Effect of Combined Application of Foliar Sprays of Orthosilicic Acid (OSA) with Basal NPK Fertilizer on Growth and Yield of Rice (Oryza sativa L.)

Benefits of silicon to plant growth and yield in higher plants has been explored recently. This study was conducted to assess effects of application of foliar application of stabilized Orthosilicic Acid (OSA) with NPK fertilizer on growth and yield of rice. The study was conducted in Ntende site at Rwagitima marshland, Gatsibo district in the Eastern province of Rwanda. The field experiment was laid in randomized complete block design with three replications across two cropping seasons in 2019 and 2020. Two recommended doses of fertilizer (RDF); 100 % RDF (200 kgha− 1 NPK and 100 kgha− 1 Urea) and 75 % RDF (150 kgha− 1 NPK and 75 kgha− 1 Urea) were used in combination with different doses of stabilized orthosilicic acid (OSA), a commercial formulation is named as Silixol OSA (referred as OSA in the text). The Si fertilizer was applied in at panicle initiation and grain filling stages, at three different doses (2 ml/l, 3ml/l and 4 ml/l). Combined application of Silixol OSA with RDF produced better growth attributes (plant height, number of tillers, root length, flag leaf length and width) compared to that of RDF only. Similarly, OSA application showed higher yield components, number of panicles, panicle weight, grain weight per panicle and 1000-grain weight. Highest yield of 5.81 t/ha which equates to 24 % increase was obtained when 100 % RDF + 4ml/l OSA was applied. Interestingly, when OSA was used at 4ml/l dose, with 75 % RDF, it attributed to significant increase in yield. Rice fertilization with OSA helps to stimulate plant growth, yield attributes and yield. Therefore, potential of Si could further be explored among rice farmers in Rwanda. Further saving of 25 % of fertilizer dose would be a good economic benefit to farmers as well as government of Rwanda. Lesser use of fertilizers would also reduce the risk of degradation of environment following the application of OSA fertilizer.


Introduction
Silicon (Si) is the second largest element in earth crust constituting about 27.8 − 32 % [1,2]. Si is present in the earth's crust predominantly as quartz (SiO 2 ), silicates or metasilicates [3]. Silicon from these complex forms cannot be utilized by plants [4]. Plants absorb Si from soil solution in form of Mono silicic acid (H 4 SiO 4 ), which is often synonymously named as ortho silicic acid (OSA) [5]. Silixol OSA is a commercial formulation of stabilized OSA, and has been used as a Si fertilizer to enhance better plant growth and yield in several crops [6,7].
Si is the only beneficial nutrient that is included in the guidelines for rice fertilization [13]. Si fertilization has been reported beneficial to rice plant growth and yield [14][15][16]. Si increases photosynthetic rate [17], plant leaf area [18] and chlorophyll content [19]. In addition to its role in improving yield and yield attributes, Si play a vital role in limiting the adverse effects of abiotic and biotic stresses in rice. Si application have reduced incidence of pest and diseases viz. brown spot [20] and blast [21]. Yield losses due to abiotic stresses such as radiation [22], salinity [23], drought [24], accumulation of toxic arsenic [23] and cadmium [24] in rice grains have been reported to reduce significantly following Si application.
Despite these numerous benefits of Si to the rice crop, farmers in Rwanda have not explored the potential Si as an exogenous fertilizer for sustainable rice production. This study was conducted to assess effects of combined application of different doses of foliar sprays of Silixol OSA with basal and top dressing synthetic fertilizers on agronomy and yield attributes of rice.

Treatment Structure and Experimental Design
Experiments were laid in a randomized complete block design with three replications and eight treatments. Silixol OSA (a commercial formulation of stabilized ortho silicic acid (OSA) with 0.6 % concentration) was applied as a source of Si element. Treatments were: T 1 (recommended fertilizer dose (RDF); 200 kg ha − 1 NPK (17-17-17) and 100 kg ha − 1 Urea), T 2 (75 % of RDF; 150 kg ha − 1 NPK and 75 kg ha − 1 Urea), T 3 (T 1 + 2.0 ml of OSA per liter of water), T 4 (T 1 + 3.0 ml of OSA per liter of water), T 5 (T 1 + 4.0 ml of OSA per liter of water), T 6 (T 2 + 2.0 ml of OSA per liter of water), T 7 (T 2 + 3.0 ml of OSA per liter of water) and T 8 (T 2 + 4.0 ml of OSA per liter of water) ( Table 2). NPK (17-17-17) was applied during planting while urea was used for top-dressing at both mid-tillering and at panicle initiation stages. The OSA liquid was applied as foliar spray at panicle initiation and grain filling stages using a handheld sprayer. Other recommended crop management practices were adopted uniformly for all treatments until harvesting.

Growth, Yield and Yield Components
Plant height, number of tillers per hill and root length data were collected from ten randomly selected hills. The plant height was measured from the bottom to the top most leaf, numbers of tillers were counted and root length was recorded from the base of the plant to the tip of the longest root. For

Statistical Analysis
Data were analyzed using analysis of variance (ANOVA), GenStat data analysis software. Treatment effects were significant at p ≤ 0.05 and means were separated by Fisher's protected least significant difference (LSD).

Growth Parameters
A significant impact of foliar application of Si fertilizer has been recorded on different agronomic parameters.
Pooled data of two seasons have been presented in the  Flag leaf is an important agronomic trait contributing towards the yield potential of rice. Foliar application of OSA had improved the length and width of flag leaf with the maximum numerical value in treatment T5 (100 % RDF + 4.0 ml/l OSA, Fig. 1). These results are in agreement with previous reports where an increase in growth attributes had been recorded following application of Si fertilizers in rice [21] and wheat [25]. Plant height is an important indicator for grain yield potential [26]. Increase in plant height has been attributed to better nutrient assimilation by plants as OSA has a proven role in improving nutrient uptake and assimilation [15] et al. Silicon has a property to deposit itself within the plant cells, thus imparting erectness and rigidity to crop [27]. Improvement in tillering following Si application has also been reported earlier [28,29]. Likewise increase in root length and flag leaf are in agreement with previous reports of Hattori et al. [30] and Dorairaj et al. [31].

Yield Attributes and Yield of Rice
In addition to agronomic traits, various yield attributes viz. number of panicles, panicle weight, grain weight per panicle and 1000-grain weight were also recorded for all treatments ( Table 4). Number of panicles exhibited higher numerical values in all treatments (T3 to T8) where foliar applications of OSA were done compared to controls (T1 and T2). Highest value (279) was recorded for T5 (100 % RDF and 4.0 ml/l OSA) which was significantly different from that of control (230; T1). The number of panicles for T6 (75 % RDF + 2.0 ml/l OSA; 221) was more than of corresponding control (T2; 75 % RDF). Panicle length also exhibited the similar trend in the numerical values, though they were not statistically significant. For panicle weight all treatments (T3-T8) with OSA applications had numerically higher values to that of controls (T1 and T2). In fact, T6 (75 % RDF + 2.0 ml/l OSA) had numerical value more than T1 (100 % RDF). About 24 % increase in panicle weight was recorded for T5 over T1. Increment in numerical values of other traits (grain weight per panicle and 1000 grain weight) were recorded in all treatments (T3 to T8) where foliar applications of OSA was done compared to controls (T1, T2).
Though grain yield was not statistically significant within treatments but numerical difference in values have been recorded for all treatments (T3 to T8) with that of control (T1 and T2). Almost similar yields were recorded for T1 (100 % RDF) and T6 (75 % RDF and 2.0 ml/l OSA), indicating that 25 % of fertilizer can be reduced during rice production without compromising on yield with foliar application of OSA. Foliar application of OSA had attributed to a yield increment of 20 % in both doses of fertilizers (100 and 75 %) following foliar sprays of 3.0 ml /l (T4) and 4.0 ml/l (T8). Similar increment in rice yields following application of Si fertilizers have been reported [15,32]. Role of Si in improving plant growth, nutrient uptake and tolerance to abiotic and biotic stresses have been identified as the major factors resulting in higher yields [33,34].
Average yield of rice in Rwanda has been reported to be 4.0t/ha [35]. In that pretext, the results of present study are very promising towards boosting overall rice productivity in Rwanda that too at reduced fertilizer dose by 25 %. The foliar spray of 3.0 ml/l at 75 % fertilizer had attributed to overall yield increment of 15 % while it increased to 39 % in treatment where higher dose (4.0 ml/l) of OSA was used as foliar application. Similar at 100 % RDF, the lowest dose of OSA (2.0 ml/l) has resulted in 27 % increase in productivity.

Conclusions
Results of this study indicate that foliar application of Si in combination with RDF resulted in positive effects agronomic and yield-related attributes of rice. Results of OSA fertilizer are very promising as they indicate 25 % saving of fertilizers without yield compromise. This is a huge economical saving for farmers as well as for Government of Rwanda for cost incurred in importing fertilizers. Furthermore, the reduced use of fertilizer would have a profound impact on reducing the degradation of environment. Pollution of water bodies and land due to excess use of fertilizer is a emerging as one of the major concerns globally, therefore we believe that adoption of OSA in rice cultivation program would ensure better sustainability for commercial rice production in the country.