The Effectiveness of Dolomite Phosphate Rock on Growth and Yield of Black Sesame (Sesamum indicum L.) in Paddy Field

Sesame, scientifically known as Sesamum indicum L., belongs to the Pedaliaceae family, is an annual plant and is one of the oldest cultivated plants in the world (Wei et al., 2022). The sesame plant is known as the “Queen of Oil Crops” and has high nutritional value (Wang et al., 2014). Sesame seeds contain 57%-63% oil, 23%-25% protein (Brewer et al., 2016), iron, magnesium, manganese, copper, calcium and essential vitamins, such as vitamins B1 (thiamine) and E (tocopherol) (Najeeb et al., 2011). Therefore, sesame is one of the sources of nutrients that improve human health. In the Mekong Delta, Vietnam, sesame is an attractive crop to grow in rotation with rice due to its relatively low nutrient requirements, resilience to low soil moisture (as it is not dependent on post germination irrigation) and heat tolerance (Khuong et al., 2022). However, the sesame area in Vietnam is about 34,683 hectares with an output of 28,774.67 tons and our country’s sesame yield is still relatively low (0.83 tons/ha) (FAO, 2023) because of poor planting and acidic soil caused by the extensive use of chemical fertilizers (Ning et al., 2017; Bhatt et al., 2019), which causes the sesame plant roots to grow poorly developed. Phosphorus is one of the essential macronutrients for sustainable plant growth (Yang et al., 2012), plays a crucial role in reproductive growth (Kim and Li, 2016; Uygur and Şen, 2018). Due to phospho’s low availability and poor recovery from applied fertilizers, phosphorus is the least available macronutrient and hence the most frequently deficient nutrient in most agricultural soils. Phosphorus has a limited availability because, in acidic soil conditions, it easily forms insoluble complexes with cations like aluminum and iron (Hellal et al., 2019). In acidic soil, applying phosphorus helps improve pH and reduce aluminum toxicity. Phosphate rock is relatively slow to release soluble P; however, compared to industrial P fertilizers, phosphate fertilizer is low in price, making it a very appealing fertilizer (Hellal et al., 2019). Moreover, dolomite is rich in calcium and magnesium and it helps improve soil conditions, such as neutralizing pH and soil structure and helps plants develop good roots (Krismawati et al., 2022). According to Chutichude et al., (2010), dolomite is frequently utilized as soil fertilizer in a variety of soils for agricultural applications.. Therefore, the study was conducted to determine the effect of the optimal amount of dolomite phosphate rock (DPR) to increase the growth and yield of black sesame.

A local black sesame variety with traits such as short duration growth (approximately 75-81 days), drought tolerance, high yield (1.2-2.0 t ha^-1) and oil content (47.5%) was used. The DPR comprised 84% dolomite and 16% phosphate rock (21.43% CaO, 5.0% SiO₂). The experiment was conducted at the My An Hung commune (latitude: 10.390361, longitude: 105.631883), Lap Vo District, Dong Thap Province, Vietnam, from February to June 2022. The characteristics of the initial soil were pH_H2O (5.25), pH_KCl (4.13), NH₄⁺ (24.64 mg kg^-1), NO₃  (20.02 mg kg^-1), N_Total (0.19%), P_Total (0.06%) and P_Available (45.83%). The textural class of experimental soil is silty clay. 
               
The field experiment was carried out in a randomized complete block design, which included four treatments, each with four replications (Fig 1). The plot size was 25 m². Treatments were applied without the DPR mix (control) and with 0.25, 0.5 and 1.0 t ha^-1 DPR. The DPR was applied after soil plowing. Mineral fertilizers were applied in this research at the recommended rate of 60 kg N, 60 kg P₂O₅ and 30 kg K₂O per hectare and fertilizing according to farmers. They were divided in two and applied to sesame plants 10 days and 20 days after sowing.


 
Data collection
 
Plant height (cm) of 20 plants per plot was measured from the soil surface to the highest growth peak and the number of leaves was counted all leaves of plant 70 days after sowing. The chlorophyll content in a leaf (µg Chl cm^-1) was measured 45 days after sowing, according to the N,N-dimethylformamide method (Moran,1982). The number of pods per plant, pod weight, the number of seeds per pod, the weight of 1.000 seeds and yield were determined according to Khuong et al., (2022). Nitrate, phosphate and potassium contents in the stem and leaves were measured 50 days after sowing and in seeds, according to Temminghoff and Houba (2004). The 5 sesame plants per replication were collected then dried and mixed for Nitrate, phosphate and potassium analyzation. Furthermore, pH_H20 (1, soil : 2.5, water), pH_KCl (1, soil : 2.5, KCl solution), nitrate, phosphate and potassium values in soil samples (0-20 cm) were measured, as described by Sparks et al., (1996). The soil samples were collected from five places in plot then dried and mixed together for pH and nutrion analyzation.
The cost of raw materials (inorganic fertilizers, DPR, plant protection drugs, seeds, water irrigation), labor and product sales were calculated according to 2022 prices.
 
Data analysis
 
The data presented in this research are the mean values of five replications. All data were analyzed using one-way analysis of variance (ANOVA) using SPSS software package version 13.0 and were compared for significant differences in treatment effects using Duncan’s test at P<0.05.

Effects of DPR on pH, nitrate, phosphate in soil
 
The results in Table 1 show that the pH levels and the percentage of PAvailable increased in the soil where DPR was applied compared to those in the soil where DPR was not applied. Similar to the results by Yang et al., (2012), in this study, the pH was increased up to three units when the acid sandy soil was applied with DPR. In the work of Suksri (1998), acidic soil was used with dolomite, which also increased the soil pH. According to the study of Lestari et al., (2016), the higher the rate of dolomite applied to acid sulfate soil, the greater the pH increase. The increase in soil pH might be due to the high pH of DPR application. The percentage of total nitrate in soil was not significantly different in all the treatments. However, the amount of NH₄⁺ and NO₃^- in the soil was higher in all treatments with the application of DPR. According to Silber et al., (2010), the soil pH affects both the availability and uptake of nutrients by plants and the release of PO₄^3- and NH₄⁺ from soil particles (Zheng et al., 2013).
 

 
 Effects of DPR on plant height, number of leaves and chlorophyll content in leaves
 
Plant height was measured at harvest time and DPR application significantly increased (Fig  2). The greatest plant height (mean value of 129.7 cm) was recorded at the treatment of 0.5 t ha^{-1 D}PR and this was significantly higher than the control and 0.25 t ha^-1 DPR (121.6 and 126.2 cm, respectively). There was no significant difference in plant height when a higher percentage of DPR was applied. The number of leaves on the plant was the highest with a treatment of 1.0 t ha^-1 DPR and when the application of the DPR was lower, the number of leaves per plant was not significantly different. The results were similar to those obtained by Soeparjono and Kadiyasari (2021) on black soybean, Suntoro et al., (2018) and Krismawati et al., (2022) on maize. When DPR was applied to ryegrass, the dry weight increased (Yang et al., 2012). 
 

               
The application of DPR was observed to have significant effects on chlorophyll b and total chlorophyll in leaves (Fig  3). However, DPR did not affect the content of chlorophyll a. The total chlorophyll and chlorophyll b were significantly higher than those in the control plants and those treated with 0.25 t ha^-1 DPR. The chlorophyll content increased in corn plants when they were treated with 0.2 t ha^-1 dolomite (Suntoro et al., 2018). The higher chlorophyll content in leaves treated with DPR might be due to high phosphate uptake in plants (Fig 4). Phosphorus plays a vital role in chlorophyll production and regulation (Billah et al., 2020).
 

 

 
Effects of DPR on nitrate, phosphate and potassium in stem, leaves and seeds
 
Nitrate and potassium contents were not significantly different in stem, leaves and seeds in all treatments (Fig  4). These results were inconsistent with the content ammonium and nitrate in the soil (Table 1). The phosphate content significantly differed in the stem when sesame plants were treated with DPR; however, it was not different in leaves and seeds. Suntoro et al., (2018) found that when corn was treated with dolomite, the phosphate concentration in plant was significant higher but dolomite application did not affect to nitrate and potasium contrations. Similar the study of Damrongrak et al., (2015) on rubber trees nitrate concentration in leaves was not affected by application of dolomite. According to Rastija et al., (2014) the availability of potassium in the soil was independent of dolomite application. It might be a reason of the concentration of potassium in sesame plants was not different in all treatments with or without application of DPR.    
 
Effects of DPR on yield components and seeds yield of sesame
 
Fig 5 shows that the number of pods per plant, pod weight and the number of seeds per pod were statistically significantly different at 5% among all treatments. The number of pods per plant was the highest in the plants treated with 0.5 t ha^-1 DPR (47.0 pods per plant). Similarly, the number of seeds per pod was the highest in plants treated with 0.5 t ha^-1 DPR  (140 seeds). In this study, the weight of 1,000 seeds was not affected by the DPR application. The higher number of pods per plant might be the higher phosphate uptake in plants. According to Kashani et al., (2015), phosphorus is an essential nutrient for flower formation and seed production.
 
  
               
Fig 6 shows that the application of the DPR with an amount of 0.5 t ha^-1 had the highest yield, with 1.61 t ha^-1 and there was a statistically significant difference at 5% compared to the control treatment (1.29 t ha^-1).  Significantly, the lowest seed yield was recorded in sesame plants with no DPR treatment and the yield increase rate compared to the control treatment was 24.8%. Similar to the study of Cahyono et al., (2022) on soybean planted on Alfisols in Jumantono, Karanganyar, Indonesia, the seed yield increased by double with the application of dolomite. There were no significant differences in sesame yield between plants fertilized with 0.5 and 1.0 t ha^-1 DPR. The soybean yield increased when dolomite or phosphate rock was applied in a single form or in combination (Minardi et al., 2021).
 

 
Financial efficiency
 
Table 2 indicates that the profit from growing the sesame is high (above 18 million VND/ha). Adding DPR increases the cost of labor and DPR. Application of 0.5 and 1.0 t ha^-1 DPR for sesame helped increase yield significantly higher than without application or 0.25 t ha^-1 of DPR. However, the profit of the treatments of 0.25 and 1.0 t ha^-1 DPR was lower than the profit of the control treatment. Application of 0.5 t ha^-1 DPR gave a higher profit than the control treatments (4.5 million VND/ha).
 

Sesame was fertilized with doses 60 N - 60 P₂O₅ - 30 K₂O kg ha^-1 and the addition of  0.5 t ha^-1 DPR increased plant height, number of leaves and pods per plant, total chlorophyll content in leaves, number of seeds per pod and seed yield (1.61 t ha^-1). The seed yield increase rate in comparison with the control treatment was 24.8%. The phosphate content increased in the stem when sesame plants were applied with DPR but not in the leaves and seeds. DPR application to paddy soil (slight acid soil) could be practiced to raise the pH of the soil (0-20 cm) and elevate P_Available amount. The suitable application amount of DPR to sesame plants gave good financial efficiency.

The authors would like to thank Assistant Professor Akagi Isao, Faculty of Agriculture, Kagoshima University, Japan, for the consultant to collect field soil samples in this research.

This research was funded by Dong Thap Department of Science and Technology under Grant no. 138/2019/HÐ-SKHCN.

None.