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Article

Preliminary Results on the Application of Phosphorus and Silicon to Improve the Post-Transplantation Growth of High-Density Nursery Seedlings

by
Hisashi Nasukawa
1 and
Ryosuke Tajima
2,*
1
Faculty of Agriculture, Yamagata University, Tsuruoka 997-8555, Japan
2
Graduate School of Agricultural Science, Tohoku University, Sendai 980-8572, Japan
*
Author to whom correspondence should be addressed.
Agronomy 2025, 15(4), 937; https://doi.org/10.3390/agronomy15040937
Submission received: 18 March 2025 / Revised: 7 April 2025 / Accepted: 9 April 2025 / Published: 11 April 2025
(This article belongs to the Special Issue Effects of Agrotechnical Factors and Farming Systems on Soil Properties and Plant Productivity)
Browse Figures
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Abstract

:
Transplanting high-density nursery (HDN) seedlings reduces seedling raising costs by increasing the sowing density compared to regular seedlings and shortens the seedling growth period. However, there is a concern that the poor early growth of HDN seedlings hinders rice production in cold regions, such as northern Japan. This study conducted a growth comparison test using a combination of phosphorus (P) and silicon (Si), which stabilizes early rice growth. The results showed that the application of P or Si alone increased the shoot dry weight at the heading stage. The application of 60 mg kg−1 or more P fertilizer increased the HDN shoot dry weight, while 2 g kg−1 Si was effective in increasing the shoot dry weight. However, the joint application of P and Si did not increase plant height or stem number or synergistically increase the shoot dry weight compared to the application of P or Si alone. As the P concentration in the soil increased owing to the P treatment, Si suppressed the excessive uptake of inorganic P and maintained its concentration at an appropriate level. The application of P and Si is important to increase the shoot dry weight of rice. In addition, Si may play a role in regulating the appropriate level of P uptake by HDN. The application of P and Si stabilized the initial growth of the HDN.

1. Introduction

Rice is a major staple cereal, with more than 3.5 billion people depending on it for more than 20% of their daily calorie intake. Rice production is estimated to increase by 114 million tons by 2035, but this must be achieved under climate change coupled with decreasing amounts of available agricultural land, labor, and water and increased input costs [1]. In 2022, the total paddy rice production in Japan was 7.27 million tons, with Yamagata Prefecture—a major rice-producing region—contributing approximately 0.37 million tons, accounting for 5% of the national output [2]. In Japan, the population involved in agriculture decreased from 11.7 million in 1960 to 2.1 million in 2015, a reduction of 86% [3]. With the increase in the average area operated by a single farmer in Japan, it becomes more difficult to consolidate fields. This trend is projected to continue, and labor-saving and cost-effective technologies are expected to gain prominence as the cultivation of land per farmer continues to increase [4]. Reducing production costs is essential for improving the agricultural income of rice cultivation; thus, the aggregation and consolidation of farmland and reduction in production material costs are recommended. Transplanting is carried out in over 99% of the planted rice area in Japan, and labor and cost input reductions are required for raising seedlings and transplants [5,6].
High-density nursery (HDN) technology sows 250–300 g of dry seeds, compared to 100–150 g of dry seeds for conventional seedlings. HDN are seedlings with lower plant age than those conventionally cultivated in Japan. HDN can reduce the number of nursery boxes required and can contribute to cost reduction because they have a higher seeding density. The growth period for HDN to use dense age seedlings is usually from 5 to 14 days, which is shorter than normal seedlings; therefore, labor and material inputs can be reduced [4]. Seedlings sown in high-density conditions are called “Mitsunae” in Japan and are recognized for their labor-saving properties [7]. However, their productivity has not been fully investigated, and there is concern that initial growth may become unstable.
A past study found that high-density seeding aggravated transplanting injury, and the growth rate stagnated after transplanting at the 2.8–3.0 plant age [8]. In cold regions, the suppression of root growth inhibits initial growth, and poor initial growth is a major factor hindering stable paddy rice production [9]. In northeastern Japan, rice productivity is severely limited by low temperatures. This is associated with its climatic conditions, which is located in a cool temperate zone [10]. If seedlings are transplanted into deep water, there is a risk that they will be submerged after transplanting, which may cause the rice to grow longer [11,12,13]. These conditions reduce tillering, increase lodging, and cause partial mortality [14]. In contrast, a past study found that the initial growth of seedlings after transplanting is improved when submerged because seedlings retain nutrients within the seed [5]. When HDN seedlings are transplanted, tillers form from the lower internodes and the number of stems increases [15]. Although many aspects of the growth characteristics of the HDN seedlings are not fully understood, controlling early growth is critical for ensuring a stable yield.
To stabilize the initial growth of HDN seedlings, it is important to establish seedlings after transplantation and ensure initial growth [16]. Phosphorus (P) improves the initial growth of paddy rice and is expected to promote tillering, root development, and flowering [17]. In paddy rice cultivation, insufficient P fertilization delays the development of the root system of paddy rice and the construction of the nutrient absorption system of paddy rice. P deficiency reduces shoot growth, which is associated with reduced tiller production and a longer phyllochron [18,19]. Silicon (Si) accumulates as opaline Si in rice plant bodies and is essential for growth and production. Deposited Si in plants alleviates water stress by decreasing transpiration, improving light interception characteristics by keeping the leaf blades erect, increasing resistance to diseases and pests, avoiding lodging, and remediating nutrient imbalances [20,21,22,23]. Si is especially important for the healthy growth and sustainable rice production, which is a typical Si-accumulating plant [21]. The high Si content of the shoot is important for the healthy growth and stability of rice production [24]. Si regulates P uptake by promoting it under P-deficient conditions and restricting it when P is high. Si enhances P availability in deficient soils and improves plant uptake efficiency, whereas it reduces P accumulation under excess conditions by limiting its transport [25]. These findings indicate the need to further investigate the interaction between P and Si to optimize nutrient management in HDN seedlings. While the positive effects of P and Si on rice growth have been widely studied, most of the existing research has focused on standard seedling densities or field-grown rice. The distinct physiological features of HDN seedlings—such as limited root volume, intensified nutrient competition, and stress susceptibility—remain underexplored. Therefore, this study aims to evaluate whether the combined application of P and Si can improve the initial growth and establishment of HDN seedlings after transplanting by enhancing nutrient uptake and stress tolerance.
Therefore, this study evaluated the application of P and Si to improve the growth of HDN. HDN seedlings were prepared and transplanted into soil to which P and Si were applied at multiple levels. HDN were cultivated in pots until the heading stage, and in field plots with embedded frames until harvest, to investigate the effects of P and Si on growth and yield. The results of this study are expected to contribute to the development of stable production technology for HDN technology and labor and cost savings in rice cultivation. This study investigates whether combined P and Si application enhances HDN seedling growth post-transplantation by improving nutrient uptake and stress resistance.

2. Materials and Methods

2.1. Raising Seedlings

Seedlings were raised at the Field Science Center, Faculty of Agriculture, Yamagata University, Tsuruoka City, Japan (38°41′ N, 139°49′ E) in 2021 and 2022. The cultivar ‘Haenuki’ was used. ‘Haenuki’ is a major rice cultivar grown primarily in Yamagata Prefecture, Japan, and is particularly adapted to the cool climate of the Tohoku region. Seeds were sown in a nursery box with inner dimensions of 58 cm in length, 28 cm in width, and 3 cm in height. The bed soil was prepared by fertilizing 3.8, 3.4, and 3.8 g box−1 of nitrogen (N), P, and potassium (K) to commercially available nursery soil. In each nursery box, 300 g of seeds were sown. In 2021, seeds were sown on 30 April, and the seedlings were raised until 17 May (17-day seedlings). In 2022, they were sown on 6 May and raised until 17 May (11-day seedlings). The quality of the raised seedlings is shown in Table 1. During the nursery period, the average air temperature was 15.7 °C in 2021 and 16.1 °C in 2022. The average relative humidity was 67.6% in 2021 and 61.3% in 2022, while the average duration of sunshine was 6.4 h/day in 2021 and 8.6 h/day in 2022.

2.2. Pot Cultivation Test

The test soil was air-dried and crushed paddy soil obtained from the Takasaka Farm of the Yamagata Field Science Center, Yamagata University. The test soil was classified as an Inceptisol according to the USDA soil taxonomy using the Harmonized World Soil Database Version 2 (HWSD V2). The physical and chemical properties of the soil samples are listed in Table 2. pH was measured in a 1:2.5 (soil–water) ratio. Electrical conductivity (EC) was measured in a 1:5 (soil–water) ratio. Available N was determined using the hot water extraction method. Available P was determined using the Bray 2 method. Cation exchange capacity (CEC) was determined by the Schollenberger method. Exchangeable bases were extracted with 1 M ammonium acetate (NH₄OAc) at pH 7.0. Organic matter content was determined using the Walkley–Black method.
The following treatments were applied to 3 kg of dry soil. In 2021, six treatments were applied with three replicates: 0 mg kg−1 P (P0), 60 mg kg−1 P (P60), 90 mg kg−1 P (P90), 2 g kg−1 Si (Si2), 60 mg kg−1 P + 2 g kg−1 Si (P60+Si2), and 60 mg kg−1 P + 4 g kg−1 Si (P60+Si4). To further examine the interaction between P and Si application, an additional treatment—90 mg kg−1 P + 2 g kg−1 Si (P90+Si2)—was introduced in 2022. Thus, a total of seven treatments were conducted in 2022, each with three replicates. The differences in treatment setup between years were designed to better elucidate the synergistic effects of higher P and Si combinations on seedling growth. Dicalcium superphosphate was used as the P fertilizer, and silica gel was used as the Si fertilizer. Ammonium sulfate and potassium chloride were used as the N and K and the amount of each component was 60 g kg−1, respectively. The treated soil was filled into 1/5000 a Wagner pots, and each treatment was replicated three times. The soil was puddled, and the seedlings were transplanted with three seedlings per plant. After transplantation, the seedlings were submerged and cultivated until the heading stage, with standard chemical controls applied for weeds and pests.
During the cultivation period, watering was performed regularly to maintain the paddy conditions. Plant growth parameters were measured every two weeks during the cultivation period. Each pot contained a single plant, which was used for all measurements. Plant height was measured from the base to the tip of the longest leaf. Leaf color was assessed using a SPAD meter (SPAD-502Plus, Konica Minolta, Tokyo, Japan), and the number of stems and plant age were recorded for the same plant. After harvesting, the shoot dry weight was measured. The harvested aboveground parts were dried at 75 °C for 48 h and then pulverized. The pulverized sample was weighed (0.5 g), and 4 mL of sulfuric acid was added. Then, it was heated to 300 °C, and hydrogen peroxide was added, which was repeated until the sample was completely decomposed. The decomposed solution was filtered, and the filtrate was used to quantify and calculate N using the indophenol method and P using the molybdenum blue method. Si was determined by the gravimetric method after the filter paper was incinerated at 550 °C for 2 h.
The soil was collected at the same time as harvesting, and after air-drying and crushing, available P and Si were analyzed. Available P was obtained by adding 20 mL of the extract (0.03M NH4F 0.1M HCl) to 1 g of air-dried crushed soil using the Bray 2 method, shaking for one minute [26]. The sample solution was filtered through ash-free filter paper (No. 5C, Advantec, Tokyo, Japan). A portion of the filtrate was colored and quantified, and the available P was calculated. Si was obtained by adding 60 mL of demineralized water to 10 g of air-dried crushed soil, standing the mixture at 40 °C for 7 days [27]. The sample solution was filtered through ash-free filter paper (No. 5C, Advantec, Tokyo, Japan). The filtrate was colored and quantified, and the concentration of available Si was calculated.

2.3. Field Cultivation Test

A 30 cm square frame was embedded in a paddy field at the Takasaka Farm of the Yamagata Field Science Center (38°41′ N, 139°49′ E) to conduct a rice cultivation experiment in 2022. Because the field cultivation was conducted in the same field where the soil for the pot experiments was collected, the physicochemical properties of the soil shown in Table 2 are the same. Each frame served as a plot for the application of P and Si fertilizers, thoroughly mixed with the soil. The fertilizer application rates followed those used in the pot experiment, and treatments were applied in randomized replicates (n = 3). Ammonium sulfate and potassium chloride were used as the sources of N and K, respectively, at a rate of 6 g m−2 for each component. The seedlings used in the field cultivation were the same as those used in the 2022 pot experiments, and planting was carried out on 17 May. Two hills of rice (three seedlings per hill) were transplanted per frame (22.2 hill m−2), with conventional practices followed for water management, pest control, and fertilization throughout the growing season. At harvest, the following yield parameters were measured: number of panicles, grains per panicle, ripening percentage, and thousand-grain weight.

2.4. Statistical Analyses

Analysis of variance (ANOVA) was used to assess the effects of P and Si addition on both crops and soils. Prior to ANOVA, normality and homogeneity of variance were verified using Shapiro–Wilk and Levene tests, respectively. Tukey’s post hoc test (p < 0.05) was used for mean comparisons after ANOVA. All experiments were nested by year, with the year considered a fixed effect in the ANOVA model. Interactions between P and Si were also tested in the ANOVA model to assess potential synergistic effects. Additionally, linear relationships among crop status variables were analyzed. Regression analysis was conducted to evaluate dose–response effects where appropriate. Effect sizes for significant factors were also calculated. All analyses were performed using R statistical software, v 4.3.2 (R Core Team, 2023) [28].

3. Results

3.1. Effect of P and Si on the Growth

Plant height, number of stems, plant age, and leaf color were measured every two weeks. No differences were observed between the treatments in 2021 and 2022. Table 3 shows the plant height, number of stems, and shoot dry weight at the time of cutting. The average plant height and number of stems were 75.7 cm and 31.8 in 2021 and 72.2 cm and 41.7 in 2022, with no significant difference between treatments. The shoot dry weight tended to increase with P and Si application. The shoot dry weights under P0, P6, and P9 were 31.1, 36.3, and 35.7 g pot−1 in 2021 and 45.8, 52.1, and 56.1 g pot−1 in 2022, respectively. In 2021, no significant increase in shoot dry weight was observed with P alone, although there was a trend of increase. In 2022, however, P90 significantly increased shoot dry weight compared to P0. While there were annual effects, the application of P alone still resulted in an increase in shoot dry weight. Regarding Si, a tendency for increased shoot dry weight was observed with Si applied alone, and a significant increase was noted in 2021, particularly at 2 g kg−1 or more. There was no significant difference between the 2 and 4 g kg−1 Si treatments (P60+Si2 and P60+Si4). As for the combined application of P and Si, the effect was not clear; while the P90+Si2 treatment in 2022 showed a trend of increased shoot dry weight, the interaction between P and Si remains uncertain. It is possible that a synergistic effect exists, but further investigation is needed to clarify the relationship. The results for yield and yield components are shown in Table 4, which further illustrate these trends. Although the yield tended to increase with the application of P and Si, either alone or in combination, compared to the control plot, these differences were not statistically significant. The increase in yield can be attributed to the higher number of panicles resulting from the application of P and Si. While no statistically significant differences were observed in the growth measurements, it became clear that the application of P and Si led to an increase in the number of stems, which in turn resulted in more panicles and a subsequent increase in yield. However, the increased number of panicles may have contributed to a reduction in the number of spikelets and filled spikelets.

3.2. Uptake of P and Si in Rice

Table 5 shows the available P and Si content in the soil after harvest. The available P content was higher in 2021 than in 2022. Available P in the harvested soil increased significantly with P application in 2021 and 2022. There was a tendency to increase from 286.9 g kg−1 under P0 to 289.2 g kg−1 under Si2 in 2021 and 194.6 g kg−1 under P0 to 198.2 g kg−1 under Si2 in 2022. Although there was a tendency for available P to increase with the application of Si, the difference was not significant. Available Si tended to increase with Si treatment; however, the difference in values between treatments was not significant.
There were no significant differences in P concentration in rice among the treatments. The P concentration in rice was high in 2021, with an average of 0.34% in 2021 and 0.29% in 2022. It increased from 0.36% under P0 to 0.38% under P60 in 2021 but was almost equal under P60 and P90; a similar trend was observed in 2022 (Figure 1). The P concentration in rice tended to decrease with the addition of Si but did not increase with the addition of P, especially in 2021. By 2022, the P concentration in rice is expected to decrease hardly.
In 2021 and 2022, Si application significantly increased the Si concentration in rice (Figure 1). P application did not increase the Si concentration in rice in 2021 or 2022. The concentration of Si in rice was 0.99% without Si treatment (average of P0, P60, and P90), 1.61% under Si 2 g kg−1 (average of P0+Si2 and P60+Si2), and 1.79% under P60+Si4 in 2021. The results were similar in 2022: it was 0.83% without Si treatment (average of P0, P60, and P90), 1.25% with Si 2 g kg−1 (average of P0+Si2, P60+Si2 and P90+Si2), and 1.53% with P60+Si4.

3.3. Relationship Between Nutrient Uptake and Growth

Figure 2 shows the correlations between the items investigated. There was a strong correlation between the number of stems and shoot dry weight. There was also a high correlation between P uptake and shoot dry weight and P uptake and tiller number, with correlation coefficients of 0.88 and 0.73, respectively. There was a strong correlation between Si uptake and shoot dry weight; however, this was not statistically significant (r = 0.53). There was also a significant correlation between stem number and P uptake.
The relationship between shoot dry weight and P uptake was analyzed, with shoot dry weight increasing as P uptake increased. The regression analysis showed a significant linear relationship, with a coefficient of determination (R2) of 0.75 (Figure 3). Overall, P uptake was higher in 2022 than in 2021. Under the P0 treatment, P uptake was lower than other treatments in both years. However, treatments with P (P60 and P90) did not show a significant linear increase in P uptake, as there was no significant difference between these two treatments. Interestingly, although the Si2 treatment did not include P, P uptake was significantly higher than in the P0 treatment, indicating that Si application may influence P uptake. No significant difference in P uptake was observed between the 2 g kg−1 Si and 4 g kg−1 Si treatments, suggesting that the effect of Si application plateaued beyond the 2 g kg−1 rate.
For Si uptake, there was a tendency for shoot dry weight to increase as Si uptake increased; however, the coefficient of determination (R2) was 0.22, and no statistically significant difference was found. In P treatments, shoot dry weight increased, suggesting that P application enhanced Si uptake. Although Si application did increase Si uptake, it did not result in a substantial increase in shoot dry weight. The increase in Si uptake may have been due to higher Si concentrations in the soil, which allowed plants to access more Si from the early stages of growth. Additionally, Si application at 2 g kg−1 may have provided sufficient Si, as no significant increase in shoot dry weight was observed with the higher Si application of 4 g kg−1. As for the interaction between P and Si, no clear differences were observed across the treatments.

4. Discussion

4.1. Effect of P and Si Application on Growth

The plant ages of the grown HDN seedlings were 3.12 of 17-day seedlings in 2021 and 2.95 of 11-day seedlings in 2022 (Table 1). In Japan, the youngest seedling transplanted by machine is defined as 2 to 3 plant ages, and younger seedlings (<25 days) can positively impact grain yield [5,29]. The seedlings of HDN in our study have enough growth by the transplanting period. Following transplanting, plant growth often slows due to environmental shock with transplanting injury [30]. Especially in northern Japan, seedlings are often submerged because they are managed in deep water to protect them from cold weather conditions. Shoot biomass and root growth are suppressed when seedlings are submerged in deep water [31]. The early growth of HDN technology must be stabilized with management practices such as fertilization to ensure high productivity in northern Japan.
Although P and Si were applied to improve the initial growth of HDN, Plant height and the number of tillers did not increase with P application (Table 3). When P is depleted in the soil, it affects the number of tillers [32]. However, when there is sufficient P, the effect of P is masked. Si also did not affect the growth of HDN. Si had little effect on tiller number [33]. Effects of silica gel may be small under optimized growth conditions but become obvious under stress conditions, such as mineral stresses such as deficiency of P and excess P, Na, Mn, N, and Al [34]. As this study was conducted under low environmental stress conditions, it is possible that the Si had a smaller effect to plant height and increasing tillers on the HDN technology.
The application of P in 2021 increased the shoot dry weight of the HDN, and P9 in 2022 increased the shoot dry weight compared with no fertilization (Table 3). The effect of improving the initial growth was remarkable because the seedlings were of low quality in 2022 (Table 1). P uptake commences at the earliest stages of seedling development in rice [35]. In rice, autotrophy reportedly occurs approximately 15 days after germination [36]. In 2022, the HDN seedlings had a short growing period of 11 days, and root development may have been delayed. Because the HDN seedling, which has a very short seedling-raising period, was planted in an environment with a high soil P concentration, it may have absorbed P immediately after rooting and increased its dry weight. The shoot dry weight could be largely attributed to Si application (Table 3). Si fertilization in paddy fields with inherently low plant-available Si is an option for increasing rice yields. High Si increases canopy photosynthesis by keeping the leaf blades erect, thereby improving the light interception characteristics [37]. The increase in shoot dry weight observed could be related to improved plant growth conditions, although direct evidence of its effect on photosynthesis was not measured in this study.
This study focused on early growth up to heading and yield, and no root morphological trait was measured. However, rice nursery roots must be important in HDN technology, as they play a key role in seedling establishment. Early P uptake enhances root activity and improves plant growth [38,39]. The application of Si improves growth by boosting root development and increasing root thickness and volume, while also enhancing stress tolerance, thereby supporting its role in plant resilience [40,41,42]. In HDN technology, these findings underscore the importance of researching rice roots, particularly with P and Si applications, for improving rice growth and resilience. Future studies should address this aspect.
Although P and Si fertilization tended to increase rice yield, the effects were not statistically significant (Table 4). This may be due to the pre-fertilized, well-managed soil used in the field experiment, which reduced the need for additional P application. Additionally, Si has a greater impact under extreme weather and stressful conditions, which were absent in 2022 [33]. However, transplanting young HDN seedlings into paddy fields exposes them to stresses like flooding and cold in the early growth stage, especially in northern Japan. Thus, P and Si applications may bolster these seedlings’ resilience, supporting stable growth and enhancing productivity potential under the unique challenges of northern Japanese paddy cultivation. HDN technology is a new technique that has been the subject of limited prior research. As it has been introduced into farmers’ fields in Japan without validation through extensive research, this study offers important insights into its potential in rice cultivation. The trend of increased yield with P and Si application observed in pot trials was not statistically significant in the field trials. This discrepancy may be due to the controlled environment of the pot trials in contrast to the more variable conditions in the field. While pot trials eliminate external factors, field trials are influenced by diverse soil and weather conditions, suggesting the need for further detailed investigation.

4.2. Effect of P and Si Application on P Absorption

The P concentration in the HDN tended to increase with the use of P fertilizer, but the difference was not significant (Figure 1). With increased P fertilizer input, rice P uptake reaches a point at which it can no longer increase [43]. The P uptake efficiency was highest at low levels of soil P and decreased with increasing soil P levels [17]. P uptake by plants is mediated by a negative feedback mechanism that assesses the P status of the entire plant and transmits a systemic signal via the shoots to all parts of the root system [44,45]. The physiological functioning of cells requires that P concentrations in the cytosol be maintained within a narrow range [46]. The increase in P uptake via a negative feedback mechanism maintains a constant P concentration to fulfill the normal physiological functions of the cells [47]. Therefore, the P concentration in the HDN remained constant, regardless of fertilization. There was a positive correlation between P uptake and the shoot dry weight of HDN in both years (Figure 2 and Figure 3). The increased uptake of P was believed to be due to the low shoot dry weight without P application and the increased shoot dry weight under P treatment. Therefore, the relationship between P uptake and shoot dry weight was unclear among the treatments.
There was no significant difference in the P concentration when Si was applied to the HDN, but the P concentration in the HDN tended to decrease (Figure 1). The application of Si may inhibit P uptake in rice. The inorganic P content in plants grown in soil without Si was higher than that in soil with Si [48]. The application of Si was thought to reduce the P concentration in the HDN because there was sufficient P in the soil in 2021. In contrast, better response of rice crops to P addition was observed in Si-fertilized soils [49]. In 2022, there was no significant difference, but the P concentration in the HDN tended to increase after treatment with P and Si compared with the no treatment (Figure 1).
This may be because the P concentration in the soil was low and there was no antagonistic action of Si to promote P uptake by the HDN. The application of Si without P application increased P uptake in the HDN, but the relationship between the shoot dry weight and P uptake of HDN did not differ from that of the simultaneous treatment with P and Si (Figure 3). Better response of rice crops with respect to P nutrition was observed in Si-fertilized soils, which might be due to increased root growth and enhanced soil P availability with Si application, decreased P-retention capacity of soil, and increased solubility of P, leading to increased efficiency of phosphatic fertilizers [49]. Meanwhile, as the soil P concentration increased, P uptake was inhibited by Si. The inorganic P concentration in plants depends on the levels of P and Si in the nutrient solution [50]. Compared with inorganic P, organic P is much less dependent on the levels in the nutrient solution. Inorganic P within plants is necessary for metabolism and storage; however, high P concentrations inhibit enzyme reactions and create abnormal osmotic pressure in cells [25]. Tuning the P uptake of HDN using Si may help optimize the rice yield.

4.3. Effect of P and Si Application on Si Absorption

Regarding the Si concentration, the application of P did not increase the Si concentration (Figure 1). Si accumulation has been attributed to the ability of roots to take up Si [51]. Increased application of P in rice has been shown to enhance the Si content in the stem sheath and increase water-soluble Si in the soil [52]. However, increasing the amount of P applied did not significantly increase the Si concentration in the stem sheaths of rice in the present study. Although the shoot dry weight of the HDN tended to increase as the absorbed amount of Si increased, the shoot dry weight did not increase linearly and the rate of increase in the shoot dry weight stagnated (Figure 3). The effect of P application on Si uptake depends on the environmental conditions. In this study, the addition of P increased Si uptake, possibly due to the increase in the shoot dry weight of HDN.
Si concentration increased significantly with increasing Si application (Figure 1). The Si concentration in the HDN increased with the application amount. Rice requires Si for its better growth and productivity and can accumulate several times more Si in the shoot than other macronutrients, such as N, P, and K [21]. As the Si concentration in the HDN increased, Si uptake also increased significantly (Figure 2 and Figure 3). Increased Si uptake upon Si application might be related to the increased Si availability in soil and enhanced root system, which stimulates the plant to absorb more Si from soils [49]. HDN transplant seedlings have an extremely short growing period; therefore, it is expected that their root development will be delayed compared to that of normal seedlings. For stable production using HDN technology, root growth should be promoted.
The effect of Si application on improving growth during the harvest period was not significant, and no increase in rice yield was observed (Table 4). Number of panicles, 1000-grain weight, and yield increased under Si fertilization [48,53]. In while rice biomass production was not affected by Si application, probably because biotic stress levels were generally low [54,55]. Because of the role of Si in alleviating abiotic and biotic stresses, the effect of Si on plant growth becomes more pronounced under stress conditions but may not be noticeable under non-stressed conditions [33]. The application of Si effectively stabilizes rice yields when the rice is subjected to severe environmental stress. This is important as HDN technology will be subjected to severe environmental stress due to transplantation. In addition, their transplantation in northeastern Japan will subject them to severe stress owing to the cold climate. To better understand these mechanisms, future studies could consider measuring stress markers, such as antioxidant activity and lignin content, to assess how Si influences plant stress responses during such conditions.

5. Conclusions

This study advances the understanding of nutrient management in HDN technology by demonstrating how P and Si application can improve early rice growth under transplanting stress, a unique challenge for HDN systems. Unlike general rice nutrition studies, this paper focuses on optimizing nutrient use specifically for HDN conditions, in which early seedling establishment is critical. The findings suggest that a combination of P60 and Si2 is most effective for improving early growth and stability under high-density transplanting, offering a tailored approach to nutrient management in HDN technology. Given that no significant benefit was observed from the higher Si application (4 g kg−1) compared to 2 g kg−1, and considering the high cost of Si fertilizers, the P60 and Si2 combination is economically more viable. These results contribute to the growing body of knowledge on optimizing fertilization practices in high-density cultivation, with potential applications in improving seedling establishment and enhancing rice yields in such systems. Future studies should examine the effects of P and Si application under low-temperature stress conditions typical of HDN systems in northern Japan.

Author Contributions

Conceptualization, R.T.; methodology, R.T.; software, H.N. and R.T.; validation, H.N. and R.T.; formal analysis, H.N.; investigation, H.N.; resources, H.N.; data curation, H.N.; writing—original draft preparation, H.N.; writing—review and editing, R.T.; visualization, H.N.; supervision, R.T.; project administration, R.T.; funding acquisition, H.N. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by JSPS KAKENHI Grant Number 22K14874.

Data Availability Statement

Dataset available on request from the authors.

Acknowledgments

We sincerely acknowledge the members of the Laboratory of Crop Science, Faculty of Agriculture, Yamagata University, for their invaluable support in this study. We also extend our gratitude to the staff of the Field Science Center at Yamagata University for their dedicated efforts in field management.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Effect of phosphorus (P) and silicon (Si) treatments on shoot P (a) and Si (b) concentrations after transplanting of high-density nursery seedlings in 2021 and 2022. Data are presented as mean ± SE (n = 3). Multiple means within each treatment were compared using ANOVA followed by Tukey’s honest significant difference test. Different letters indicate significant differences (p < 0.05) within each year. Treatment abbreviations: P0 = 0 mg kg−1 P, P60 = 60 mg kg−1 P, P90 = 90 mg kg−1 P, P0+Si2 = 2 g kg−1 Si, P60+Si2 = 60 mg kg−1 P + 2 g kg−1 Si, P60+Si4 = 60 mg kg−1 P + 4 g kg−1 Si, P90+Si2 = 90 mg kg−1 P + 2 g kg−1 Si.
Figure 1. Effect of phosphorus (P) and silicon (Si) treatments on shoot P (a) and Si (b) concentrations after transplanting of high-density nursery seedlings in 2021 and 2022. Data are presented as mean ± SE (n = 3). Multiple means within each treatment were compared using ANOVA followed by Tukey’s honest significant difference test. Different letters indicate significant differences (p < 0.05) within each year. Treatment abbreviations: P0 = 0 mg kg−1 P, P60 = 60 mg kg−1 P, P90 = 90 mg kg−1 P, P0+Si2 = 2 g kg−1 Si, P60+Si2 = 60 mg kg−1 P + 2 g kg−1 Si, P60+Si4 = 60 mg kg−1 P + 4 g kg−1 Si, P90+Si2 = 90 mg kg−1 P + 2 g kg−1 Si.
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Figure 2. Correlations between rice growth factors and nutrition absorption. The color of each plot indicates the correlation between corresponding rice growth factors and nutrition uptake. Red color means a highly positive correlation, and blue color means a highly negative correlation. The numbers in each plot are the correlation coefficient (r). The rice growth factors include shoot dry weight (SDW), plant height (Hight), and tiller number (Tiller). The nutrient absorption includes P concentration (P conc.), P uptake, Si concentration (Si conc.), and Si uptake. Asterisks indicate significant differences (* p < 0.05, ** p < 0.01, *** p < 0.001).
Figure 2. Correlations between rice growth factors and nutrition absorption. The color of each plot indicates the correlation between corresponding rice growth factors and nutrition uptake. Red color means a highly positive correlation, and blue color means a highly negative correlation. The numbers in each plot are the correlation coefficient (r). The rice growth factors include shoot dry weight (SDW), plant height (Hight), and tiller number (Tiller). The nutrient absorption includes P concentration (P conc.), P uptake, Si concentration (Si conc.), and Si uptake. Asterisks indicate significant differences (* p < 0.05, ** p < 0.01, *** p < 0.001).
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Figure 3. The relationship between phosphorus (P) uptake and shoot dry weight (a) and silicon (Si) uptake and shoot dry weight (b) after transplanting of high-density nursery seedlings in 2021 and 2022. Vertical bars indicate the standard error of three replicates. Dashed lines represent regression lines, and gray lines indicate the upper and lower bounds of the 95% confidence interval. Treatment abbreviations: P0 = 0 mg kg−1 P, P60 = 60 mg kg−1 P, P90 = 90 mg kg−1 P, P0+Si2 = 2 g kg−1 Si, P60+Si2 = 60 mg kg−1 P + 2 g kg−1 Si, P60+Si4 = 60 mg kg−1 P + 4 g kg−1 Si, P90+Si2 = 90 mg kg−1 P + 2 g kg−1 Si.
Figure 3. The relationship between phosphorus (P) uptake and shoot dry weight (a) and silicon (Si) uptake and shoot dry weight (b) after transplanting of high-density nursery seedlings in 2021 and 2022. Vertical bars indicate the standard error of three replicates. Dashed lines represent regression lines, and gray lines indicate the upper and lower bounds of the 95% confidence interval. Treatment abbreviations: P0 = 0 mg kg−1 P, P60 = 60 mg kg−1 P, P90 = 90 mg kg−1 P, P0+Si2 = 2 g kg−1 Si, P60+Si2 = 60 mg kg−1 P + 2 g kg−1 Si, P60+Si4 = 60 mg kg−1 P + 4 g kg−1 Si, P90+Si2 = 90 mg kg−1 P + 2 g kg−1 Si.
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Table 1. Characteristics of high-density nursery seedlings in the experiment conducted in 2021 and 2022.
Table 1. Characteristics of high-density nursery seedlings in the experiment conducted in 2021 and 2022.
YearPlant HeightPlant AgeLeaf ColorDry WeightSeedling Index
(cm) (SPAD)(mg)
202111.623.1229.789.962.68
202211.282.9529.678.222.15
Leaf color was measured using soil–plant analysis development. Seedling index was calculated by multiplying the plant age per plant height by the seedling dry weight.
Table 2. Physicochemical properties of the soil used in the experiment.
Table 2. Physicochemical properties of the soil used in the experiment.
pHECAvailable NAvailable PCECExchangeable CationHumus
CaMgK
(mS cm−1)(mg kg−1)(mg kg−1)(cmolc kg−1)(cmolc kg−1)(cmolc kg−1)(cmolc kg−1)(%)
5.10.0663.9250.1212.894.371.180.252.9
EC, electrical conductivity; Available N, available nitrogen, Available P, available phosphorus; CEC, cation exchange capacity.
Table 3. Effect of phosphorus (P) and silicon (Si) treatments on plant height, tiller number, and shoot dry weight after transplanting high-density nursery seedlings in 2021 and 2022.
Table 3. Effect of phosphorus (P) and silicon (Si) treatments on plant height, tiller number, and shoot dry weight after transplanting high-density nursery seedlings in 2021 and 2022.
Year TreatmentPlant HeightTillerShoot Dry Weight
(cm)(pot−1)(g pot−1)
2021P074.3±1.2a29.0±2.3a31.1±1.1b
P6074.5±3.5a33.3±2.6a36.3±1.1ab
P9073.0±2.1a31.3±2.2a35.7±1.9ab
P0+Si277.2±1.7a32.7±2.2a39.2±1.5a
P60+Si275.3±1.5a33.7±1.5a38.4±1.4a
P60+Si479.7±2.2a30.7±0.3a41.8±0.4a
2022P072.3±0.6a43.0±5.5a45.8±0.4b
P6072.5±0.9a40.0±1.2a52.1±2.4ab
P9071.5±1.3a40.3±1.5a56.1±1.4a
P0+Si271.0±1.5a43.0±2.5a55.1±2.1ab
P60+Si272.2±1.1a43.7±2.6a55.2±2.1ab
P90+Si273.0±0.9a39.7±1.7a56.1±3.4a
P60+Si472.8±1.9a42.0±1.5a57.0±1.0a
ANOVAYear*********
Treatmentn.s.n.s.***
Year × Treatmentn.s.n.s.n.s.
Data are presented as mean ± SE (n = 3). Multiple means at treatment were compared using ANOVA with Tukey’s honest significant difference (HSD) test. Different letters indicate significant difference (p < 0.05) in each year. Asterisks indicate significant differences (*** p < 0.001); ns, not significant. Treatment abbreviations: P0 = 0 mg kg−1 P, P60 = 60 mg kg−1 P, P90 = 90 mg kg−1 P, P0+Si2 = 2 g kg−1 Si, P60+Si2 = 60 mg kg−1 P + 2 g kg−1 Si, P60+Si4 = 60 mg kg−1 P + 4 g kg−1 Si, P90+Si2 = 90 mg kg−1 P + 2 g kg−1 Si.
Table 4. Effect of phosphorus (P) and silicon (Si) treatments on grain yield and yield components after transplanting high-density nursery seedlings in 2022.
Table 4. Effect of phosphorus (P) and silicon (Si) treatments on grain yield and yield components after transplanting high-density nursery seedlings in 2022.
Grain YieldPaniclesSpikeletsFilled Spikelets1000-Grain Weight
(Mg ha−1)(m−2)(panicle−1)(%)(g)
P05.73±0.30a470.4±41.2a59.1±2.6a90.3±0.4a23.0±0.1a
P606.74±0.11a590.7±12.3a55.9±0.9a88.5±0.6a23.1±0.1a
P906.40±0.21a598.1±19.5a54.0±2.3a86.1±1.7a23.1±0.2a
P0+Si26.69±0.08a600.0±21.1a54.8±1.0a88.8±1.9a23.0±0.3a
P60+Si26.39±0.12a533.3±33.9a60.8±1.4a86.5±1.2a22.9±0.2a
P90+Si26.22±0.68a518.5±35.3a57.4±2.0a90.1±0.1a23.0±0.2a
P60+Si46.51±0.32a607.4±19.6a53.8±1.4a86.6±3.5a23.0±0.1a
Data are presented as mean ± SE (n = 3). Multiple means at treatment were compared using ANOVA with Tukey’s honest significant difference (HSD) test. Different letters indicate significant difference (p < 0.05). Treatment abbreviations: P0 = 0 mg kg−1 P, P60 = 60 mg kg−1 P, P90 = 90 mg kg−1 P, P0+Si2 = 2 g kg−1 Si, P60+Si2 = 60 mg kg−1 P + 2 g kg−1 Si, P60+Si4 = 60 mg kg−1 P + 4 g kg−1 Si, P90+Si2 = 90 mg kg−1 P + 2 g kg−1 Si.
Table 5. Effect of phosphorus (P) and silicon (Si) treatments on available P and Si in soil after transplanting high-density nursery seedlings in 2021 and 2022.
Table 5. Effect of phosphorus (P) and silicon (Si) treatments on available P and Si in soil after transplanting high-density nursery seedlings in 2021 and 2022.
YearTreatmentPSi
(mg kg−1)(mg kg−1)
2021P0286.9±4.32b51.2±2.52a
P60301.7±4.78ab51.9±0.53a
P90318.1±2.74a50.2±1.16a
P0+Si2289.2±5.03ab52.4±0.51a
P60+Si2298.6±4.03b54.5±0.39a
P60+Si4302.5±3.41ab57.2±2.72a
2022P0194.6±4.11d41.6±0.49a
P60208.7±1.44b43.3±1.35a
P90222.1±2.17a42.0±1.50a
P0+Si2198.2±1.89cd48.1±2.26a
P60+Si2206.5±2.41bcd45.1±0.75a
P90+Si2214.6±3.33ab46.8±3.90a
P60+Si4209.8±0.52bc50.5±1.50a
ANOVAYear******
Treatment*****
Year × Treatmentn.s.n.s.
Data are presented as mean ± SE (n = 3). Multiple means at treatment were compared using ANOVA with Tukey’s honest significant difference (HSD) test. Different letters indicate significant difference (p < 0.05) in each year. Asterisks indicate significant differences (**p < 0.01, *** p < 0.001); ns, not significant. Treatment abbreviations: P0 = 0 mg kg−1 P, P60 = 60 mg kg−1 P, P90 = 90 mg kg−1 P, P0+Si2 = 2 g kg−1 Si, P60+Si2 = 60 mg kg−1 P + 2 g kg−1 Si, P60+Si4 = 60 mg kg−1 P + 4 g kg−1 Si, P90+Si2 = 90 mg kg−1 P + 2 g kg−1 Si.
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Nasukawa, H.; Tajima, R. Preliminary Results on the Application of Phosphorus and Silicon to Improve the Post-Transplantation Growth of High-Density Nursery Seedlings. Agronomy 2025, 15, 937. https://doi.org/10.3390/agronomy15040937

AMA Style

Nasukawa H, Tajima R. Preliminary Results on the Application of Phosphorus and Silicon to Improve the Post-Transplantation Growth of High-Density Nursery Seedlings. Agronomy. 2025; 15(4):937. https://doi.org/10.3390/agronomy15040937

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Nasukawa, Hisashi, and Ryosuke Tajima. 2025. "Preliminary Results on the Application of Phosphorus and Silicon to Improve the Post-Transplantation Growth of High-Density Nursery Seedlings" Agronomy 15, no. 4: 937. https://doi.org/10.3390/agronomy15040937

APA Style

Nasukawa, H., & Tajima, R. (2025). Preliminary Results on the Application of Phosphorus and Silicon to Improve the Post-Transplantation Growth of High-Density Nursery Seedlings. Agronomy, 15(4), 937. https://doi.org/10.3390/agronomy15040937

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