Silicon Application Methods Influence the Nutrient Uptake of Maize Plants in Tropical Soil

The benefits of applying silicon to plants under stressful conditions are recognized. However, few studies have shown the effect of supply and form of application of silicon on the nutrition of plants grown under ideal conditions. This study aimed to verify the effects of different methods of silicon application on the nutrient uptake of maize in two tropical soils. Silicon was supplied in three application methods (in the planting furrow, in the total pot area, and spraying) at two rates (30 and 120 mg Si kg−1 in soil; 2.56 and 7.68 mg Si pot−1 in leaf spraying) in maize plants in two tropical soil types (loamy sand textured soil and sandy clay loam textured soil). Thirty days after emergence, the concentrations of macronutrients, micronutrients, and silicon were evaluated. In sandy soil, potassium silicate application contributed to an increase in N, P, K, Mg, Si, and Cu concentrations, whereas in clay soil, there was an increase in the plant concentrations of P, K, Ca, Mg, S, Cu, Si, and Mg. It was concluded that silicon application contributed to greater nutrient uptake in maize plants. Our study suggests that silicon application could be an important tool for increasing mineral fertilization in tropical soil conditions.


Introduction
The maize crop represents 30% of food sources in the Americas, 38% in the African continent, and 6.5% in the Asian continent [1].Globally, over one billion tons of maize are produced each year [2] for human consumption and biofuel production, and it is estimated that by 2050, this production will double [1].Maize crops are highly important for the human diet, mainly because of their nutritional value and the presence of vitamin E, phenolic compounds and carotenoids that help in chronic disease prevention [3].This crop is also an important food for the animal diet, in developed countries, 70% of the corn grown is intended for livestock feed [4].Silicon (Si) is a beneficial element for crops, as it reduces biotic and abiotic stresses [5][6][7][8][9][10].In addition, recent studies have reported that silicate application may increase the uptake efficiency of different crops, contributing to high nutrient concentrations in plant shoot biomass [11][12][13].Potassium silicate application, when sprayed at a concentration of 6.3 mg Si/10 plants, increased N, P, and K in wheat crops under saline stress [14].Studies by Elrys and Merwad [15] and Merwad [16] showed that spraying Si increases leaf nutrient content in pea crops.
Some studies have been undertaken to assess the impact of Si fertilization on nutrient uptake in various crops, providing different findings.Pati et al. [17] observed that Si application in alkaline soil increased the nutrient concentration of rice crops, for example, Si (33%), N (44%), P (29%), and K (16%).In contrast, Jang et al. [18] conducted a study in nutrient solution and verified that Si application decreased Ca uptake by 11-18% in rice crops.Hu et al. [19] showed that Si application reduced P uptake in rice crops due to the negative regulation of a transporter gene (OsPT6), which occurred when Si accumulated in the shoot biomass.A few studies have shown that the greatest use of Si by plants occurs when this element is absorbed by the roots [20,21], while other reports have stated that high Si absorption occurred with spraying [22,23].While these studies provided data on nutrient uptake in plants with Si application, they did not determine the most efficient method of Si application.Therefore, the effects of Si application via planting furrow, spraying, and over the total pot area regarding maize nutrient uptake, especially in tropical conditions remain unclear.
Soils of tropical regions are highly weathered, with predominance of clay minerals of type 1:1 (for exemple, kaolinite) and Fe, Al and Mn oxides, with low natural fertility, low pH and low levels of Si available for plants [24][25][26][27].FAO estimates indicate an annual loss of 210 to 224 million tons of Si from arable soils globally [28].So, supplementation with Si is important to return the Si extracted from these soils.
As a result of the weathering process, the low fertility of tropical soils demands the application of large amounts of fertilizers, in order to build their fertility and make possible the cultivation of plants, however, even with nutritional supplementation, the efficiency of fertilizers in these soils is considered low, because some nutrients, such as P, for example, have ease in forming complexes with Fe and Al oxides, or others how the K, Mg and Ca can also be leached [29,30].These factors reduce the availability of nutrients for plants and increase production costs, because to meet the need for crops in these soils farmers have doubled the use of fertilizers.Considering that tropical soils have low Si contents and that the supply of Si has been shown to improve the efficiency of nutrient absorption in alkaline soil [17] and under conditions of nutritional deficiency [31,32], it is necessary to study the effect of Si on nutrient absorption in tropical soils, especially in soils weathered.
Furthermore, the positive effect of Si on nutrient uptake has been reported under stress conditions, for example, low nutrients, hydric deficiency, and salinity stress [7,31,33].Currently, the effect of Si on plant nutrient uptake under optimal conditions as a factor in nutrient uptake by crops in tropical soil conditions is undetermined.
In this context, we hypothesized that: i) Si application methods would influence maize nutrient uptake in tropical conditions; ii) soil Si application would increase this uptake to a greater extent than the spraying application, and iii) Si concentration could influence nutrient uptake by maize plants under ideal conditions.This study aimed to verify the efficiencies of different methods of Si application on the nutrient uptake of maize in two tropical soils.

Material and Methods
Two experiments were performed, one in loamy sand textured soil (referred to as sandy soil) and the other in sandy clay loam textured soil (referred to as clay soil), at the Department of Forest Science, Soil and Environmental Resources, Faculty of Agronomic Sciences, São Paulo State University (FCA/ UNESP), located in Botucatu, São Paulo, Brazil.Both experiments occurred in a tunnel-type plastic greenhouse.

Experimental Design and Treatments
The experiments were conducted in randomized blocks with four replicates.The treatments consisted of three forms of potassium silicate application (in the planting furrow, in the total area, and by spraying) and two rates of Si (Si1 and Si2).A total of seven treatments were applied: control (no Si application), Si1 PF (30 mg Si kg −1 in planting furrow), Si2 PF (120 mg Si kg −1 in planting furrow), Si1 TA (30 mg Si kg −1 in total area of pot), Si2 TA (120 mg Si kg −1 in total area of pot), Si1 Leaf (spraying of 2.56 mg Si pot −1 ), and Si2 Leaf (spraying of 7.68 mg Si pot −1 ).Each experiment had 28 experimental units, represented by pots with a capacity of 4 L of soil (23 × 26 × 19 cm).

Greenhouse Experiments
The soils used in the two experiments were classified as Ustox [34], with low natural fertility.One experiment used soil with a sandy texture and the soil in the other had a clayey texture.Chemical and granulometric results of soils are presented in Table S1.Dolomitic limestone (PNV = 95%) was applied and incorporated into the potted soil and the soil was incubated for a period of 30 days with the objective of increasing the base saturation to 70%, as recommended for maize crops [35].After this period, fertilization was performed as recommended by Raij et al. [35].
The precocious hybrid maize seeds (FS521 cultivar) used possessed the Bacillus thuringiensis (Bt) technology and were treated with clothianidin + chlorantraniliprole.The sowing was on February 4, 2021, and was sowed four seeds per pot.Emergence occurred on February 9, 2021, when thinning was performed, leaving two seedlings per pot.
At sowing, 0.13 g of nitrogen (N), 0.43 g of phosphorus (P), and 0.17 g of potassium (K) were applied to each pot in the form of urea, superphosphate, and potassium chloride (KCl), respectively.Also, 0.53 g of N was applied as a topdressing 15 d after plant emergence.The soil moisture was maintained at 70% of field capacity.
The treatments with the lowest concentration of Si in the soil (30 mg kg −1 of Si) were supplemented with KCl to reach the K values recommended by Raij et al. [35].In the experiments with the highest Si concentration (120 mg kg −1 of Si), K was supplied as potassium silicate, at a level higher than that recommended by Raij et al. [35].The treatments that received spraying of potassium silicate or application of K in soil were supplied with KCl [35].
The compositions of the potassium silicate sources used are listed in Table S2.Potassium silicate was applied in the soil for fertilization.The potassium silicate supply in the planting furrow was added five centimeters below and beside the seeds; for application in the total area, potassium silicate was incorporated throughout the soil of the pot.
Potassium silicate was sprayed twice when the plants presented four and six fully expanded leaves (V4 and V6 stages) at 11 and 23 days after emergence, respectively.An accumulated manual pressure sprayer with a capacity of two liters of water was used for this application.
After 30 days of plant emergence, two plants were cut close to the ground for nutritional evaluation of macro-and micronutrients and Si in the shoot.The plants were washed and oven-dried in a forced-air circulation oven at 65 °C until a constant weight was achieved.Afterwards, they were ground in a Wiley-type knife mill.Macro-and micronutrient analyses were performed according to the methodology described by Malavolta et al. [36], and the Si content was quantified according to Korndörfer et al. [37].
The data were subjected to the Shapiro-Wilk normality test, followed by one-way analysis of variance (ANOVA) at a significance level of p < 0.05, and the Scott-Knott test was applied for comparison of means.Agroestat [38] was used for the ANOVA and the test of means, and Minitab [39] was used for the normality test.

Results
There were significant differences in nutrient concentration of the shoot biomass under the different application methods of potassium silicate by the Scott-Knott test at 5% (Fig. 1).In the sandy soil, the highest values of N were found in Si1 PF (51% higher than control), and this was resembling to the other treatments that received applications of Si (Fig. 1a).For P, the highest values were observed in Si1 Leaf (43% higher than control), Si2 TA (36% higher than control), and Si1 PF (36% higher than control) (Fig. 1b).For K, the highest increase was noted in Si2 TA (127% higher than the control) (Fig. 1c).Mg concentrations were greatest in Si1 TA (50% higher than control), Si1 PF (44% higher than control), Si1 Leaf, and Si2 Leaf (43 and 35% higher than control, respectively) (Fig. 1d).S concentrations were higher when Si was applied, regardless of the application methods or rates of Si (Fig. 1e).Cu showed the highest values in the Si1 PF, Si1 TA, Si2 TA, and Si1 Leaf (48%, 48%, 48%, and 41% higher than control) (Fig. 1f).Finally, for Si concentrations (Fig. 1g), the greatest values were found in Si1 PF (993% higher than control), Si1 TA (913% higher than control), Si Leaf, Si2 PF and Si2 TA (993%, 891%, and 800% higher than control, respectively).In the sandy soil there was no significant difference between the potassium silicate application methods for the levels of Ca, B, Fe, Mn and Zn (means of 4.3, 12.5, 115.1, 34.0 and 9.2 respectively) in the shoot of the maize plants (Fig. 1d, g, i, j and k).
When examining the effect of rate and Si application form (potassium silicate) in clay soil, there was a significant difference in the concentrations of P, K, Ca, Mg, S, Cu, Mn, and Si (Fig. 2).Independent of rate and silicon application method, an increase in P concentration, on silicon application method in planting furrow and with low rate of Si (Si1 PF), was noted an increase of 41% of P in relation the control (Fig. 2a).For K concentration, the highest values were found in Si2 TA (49% higher than control) (Fig. 2b).For Ca concentration, the highest values were found in Si2 PF, Si1 TA, Si2 TA, Si1 PF and Si1 Leaf (56%, 49%, 49%, 36%, and 36% higher than control, respectively) (Fig. 2c).For Mg concentration, the greatest values were observed in Si1 PF, Si1 TA, and Si1 Leaf (41%, 41%, and 34% higher than control, respectively) (Fig. 2d).For S, the highest concentrations were found in Si1 PF (42% higher than control), Si2 TA (40% higher than control), and Si1 PF (35% higher than control) (Fig. 2e).Cu concentration was similar among the treatments that received Si, regardless of the application method and doses of Si however these differed from the treatment that did not receive Si application (Fig. 2f).Mn concentrations were greatest in Si1 TA (54% higher than control), Si1 PF (48% higher than control), and Si2 TA (39% higher than control) (Fig. 2g).For Si, the highest values were found in Si2 TA (488% higher than control) and Si1 PF (432% higher than control) (Fig. 2h).
In clay soil, the potassium silicate application methods did not influence on the concentration N, B, Fe, and Zn.The shoot of the maize plants presented mean of 28.1, 17.3, 140.5 and 10.1, for concentration of N, B, Fe and Zn, respectively (Fig. 2a, g, i, and k).

Discussion
Plant nutrient uptake was improved in all treatments that received Si, confirming our hypothesis.Essentially, these data showed that Si rate and application form may improve nutrient uptake by maize crops in both soil types.The results showed that six (in sandy soil) and seven (in clayey soil) essential nutrients had increased uptake in maize plants when Si was applied in the total pot area and planting furrow.
Silicon application in planting furrows and total pot area promoted P uptake in the maize plants.Recently, some reports have shown the role of Si in nutrient availability, mainly due to soil acidity correction [40,41] and competition for adsorption sites with anions (e.g., phosphate).However, due the limited diffusion of P in the soil and low use-efficiency in tropical soils (~ 10-20%), it is possible that Si application increases the physiological traits of crops.According to Lata-Tenesaca et al. [42], Si plays an important role in photosynthesis and transpiration, increasing the expression of the gene responsible for plant nutrient uptake.In addition, higher P concentrations using the planting furrow and total area methods compared to spraying provide evidence that uptake of Si by roots can be linked with transport from the root to plant shoots.According to Pavlovic et al. [32], Si application may regulate some genes responsible for P transporters, such as TaPHT1 and TaPHT2, under low availability and acidic soil conditions.These findings suggest a possible role of Si in plant physiology.
The role of Si in plant stress attenuation (e.g., salinity, low nutrient availability, and hydric stress) has been reported, but under ideal conditions in tropical soils, the role of Si in relation to increased nutrients is poorly understood.Silicon has been suggested to equilibrate the K + : Na + ratio, and regulate the plant hormone-related AKT2, AKT2/3-like K channel [43].Recently, evidence obtained by Yan et al. [44] showed that Si has a regulatory role in nutrient uptake by reducing abiotic stresses, such as salinity, and increasing K uptake.According to those authors, Si application improved the expression of genes (OsAKT1 and OsHAK1) responsible for K uptake and transport by xylem.The Si application source is also relevant, and in this study, the source was enriched with K, which was expected to result in a high K concentration.Furthermore, the Fig. 1 Effects of potassium silicate application methods and rates of silicon on concentration of nitrogen (a), phosphorus (b), potassium (c), calcium (d), magnesium (e), sulfur (f), boron (g), copper (h), iron (i), manganese (j), zinc (k) and silicon (l) in shoot of maize plants.Experiment with sandy soil.Means followed by the same letter do not differ from each other by the Scott & Knott test.Error bars represent the standard error (n = 4).Treatments abbreviations: control, no Si application; Si1 PF, 30 mg Si kg −1 in planting furrow; Si2 PF, 120 mg Si kg −1 in planting furrow; Si1 TA, 30 mg Si kg −1 in total area of pot; Si2 TA, 120 mg Si kg −1 in total area of pot; Si1 Leaf, spraying of 2.56 mg Si pot −1 ; and Si2 Leaf spraying of 7.68 mg Si pot.−1 .** = significant at 1 and 5% level.* = significant at 5% level.ns = no significant (Scott-Knott's test) high K concentration found with Si applied to the total area at 120 mg kg −1 , may be explained by the Si source used, which promoted ideal rates of K, as described by Sarah et al. [45].In addition, the potassium silicate application using spraying was lower than that via planting furrows and the total area, which is in agreement with the findings of Ali et al. [46].
In sandy soil, the Mg concentration was increased by Si application.These results agree with studies by Merwad [16].According to the findings of those authors, the Si application increased Mg uptake to 237% in sandy soil conditions.However, when Si was applied to the total area and sprayed at high rates, low Mg uptake was found in maize plants.This may be related to the source used in that study (KSiO 3 ).According to Xie et al. [47] and Sirisuntornlak et al. [48], excessive K application increases the risk of Mg deficiency in plants due to higher K in the soil solution reducing Mg uptake, since these interactions can occur during root uptake, translocation, distribution, and utilization by plants.In addition, Si application promoted higher Ca uptake than the control in both soil conditions.According to Buchelt et al. [49], the role of Si in calcium uptake and efficiency may be related to the effect of this beneficial nutrient on leaf structure and cell wall components (e.g., cell polymers, lignin, and carbohydrate concentrations), and Si application may reduce lipid peroxidation through a There were no significant differences in the effects of S on plant uptake in sandy soil, independent of the rates and application methods.However, for the clay soil, the application in the planting furrow and total area produced the highest S concentration.This may be related to the capacity of clay minerals and iron oxides to adsorb S in tropical soils, thus reducing soil S availability.This condition may create S deficiency in crops [50].For the Si application in the total area, this phenomenon may be reduced by soil pH correction, competition with S by adsorption sites, or by genetic factors.According to studies conducted by Lainé et al. [51], Si application may regulate the genes responsible for S transport (e.g., BnaSultr1), which may be substantially reduced in rice plants under stress.
In sandy soil, the application of Si resulted in an increase in the Cu concentration in maize shoots, irrespective of Si application methods.However, in clay soils, the Cu concentration varied depending on the method of Si application.To date, studies have reported only on Si application as a function of toxicity scenarios of crops by Cu [52].In this present study, it was observed that Si may contribute to an increase in the Cu content in plants, potentially serving to enhance Cu absorption efficiency.However, further studies need to be conducted to substantiate this hypothesis.
Our results showed that Si application may increase Mn uptake when applied to the total area and planting furrow in clay soil.According to Oliveira et al. [53], Si application through the root was more beneficial than leaf spraying due to the high capacity of Poaceae plants to accumulate Si; thus, high uptake was expected.The main role of Si application in alleviating Mn stress may be linked to the reduction of oxidative stress on plants (by improving enzyme regulation), thereby increasing plant photosynthetic efficiency, leaf area, Mn use-efficiency, and dry matter.
Our findings suggest that Si application could be an important tool to increase mineral fertilization in tropical soils, as the main nutrients requested by plants had higher uptake by crops in the Si application plots than those in the control plots under tropical conditions.

Conclusion
In sandy soil, potassium silicate application contributed to an increase in N, P, K, Mg, Si, and Cu concentrations, whereas in clay soil, there was an increase in the plant concentrations of P, K, Ca, Mg, S, Cu, Si, and Mg.For these nutrients, greater increases were observed with the supply of Si through the planting furrows and in the total pot area.In general, the two rates of Si used contributed to an improvement in nutrient uptake.This study indicates that Si application in the form of potassium silicate, even under ideal conditions, improves nutrient uptake by maize crops in tropical soils; however, it is worth noting that our study was conducted for up to 30 days of the plant cycle.
More studies with Si application methods evaluating the complete cycle of maize culture should be performed, as they may be the key to increasing the nutrient concentration of edible parts of plants and contributing to food security.

Fig. 2
Fig. 2 Effects of potassium silicate application methods and rates of silicon on concentration of nitrogen (a), phosphorus (b), potassium (c), calcium (d), magnesium (e), sulfur (f), boron (g), copper (h), iron (i), manganese (j), zinc (k) and silicon (l) in shoot of maize plants.Experiment with clay soil.Means followed by the same letter do not differ from each other by the Scott & Knott test.Error bars represent the standard error (n = 4).Treatments