and inserted vertically 10 cm into the soil. The first harvest of foliage was made at 60 days and the second harvest 90 days later.

Biochar was produced from rice husk combusted in a gasifier stove (Photos 1 and 2). It was mixed into the top 10 cm of soil according to the treatments.

Urea was applied as fertilizer at the rate of 50 kg N/ha. The plots were irrigated every evening. Weeds were removed on days 31 and 91.


Photo 1. Burning rice husk to make biochar	Photo 2. Biochar from rice husk


Photo 3. Arrangement of the plots	Photo 4. Appearance of the soil with biochar levels from 0 to 2 kg/m²


Photo 5. Bitter cassava with biochar (2 kg/m²) after 60 days of growth	Photo 6. Sweet cassava with biochar (2 kg/m²) after 60 days of growth	Photo 7. Filtering of biochar after 24h suspension in water

Data collection

Samples of soil were analyzed at the beginning and end of the trial for DM (drying at 100^oC for 24h), pH and water holding capacity (WHC). WHC of the soil and water retention rate (WRR) of the biochar were measured by suspending 1 kg of soil/biochar in 5 liters of water for 24h. The suspension was then filtered (Photo 7) to determine the amount of water retained by the soil/biochar. WHC was exprssed as percentage of water in the saturated soil. WRR was expressed as liters of water/kg biochar.

Observations were made of germination of the stems and the height of the plants. At harvest the above-ground biomass was separated into stems, leaves and petioles. AOAC (1990) methods were followed for analysis of : DM and crude protein in each component of the cassava foliage, and of ash and N in the soil and the biochar.

Statistical analysis

The data were analyzed using the GLM option of the Minitab (2000) ANOVA software. Sources of variation were; biochar level, sweet or bitter cassava, peeling or no peeling the stems; interaction biochar level*sweet or bitter cassava, and error.

Results

Chemical composition of the soil and the biochar

The soil was acid and low in organic matter and nitrogen (Table 1).The water retention rate (WRR) of the biochar was similar to the value reported by Orosco et al (2018) for biochar produced from rice husks.

Table 1. Composition of soil and rice husk biochar.
	DM, %	pH	WHC, %	WRR	OM, %	N, %

Soil	86.6	5.40	42.5		0.80	0.05
Biochar	93.5	8.61	-	4:1		-

Germination

The rate of germination was much higher for stems from the bitter compared with the sweet variety of cassava and was higher when the stems were partially peeled (Table 2; Figures 1 and 2) but was not affected by soil amendment with biochar.

Biomass yield

Yields of all components of the above-ground biomass were much higher for the bitter than the sweet cassava variety. There were no effects on foliage yield due to level of biochar (Tables 2 and 3, Figures 3 and 4) and no interaction (p= 0.71) between variety and level of addition of biochar.

Table 2. Mean values for effects of variety and peeling of the cuttings on germination, height and biomass yield of bitter and sweet cassava
	Cassava		p	Peeling		p	SEM
	Bitter	Sweet	p	No	Yes	p	SEM

Germination, %	96.3	55.6	0.001	71.9	80.0	0.05	2.8
First harvest
Height, cm	72.4	60.3	0.001	67.3	65.4	0.62	2.69
Fresh biomass, kg/m²
Total	2.65	0.99	0.001	1.94	1.7	0.23	0.054
Stem	0.89	0.31	0.001	0.65	0.55	0.23	0.048
Petiole	0.74	0.31	0.001	0.58	0.47	0.11	0.060
Leaves	1.03	0.37	0.001	0.72	0.68	0.62	0.20
Second harvest
Height, cm	237	230	0.33	233	234	0.82	4.9
Fresh biomass, kg/m²
Total	12.5	9.16	0.001	10.5	11.1	0.50	0.67
Stem	6.27	5.00	0.02	5.43	5.84	0.46	0.38
Petiole	2.29	1.41	0.001	1.66	2.04	0.15	0.19
Leaves	3.62	2.50	0.001	3.11	3.01	0.70	0.19

Table 3. Mean values for effect of biochar on germination, height and biomass yield of cassava grown for forage (average of bitter and sweet varieties)
	Biochar, kg/m²					p	SEM
	0	0.5	1.0	1.5	2.0	p	SEM

Germination, %	80.6	80.6	80.6	80.6	80.6	0.77	0.40
First harvest
Height, cm	63.8	69.8	67.9	62.1	68.1	0.68	4.25
Fresh biomass yield, kg/m²
Total	1.92	1.95	1.83	1.67	1.71	0.87	0.23
Stem	0.66	0.63	0.63	0.53	0.55	0.78	0.09
Petiole	0.53	0.52	0.58	0.5	0.5	0.29	0.08
Leaves	0.74	0.81	0.63	0.65	0.67	0.52	0.09
Second harvest
Height, cm	232	231	237	222	245	0.35	7.7
Fresh biomass yield, kg/m²
Total	11.1	10.7	11.2	9.55	11.6	0.71	1.07
Stem	5.8	5.3	5.93	5.03	6.12	0.68	0.60
Petiole	1.75	1.67	1.85	1.72	2.27	0.64	0.30
Leaves	3.30	3.19	2.97	2.38	3.48	0.12	0.31


Figure 1. Germination was much higher for the bitter variety of cassava than the sweet one and was higher when the stem was partially peeled	Figure 2. Germination was not affected by application of biochar to the soil


Figure 3. Yield components of cassava were not affected by biochar	Figure 4. Biomass yield of cassava was higher for bitter than sweet variety

Soil fertility

Soil pH, water-holding capacity, content of organic matter and of nitrogen were all enhanced with linear trends as the level of biochar added to the soil was increased (Table 4; Figures 5, 6, 7 and 8).

The soil pH was increased linearly from 5.40 to 6.92 as the level of biochar was increased from 0 to 2.0 kg/m² (Tables 1 and 4, Figure 5). Similar responses have been reported by many researchers (eg: Rodríguez et al 2009; Southavong et al 2012; Dao et al 2013). Water holding capacity was increased by application of biochar from 42.5 to 54.4% (Tables 1 and 4, Figure 3). The result was similar to that reported by Sokchea et al (2011) and Southavong et al (2011). The organic matter in the soil was increased from 0.80 to 1.43% at the biochar level of 2 kg/m² (Tables 1 and 4 and Figure 4). The soil nitrogen was increased ten-fold (from 0.12 to 1.53%) when biochar was added to the soil at 2 kg/m ² (Table 4; Figure 8).

Table 4. Mean values for effect of biochar on soil pH, water-holding capacity (WHC), organic matter and nitrogen content
	Level of biochar, kg/m²					p	SEM
	0	0.5	1.0	1.5	2.0	p	SEM

pH	5.95	5.98	6.56	6.59	6.92	0.04	0.25
WHC, %	42.3	46.3	47.3	50	54.4	<0.001	1.44
OM, %	0.85	1.1	1.17	1.4	1.43	<0.001	0.05
N, %	0.12	0.57	0.89	1.09	1.53	<0.001	0.11


Figure 5. Biochar increased the soil pH	Figure 6. Biochar increased the water holding capacity of the soil

Figure 7. Biochar increased the organic matter content of the soil	Figure 8. Biochar increased the nitrogen content of the soil

Discussion

The positive effects of adding biochar to the soil on pH, water-holding capacity, organic matter, nitrogen and pH are in line with the report of Chittavong et al (2017) who recorded similar effects of biochar amendment in the same soil type. However, while these soil improvements were associated with increased biomass yields - in the case sugar cane (Chittavong et al 2017) - in our experiment, biomass yield of cassava was not affected. The quality of the biochar was not considered to be a factor as the water retention rate (WRR = 4:1), assumed to be indicative of the capacity of the biochar to adsorb nutrients/microbes, was of the same magnitude as was reported by Orosco et al (2018) for biochar made from rice husks.

We have no explanation for our results in which soil fertility was improved by addition of biochar but this improvement was not reflected in biomass yield of the cassava grown for forage. Clearly this is an area of research that should be studied comprehensively in view of the increasing importance of cassava cultivation in Lao PDR.

Conclusions

The rate of germination was much higher for stems from bitter compared with sweet variety of cassava and was higher when the stems were partially peeled. Germination rate was not affected by soil amendment with biochar.
There were linear increases in pH, water-holding capacity, and content of nitrogen and organic matter in the soil as the application rate of biochar was increased from 0 to 2 kg/m².
Yields of all components of the above-ground biomass were much higher for the bitter than the sweet cassava variety. There were no effects on foliage yield due to soil amendment with rice husk biochar.

Acknowledgements

This research was done by the senior author as part of the requirements for the MSc degree in Animal Production "Improving Livelihood and Food Security of the people in Lower Mekong Basin through Climate Change Mitigation" in Cantho University, Vietnam. The authors would like to express their appreciation to: the MEKARN program funded by SIDA for providing the opportunity and budget to carry out the study; and to Savannakhet University for the facilities to conduct the experiment. We thank the BSc students and staff of Savannakhet University for their help in facilitating the execution of the experiment.

References

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Received 10 January 2018; Accepted 28 February 2018; Published 1 April 2018

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