Soil salinity and nutrient availability influenced by silicon application to to-mato irrigation with different saline water

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INTRODUCTION
Agriculture in arid and semi-arid regions faces the problem of insufficient pure water resources for irrigation caused by anthropic climate changes and human activities. Using poor quality water in such regions is a common practice where there is limited good quality water supply for irrigation. The reduction in yields of 25-30% is attributed to waterlogging and salinization results from poor agriculture, soil and water management 1 . Tomatoes are an important vegetative crop in Basrah province, with a total cultivated area of 15372 Donums in 2021. Tomatoes are classified as moderately tolerant to salinity. The percentage reduction of fruit yield was 4, 18, 25 and 31% for EC of irrigation water of 2.4, 4.8, 7.2 and 9.6 dSm -1 , respectively 2 . 3 also found fruit yield of tomato reduced by about 50% when EC of irrigation water rise from 0.6 to 6 dSm -1 . Silicon(Si) is abundant in soils and is the second element in the soil. Recently, several studies stated that treating soil or plants with silicon will alleviate environmental stresses such as drought, salt, freezing, heavy metals, and biotic stresses. Improvement of salt tolerance by adding Si may start from enhancing soil properties related to salinity stress. It has been reported that the addition of Si significantly reduced salt concentration in soil and that could be due to the low level of cations and anions in soil solution resulting from either Si increased the solubility and mobility of nutrients in the soil making it readily to uptake or to improvement in the plant growth which enhanced nutrient uptake 4 . 5 stated that the accumulated Na in the soil due to root absorption selectivity under silicon treatment could be leached and weakens bond outside the root zone. However, some studies obtained increasing 6 or no effect 7 of soil salinity due to silicon application. Silicon substances usually have very high adsorption capacity due to their large surface area, so increase the soil adsorption capacity. The specific surface area of sandy loam soil increased by 80% due to using nano-silica, leading to changes in many physicochemical properties 4 . Silicon application reduced the leaching of mobile nutrients from the soil, such as phosphorus 8 , nitrogen 9 and potassium 10 . Sadgrov(2006) found a reduction in leaching of N, P and K for 60,30 and 6%, respectively, after using Si-rich organic materials. The enhancement of nutrient availability can justified by increasing soil CEC, improving water and air regimes and changing soil mineral composition. Nitrogen and phosphorus are weakly adsorbed on Si-rich materials and remain in available form. Locally, few and limited studies were found on the effect of Si application on soil properties. So, this work was conducted to study the effect of applying Si(nanosilica or potassium silicate) through the soil on soil salinity under different irrigation water salinity levels and the availability of some macronutrients for tomato plants.

The experimental Site
An experiment was conducted in greenhouses at Agricultural Research Station -College of Agriculture -University of Basrah, Iraq located in Karmat Ali region ( 47⁰44'40"E and latitude 30⁰33'44" N) rising 3 m above sea level and 9.78 km from the city center in 2020-2021 to clarify the role of nano-silica in improving the resistance of tomato plant REDFLORA F1 hybrid to salinity of irrigation water and its comparison with conventional silicon. Random samples were taken from the soil layers (0-15, 15-30 and 30-45) cm, mixed well, air-dried, then crushed and passed through a sieve of 2 mm diameter for analysis of some chemical and physical properties according to the standard methods mentioned in 11 and 12 and included in table 1.

Treatments
The experiment includes two factors : The first factor is the salinity of the irrigation water, which includes four treatments: 1. Freshwater with a salinity of 1.65 dSm -1 (W1) 2. Water with a salinity of 3 dSm -1 (W2) 3. Water with a salinity of 6 dSm -1 (W3) 4. Water with a salinity of 9 dSm -1 (W4) Salinity levels of 3, 6 and 9 dSm -1 are prepared from the dilution of drainage water with tap water using the following equation 13  Where: EC1 = electrical conductivity of the water to be obtained (dS m -1 ) ECa = electrical conductivity of water used in dilution (dS m -1 ) ECb = electrical conductivity of drainage water (dS m -1 ) a = percentage of water used for dilution (liters)

Experimental design
The field was plowed thoroughly and divided into 6 rows extending along the length of the plastic house, with a distance of 1m between the rows. Rows were fertilized with cattle manure at a rate of 5 tons ha -1 . The field was divided into 3 blocks, and the individual plots within a block were designed according to the treatments at a row size of 3.5 × 0.5m with a factorial experiment. The drip irrigation system was designed to be connected to four plastic tanks of a capacity of 3 tons dedicated to each type of irrigation water salinity from the middle of the plastic house to supply the drip holders on both sides. A leaching requirement of 20% was used for all treatments. Tomato (Solanum Lycopersican Mill.) seedlings, REDFLORA F1 hybrid, were transferred to the field in October 2020 at a rate of 16 plants for each plot, with a distance of 0.4 m between plants. Nitrogen in the form of urea (46% N), phosphorous in the form of DAP (21% P) and potassium in the form of potassium sulfate (43% K) were added at the rate of 300 kg N ha -1 , 65 kg P ha -1 and 250 kg K ha -1 , respectively. All fertilizers were added to the soil along the plant line under drippers. Nano-silicon and potassium silicate were mixed with distilled water by a mixer for 30 minutes, then mixed with a small amount of field soil to ensure the homogeneity of nano-silica particles with the soil. After that, each level of nanosilicon or potassium silicate was added to the soil in two doses after 2 and 4 weeks of transplanting the seedlings. Other agricultural practices were carried out according to local recommendations of the region.

Soil samples and analysis:
Soil samples were collected at different depths ( 0-15, 15-30 and 30-45 cm) at the end of the season by taking composite samples from the center of the plots, airdried, crushed and passed through a sieve of 2 mm diameter. Electrical conductivity (EC) was determined in 1:1 soil extract according to 12 . The available amounts of N, P and K were also estimated in soil samples of surface layer (0-15 cm). For nitrogen, samples were extracted with 2M KCl according to the method of 14 and then determined by steam distillation 15 . For phosphorous, samples were extracted with a solution of 0.5 MNaHCO3 according to the Olsen method. Then P was estimated by a Spectrophotometer at a wavelength of 700 nm, according to the blue color method 16 . Available potassium was extracted with 1N NH4OAC and measured by a Flame photometer as stated in 12 .

Statistical analysis
The experiment was a factorial experiment with two factors (irrigation water salinity levels × silicon treatments) in a random complete block design RCBD with three replications. Data were subjected to analysis of variance (ANOVA) using the GenStat procedure Library Release PL 18.2 program. Differences among means were compared using the least significant difference (RLSD) test at a probability level of 0.05 17 .

RESULTS
Soil salinity (Electrical conductivity) of different soil depths (0-15,15-30 and 30-45 cm) as influenced by irrigation water salinity and silicon application at the end of tomato season are presented in Tables 3,4, and 5. Generally, increasing the salinity of irrigation water significantly increases soil salinity for all depths. Irrigated tomatoes with irrigation water 9dSm -1 (W4) recorded the highest soil salinity values of 3.19, 3.52 and 5.18 for 0-15,15-30 and 30-45cm depths, respectively. Increasing soil salinity values may be due to soluble ions in irrigation water, which leads to an increase in soil solution because the soil retains a portion of the water equivalent to the field capacity. This result is supported by 18,19 . 20 stated that increasing sodium chloride salt led to an increase in the osmotic pressure of soil solution.

Silicon treatment
Irrigation water Salinity(dSm -1 ) Mean W1 W2 W3 W4 S1 1.   For silicon application, soil salinity of the different soil depths decreased as compared with corresponding control (S1) tables 3,4 and 5. The lowest values were recorded at the treatment of nano silica at a rate of 300kg Si ha -1 (S3), confirming the superiority of nano-silicon in reducing soil salinity. Analysis of variance showed the significant effect of the interaction of water salinity and silicon treatments in soil salinity at depths of 0-15 and 15-30 cm Tables 3 and 4. Nano silicon at a rate of 300kg Si ha -1 (S3) reported the lowest values of soil salinity at all water salinity levels with a decrease of 1.4-61.7% for depth 0-15cm and 9.9-73.6% for depth 15-30 cm. That means using nano-silica in reducing soil salinity did not change with changing salinity levels of irrigation water.

Available Nitrogen
Data in Table 6 showed the effect of irrigation water salinity and silicon's addition on the soil's available nitrogen. The results showed that the increase in soil salinity led to a significant decrease in the amount of available nitrogen in the soil; the values were 78.34, 71.62, 70.98 and 65.41 mg kg -1 soil for treatments W1, W2, W3 and W4, respectively, with significant differences between all levels, except for the W2 and W3 treatments. The accumulated salts in soil solution due to irrigation with saline water could be reduced.  Table 6. The effect of irrigation water salinity and silicon treatments on soil's available nitrogen (mg kg -1 soil ± SD).W1 : 1.65 dSm -1 ; W2: 3 dSm -1 ; W3 : 6 dSm -1 ; W4: 9 dSm -1 ; S1: 0 kg Siha -1 ; S2: 150 kg Si ha -1 as nano-silica; S3: 300 kg Si ha -1 as nano silica; S4: 300 kg Si ha -1 as potassium silicate. Table 6 showed that adding silicon significantly increased the available nitrogen compared to the no-addition treatment. The increase percents were 15.8, 19.14 and 5.25% for S2, S3 and S4 treatments, respectively. Treatment S3 showed a significant recorded available nitrogen compared to S2 and S4 treatments. This could be since the large surface area of silicon can absorb water, nutrients and heavy elements, thus saving nitrogen from loss. Sadgrov(2006) obtained a decrease in nitrogen leaching rates of 60% when treated with silicon from an organic source because of its good porous structure.

Available phosphorus:
The increase in the salinity of the irrigation water led to a significant decrease in available phosphorus content Table 7. These results were similar to those of 32 and 33 , who found a decrease in available phosphorus with an increase in the salinity of irrigation water. Twenty-four indicated that the low concentration of phosphate in the soil solution is not only due to the effects of ionic strength that reduce phosphate activity but also to the high control of phosphorous concentrations in the soil solution through the absorption processes and low solubility of Ca-P compounds.  The results shown in Table 7 demonstrated that the addition of silicon significantly increased the available phosphorus compared to the non-addition treatment, with an increased percent of 4, 12 and 7% for treatments S2, S3 and S6, respectively. Moreover, Sadgrov(2006) concluded that the decrease in phosphate losses after the addition of Si-rich materials is due to the highly porous structure of these materials, which preserves water in large quantities.
Regarding nano-silica superiority, 30 explained the superiority of nano-silica over sodium silicate in increasing the available phosphorus in soil. The fact that nanoparticles have a large surface area and reduced concentration of soluble calcium (Data not published) could justify their high effect in increasing the amount of phosphorus in soil compared to potassium silicate in the present study. Data analysis of the effect of water salinity along with silicon treatments showed that the highest values were obtained with S3 treatment at all salinity levels, with the highest value of 55.68 mg kg -1 soil when interacted with W1 level, which is 8-42% higher than others. Table 8 indicates a significant decrease (p ≤ 0.05) in available potassium content by increasing the salinity level of irrigation water. 39 found that irrigation with dilute seawater decreases the proportion of nutrients such as calcium and potassium due to an increase in sodium concentration in the soil. The available potassium in soil significantly increases from 241mgkg -1 soil for control to 294.26, 441.26 and 349.92 mg kg -1 soil for S2, S3 and S4 treatments, respectively. Thus, the available potassium is about doubled in the presence of 300 kg Si ha -1 at nano-silica. 7 indicated a significant difference between soil treated with silicon and those that did not increase the potassium concentration. The role of silicon in increasing the availability of nutrients, including potassium, can be summarized by increasing the ability of soil to retain water, increasing CEC and holding ions 38 . Sadgrov(2006) obtained a decrease in potassium leaching by 60% when using Si-rich materials due to their highly porous structure, resulting from increasing water retention.  Table 8. The effect of irrigation water salinity and silicon treatments on soil's available potassium (mg kg -1 soil ± SD). W1 :
S3 treatment had the highest available potassium of all water salinity treatments, and this superiority could be a result of the highly active surface of nanoparticles and their impact on soil properties such as water retention, CEC and retention in the form of colloids. 7 indicated that the available amount of nutrients such as calcium and sulfur increased with the addition of silicon to the soil due to enhanced soil CEC and water holding, resulting in higher uptake by the plant. 4 also indicated that the addition of silicon decreased soil salinity due to Si increasing the solubility and mobility of nutrients, making it readily available for plant uptake. Si also improves plant growth and metabolism, increasing nutrient uptake by plant roots. However, decreasing the activity of some ions, such as calcium, due to forming Si-Ca complexes may reduce the soluble amount in soil solution and decrease soil salinity 21 . 22 stated that silica nanoparticles are small and have a highly specific surface area, so they have great absorption capacity by plants and reducing ions in soil solution. Silica minerals are basic in nature, and it is easy to adsorb cations with higher adsorbing capacity for nano-silica compared with conventional silica due to higher density 23 . From our results, it can be noticed that soil salinity increased with increasing soil depth. This may be attributed to the leaching rate of ions to the deeper layer (30-45cm), confirmed by using the leaching requirement in the experiment and by soil properties treated with manure. This result is in harmony with those of 19 who found that soil salinity increased with increasing soil depth from 20 to 60 cm under corn or bean covers. 24 attributed this decrease to the competition of Na + and Clwith nutrients such as K + , Ca +2 and NO3in saline soils. 25 indicated that the excessive increase of soluble ions due to salinity would provide osmotic stress and thus uptake of certain ions compared to other ions. It is well documented that Cl -1 reduces the uptake of NO3by plant 26 . Organic N mineralization 27 and urease activity 28 results in low available nitrogen in the soil. The enhancement of available nitrogen in soil may be attributed to silicon's positive effect on the microorganisms in the soil 40 and to the positive effect of silicon in reducing NO3leaching 9 and ammonia volatilization 29 due to increasing CEC and retaining more water. Treatment S3 showed a significant recorded available nitrogen as compared with S2 and S4 treatments; this may be because nano-silica is converted to silicon colloids in soil because of hydrogen bonds, making it more active molecules as well as does not affect soil pH, maintaining optimum pH for microorganisms growth and availability of NPK 30 . Moreover, the presence of a suitable water layer on the surface of the nano-silica could facilitate the attraction of silicon on the microbial surface 31 , which results in an increase in the activity of microbes and an increase in the availability of nutrients, including nitrogen. 34 indicated a significant correlation between soil salinity and Ca-P compounds, which results in a significant negative correlation (r = -0.64) between soil salinity and available phosphorus. Calcium and other salt ions accompany the orthophosphate ion and adsorbate on clay particles 35 . 36 justified the relationship of salinity with the availability of phosphorus to the fact that mono-salts such as sodium chloride encourage the dissolution of calcium carbonate in calcareous soils, releasing more calcium, which accelerates phosphorus adsorption. 9 indicated an increase in the availability of phosphorus in soil after silicon fertilization due to a series of reactions that include monosilicic acids adsorption on slightly soluble phosphates of calcium, magnesium, iron or aluminum, then exchange of phosphate-anion by silicate-anion followed by desorption of phosphate-anion leading to increase phosphorus in soil solution. This also could be because application of Si-rich material could decrease P leaching by 40-70 %, and phosphorus absorbed by this material remained in plant-available form 8 . 37 who found that potassium in the soil decreased after irrigation with drainage water compared to river water. He explained by the fact that the increase in the concentration of Ca, Mg and Na was greater than the increase in potassium concentration, which allows displacing potassium from the exchange sites and may be subjected to leaching. The result is in agreement with the findings of 30 , who obtained an increase in the available potassium as a result of adding nano silica compared to sodium silicate.`

CONCLUSIONS
It can be concluded from this study that the addition of nano-silica or potassium silicate led to a reduction in soil salinity at all soil depths and for a wide range of irrigation water salinity (1-9 dSm -1 ), Proving its important role in improving soil properties. Nutrient ( N, P and K ) availability were positively affected by silicon application with higher value at nano-silica compared to potassium silicate under different level of irrigation water salinity. It is possible that the addition of nanosilica at a level of 300 kg Si ha -1 is the best level for reducing soil salinity and preparing a suitable medium for tomato plants with sufficient available quantities of nutrients, especially since the cost of obtaining the nano-source is low and does not constitute a burden on the inputs of the agricultural process.