Vermicompost and Rice Husk Biochar Interaction Ameliorates Nutrient Uptake and Yield of Green Lettuce Under Soilless Culture

This paper is based on the results of a pot experiment conducted in the polyethylene house of the Faculty of Agriculture, Islamic University of Malang, Lowokwaru District, Malang City, East Java, Indonesia with an altitude of approximately 550 meters above sea level. The daily average temperature was in the range of 23–30 °C. This study was performed from the beginning of August 2020 till the end of December 2020. The making of vermicompost was carried out at the Compost Laboratory of the Faculty of Agriculture, Islamic University of Malang.

The vermicompost was made in a bin of size 80 cm × 120 cm × 30 cm. The materials used for making vermicompost were cow dung, leaf litter, fresh vegetable residue, spent mushroom media, Lumbricus rubellus earthworms, and additives consisting of fish bone meal, Tithonia diversifolia leaves, and eggshell flour. The complete processes consist of different stages, which are organic matter preparation, mixing the media, inoculating Lumbricus rubellus earthworms, vermicomposting, and composting process. The duration kept for vermicomposting and composting was one month and two weeks, respectively (Nurhidayati et al. 2017a). The vermicompost produced in this study contained total nitrogen (2.05%), total organic C (29.14%), C/N ratio (14.2), pH (8.81), total P (0.92%), total K (1.55%), total Ca (7.96%), total Mg (2.27%), and total Na (1.14%).

Two factors were studied. The first factor was the composition of the growing substrate, where M1 consisted of 55% cocopeat, 15% rice husk biochar, and 30% sand, and M2 consisted of 55% cocopeat, 30% rice husk biochar, and 15% sand. The size of the pots used was 14 cm in height and 18.5 cm in diameter, thus the volume of the pot was 3,761.3 cm³. The weight of the growing media material in the pots was 1 kg. The second factor was the amount of vermicompost in the substrate: V1 – 50 g per pot, V2 – 100 g per pot, V3 – 150 g per pot, V4 – 200 g per pot, and V5 – 250 g per pot. Each treatment involved five pots/plants in three replications.

Seeds of green lettuce (Lactuca sativa L.) ‘Grand Rapids’ were sown into seedbeds, filled with a mixture of coconut husk media and cow dung. The substrate was sprinkled with water every day morning for 3 weeks. The growing substrates were mixed evenly and incubated for one week in the pots. After that, two seedlings of green lettuce were transplanted into each pot. Watering was done twice a day in the morning and evening with 100 ml of water per pot each time. After two weeks, the water volume increased to 150 ml per pot.

At the harvesting date, 30 days after transplanting seedlings (DAT), the variables were analyzed in terms of nitrogen (N), phosphorus (P), and potassium (K) nutrient uptake in plant and plant yield. Plant yield was evaluated as total fresh and dry weight of the plant, fresh weight of marketable yield, and fresh and dry weight of roots. Nutrient uptake is calculated by multiplying the nutrient content (%) by the dry weight of the plant to obtain nutrient uptake (gram per plant).

The content of N, P, and K in leaves was measured by using the wet combustion method. N content was analyzed using sulfuric acid (H₂SO₄) by adopting the Kjeldahl method. The plant samples of 0.250 g (< 0.5 mm) were taken into a digestion tube. A mixture of 1 g sulphuric acid and selenium powder and 2.5 ml H₂SO₄ p.a. was added to the tube. The digestion process was carried out at 350 °C (4 hours) until white vapor was released and a clear extract was obtained. Measurement of N content was carried out by distillation process. P content was analyzed by a spectrophotometer using nitric acid (HNO₃) and perchloric acid (HClO₄). A sample extract of 1 ml was taken into a chemical tube. Deionized water (9 ml) was added to the tube and shaken (10x dilution). 2 ml of the aqueous sample extract was pipetted and calculated the standard series P (0–20 ppm PO₄) into the test tubes. 10 ml of reagent P was added to the sample extract and shaken with a tube shaker till it become homogeneous, thereafter, left for 30 minutes. P content in the solution was measured by means of a spectrophotometer at a wavelength of 693 nm. The K content was analyzed by using nitric acid (HNO₃) and perchloric acid (HClO₄) with a flame photometer with a standard series as a comparison. 1 ml of extract samples were pipetted into a chemical tube and 9 ml of 0.25% La solution was added. The solution was shaken using a tube shaker until it became homogeneous. The K content was also measured with a flame photometer as well compared to a standard series. The element uptakes by the plants were calculated by multiplying the nutrient content with the total dry weight of the plant.

The study was conducted as a 2×5 factorial in a randomized complete block design (RCBD) with three replications. The obtained data were tested by using analysis of variance (ANOVA) at the significance level of 5%. If the results of the variance analysis showed a significant effect, then the Tukey test was employed with a level of 5% significance to determine the differences between treatments.

Significant differences derived from the composition of the growing substrates were observed between the yield-related averages. They were especially important in the fresh and dry weight of the total yield and fresh weight of the marketable yield. Increasing the proportion of rice husk biochar from 15% to 30% resulted in a significant increase in the values of all yield parameters. Similarly, these yield elements concerning aboveground parts were positively influenced by increasing the share of vermicompost to 250 g per pot in the M1 substrate and to 200 g per pot in the M2 substrate. The higher proportion of rice husk biochar also increased the fresh and dry mass of the roots, but for these features, the content of vermicompost was effective only up to 150 g per pot. For fresh plant weight, a medium containing more rice husk biochar and 200 g per pot of vermicompost was the most preferred (Table 1).

The average total fresh weight of green lettuce grown on the M2 (61.89 g per plant) was higher than on the M1 substrate (47.72 g per plant). In addition, the fresh weight of marketable yield in the M1 (43.50 g per plant) was lower than in the M2 substrate (56.23 g per plant). The addition of 30% rice husk biochar proportion (M2) to the growing media was able to increase total crop yields by 29% compared with 15% (M1). An increase in rice husk biochar from 15% to 30% in the mixture resulted in a reduction in the amount of sand from 30% to 15%, which certainly changed the physical and chemical properties of the substrate. The rice husk biochar reduces the porousness and extremely low holding capacity of the sand. It removes a barrier to soilless cultivation. Sand has a low cation exchange capacity, which makes it simple for nutrients in the media to drain. When irrigated, sand gets moist quickly and, thanks to evaporation, also dries out quickly (Rosalina et al. 2019). Biochar is able to increase nutrient availability in the substrates, leading to enhance nutrient uptake by plants (Lehmann et al. 2003). Biochar enhances water and nutrient retention and promoted beneficial microbial activity, suggesting high input of biochar can lead to an increase in the yield of green lettuce (Sohi et al. 2010). Vaughn et al. (2013) examined different contents of biochar from 0% to 15% in the soilless substrate in the greenhouse. They found that increasing biochar amount increases the residual nitrate and phosphate release. The application of rice husk biochar alone and in combination with perlite has been tested on cabbage, red lettuce, dill, and mallow plants and has shown high plant yields (Awad et al. 2017). Graber et al. (2010) reported that biochar enhances the growth and productivity of pepper (Capsicum annuum L.) and tomato (Lycopersicum esculentum Mill.). The 50 g per pot of vermicompost in combination with 30% rice husk biochar was found to be beneficial for total fresh weight (M2). The higher quality of plants in the marketable yield was observed for the amendment of vermicompost ≥ 100 g per pot in M2 growing medium. Vermicompost supplies plants with N, P, K, Ca, and Mg, increases the amount of organic matter in the soil, improves soil quality, gives plant development hormones, and acts as soil support. It shows that vermicompost directly influences plant growth and yield (Lazcano & Domínguez 2011). Vermicompost has been shown to improve soil water retention, particularly in porous soils like the growing medium used in this study (Jouquet et al. 2010; Jouquet et al. 2011). The growth of organically grown vegetable plants in pots, such as Phak-coi mustard and broccoli, was accelerated by the use of vermicompost (Nurhidayati et al. 2015, 2016; Nurhidayati et al. 2017b).

The amount of dry matter in shoots and roots is determined by environmental factors, especially the availability of nitrogen nutrients. Low nitrogen concentrations in the soil reduce the shoot-to-root ratio (Laghari et al. 2004). They found that high rates of vermicompost can increase dry matter; meanwhile, lower vermicompost rates increases more fresh and dry weight of roots. Nitrogen supply increases shoot and root growth of the plant, but usually shoots grow relatively much more than the roots (Marschner 1988; Laghari et al. 2004).

It suggests that an increase in biochar to 30% in growing media provides a more efficient at a lower amount of vermicompost. This is probably because biochar is able to retain water and nutrients so that it can reduce nutrient leaching on porous growing media such as soilless culture. Nevertheless, the application of biochar combined with vermicompost may enhance and sustain the biophysical and chemical characteristics of growing media, so that plant growth and yields increase significantly.

N, P, and K uptake in lettuce plants were dependent on substrate composition and was significantly higher in the substrates containing 30% of rice husk biochar compared with 15% (Fig. 1A–C). Also, an increase in vermicompost content in the substrates increased N, P, and K uptake, being the highest in the treatments with 200 and 250 g per pot (Fig. 2), although the reaction to the higher rate of vermicompost was not significant in the substrates with 15% rice husk biochar (Fig. 1A–C).

Figure 1

Figure 2

The magnitude of the increase in nutrients up-take with the higher proportion of rice husk biochar 30% versus 15% was 52%, 67%, and 117% for N, P, and K, respectively. This may be dependent on higher water or nutrient retention in M2 compared to the M1 substrate.

This is consistent with the earlier report of Wiedner et al. (2013). They showed that the change in water and nutrient retention in substrates affect the ability of the plant to absorb the nutrient, which enhances plant growth. Cai et al. (2016) reported that biochar can be produced from several organic matters at a higher temperature. It showed excellent retention ability in holding cations such as NH₄⁺, which is related to the carboxyl and keto groups present in biochar-making process at a temperature of 200 °C. Biochar acts as a slow-releasing agent of nitrogen which is absorbed via the roots. Furthermore, biochar can indirectly affect nutrient availability by reducing nutrient leaching so that fertilizer application becomes more efficient (Lehmann et al. 2003; Major et al. 2009; Yao et al. 2012). Along with N, biochar increases the availability of P (Li et al. 2019). In addition, biochar provides a habitat for beneficial micro-organisms because it increases pore size, and surface area (Ajema 2018). Moreover, biochar was reported as a useful carrier for increasing the population of Enterobacter cloacae (Hale et al. 2015) and Azospirillum lipoferum (AZ 204) (Saranya et al. 2011).

In the uptake of N, P, and K in lettuce plants, significant differences (p < 0.05) were observed, resulting from vermicompost rates. There was a tendency that the higher the vermicompost rates, the greater nutrients uptake. The N uptake in the plants grown in the substrates possessing 200 and 250 g per pot vermicompost was higher compared with lower vermicompost rates but they differed significantly only at the higher dose of rice husk biochar (Fig. 1A). Similar pattern was observed in P and K uptake (Fig. 1B, C). Generally, the increase of biochar dose increased significantly N, P, and K uptake (Fig. 2), which means that macro elements uptake was stimulated by biochar. Also, increased doses of vermicompost were accompanied with increasing uptake of macroelements (Fig. 3).

Figure 3

Vermicompost contains complete nutrients, the amounts of which vary depending on the raw materials used (Nurhidayati et al. 2017a). The vermicompost used in this study contained 2.05% N, 0.92% P, and 1.55% K. The nutrient contents in vermicompost are lower compared to inorganic fertilizers, but the advantage of this source is that they release nutrients gradually, up to four planting periods (Nurhidayati et al. 2018). Vermicompost contains higher nutrient contents compared to conventional compost and they are more balanced so their uptake by plant roots is more effective for plant growth stimulation (Vinothini et al. 2016).