The Use of Si‑Based Fertilization to Improve Agricultural Performance

Silicon (Si) is a “ quasi-essential ” element, associated to stresses/limitations alleviation in crops. However, stressful situations are becoming the norm, due to climate change, human exhaustion of natural resources, land degradation and loss of soil biodiversity . In this context, Si becomes a critical element, capacitating crops to grow more and better with less. A total of 467 articles up to 2021 were selected, reporting 501 experiments (lab-oriented and field trials) resulting in 682 cases where different Si-based fertilizers were used to study crops/plants with agronomic value to evaluate its physiological and/ or agronomic performance under a specific motivation. Results show that 63% of cases testing Si-fertilization show increase in productivity, while 13% refuted this observation and 24% did not evaluate plant growth. Crop physiological responses was registered in 42% of the cases and only 3% did not report any alteration, although 55% did not evaluate these indicators. Symptom alleviation, in cases where stresses/limitations were studied, was registered in 74% of the cases with a low number of negative outcomes (9%). Field trials where crop yield was measured after application of Si-fertilization show that 69% of different measurements (599) register a yield improvement ≥ 5%, while 18% of the measurements registered between 0 and 5% and only 14% reported yield loss. Si-fertilization is therefore and important factor to improve crop yield and capacitate crops with resilience to endure future limitations. Its inclusion in modern agriculture should be considered in larger scales to adapt crops to the current challenges of sustainable agriculture and food provision.


Introduction
Silicon (Si) is an element frequently associated with minerals, due to its high representativity in the terrestrial systems in the form of quartz and feldspar. Lately, the importance of Si in the plant systems has grown and became an independent field of study. First, scientists acknowledge that soils act as Si filters (Struyf and Conley 2012;Schäller et al. 2021), recycling the weathered material through plants and impacting on the quantity of dissolved Si reaching the water zones. Studies showed that land use (Barão et al. 2014;Vandevenne et al. 2015), soil type (Georgiadis et al. 2014;Ronchi et al. 2015) and climate (Blecker et al. 2006;Unzué-Belmonte et al. 2017) among other factors influence vegetation abundance and variety and therefore the amount and type of Si-accumulation in plant cells in the form of phytoliths (Piperno 2006;Saccone et al. 2007). The accumulation of Si in plant cells showed that this element plays an essential role in ecosystems (Katz et al. 2021), by coating plants with a defensive weapon against herbivory (Bakhat et al. 2018;Hartley 2015;Hummel et al. 2011). Advancements in this field pinpointed plants, which are actively able to accumulate Si (Deshmukh and Bélanger 2016), with better performance in stressful, toxic or limited situations (Meharg and Meharg 2015), compared to plants which are not able to accumulate Si (Bhatt and Sharma 2018;Guntzer et al. 2012).
Among the highest Si-accumulators (Ma and Takahashi 2002), figure plants with an Si concentration over 1% and a [Si]/[Ca] > 1, such as grasses and crops like rice, wheat, barley, maize, sugarcane, sugar beet or tomatoes (Hodson et al. 2005). It was interesting to conclude that accumulators were mostly crops with significant value for the agricultural sector, contrarily to the Si "excluders" which are mainly dicots. This fact led many scientists to investigate in detail the advantages of such uptake in anthropogenically modified 1 3 soil ecosystems. Highlighted benefits have been listed during the last years in many studies and included crop resistance to a variety of pests and diseases (Adrees et al. 2015;Bakhat et al. 2018;Fauteux et al. 2005;Reynolds et al. 2016), ability to tolerate higher levels of metal concentrations in soils (Adrees et al. 2015;Yu et al. 2016;Khan et al. 2021), ability to resist to drought and saline stressful situations (Coskun et al. 2016;Liu et al. 2019;Rios et al. 2017), improvement of nutrient uptake (Sheng et al. 2018) and alleviation of phosphorus deficiency (Schaller et al. 2019;Taylor 1961).
Despite the knowledge gathered about the potential beneficial effects of Si in plants listed before, consistent information about which plants benefit more from this supplementation and how attainable are yield improvements when Si-based fertilization is added is still scattered information (Tayade et al. 2022). Studies are often plant-specific and provide detailed results, but the global overview is missing (de Tombeur et al. 2021). Other findings resulting from recent studies suggest that Si beneficial potential for plant survival and growth go beyond situations where stress, limitation or toxic environment were induced on purpose (Besharat et al. 2020).
Nowadays, agriculture faces a new paradigm to produce sustainable food to successfully achieve zero hunger while preserving life on land (United Nations 2015). Part of this strategy includes adaptation/mitigation of the present conditions, and the ability to produce food efficiently, using less resources in limited and rapidly changing conditions. In this context, Si might be no longer a quasi-essential element (Epstein 1999), but may become a critical one, with potential to capacitate crops to grow more and better with less (Li et al. 2018). This work aims to move a step further in this field by providing clear answers regarding the full potential of Si-based fertilization. Studies where experiments were set up to evaluate the effect of Si-based fertilization in plants in a myriad of different situations were gathered. The goals set here were to (1) provide an overview on the Si-based fertilization research during the last decades, and (2) establish the current main findings about Si application potential such as (a) the percentage of experiments where Si supplement had a positive effect (either in growth, stress/limitation symptom alleviation and/or physiological response), (b) which stress/ limited situations are more easily overcome with Si-based fertilization and (c) how much yield improvement can be expected with this supplementation.

Manuscript Selection and Characterization
Manuscripts published up to 2021 in international peer reviewed journals with impact factor were selected when reporting studies where different types of Si-based fertilizers or Si-sources were tested on the growth and/or performance of crops/plants from the agro-food sector. Queries used in the search were the following combinations: "silicon fertilization", "silica fertilization", "silicon fertilizer", "silica fertilizer". The search was performed in different days along several months, to cover all the available publications. Publications such as reviews were excluded because they did not fill the criteria as mentioned previous, as well as results from experiments using plants outside the agro-food sector such as trees from forests, grasses, flowers, etc.
Each selected publication was added to a database with the following information: (a) Year of publication; (b) Title of the publication, (c) First author name, (d) Journal of publication. Additionally, information concerning specifically the experiment conducted in the publication was also inserted such as (e) Type of experiment performed, (f) Crop/ plant under study, (g) Motivation for using the Si-source fertilization, (h) Si-based fertilizer used in the study, (i) type of fertilizer application.
Four types of experiments were considered depending on the set up: petri dishes, hydroponic experiments, pot experiments conducted in the greenhouse or outside and field trials, with crops growing in bare soil. Crops identified in these studies were categorised into cereals, vegetables, fruits, leguminous crops or others. The motivations for the studies identified was based on previous identification from (Guntzer et al. 2012) about the effects of silicon in plants: Alleviation of toxicities (including all different metals such as Al, As, Cd, Pb, Zn, Cu etc.), alleviation of stresses (drought and salt mainly but also wind, cold and high temperature stresses), resistance to pests and diseases, improve nutrient uptake and balance, improve crop growth and ameliorate environmental impacts. Finally, the fertilizer application was considered as soil application, if directly applied in the field or in the pot in the solid or liquid form, foliar application when applied directly in the leaves, seed application when the product was used in the seed coting before seedling or plantation and nutrient solution, when in the hydroponic experiments the fertilization was supplied through the solution in the liquid form.
Single publications often reported studies on more than one crop/plant, different experiment types and/or different fertilizers used and different motivation conducted in separated or joint experiments, which was considered individually during the analysis.

Evaluation of Silicon-Based Fertilization Impact
Each single experiment reporting a crop growing under a specific silicon-based fertilizer (case) was evaluated on the following topics: (a) increase in the productivity, (b) alleviation of limitation/stressful symptoms and (c) alteration of the crop physiological indicators. For each of these fertilizers' application and evaluation, the possible answers were "Yes", "No" and not available "NA". To evaluate the increase of productivity and the alleviation of stress/limitation symptoms, quantitative results were taken into consideration from the Results section of the manuscript and/or the descriptive conclusions written by the authors in the abstract or the Conclusion section. Depending on the type of experiment, productivity was considered as the growth of seeds, plant heigh, dry biomass, etc. The symptoms were also dependent on the type of experiment motivation, i.e. number of lesions, concentration of different elements (As, Cu, Cd, etc.), concentration of saline elements in the leaves (Na + ), concentration of nutrients (N, P, K, Ca) among many others. Finally, the physiological response was also evaluated based on the reported values or descriptive conclusions concerning the alterations on chlorophyll, electrolyte leakage, net photosynthetic rate, stomatal conductivity, intercellular CO 2 concentration, transpiration rate, enzymatic activity, phenol content, etc.

Yield Improvement in the Field Trials
The field trials were selected among the other experimental studies and the yields reported in the Results section compared to the control plot reported. It was made sure that each comparison pair differ solely on the silica fertilization added. The improvement was therefore reported in percentage as stated in Eq. 1, where i is the number of different fertilizers/ dosages applied.
Results were collected directly from tables and indirectly from figures (estimated). Calculations were only done with available data; if yields were not provided in the study, they were not considered in the analysis. Finally, results from all the available yield improvements were grouped together in a histogram, to understand which improvements are more likely to be obtained when using Si-based fertilization. The analysis was done by plant category (cereals, vegetables, fruits, leguminous and others) and individually for valuable crops such as rice, wheat, maize and barley. experiments where silicon-based fertilizers were tested to improve crops/plants performance (Table S1). The number of articles published follows an exponential tendency, with year 2020 reaching a mark of 94 annual articles released ( Fig. 1). Within these articles, a total of 501 different experiments were identified (Table 1), ranging from lab-oriented conditions such as petri dishes (2%) and hydroponic experiments (34%) to soil cultivated crops in pots (41%) and in the field (21%). The fertilizer application method greatly differed within the experiments type (Table 1). While in petri-dishes fertilizer was mainly tested through seed coating (83%), in hydroponic experiments it was supplied by the nutrient solution (> 86%). In both pot experiments and field trials, the most used method to apply the silicon-based fertilizer was through soil application (> 79% and 58% respectively). However, foliar application of the tested fertilizer was quite abundant in field trials as well (27%).

Overview on Silicon-Based Fertilization Effects Studies
The most tested crops along the experiments identified were, by far, cereals, with special emphasis on rice, wheat, maize, barley and sorghum, reaching a total of ~ 60% in both hydroponic and pot experiments and field trials. Secondly, vegetables such as cucumber, tomato and sugar beet were also studied, especially in hydroponic experiments (26%) comparing to 14-15% in the other experiments. Additionally, sugarcane was also the focus on many studies, but mainly in pot experiments, while fruits and leguminous crops were targeted in a lower number of studies (4-7%). Melon and soybean were respectively the most studied crops in these categories (Fig. 2, Table S2).
The motivation for conducting the study and test the silicon-based fertilization changed according to the type of experiment. Hydroponic and pot experiments share the same distribution, with the alleviation of toxicity being the most usual (30-38%), follow by alleviation of stresses such as drought and salt (21-29%), improve resistance to pests and diseases (17-21%) and improve growth (12-17%). However,  Table S3).

Productivity Increase
Silicon-based fertilization in drought and saline conditions contributed to a significant increase in productivity, with 75-88% of cases in both pot, hydroponic and field experiments returning positive answers (Fig. 3A) and reduced number of negative outcomes (0-9%). Studies focused on alleviation of other stresses and limitations were not as successful as this one. In the alleviation of toxicities cases reporting positive responses in productivity after Si-fertilization represented 63-72% in hydroponic and pot experiments but were drastically reduced to 24% in field trials. Also, the number of negative outcomes was higher and reached 50% cases in also field trials (Fig. 3D). In nutrient uptake improvement, positive answers were more homogeneous within experimental types, always between 50 and 75%, but negative answers represented also between 15 and 25% of the experiment tests (Fig. 3G). In studies were pests and diseases alleviation were the focus, the number of positive outcomes for productivity increase was the lowest registered (15-33%), but the number of negative outcomes was not very high as well (4-21%), meaning that productivity was not quantified in most of studies with this motivation (Fig. 3J). Finally, studies where the cases focus was the growth potential of crops under Si-fertilization, results were mainly positive (79-89%), although negative answers, reflecting no improvement of yield or maintenance of the same yield represented still 6-12% along the different experimental types.

Crop/Plant Physiological Performance
The observation of physiological alterations in crops when silicon-based fertilization was used was generally positive (42%), meaning that measurements of physiological parameters show an alteration when the crop was exposed to a silicon-source, especially for studies focused on stress alleviation and toxicities (Fig. 3B, E). However, the number of experiments/cases where these parameters/indicators were not assessed was high, especially in pot experiments and field trials compared to hydroponic, and in studies where the motivation was the nutrient uptake improvement, the increase resistance to pests and diseases and improve crop growth (Fig. 3H, K, N) where the number of cases with no assessments could be > 50%.

Alleviation of Stress/Limitation Symptoms
Except for the studies focused on the stress alleviation, where the symptoms were not generally evaluated (percentage of N/A cases answers was ~ 50%- Fig. 3C), other experiments show that symptoms were generally evaluated and positive case answers in the alleviation was frequently high (< 75%), even reaching 96% in pot and field trials of studies where the motivation was the resistance to pests and diseases (Fig. 3L). Interestingly, the highest number of negative answers was achieved in the studies where the motivation was the nutrient uptake (50% negative answers in pot experiments) suggesting that the fertilization with silicon failed in turning nutrients more available for the crop (Fig. 3I).

Yield Improvement with Silicon-Based Fertilization
Results obtained from the field trials (Fig. 4) show that most measurements report yield improvement/stability (88%) compared to a small number of cases where there was yield regression (12%). The yield regression was mainly observed in cereals, fruits and other (sugarcane and rapeseed).
Within the positive measurements of yield improvement (≥ 0%), the highest frequency of observation was in classes of yield between 0 and 5%, 5 and 10% and 10-20% with 18%, 18%, 17% of measurements respectively. Classes including the report of higher yield improvements were less frequent, although yield improvements between 20 and 30% and between 50 and 100% still represented 9% and 12% of the measurements respectively.
When looking at the results per crop category, it is possible to conclude that cereals, fruits and other crops (mainly sugarcane and rapeseed) registered yield improvements in almost all classes, from yield regression to 100% yield improvement. For cereals, the highest frequency observed was for classes of yield between 0 and 5% (21%) followed closely by 5-10%, 10-20% and 20-30% with 19%, 15% and 14% respectively. Similarly, sugarcane and rapeseed (category others) follow a similar pattern for the highest frequency of measurements. But for vegetables, the highest frequency registered was for yield improvements between 50 and 100% (37%) and even 10% of the measurements returned yield improvements higher than 100%. Fruits also had the highest frequency of measurements in the class of yield improvement 10-20%, representing 45%, while leguminous crop registered 24% of measurements in the class of 30-40% yield improvement and 27% in the class of 50-100% yield improvement.
Analysis of the yield improvement for specific and important crops such as some cereals indicate that rice, wheat, maize and barley yield improvements with Si-based fertilization were rather different (Fig. 5). While studies for all crops reported some measurements from trials where yield improvements were negative or neutral, rice and maize also reported a high number of measurements with yield improvements between 5 and 30%. Wheat was the one showing a different pattern, with the number of measurements reporting yield losses or small yield gains (0-5%) practically equal to trials where gains were set between 5 and 30%. Barley measurements display no clear trend due to the low number of available data.

The Shape of the Si-Fertilization Experiments
From this study, we can conclude that majority of experiments testing the effects of Si fertilization are still conducted on a laboratorial scale-hydroponics or pot experiments and only a limited number are tested in field conditions (Table 1). Most crops/plants tested are cereals, mainly rice, and only limited amounts of vegetables, fruits and leguminous crops are tested to see the influence of Si fertilization on their performance, despite their ability to accumulate Si. This research reality is directly correlated with some current agronomic facts-rice is a very important crop (Khush 2003), especially in Asian countries where the need to feed an overgrowing population is big worry and it is constantly affected both with pests and As contamination (Ali et al. 2020). The dominance of hydroponics and pot experiments maybe be related to the convenience to conduct the experiments and the fact that conditions are easier to be controlled. Also, many researchers are pursuing answers related to the physiological effect that Si has in the plant, as observed in the significant number of physiological measurements conducted and therefore a laboratorial-scale controlled environment is the most suitable option (Fig. 3). The search for the exact mechanisms of the plant cells affected by Si and how this element helps plants cope with the stresses and limitations is of extreme importance for future adaptations Fig. 4 Histogram showing the frequency of measurements for different yield improvement classes (%) in field trials for cereals, vegetables, fruits leguminous and others where Si-fertilization was applied. Cereals include data from rice, wheat, maize, barley and millet.
Vegetables include data for onion, pumpkin, cucumber, potato and sugar beet. Leguminous include data from soybean, pea, bean. Fruits include data from mango, okra, melon and grape. Others include data from sugarcane and rapeseed  (Thorne et al.). While agronomic measurements can immediately provide an outcome related to the crop/plant performance in the presence of a Si fertilizer, it is critical to understand how is that fertilizer improving the crop/plants odds of adaptation. The ideal situation would be a gradient of testing scales, where a crop/plant test would go from the specific controlled added Si source, where the occurring processes are identified and described and gradually soil and climatic conditions would be added to fully understand the potential of adding Si in the crop/plant adaptation.

Does Si-Fertilization Matter?
Our results show that fertilization with Si had generally a positive influence on the crop/plant. When looking at the increase in productivity, 63% of the cases observed show improvement after Si fertilization, while 13% reported loss or maintenance and 24% of the cases did not evaluate it (Table 2(A)). Alteration and observation of physiological patterns in the crop was also highly detected, with 44% of cases reporting alterations and only 3% did not show any change when fertilized with Si (Table 2(B)). Unfortunately, positive cases cannot be higher due to the higher number of cases where these parameters were not measured (55%). The alleviation of limitation and stresses symptoms was 74% and the negative cases reporting no alteration on the crop and/or aggravation of the situation was as lower as 9% (Table 2(C)).
While pot and hydroponic experiments are not ideal to estimate the Si influence on the yield produced, it was possible to see that productivity reported in the form of dry weight and/or plant height collected a high number of positive answers from the cases in the experiments analysed (Fig. 3A, D, G, J and M) resulting in 63% of positive outcomes, 13% negative and 24% not evaluated.
The highest number of positive responses occurred in experiments testing the alleviation of stresses (mainly drought and saline), while in these same experiments alleviation of symptoms was not evaluated in 47% of the cases (Table 2(A)). This means that in these experiments, researchers used the plant growth as a response to drought and saline stress alleviation and by supplementing the crop with Si fertilization, they saw crops could cope easily with the stress and grow more, even in field trials.
On the opposite, when researchers were analysing the Si influence on the crop potential to deal with pests and diseases, they mainly assessed the symptoms alleviation such as the number of lesions, etc. and cared less for the increase of productivity, which is patent in ~ 60% of no evaluation of productivity indicators in the experiments (Fig. 3J). Anyway, the lower number of experiments available that did evaluate productivity in such conditions shows a higher number of negative/neutral answers to productivity (Table 2(A)) by comparison with higher number of symptom alleviation (Table 2(C)), suggesting that the elimination of pests and diseases is significantly alleviated through silica fertilization, but that does not translate in high yield improvements. In this case, the silica action is to protect the crop and guarantee that the yield obtain is not reduced. Silica acts in a protective role, and fertilization with silica ensures security for future attacks. The alleviation of toxicities using silica fertilization proved to be also efficient, with high positive answers in productivity increase and alleviation of symptoms (Table 2(C)). In these cases, silica fertilization serves two purposes, since it decreases the toxic concentration in the crop, eventually contributing for a healthier food consumption pattern, but also that leads directly and indirectly for a higher crop growth. However, these consistent results fail significantly in the field trials, where the number of experiments where no crop improvement in productivity was registered is higher than the positive. This means that the conclusion supported by the pot and hydroponic experiments suggesting that the high alleviation of toxic compounds makes the crop increase growth is not supported by in situ evidence. Field trials show that the main silica "field of action" is the reduction of toxic compounds in the crop and that it should be used only for this goal. This disparity of results from lab-scale results and field trials may be explained by the fact that in the first one's productivity is lightly assessed through dry mass and/ or other agronomic traits, while in field trials the yield is the only important parameter. While without toxic compounds in these leaves cells the crop can produce a higher number of leaves, higher stalks, that it is not enough to produce more agronomic yield.
The worst results in terms of productivity and symptom alleviation were registered in experiments testing the improvement of nutrient uptake and nutrient balance (Fig. 3G). The high number of negative responses to productivity increase indicate that even when the crop does uptake more nutrients (normally nitrogen, and/or calcium) after being exposed to silica fertilization, that does not influence its growth potential. Probably, it capacitates the crop with heathier tools to deal with attacks and limitations, but it does not translate necessarily into a direct growth. Moreover, the number of experiments reporting no improvement on the nutrient uptake in pot experiments highlights once more the necessity to understand the mechanism behind the link between Si and the N and P nutrients. Its interaction will occur primarily in the soil, then, it is highly probable that the soil biogeochemical characteristics (pH, organic matter, texture, parent material, microbiome, etc.) would conditionate the effect that Si has on the other element availability Rajput et al. 2021).
When looking at the crops that can mostly benefit from Si fertilization, it is possible to conclude that all crop categories under test show positive yield improvements in field trials. However, some publication bias must be considered since negative results are often not reported and not published (Murad et al. 2018). Most of the crops tested in field conditions show improvements that fall between 0 and 20% of yield improvement. While the improvements within the class of 0% to 5% indicate a quasi-neutral effect of the Si fertilization in the plant yield, it does not compromise the yield normally achieved in a control situation while still adding benefits to the crops. Moreover, the following classes highlight that Si fertilization can contribute to a significant increase in the yield achieved compared to reference situations. Cereals analysed in these studies such as rice, maize, wheat and barley are already known to be high Si accumulators and therefore their good performance under this fertilization is not a surprise. The good results shown by vegetables in the field can also be explained by the fact that cucumber, potato or sugar beet also accumulate high quantities of Si. Nevertheless, these results highlight the potential of using Si-fertilization as a supplement for growth improvement in larger scales. Finally, the positive results for both leguminous crops and fruits suggest that the potential to use Si-based fertilization goes being the Si accumulator crops (Putra et al. 2020). Especially the fruits analysed in these field trials results (mango, okra, melon and grape) show a high frequency of 10-20% yield improvements, with is significant and should be encouraged (Fig. 4).
Future research should also include meta-analysis studies that target specifically the Si-based fertilizers effective potential for plant growth. This would be dependent on important factors such as the local of production (soil type and climatic conditions), fertilizer type, application method and dosages. However, to obtain such results individually for each crop, it is also important that research manuscripts reporting data on experiments with Si-based fertilization provide all the information related to the experiment conditions and fertilization used, which was often not the case.

Where to Go from Here?
Research conducted up to now tell us that Si is a very important element to help crops cope with stresses and limitations and other types of attacks. It is stated already that also other non-Si accumulator crops can also benefit from the Si presence in soils and that even when no stress or limitation is induced, the Si added to the plant improves growth (Besharat et al. 2020). This later finding is probably related to the fact that there is always some limitation present and that plants do not grow normally in perfect conditions, which will even be more compromised in the future with climate change effects on agriculture and human pressure to produce food. There are several pathways for the future research in this area: 1. Improve the lab-controlled experiments on specific and agronomically significant crops/plants (both local, regional or globally) to clearly define the mechanisms and the processes by which the addition of Si is helping the plant (Riaz et al. 2021;Sahebi and Hanafi 2016). This involves not only the processes occurring in the plant, but also in the soil. By knowing the parameters affected by the Si-addition and the natural processes occurring, it will become easier to make predictions for other type of environment and the adaptation of the crop in a larger scale-environment. 2. Increase the number of field trial experiments for crops/ vegetables that have returned good results in the labcontrolled environments for the already known Si-based fertilizers to test the adaptation to climate and soil variances. The availability of water and the temperature as well as soil parameters such as pH, organic matter and CEC can greatly alter the Si effect on the plant (Haynes 2014;Samaddar et al. 2019). 3. Increase the dissemination projects to show to farmers that for the already in situ tested crops, the addition of Si to the soil will bring more income through yield improvement while turning the crops more resilient to the present and future limitations. Many farmers are already struggling with adaptation problems and making them aware of new solutions is a step further to make sure that the Si research conducted during the last 4 decades truly becomes significant. 4. Conduct life cycle assessment (LAC) and economic viability studies in the Si fertilizers already in use and on the main Si-sources used in the experiments. Up to now, the research is dominantly focused on the performance that the Si-based fertilizer has on the crop, but one need to have in mind that to be widely used the production of that fertilizer should also have economic advantages (Alvarez and Datnoff 2001) and their production and consumption impacts should not he high; otherwise, the solution found is not sustainable. This branch of research has not been explored and there is a currently need to know exactly what the environmental impact of the different Si-sources is, to make the right choice when dealing with Si-fertilization (Barão and Teixeira 2015). 5. Increase the tests on different Si-based sources as well as the search for different and more efficient sources itself Babu et al. 2016). The diversity of sources will ensure that there is no exploitation of a single source, and it is always possible to find a better suitable Si-fertilizer with less environmental footprint and economic advantage (Marxen et al. 2016). This is espe-cially important for the biological Si-sources such as the microorganisms (Rezakhani et al. 2019;Etesami 2018;Shao et al. 2016). This research has not been widely explored and yet it has a great potential. The search, isolation and description of Si solubilizing bacteria that can make available the mineral Si abundantly present in soils, as well as the influence of other microorganisms that can indirectly influence the increase of dissolved Si in the pore water for crop uptake. Lastly but not least is the significance of mycorrhiza in the Si uptake by plants. Some studies have already pinpointed this influence but is has not been explored to its full potential (Frew et al. 2017;Garg and Bhandari 2015;Van Hees et al. 2004;Verma et al. 2020). 6. Use models to predict the Si-based fertilizer effect on the crops. One way to avoid the multiplication of experimental trials, especially in the field which can consume a significant amount of time and money. Labcontrolled experiments are fundamental to understand the mechanism, as mentioned before, but once the process is biologically and mathematically described, the use of a model can contribute to predict easier and faster the conditions in which the fertilization will be successful and how much resistance and growth we can except (Cock and Yoshida 1970;Nguyen et al. 2016;Ronchi et al. 2013). By doing this, it will also be possible to shed some light on other topics such as the line between Si and the other nutrients (Keeping et al. 2014;Li et al. 2018;Murozuka et al. 2014;Xu et al. 2020) and how can we adopt agricultural management practices that are sustainable and potentiate the uptake of Si (Klotzbücher et al. 2018). 7. Other research lines related to Si in agriculture remain poorly studied but will certainly have a high impact in the future. It will be extremely important to understand exactly how Si impacts on human health (Farooq and Dietz 2015); this includes not only the benefits from having Si in the food, but also the potential damages, if any, of consuming it in excess. This is also extended to animals, and it is already a concern for farmers growing pastures amended with Si fertilization-what is the consequence for animals of a rich Si diet?

Conclusions
During the last decades, the shape of Si-research has been altered and it is nowadays more focused on agriculture and the benefits offered by a Si-based fertilization. In this work, an extensive list of published manuscripts was grouped and analysed to systematize results from experiments where Si fertilization was tested on different crops/plants.
Results show that the interest of researchers in testing the Si-based fertilization effects has increased exponentially during the last years and up to 2020. Their main motivation for the setting up of the experiments in field trial is to test the plant growth improvement when supplemented by Si, which shows the natural motivation of the researchers to reach the full application of this fertilization and turn it applicable for farmers. However, in lab-conditions, the experiments carried out were main focused on the alleviation of toxicities (30-38%), pinpointing the future expectations towards the potential of Si fertilization. The main conclusion from these facts is that Si-fertilizers as a technology are mainly at an intermediary readiness level, as they are currently being tested at large scales in real conditions. It was also possible to conclude that the tested Si fertilization in these lab experiments effectively contributed to plant productivity increase while also reducing stresses/limitations symptoms, especially when crops were subjected to pests and diseases and experiments where soil was contaminated with toxic metals. These findings highlight the importance of Si supplementation other than just improve plant production. These results suggest that Si supplementation can reduce food toxicity and improve plant survival when under attack by diseases with large implications for modern agriculture. One can conclude additionally from the results that Si supplementation helps plants cope with drought and saline stress, i.e. although it does not alleviate symptoms, it enables plants to grow more in such limited conditions. Finally, it was possible to conclude that Si supplementation has real positive effects on yield improvement from crops, with 69% of the total measurements gathered registering a yield improvement higher than 5% when only 14% of cases reported yield loss. Si-based fertilization application in agriculture is therefore efficient and should be largely disseminated, emphasizing to farmers and other stakeholders its multiple benefits. Given our current challenges with climate change, natural resources exhaustion and land degradation, Si fertilization can provide an efficient answer to capacitate plants with resilient ways to face adversities.

Competing Interests
The author declares no competing interests.
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