Assessing the Optimum Harvesting Stage of Tithonia diversifolia as Climate Smart Soil Amendment for Coconut Plantations ^†

Abstract

Tithonia diversifolia is often grown as a cover crop or as a green manure crop in climate-smart agriculture practices. This plant can be harvested at various growth stages, and the biomass can be incorporated into the soil. The decomposition of plant biomass enhances the soil’s nutrients, organic matter content, and crop productivity. This study aimed to determine the best harvesting stage of T. diversifolia to be used as an efficient soil amendment for coconut plantations. Samples were collected at one-, two-, three-, and four-month harvesting stages from an existing T. diversifolia field at Rathmalagara Research Station of the Coconut Research Institute of Sri Lanka. In the study, both plant-growth parameters and the nutrient composition of each plant part were individually evaluated for every section of the plant. Biochar was prepared from hardwood stems of T. diversifolia using them as the feedstock under five different temperatures from 300 ℃ to 700 ℃, and a proximate analysis was performed for the characterization of produced biochar. The mean values of measured parameters of T. diversifolia and the properties of biochar were significantly different (p < 0.05) at different growth stages and temperatures, respectively. Considering all the measured parameters of T. diversifolia, the three-month harvesting stage can be suggested as the best growth stage for it to be used as green manure. According to the proximate-analysis results, and by observing the half-burning of produced biochar, 500 ℃ can be proposed as the ideal temperature to produce biochar from hardwood stems.

1. Introduction

Tithonia diversifolia is a multipurpose shrub from the Asteraceae family, and known as the Mexican sunflower, wild sunflower, or tree marigold. Typically, this is a woody herb or succulent shrub that can reach up to 10 feet in height [1]. T. diversifolia is popularly known for its large, bright orange or yellow flowers which resemble those of the sunflower. Tithonia originated in Mexico and is now extensively distributed throughout Central and South America, Asia, and Africa. This species frequently grows along roadsides, farm boundaries, and native fallow systems [2]. It is a common plant in Sri Lanka in upcountry areas and is considered a weed [3]. T. diversifolia is frequently used as an ornamental plant. In addition, it has been used for various purposes around the globe, including as a source of fuel, a raw material to produce compost, for land demarcation, to control soil erosion, as building materials, as a shelter for livestock animals, and as fodder for ruminants [4]. T. diversifolia is important for its high nutrient content. It has a considerable amount of potassium (K) compared to nitrogen (N) and phosphorus (P) [5]. This high nutrient content is reported due to its cluster-root-like formations, which allows it to scavenge for nutrients in the soil [6]. K is an essential plant macronutrient that is necessary for plant growth, development, metabolism, and yield [7]. Plants absorb K from the soil through their root system. In many agricultural systems, the supply of K from the soil is insufficient to fulfil the plant requirement. Moreover, coconut is a high-K-demanding crop, because the nuts and husk of the coconut contain comparatively higher K content than other parts. Harvesting of nuts for processing or consumption causes removal of K from the field, resulting in low K levels in the soils [8]. Therefore, the application of fertilizer is a must to meet the nutrient requirements. Even though the most common practice of providing nutrients is the application of inorganic fertilizer, it has several limitations and negative impacts. In the current scenario, the main restrictions to access inorganic fertilizer are government regulations and the high price of fertilizer. As a result, there is a huge trend for organic manure, including compost and green manure. T T. diversifolia has been identified as a K-rich plant that has promising character for enhancing soil fertility [2]. Although K concentration in T. diversifolia has been mentioned in many of the literature studies, the best lopping stage with the optimum nutrient content is still not clearly identified. Other than that it is used as a green manure, T.diversifolia performs well if biochar can be produced using its mature stems. Biochar is a carbon-rich product that can enhance soil productivity, carbon storage, and possibly the filtration of percolating soil water [9]. Therefore, this study was aimed at identifying the best stage of T. diversifolia for optimum nutrient content, and also to evaluate the feasibility of using it as a feedstock for biochar production.

2. Materials and Methods

2.1. Location and Sample Collection

The experiment was carried out between November 2022 and March 2023 at the Rathmalagara research station and the Agronomy Division of the Coconut Research Institute of Sri Lanka, Lunuwila (7 20° 37 N, 79 51° 42 E). This research site was situated within the Northwestern Province of Sri Lanka, specifically within the agro-ecological zone designated as the Low Country Intermediate Zone (IL1a).

2.2. Treatments and Experimental Design

The experiment utilized an established mature field of T. diversifolia planted in a double-row system with inter-row spacing of 6 feet and intra-row spacing of 3 feet. Four distinct growth stages, namely one, two, three, and four months of lopping, were considered as treatment factors. These treatments were arranged following a randomized complete block design (RCBD) with three replicates.

2.3. Sampling Procedure and Data Collection

Before initiating the experiment, all the plants were cut at a height of one foot above the ground level and allowed to grow without carrying out agronomic practices. Then, the regrown stems were harvested after one month, two months, three months, and four months as four different lopping stages. Five random samples were taken from each plot. Height of the plant, number of stems, diameter of the stems, fresh weight of leaves, and semi hardwood stems and dry weight of leaves and semi hardwood stems were recorded as growth parameters. All the samples were dried at a 60 °C of temperature until reached to a constant weight. Dried samples were analyzed for their nutrients: N, P, and K. N was determined by Kjeldhal digestion and distillation techniques, and the P was determined by the colorimetric method. An atomic absorption spectrophotometer was used for determining the K [10]. Mature stems of T. diversifolia were subjected to five different temperatures to produce biochar. Stems were cut into small pieces and kept in a muffle furnace at 300 ℃, 400 °C, 500 °C, 600 °C, and 700 °C for 1 h. The produced biochars were analyzed for their conversion efficiency, pH, EC, and available K contents [11].

2.4. Statistical Data Analyses

Statistical analyses were performed using the Minitab 19 statistical software. Normality of all measured samples was checked using the normality and outlier test. The mean, minimum, maximum, standard deviation (SD), standard error (SE), and coefficient of variation (CV) were calculated under descriptive statistics. Finally mean values of the data were compared statistically using the one-way analysis of variance (ANOVA) at 5% significance and the Tukey’s pairwise comparison test.

3. Results and Discussion

3.1. Measuring of Growth Parameters

Table 1. Mean values of growth parameters in T. diversifolia at different harvesting intervals.

Lopping Stage	Height (cm)	No of Stems	Stem Diameter (mm)	Fresh Weight (g)		Dry Weight (g)
				SW	Leaves	SW	Leaves
1 month	74.56 c	61.87 a	7.65 c	289.20 b	408.40 c	22.66 c	38.73 c
2 month	135.73 ab	39.53 b	10.09 b	901.89 a	757.44 a	174.72 b	115.78 ab
3 month	151.73 a	36.33 b	12.02 a	1159.02 a	674.22 ab	319.37 a	135.82 a
4 month	135.73 b	29.73 b	11.40 ab	924.70 a	484.86 bc	327.79 a	80.78 b
p-Value	< 0.001	<0.001	<0.001	< 0.001	<0.001	<0.001	<0.001

Means that do not share a letter are significantly different; (SW—Semi hardwood).

shows the mean values of measured growth parameters. There is a significant effect (p < 0.05) in the different lopping stages on height, number of stems, stem diameter, fresh weight, and dry weight of semi hardwood stems and leaves. The maximum height and the stem diameter were recorded at the three-month lopping stage, and the highest number of stems were recorded at the one-month lopping stage. The minimum height and stem diameter were recorded at the one-month lopping stage. The maximum number of stems were recorded at the one-month lopping stage, while the lowest was recorded at the four-month lopping stage. The highest fresh weight of semi-hardwood stems and leaves were recorded at the three-month lopping stage and the two-month lopping stage, respectively, while both parameters were lowest at the one-month lopping stage. In semi- hardwood stems, an increase was observed until the three-month lopping stage, and it was decreased at the four-month lopping stage. No pattern was observed in the fresh weight of leaves. The dry weight of semi-hardwood stems increased with increasing maturity, and the highest was recorded at the four-month lopping stage, while the lowest was recorded at the one-month lopping stage. The highest dry weight of leaves was recorded at the three-month loping stage and the lowest was recorded at the one-month loping stage. Here, also no trend was observed in the dry weight of the leaves with the increasing maturity. According to the findings presented in reference [2], cultivating T. diversifolia as a green manure crop can yield 5 t/ha⁻¹ of dry matter.

3.2. Measuring of Nutrient Level

Table 2. Mean values of nutrient contents in T. diversifolia at different harvesting intervals.

Lopping Stage	Semi Hardwood			Leaves
	N%	P%	K%	N%	P%	K%
1-month	0.96 a	0.17 b	8.02 a	2.37 b	0.20 b	3.59 b
2-month	0.66 b	0.19 ab	4.96 b	3.55 a	0.31 a	4.81 ab
3-month	0.53 bc	0.25 a	2.70 c	2.39 b	0.42 a	4.18 ab
4-month	0.36 c	0.15 b	1.96 c	3.04a b	0.40 a	5.09 a
p-value	<0.001	<0.004	<0.001	<0.001	<0.001	<0.021

Means that do not share a letter are significantly different.

shows the mean values of nutrients in semi-hardwood stems and leaves. The mean values were significantly different (p < 0.05) at different lopping stages. N content of semi-hardwood stems was reduced with maturity. Therefore, the highest N was recorded at the one-month lopping stage and the lowest was recorded at the four-month lopping stage. Moreover, a previous study found the nutrient concentration tends to be lower in senesced- than green leaves [2]. There was no trend observed in the P content of semi-hardwood stems with increasing maturity. The highest P was recorded at the three-month lopping stage, and the lowest was recorded at the four-month lopping stage. Similar to N content, the K content of semi-hardwood stems decreased with increasing maturity. The highest K was recorded at the one-month lopping stage, while the lowest was recorded at the four-month lopping stage. No trend was observed in the N and K contents of leaves with plant maturity. However, the highest N content in leaves was observed at the two-month lopping stage, and the lowest at the one-month lopping stage. The highest K content in leaves was recorded at the four-month lopping stage, while the lowest was recorded at the one-month lopping stage. The highest P content was recorded at the three-month lopping stage in leaves, while the lowest was recorded at the one-month lopping stage. According to reports by [2,5], the average nutrient concentrations in the green leaves of T. diversifolia were 3.5% N, 0.37% P, and 4.1% K on a dry weight basis.

3.3. Properties of Biochar

Table 3. Mean values of properties of biochar in T. diversifolia at different temperatures.

Temperature	pH	Electrical Conductivity (µs/cm)	Available K%	Conversion Efficiency %
300 °C	7.51 c	535.30 b	1.23 b	53.75 a
400 °C	8.47 b	590.70 b	3.47 ab	38.33 b
500 °C	8.76 b	699.30 b	3.42 ab	28.73 c
600 °C	9.20 a	1042.00 a	4.20 a	23.95 d
700 °C	9.28 a	1042.40 a	4.04 a	23.74 d
p-value	<0.001	<0.001	<0.018	<0.01

Means that do not share a letter are significantly different.

shows the pH, electrical conductivity, available K%, and conversion efficiency of biochar. The mean values of the measured property parameters were significantly different (p < 0.05). The pH electrical conductivity increased with increasing temperature. The highest available K content was recorded at 600 °C, and the lowest at 300 °C. The conversion efficiency decreased with increasing temperature. However, half-burned and low-quality biochar was observed at 3000 C and 4000 C. Therefore, considering the proximate composition, other properties, energy consumption, and cost of production of the produced biochar, 5000 C can be proposed as the optimum temperature to produce biochar using mature stems of T. diversifolia as the feedstock.

4. Conclusions

In conclusion, this study established the potential of T. diversifolia as a valuable multipurpose shrub for soil improvement in coconut plantations within the Sri Lankan context. The investigation revealed that the three-month lopping stage yields the highest nutrient content and biomass production, making it the optimal choice for sustainable soil amendment. Moreover, for the production of biochar from mature T. diversifolia stems, a temperature of 500 °C has been identified as the most suitable. These findings hold significance for the sustainable management of soil fertility in coconut plantations, not only in Sri Lanka, but also in other tropical and subtropical regions where T. diversifolia is prevalent. Future research endeavors could delve deeper into the potential utility of T. diversifolia as a green manure crop, exploring its effects on crop yields and soil health. Such investigations could further enhance our understanding of its practical applications in the coconut sector, thereby contributing to the development of environmentally friendly and sustainable climate smart agriculture practices.