Effects of Silicon Alone and Combined with Organic Matter and Trichoderma harzianum on Sorghum Yield, Ions Accumulation and Soil Properties under Saline Irrigation

Abstract

The action of silicon as a salt stress mitigator has been investigated in isolation, and its combined efficacy with other salt stress mitigators needs to be addressed. This work verified whether silicon, in combination with organic matter and Trichoderma harzianum, enhances the production of forage sorghum under saline irrigation and its effects on soil properties. The field experiment was conducted in Parnamirim (PE), a semiarid region of Brazil. Forage sorghum (Sorghum sudanense (Piper) Stapf) was irrigated with saline water (3.12 dS m⁻¹) and subjected to the application of non-silicon, silicon alone, and silicon combined with Trichoderma and organic matter over three consecutive cuts (every three months after germination). Silicon applied in combination significantly increased the content of nutrient ions K⁺, P, Ca²⁺, and Mg²⁺ in sorghum leaves, stems, and panicles and increased P content in the soil by 170, 288, and 92% for the first, second, and third cuts, respectively. When silicon was applied in combination, sorghum’s dry and fresh matter (total yield for the three cuts) increased to 62.53 and 182.43 t ha⁻¹, respectively. In summary, applying silicon (Si) combined with Trichoderma and organic matter promotes higher nutrient ion contents in soil and sorghum plants and a higher forage sorghum yield.

1. Introduction

Food production is directly linked to climate, and this means that crop yields, water use, and soil quality are affected by climate change [1]. Increased concentrations of atmospheric greenhouse gases are likely indicators that climate change significantly impacts soil salinization [2], by increasing air temperature and thereby increasing soil water and groundwater evaporation. Consequently, the capillary movement of groundwater and soil water will rise in dry seasons, accelerating the soil salinization process in arid regions of the world [3]. Thus, salt-affected areas have increased, mainly due to increased evapotranspiration, lower precipitation, increased use of poor-quality water, poor irrigation management, and inappropriate soil management practices [4]. Given this scenario, one of the main challenges of current world agriculture is ensuring food security under extreme abiotic stress, such as saline stress.

In much of the Brazilian semiarid region, producers already suffer from problems related to the availability of good quality water for use in irrigated agriculture [5]. This has caused an increase in groundwater use for irrigation purposes, which generally have high saline contents, with high concentrations of sodium and chlorine in their composition [6,7]. The excessive use and mismanagement of these waters, combined with poor soil management and lack of drainage [8], has limited food and fodder production and contributed enormously to soil degradation and desertification of the region [9].

Salinity affects plant production and plant metabolism at all stages of growth [10] through nutritional imbalances [11], reduction in osmotic potential [12], and ionic and oxidative toxicity [13], causing morphological [14], physiological, and biochemical damage [15]. Despite the Brazilian semiarid region’s current scenario concerning the increasing use of saline water for crop irrigation, cultivation strategies that favor mitigating saline stress on plants still need to be explored. In this context, it is essential and urgent to carry out studies investigating ways to minimize salt stress and promote crop production that meet the region’s demand.

To address this issue, using crops with agricultural potential and a tolerance to high salinity is a promising alternative to ensure plant production in the Brazilian semiarid region and around the globe [16]. In this sense, forage sorghum (Sorghum bicolor (L.) Moench) is an essential source of animal feed that has increased its importance in many semiarid regions of the globe due to its high yield and ability to use water effectively, even under water and salt stress conditions [10,17].

Although sorghum is recommended for cultivation in saline environments [18], its salt tolerance can be improved by using salt stress attenuators. Thus, it is crucial to investigate the effectiveness of salt stress attenuators in mitigating the effects of salt on different crops and increasing the range of possibilities for success in the yield of salt-tolerant plants.

Silicon (Si) has been widely studied as a saline stress attenuator [19,20,21]. The effectiveness of Si in attenuating salt stress in plants can occur through several mechanisms. Zhu and Gong [22] report that Si has a relieving effect on salt stress in plants by reducing the ionic toxicity of Na⁺ and Cl⁻; decreasing oxidative damage by increasing the activity of antioxidant enzymes; regulating the biosynthesis of compatible solutes; affecting lignin biosynthesis; and by regulating levels of plant hormones and polyamines. Although several studies report the beneficial effects of Si on crops growing in saline environments, few studies evaluate the effectiveness of Si combined with other salinity attenuators.

Organic treatments correspond to a wide variety of products made by organic compounds that can be added to the soil to increase soil fertility and favor plant growth, improving agricultural sustainability, habitats, biogeochemical cycles, and soil biological activity [23]. These correspond to organic fertilizers, applied to the soil as beneficial microorganisms (Rhizobium, arbuscular mycorrhizal fungi, Trichoderma, Azospirillum, etc.) and organic soil conditioners (manure, biochar, etc.), which tend to improve the physical properties of the soil, accelerate the leaching of salts and Na⁺, increase the percentage of aggregate stability, and reduce the percentage of exchangeable sodium (ESP) and electrical conductivity (EC) [24].

Trichoderma is an important fungus commonly found in soils, which can spread rapidly, colonizing and surviving in the rhizosphere for extended periods [25]. Trichoderma promotes root growth and has a high capacity to mobilize and absorb nutrients from the soil, increasing the efficiency of nutrient utilization by plants and promoting crop growth and production [26], thus providing greater tolerance to various environmental stresses [27]. Additionally, Trichoderma alleviates the effects of salinity on plants by increasing the activity of antioxidative defense systems [28]. On the other hand, using manure as a source of organic matter in soils is a common practice in the Brazilian semiarid region. In this sense, using Trichoderma can potentiate the mineralization of the organic matter applied as manure and increase the solubilization of nutrients for the plants, since salinity reduces the absorption of nutrients and their efficiency in crops [29].

Although the benefits of Si as an attenuator of salt stress in plants are well documented in the literature [18,20,30,31], in the Brazilian semiarid region, its use for this purpose is still incipient, and investigations are needed to assess its effectiveness in mitigating salt stress. Furthermore, its association with organic treatments still needs to be reported in the literature. Thus, we hypothesize that the combined application of Si with organic treatments can increase the performance of sorghum under saline stress. The objective of this study was to evaluate the effectiveness of the application of Si alone and combined with organic matter and Trichoderma harzianum on the yield of forage sorghum under saline irrigation, as well as to verify changes in the soil chemical properties in response to the application of saline attenuators.

2. Materials and Methods

2.1. Study Area

The study was carried out at the Parnamirim Irrigated Agriculture Station—Federal Rural University of Pernambuco (UFRPE), in Parnamirim (PE), located in a semiarid region of Brazil (latitude 8°5′08″ S, longitude 39°34′27″ W and an altitude of 390 m) (Figure 1). According to the Köppen classification, the climate in the region is semiarid, of the BSwh type. The average annual rainfall in the study area is 431.8 mm [32]. However, the rainy season in the region is short and occurs from December to March.

The region’s principal economic activity is the production of goats and sheep. It requires a forage supply in large quantities. As the rainy season in the region is short, many producers resort to groundwater use in the driest months to guarantee forage production. However, these waters do not have a good quality for irrigation use due to the high saline content, which compromises forage production.

The experiment was conducted in the field, in saline Fluvisol soil [33]. The chemical and physical properties of the soil used in the experiment at depths of 0–10, 10–20, 20–40, and 40–60 cm are shown in

Table 1. Chemical and textural characterization of soil in the experimental area.

Soil Property	Soil Layer (cm)
	0–10	10–20	20–40	40–60
Chemical property
pH	6.05	6.09	6.16	6.32
OM (g kg−1)	25.08	19.35	18.00	11.75
P (mg dm−3)	28.40	19.73	14.83	9.75
K+ (cmolc dm−3)	0.51	0.34	0.24	0.14
Na+ (cmolc dm−3)	0.30	0.37	0.52	0.82
Ca2+ (cmolc dm−3)	9.06	8.54	8.79	9.98
Mg2+ (cmolc dm−3)	5.29	5.40	5.21	5.53
H + Al (cmolc dm−3)	0.96	0.89	0.76	0.60
SB (cmolc dm−3)	15.14	14.65	14.93	16.48
CEC (cmolc dm−3)	16.10	15.54	15.69	17.08
ESP (%)	1.86	2.38	3.31	4.80
Saturation extract
EC (dS m−1)	3.56	6.21	7.22	5.77
Ca2+ (mmolc L−1)	31.88	58.88	60.45	43.85
Mg2+ (mmolc L−1)	12.03	22.78	25.15	18.00
K+ (mmolc L−1)	9.63	5.78	5.08	2.13
Na+ (mmolc L−1)	5.58	9.40	12.18	13.15
SAR (mmolc L−1)	1.20	1.50	1.83	2.23
Soil texture
Sand (%)	14.20	10.30	11.30	10.92
Silt (%)	72.12	75.62	71.62	75.48
Clay (%)	13.68	14.08	17.08	13.60

EC—electrical conductivity of saturation extract; pH—hydrogen potential; OM—organic matter; SB—sum of base; CEC—cation exchange capacity; ESP—exchangeable sodium percentage; SAR—sodium adsorption ratio.

.

2.2. Experimental Design

The experiment was conducted in a randomized block design, with five treatments and four replications. The saline stress attenuators used were silicon (Si) applied alone and silicon in combination with organic matter (goat manure) (OM) and Trichoderma harzianum (T), tested using the forage sorghum crop (Sorghum sudanense (Piper) Stapf), cultivar IPA Sudan 4202. The treatments were: sorghum without application of salinity attenuator (control); sorghum + Si; sorghum + Si + OM; sorghum + Si + T; and sorghum + Si + T + OM. The dimensions of the experimental units were 20.0 m × 16.5 m for the total area, 4 m × 4 m for the plots, and 2 m × 2 m for the useful plots (where plants and soil samples were collected). The adopted spacing was 0.50 m between rows and ten plants per linear meter.

For the study, three sorghum cuts (the first cut plus two regrowths) were carried out between June 2021 and April 2022, totaling ten months, among which it was possible to evaluate the sorghum growth responses to the application of salinity attenuators during the region’s dry and rainy seasons (Figure 2). There was no rain between the sowing and the first cut (1st cycle), while in the 2nd and 3rd cuts, the accumulated precipitation values were equal to 176.5 and 252 mm, respectively.

Irrigation was carried out using a drip irrigation system with an efficiency of 96%, the flow of each dripper set at 1.06 L h⁻¹, emitters spaced at 40 cm, and an application interval of 48 h, based on the total replacement of crop evapotranspiration (ETc) [34]. The reference evapotranspiration (ETo) was determined by the model proposed by Penman–Monteith and adapted by FAO-56 [34]. Meteorological data were collected at an automatic INMET station (National Institute of Meteorology) [Salgueiro, Pernambuco, Brazil], located 50 km from the experimental area. Rainfall data were collected in the experiment area using a manual rain gauge.

The water for irrigation came from an artesian well (

Table 2. Chemical properties of irrigation water during the experimental period.

pH	EC	Ca2+	Mg2+	K+	Na+	Cl−	HCO3−	SO42−	B	Cu	Fe	Mn	Zn	SAR
	dS m−1	----------------- mmolc L−1 -----------------							--------- mg L−1 ---------					mmolc L−0.5
7.23	3.12	9.29	9.23	0.12	11.73	26.38	5.70	0.76	0.11	0.02	0.02	0.03	0.20	3.85

pH—hydrogen potential; EC—electrical conductivity; SAR—sodium adsorption ratio.

), classified as C₄S₁, with a very high risk of promoting soil salinization and a low risk of soil sodification, according to [35].

Potassium silicate, the silicon source used in this work, is a fertilizer containing at least 10% K⁺ in K₂O and 10% silicon. It was applied twice via the soil (the first application was performed one week after sowing and the second after an interval of 15 days) and two foliar applications (the first within 15 days after the applications via soil and the second 15 days after the first foliar application), in all cycles. The spray concentrations were 5 mL L⁻¹ and 10 mL L⁻¹ for soil and foliar applications, respectively, as recommended by the manufacturer. In both conditions, 39.06 mL m⁻¹ linear was applied.

T. harzianum was obtained from the commercial product Trichodermil SC I306. In the first cut, two applications were carried out via soil, the first at sowing and the second with an interval of 30 days after the first application. In the 2nd and 3rd cuts, an application was made at the beginning of each regrowth. In all applications, the syrup concentration was 12.5 mL L⁻¹, as recommended by the manufacturer. The applications were carried out using a costal manual pump in the planting line, with a volume of 39.06 mL of mixture per linear meter.

At the time of sowing, the soil was fertilized with goat manure (source of organic matter) (

Table 3. Characterization of goat manure used as an organic matter source in the experiment.

D	C	N	P	K+	Na+	Ca2+	Mg2+	pH	EC
g cm−3	-------------------------------- g kg−1 ---------------------------------								dS m−1
0.8158	119.70	17.90	8.70	4.50	1.10	27.70	10.20	7.87	1.87

D—density of manure; C—total organic carbon; N—total nitrogen; pH—hydrogen potential; EC—electrical conductivity.

) in the proportion of 50 Mg ha⁻¹, in the treatments that included the application of organic matter. According to [36], this dose provides the best performance for the sorghum crop. The manure was previously tanned and incorporated into the surface layer of the soil.

2.3. Soil Sampling

Soil samples were collected at the end of each sorghum cut, in the “useful plots”, at depths of 0–10, 10–20, 20–40, and 40–60 cm. All samples were air-dried and sieved through a 2 mm mesh. Then, the samples were submitted for the analysis of elements in the exchange complex and the soil saturation extract (soil soluble complex).

2.4. Soil Analysis

2.4.1. Soluble Complex

The saturated paste was prepared according to the methodology described by [37]. For this, distilled water was added to 500 g of soil until complete saturation was reached [37]. After 12 h of rest, the soil solution was extracted by suction. In the soil solution, the electrical conductivity (EC) was measured at 25 °C, using a conductivity meter, as well as the concentrations of Ca²⁺, Mg²⁺, Na⁺, and K⁺ (cations) and Cl⁻ (anions). Ca²⁺ and Mg²⁺ were determined by atomic absorption spectrometry, and Na⁺ and K⁺ were determined by flame emission photometry [37]. Cl⁻ was determined by titration with AgNO₃ solution, using K₂CrO₄ as an indicator [37].

2.4.2. Exchangeable Complex

Soil pH was measured directly in the 1:2.5 soil/water mixture suspension using a potentiometer. The exchangeable cations (Ca²⁺, Mg²⁺, Na⁺, and K⁺) were determined after washing soil samples with 96° alcohol. Exchangeable cations were extracted with 1 mol L⁻¹ ammonium acetate solution. After that, Ca²⁺ and Mg²⁺ were determined by atomic absorption spectrometry, and Na⁺ and K⁺ were determined by flame emission photometry [37]. The exchangeable sodium percentage (ESP) was calculated by the ratio of exchangeable Na⁺ to the cation exchange capacity (CEC), according to [37] (Equation (1)).

ESP (%) = (Na⁺/CEC) × 100

(1)

Soil-available phosphorus was extracted with 0.5 mol L⁻¹ NaHCO₃ solution at pH 8.5, and determined by colorimetry with ammonium molybdate, using a spectrophotometer at 882 nm [38].

2.5. Content of Sodium, Potassium, Calcium, Magnesium, Chloride, and Phosphorus in the Plant

The sorghum plants were fractionated into leaves, stems, and panicles, and the dry mass of the sorghum plant fractions was ground in a Wiley-type mill to determine the elements Ca²⁺, Mg²⁺, Na⁺, K⁺, and P.

To determine Na⁺, K⁺, and Cl⁻ concentrations, we added 25 mL of ultra-pure water to 100 mg of dry matter in a closed container and boiled for 1 h at 100 °C [39]. The obtained extract was filtered, and the Na⁺ and K⁺ contents were determined by flame emission photometry. To determine Cl⁻, 10 mL of the extract was collected and titrated with silver nitrate (28 mM AgNO₃), using potassium chromate (5% K₂CrO₄) as an indicator.

To determine P, Ca²⁺, and Mg²⁺ concentrations, we added 25 mL of 1 mol L⁻¹ HCl solution to 500 mg of dry plant mass and heated it at 80 °C for 15 min [40]. After that, Ca²⁺ and Mg²⁺ were determined by atomic absorption spectrometry, and P was determined by molybdenum blue spectrophotometry at a wavelength of 660 nm [40].

With the values of these elements’ concentrations per fraction of the plants and the dry mass production of each fraction, the contents of the elements in each fraction of the aerial part were calculated. For that, we multiplied the ion concentration by the dry mass of each plant compartment [41].

2.6. Sorghum Productivity and Water Use Efficiency (WUE)

The fresh and dry yields of forage sorghum were obtained at the end of the three sorghum cuts, with representative plants of the “useful plot”. The plants were harvested and weighed at the end of each sorghum cut to obtain average fresh mass values. Then, the plants were placed in a forced circulation oven at 65 °C for 72 h until they reached constant weight, to determine the dry mass of forage sorghum. The productivity values of fresh and dry mass were multiplied by the plant stand (number of plants ha⁻¹) to obtain the productivity of shoots (dry and fresh mass in Mg of plants ha⁻¹). In this study, we had a homogeneous stand of 200,000 plants ha⁻¹ for all treatments. The total productivity of fresh and dry mass was obtained by adding the values found for the productivity of the three sorghum cuts.

From the ratio between the DMY and the sum of the ET_C for the cycle, the water use efficiency (WUE) was calculated in g L⁻¹.

2.7. Statistical Analysis

Data were subjected to an analysis of variance and normality testing, as well as a comparison of the means using the Scott–Knott test at a 5% probability level, with the statistical program Rstudio (version 4.2.2) (R Core Team, 2022). Graphs were created using SigmaPlot (version 14.0) (Systat Software Inc., San Jose, CA, USA).

3. Results

3.1. Soil Chemical Properties

3.1.1. Salinity Parameters

Applying Si, Trichoderma, and organic matter mitigated the increase in soil salinity due to the use of saline irrigation over time, which could be observed in the second and third cuts. In the first cut, there was a significant difference (p ≤ 0.05) in EC values only in the 10–20 cm layer, where the treatment with the application of Si + T obtained the lowest value. In the second cut, the application of isolated or combined Si in all soil layers efficiently attenuated the increase in EC, with isolated Si promoting the lowest values. In the third cut, the treatments promoted differentiation in EC values in the 10–20 cm and 40–60 cm layers, where the application of Si alone and Si + T had the best attenuation in the increase in EC, respectively (Figure 3A).

There was a change in pH in the soil layers 20–40 and 40–60 cm (cut 1), 0–10 and 40–60 cm (cut 2), and 10–20 and 20–40 cm (cut 3). Applying Si alone or combined promoted increases in soil pH values (Figure 3B). Regarding ESP, the effect of Si alone or combined did not enable significant differences (p ≤ 0.05) concerning the control treatment in the first sorghum cut in all layers (Figure 3C). In the second and third cuts, there was a tendency for Si alone or combined to attenuate the increase in ESP concerning the control treatment. The control treatment was the least efficient in attenuating the rise in ESP, with values varying from 43% (10–20 cm) to 28% (40–60 cm) in the second cut and from 46% (10–20 cm) to 25% (40–60 cm) in the third cut.

3.1.2. Soluble Complex

The application of combined Si promoted increases in the levels of nutrient ions (K⁺, Ca²⁺, and Mg²⁺) in the soil solution while reducing the levels of toxic ions (Na⁺ and Cl⁻) (

Table 4. Soluble ions concentrations for each studied soil layer in response to the tested saline attenuator along the three sorghum cuts. Different letters in the line indicate significant differences at p ≤ 0.05.

Soil Layer (cm)	Cut 1					Cut 2					Cut 3
	C	Si	Si + OM	Si + T	Si + T + OM	C	Si	Si + OM	Si + T	Si + T + OM	C	Si	Si + OM	Si + T	Si + T + OM
K+ (mmolc L−1)
0–10	0.79 c	0.51 d	1.50 a	0.97 b	1.53 a	0.67 b	0.67 b	0.76 b	0.92 a	0.94 a	0.36 c	0.44 c	0.40 c	0.56 a	0.49 b
10–20	0.71 b	0.64 b	0.80 b	0.65 b	1.13 a	0.31 b	0.22 b	0.34 b	0.47 a	0.31 b	0.38 a	0.24 b	0.31 a	0.32 a	0.31 a
20–40	0.42 b	0.45 b	0.39 b	0.49 b	0.60 a	0.35 a	0.24 b	0.29 b	0.36 a	0.25 b	0.34	0.29	0.36	0.30	0.23
40–60	0.39	0.47	0.52	0.36	0.36	0.30 b	0.28 b	0.34 b	0.64 a	0.33 b	0.40 a	0.43 a	0.33 b	0.30 b	0.29 b
Na+ (mmolc L−1)
0–10	124.37 a	84.69 b	144.26 a	90.22 b	119.69 a	133.57 a	72.07 c	107.30 b	90.58 c	83.41 c	60.27 c	48.98 d	74.95 b	86.12 a	76.61 b
10–20	77.42 b	111.44 a	95.03 b	87.48 b	90.82 b	122.82 a	98.4 b	116.85 a	94.76 b	114.46 a	103.56 a	68.97 b	80.24 b	71.91 b	113.73 a
20–40	43.62	50.41	56.41	42.21	67.62	81.03 a	80.43 a	62.52 b	70.28 a	55.95 b	93.08 a	74.95 b	102.57 a	63.46 b	88.76 a
40–60	46.34 c	75.67 b	57.83 c	52.01 c	99.85 a	121.33 a	63.12 b	72.07 b	61.32 b	63.71 b	51.22 b	82.59 a	52.51 b	43.57 b	57.58 b
Cl− (mmolc L−1)
0–10	64.58 a	29.58 b	88.75 a	44.58 b	73.75 a	72.08 a	20.00 b	41.25 b	44.58 b	31.67 b	13.33 c	13.75 c	27.92 a	22.08 b	32.08 a
10–20	68.75 a	56.25 b	59.58 b	44.58 c	45.42 c	53.75 a	26.25 b	45.42 a	52.08 a	52.92 a	34.58	20.63	25.63	39.58	27.50
20–40	41.25	42.50	30.42	36.25	42.08	79.38 a	37.92 c	57.92 b	59.58 b	58.75 b	50.42 a	27.08 b	56.25 a	40.42 b	29.17 b
40–60	42.08	42.92	37.92	31.25	43.75	98.33 a	47.08 b	52.08 b	58.75 b	46.25 b	84.17 a	44.17 b	50.42 b	27.08 c	44.58 b
Ca2+ (mmolc L−1)
0–10	23.36 b	18.61 b	39.50 a	26.31 b	43.30 a	29.20 a	13.19 b	33.12 a	25.09 a	25.44 a	10.54 b	10.43 b	16.93 a	14.94 a	18.77 a
10–20	35.19 b	29.75 b	46.04 a	23.14 b	30.81 b	24.50 b	14.06 c	32.72 a	29.80 a	23.98 b	17.38 a	11.11 b	20.63 a	19.18 a	20.21 a
20–40	31.42	30.75	33.69	30.43	35.71	38.26 a	23.79 b	38.18 a	42.91 a	40.45 a	26.81 a	16.56 c	31.28 a	32.68 a	22.85 b
40–60	34.28	38.26	42.30	32.26	39.28	60.73 a	30.96 b	37.45 b	63.01 a	42.03 b	38.00 a	28.54 a	43.56 a	19.64 b	32.24 a
Mg2+ (mmolc L−1)
0–10	12.07 b	9.46 b	31.87 a	22.99 a	24.11 a	15.58	7.08	12.56	14.15	10.48	5.10 c	4.11 c	9.05 a	7.46 b	9.55 a
10–20	19.34	16.95	20.27	16.48	17.06	13.77 b	6.91 b	11.08 b	21.23 a	13.60 b	9.76	4.28	5.65	5.92	5.65
20–40	17.61	20.41	16.90	15.22	18.68	28.80 a	14.32 b	22.55 a	16.95 b	16.84 b	13.28 a	8.28 b	16.95 a	7.41 b	12.29 a
40–60	33.60 a	28.36 a	10.94 b	15.99 b	20.38 b	23.26	18.82	15.63	16.62	20.52	31.43 a	14.43 b	24.91 a	9.66 b	16.79 b

Si = silicon; Si + OM = silicon + organic matter; Si + T = silicon + Trichoderma harzianum; Si + OM + T = silicon + organic matter + Trichoderma harzianum.

). For K⁺, in the first cut, the highest concentrations were observed with the application of Si + T + OM, ranging from 1.53 (0–10 cm) to 0.36 mmol_c L⁻¹ (40–60 cm). In the second cut, the highest concentration of K⁺ was observed with the application of Si + T. In the third cut, applying Si + T promoted a higher concentration of K+ on the surface (0–10 cm). There was no difference in the concentration of K⁺ in the subsoil layers (10–20, 20–40, and 40–60 cm) concerning the control treatment (Table 4).

There was a higher concentration of Ca²⁺ in the soil solution on the surface (0–10 and 10–20 cm) with the application of Si + OM and Si + T + OM in the first cut (Table 4). In the second cut, the application of Si + OM and Si + T promoted a significant increase (p ≤ 0.05) in Ca²⁺ levels in the 10–20 cm layer, while in the third cut, the application of Si + OM, Si + T, and Si + T + OM promoted increases in Ca²⁺ content in the 0–10 cm layer.

Applications of Si + OM, Si + T, and Si + T + OM also promoted increased Mg²⁺ in the soil solution on the surface (0–10 cm) in the first cut (Table 4). In the second cut, there was an increase in Mg²⁺ content in the 10–20 cm layer with the application of Si + T, while in the third cut, there was an increase in the surface (0-10 cm) with the application of Si + OM, Si + T, and Si + T + OM. There was no difference in the other layers compared to the control treatment (p ≤ 0.05).

Concerning Na⁺ concentrations, the application of Si + T most attenuated its concentration during the first cut compared to the other treatments (Table 4). In the second cut, isolated Si and its combinations effectively attenuated the increase in Na⁺ concentration in the soil solution in all layers evaluated. In the third cut, Si + OM and Si + T attenuated the rise in Na⁺ in the soil solution (Table 4).

The concentration of Cl⁻ in the soil solution was lower in the first cut for the Si and Si + T treatments in the 0–10 cm layer, while in the 10–20 cm layer, all forms of Si application proved effective for reducing Cl⁻ levels. In the second cut, isolated and combined Si presented the lowest concentrations of Cl- in soil solution in the layers of 0–10, 20–40, and 40–60 cm. In the third cut, there was an attenuation of Cl⁻ levels in the subsurface, with the application of Si, Si + T, and Si + T + OM in the 20–40 cm layer and the 40–60 cm layer, the application of Si + T was the combination that obtained the lowest concentration of Cl⁻ (Table 4).

3.1.3. Sorptive Complex

Regarding the soil assortative complex, the application of Si, alone and combined, also promoted a greater concentration of nutrient ions (K⁺, Ca²⁺, Mg²⁺, and P) and reductions in the concentration of exchangeable Na⁺ (

Table 5. Ions concentrations in the exchange complex for each studied soil layer, in response to the tested saline attenuator along the three sorghum cuts. Different letters in the line indicate significant differences at p ≤ 0.05.

Soil Layer (cm)	Cut 1					Cut 2					Cut 3
	C	Si	Si + OM	Si + T	Si + T + OM	C	Si	Si + OM	Si + T	Si + T + OM	C	Si	Si + OM	Si + T	Si + T + OM
K+ (cmolc kg−1)
0–10	1.14	1.54	1.35	1.11	1.38	0.79 c	1.23 a	1.01 b	1.25 a	1.03 b	0.58 b	0.96 a	0.65 b	0.74 b	0.63 b
10–20	0.59 c	0.60 c	0.85 b	0.69 c	1.07 a	0.45 b	0.35 b	0.54 b	0.77 a	0.69 a	0.42	0.29	0.41	0.43	0.45
20–40	0.35 a	0.26 b	0.28 b	0.24 b	0.33 a	0.31	0.26	0.26	0.37	0.35	0.22	0.24	0.21	0.21	0.21
40–60	0.21	0.22	0.19	0.21	0.26	0.33 a	0.23 b	0.26 b	0.37 a	0.23 b	0.21	0.22	0.14	0.21	0.24
Na+ (cmolc kg−1)
0–10	6.23	6.02	6.34	5.52	6.96	6.51 a	5.93 b	6.02 b	5.90 b	6.07 b	5.63	5.19	5.25	5.39	5.39
10–20	5.02	6.02	4.76	5.41	5.29	6.86	7.00	6.70	6.66	7.22	7.16 a	5.16 b	6.83 a	5.77 b	6.93 a
20–40	3.44	3.53	3.62	3.29	3.94	4.94	5.12	5.03	3.87	5.26	6.15 a	5.51 b	5.48 b	5.63 b	6.21 a
40–60	3.65	4.35	3.62	3.50	4.12	4.80 a	4.88 a	4.33 a	3.38 b	4.62 a	4.25	4.10	3.64	3.75	4.19
Ca2+ (cmolc kg−1)
0–10	14.79 b	15.31 b	18.67 a	14.19 b	16.12 b	18.12 b	16.52 b	18.23 b	19.45 b	22.79 a	18.21 b	17.88 b	18.72 a	16.78 c	19.30 a
10–20	15.28	15.41	15.84	15.13	16.05	17.41	16.74	16.18	16.75	17.09	16.38	16.04	16.59	16.56	16.08
20–40	16.75 b	16.47 b	18.67 a	18.00 a	19.20 a	18.07	18.61	18.29	18.31	19.47	16.38	16.81	16.49	16.49	17.16
40–60	20.52 a	19.70 a	17.08 b	17.96 b	19.68 a	19.32	18.34	20.12	20.04	19.45	18.02	17.25	17.21	17.52	17.44
Mg2+ (cmolc kg−1)
0–10	9.85 b	9.02 c	11.04 a	9.90 b	10.93 a	9.06 b	8.29 c	8.73 b	8.73 b	10.49 a	13.34 a	12.49 b	12.92 b	12.57 b	13.32 a
10–20	10.20	10.34	10.95	10.31	10.86	8.73 a	8.84 a	8.91 a	8.27 b	9.17 a	12.90 a	13.21 a	12.95 a	11.70 b	13.19 a
20–40	12.00 a	9.87 b	11.96 a	9.17 b	9.39 b	9.15	9.30	9.61	9.46	9.90	11.85 b	12.16 b	13.30 a	11.78 b	12.81 a
40–60	13.25 a	13.03 a	11.56 b	12.42 a	11.28 b	11.81	11.02	10.53	9.81	11.04	13.98	14.64	12.75	12.55	13.69
P (mg kg−1)
0–10	45.72 b	56.19 b	123.17 a	43.10 b	113.66 a	38.14 c	44.79 c	97.20 b	40.63 c	148.32 a	42.16 b	32.29 b	81.07 a	38.60 b	70.40 a
10–20	35.52 c	42.11 c	52.67 b	38.41 c	69.87 a	23.87	34.42	37.21	39.06	47.86	26.94 c	35.57 b	41.96 a	28.16 c	44.73 a
20–40	24.43	25.81	26.77	27.74	24.84	24.63	23.03	25.21	28.08	31.62	30.97 a	23.09 b	34.92 a	23.87 b	20.92 b
40–60	23.34	30.13	24.87	21.58	28.85	17.10 c	26.42 b	21.18 c	32.56 a	26.42 b	30.18	21.06	21.62	27.53	26.77

Si = silicon; Si + OM = silicon + organic matter; Si + T = silicon + Trichoderma harzianum; Si + OM + T = silicon + organic matter + Trichoderma harzianum.

). Applying Si + OM and Si + T + OM provided a higher concentration of K⁺ in the 10–20 cm layer during the first cut. In the second cut, all forms of Si application showed a higher concentration of K⁺ on the surface (0–10 cm). In the third cut, only the application of Si was superior in surface area (0–10 cm) (Table 5).

The Ca²⁺ concentration was higher with the application of Si + OM in the 0–10 cm layer and with Si + OM, Si + T, and Si + T + OM in the 20–40 cm layer during the first cut of sorghum. In the second cut, only the application of Si + T + OM in the 0–10 cm layer showed a higher concentration of Ca²⁺. In the third cut, the highest concentrations of Ca²⁺ were also observed on the surface (0–10 cm) with the application of Si + OM and Si + T + OM (Table 5).

Concerning Mg²⁺ in the first and second cuts, its highest concentration was observed on the surface (0–10 cm) with Si + OM and Si + T + OM. In the third cut, this increase was observed in the subsurface (20–40 cm) with the application of Si + OM and Si + T + OM (Table 5). A lower concentration of Na⁺ was observed from the second cut of sorghum on the surface (0–10 cm) with any form of application of Si, isolated or combined. In the subsurface (40–60 cm), only the application of Si + T attenuated the Na⁺ concentration. The Na⁺ concentration was attenuated in the 10–20 and 20–40 cm layers during the third cut by applying Si, Si + OM, and Si + T (Table 5).

Available P presented the highest concentration on the surface in treatments with the application of organic matter (Si + OM and Si + T + OM) during the first cut. In the second cut, similarly to the first, the treatments where organic matter was applied showed the highest concentrations of P on the surface (0–10 cm), and it was also possible to observe differences in the 40–60 cm layer with the application of Si, Si + T and Si + T + OM, which presented higher concentrations compared to the control treatment. In the third cut, treatments applying organic matter also stand out concerning the others (Table 5).

3.2. Ions Content at Plant Tissue

The total Na⁺ content in the aerial part of the plant differed only in the second cut, where the application of Si + T presented a higher content (

Table 6. Ions content in each part of the plant and all plant shoots, in response to the tested saline attenuator for the three sorghum cuts. Different letters in the column indicate significant differences at p ≤ 0.05.

Treatment	Na+ (g)			K+ (g)			Cl− (g)			P (mg)			Ca2+ (g)			Mg2+ (g)
	Cut 1	Cut 2	Cut 3	Cut 1	Cut 2	Cut 3	Cut 1	Cut 2	Cut 3	Cut 1	Cut 2	Cut 3	Cut 1	Cut 2	Cut 3	Cut 1	Cut 2	Cut 3
Leaves
Control	0.05	0.15 a	0.06	0.78 b	0.67	0.36	0.41	0.32	0.31 a	33.84	43.73 a	27.09 c	0.20 b	0.22	0.16 b	0.13 b	0.10 c	0.09
Si	0.07	0.06 b	0.04	0.90 b	0.48	0.30	0.44	0.24	0.24 b	44.47	32.49 b	30.44 b	0.26 a	0.24	0.21 a	0.18 a	0.12 c	0.10
Si + OM	0.06	0.08 b	0.05	0.85 b	0.57	0.41	0.38	0.30	0.22 b	41.27	48.72 a	23.93 c	0.21 b	0.24	0.22 a	0.17 a	0.11 c	0.10
Si + T	0.06	0.10 b	0.07	0.84 b	0.64	0.38	0.36	0.37	0.23 b	46.79	43.16 a	25.05 c	0.27 a	0.26	0.23 a	0.18 a	0.13 b	0.11
Si + OM + T	0.06	0.10 b	0.05	1.06 a	0.63	0.39	0.52	0.34	0.30 a	47.92	45.66 a	34.01 a	0.28 a	0.29	0.25 a	0.20 a	0.15 a	0.13
Stems
Control	0.16 b	0.22 b	0.07	2.65 b	2.11	0.86 b	2.07 a	1.14	0.82 b	23.75	41.42	24.33 b	0.21 a	0.22 b	0.21 b	0.22 a	0.23 a	0.21
Si	0.11 b	0.19 b	0.14	2.72 b	2.19	1.68 a	1.47 b	1.32	1.06 b	16.94	35.99	53.85 a	0.15 b	0.18 b	0.20 b	0.15 b	0.17 b	0.23
Si + OM	0.21 b	0.30 b	0.17	3.75 a	2.93	1.82 a	2.07 a	1.19	1.32 a	31.03	56.92	41.75 b	0.22 a	0.20 b	0.28 a	0.25 a	0.20 b	0.26
Si + T	0.32 a	0.80 a	0.15	4.16 a	2.35	1.92 a	2.21 a	1.73	1.27 a	19.99	56.99	64.87 a	0.21 a	0.30 a	0.26 a	0.20 a	0.25 a	0.27
Si + OM + T	0.17 b	0.39 b	0.12	3.22 b	2.82	1.79 a	2.14 a	1.46	1.30 a	24.33	60.54	67.31 a	0.23 a	0.29 a	0.27 a	0.23 a	0.29 a	0.28
Panicles
Control	0.02	0.02	0.01	0.15 b	0.14	0.05 c	0.05	0.02	0.03	6.52 c	14.36	5.31	0.01 c	0.02 c	0.03 c	0.03 c	0.02 b	0.02 c
Si	0.03	0.03	0.01	0.19 b	0.13	0.04 c	0.10	0.04	0.03	12.01 b	11.26	3.16	0.02 b	0.04 b	0.06 b	0.05 b	0.04 b	0.04 b
Si + OM	0.05	0.02	0.02	0.42 a	0.14	0.10 a	0.11	0.04	0.06	18.60 a	13.05	9.85	0.04 a	0.06 a	0.09 a	0.09 a	0.06 a	0.07 a
Si + T	0.05	0.03	0.01	0.27 b	0.13	0.04 c	0.11	0.02	0.04	20.38 a	15.21	532	0.03 b	0.06 a	0.06 b	0.06 a	0.06 a	0.06 a
Si + OM + T	0.06	0.02	0.01	0.35 a	0.14	0.07 b	0.10	0.03	0.07	20.02 a	12.72	12.24	0.03 a	0.06 a	0.07 b	0.07 a	0.06 a	0.07 a
Plant shoot
Control	0.23	0.38 b	0.14	3.58 b	2.93	1.28 b	2.54	1.49	1.16 b	64.11 b	99.50 b	56.73 c	0.42 c	0.46 b	0.40 b	0.37 c	0.35 b	0.31 b
Si	0.21	0.28 b	0.18	3.82 b	2.79	2.03 a	2.02	1.60	1.33 b	73.41 b	79.74 b	87.45 b	0.44 c	0.47 b	0.46 b	0.38 c	0.33 b	0.38 b
Si + OM	0.32	0.40 b	0.24	5.02 a	3.64	2.33 a	2.57	1.53	1.61 a	90.91 a	118.69 a	75.53 c	0.48 b	0.49 b	0.59 a	0.51 a	0.38 b	0.43 a
Si + T	0.43	0.93 a	0.23	5.26 a	3.12	2.35 a	2.67	2.12	1.54 a	87.16 a	115.36 a	95.25 b	0.51 a	0.62 a	0.56 a	0.44 b	0.44 a	0.43 a
Si + OM + T	0.30	0.51 b	0.19	4.63 a	3.60	2.25 a	2.76	1.83	1.66 a	92.27 a	118.93 a	113.55 a	0.55 a	0.64 a	0.59 a	0.50 a	0.49 a	0.47 a

Si = silicon; Si + OM = silicon + organic matter; Si + T = silicon + Trichoderma harzianum; Si + OM + T = silicon + organic matter + Trichoderma harzianum.

). We observed that the Na⁺ content in the leaf was lower in all treatments with the application of isolated or combined Si, compared to the control treatment, and that sorghum tends to concentrate more Na⁺ in the stalk, having the Si + T treatment present the highest content (Table 6).

About K⁺, the application of Si alone or combined promoted a more significant accumulation of this element in the aerial part of sorghum in the first and third cuts than the control treatment. In the leaves, the application of S + T + OM favored a more significant accumulation of K⁺. In the stalks, in the third cut, the control treatment showed a lower accumulation of K⁺ than the others. There was also a difference in the content of this ion in the panicle, with treatments with the application of organic matter (Si + OM and Si + T + OM) showing the highest values.

In the third sorghum cut, the application of Si + OM, Si + T, and Si + T + OM favored a more significant accumulation of Cl⁻ in the aerial part of the plant, so that most of the Cl⁻ was more significantly present in the plant stalks. In the leaves, the concentration of Cl⁻ was significantly higher in the control treatment and with the application of Si + T + OM. In contrast, in the stalks, the control treatment and Si alone showed lower contents of this element (Table 6).

Applying Si + OM, Si + T, and Si + T + OM also favored a more significant accumulation of P in the aerial part of sorghum in the three cuts. Si + T + OM promoted the highest P contents, with 92.27, 118.93, and 113.55 mg of P in the first, second, and third cuts, respectively. The accumulation of Ca²⁺ and Mg²⁺ in the sorghum leaf, stalk, and panicle was enhanced with the application of Si alone and combined with T and OM; however, combined Si (Si + OM, Si + T, and Si + T + OM) promoted more significant accumulation of these elements in different parts of the plant as well the aerial part of sorghum, in the three cuts (Table 6).

3.3. Sorghum Yield and Water Use Efficiency

In all sorghum cuts, applying Si combined with T and/or OM, significantly increased the fresh and dry mass of the shoot in the three sorghum cuts (

Table 7. Mean values of sorghum dry matter yield (DMY) and sorghum fresh matter yield (FMY) for each cut and the total of the three cuts, depending on the tested saline attenuator. Different letters in the column indicate significant differences at p ≤ 0.05.

Saline Attenuator	DMY (t ha−1)				FMY (t ha−1)
	Cut 1	Cut 2	Cut 3	Total Yield	Cut 1	Cut 2	Cut 3	Total Yield
Control	16.99 b	17.59 b	10.21 c	44.80 b	52.33 b	52.28 c	41.76 b	146.37 b
Si	18.09 b	19.34 b	11.71 c	49.14 b	55.42 b	49.25 c	41.58 b	146.24 b
Si + OM	22.09 a	21.06 b	18.88 a	62.03 a	62.58 a	58.78 b	61.07 a	182.43 a
Si + T	19.96 b	27.36 a	15.21 b	62.53 a	58.61 b	67.14 a	53.70 a	179.45 a
Si + OM + T	23.69 a	20.56 b	14.80 b	59.05 a	66.20 a	51.22 c	52.20 a	169.62 a

Si = silicon; Si + OM = silicon + organic matter; Si + T = silicon + Trichoderma harzianum; Si + OM + T = silicon + organic matter + Trichoderma harzianum.

). The highest accumulated yields (total of the three cuts) were also achieved with the application of combined Si, where Si + OM, Si + T, and Si + T + OM obtained, respectively, yields of 62.03, 62.53, and 59.05 t ha⁻¹ of dry matter, and 182.43, 179.45, and 169.62 t ha⁻¹ of fresh mass (Table 7). It corresponds to gains in dry mass for the Si + OM, Si + T, and Si + T + OM treatments, of 38, 39, and 31% compared to the control treatment (without application of Si) and 26, 27, and 20% compared to the treatment where Si was applied alone. Regarding fresh mass, also for the Si + OM, Si + T, and Si + T + OM treatments, the accumulated gains concerning the control treatment (without application of Si) and Si alone were similar—25, 23, and 11%.

Applying Si + OM, Si + T, and Si + T + OM increased the water use efficiency (WUE) of sorghum plants, compared to the control and Si alone, in the three sorghum cuts. Regarding the accumulated values, Si + OM, Si + T, and Si + T + OM obtained a water use efficiency of 15.3, 15.05, and 14.65 g of dry mass per liter of water, respectively, compared to 10.98 and 12.02 g L⁻¹ of dry mass from the control treatments and with application of Si alone, respectively (

Table 8. Mean values of water use efficiency (WUE) by sorghum for each cut and the total of the three cuts, depending on the tested saline attenuator. Different letters in the column indicate significant differences at p ≤ 0.05.

Saline Attenuator	WUE (g DM L−1 H2O)
	Cut 1	Cut 2	Cut 3	Total WUE
Control	4.89 b	3.55 b	2.55 c	10.98 b
Si	5.20 b	3.90 b	2.92 c	12.02 b
Si + OM	6.35 a	4.25 b	4.71 a	15.30 a
Si + T	5.74 b	5.52 a	3.79 b	15.05 a
Si + OM + T	6.81 a	4.14 b	3.69 b	14.65 a

Si = silicon; Si + OM = silicon + organic matter; Si + T = silicon + Trichoderma harzianum; Si + OM + T = silicon + organic matter + Trichoderma harzianum.

). It meant an increased water use efficiency for Si + OM, Si + T, and Si + T + OM of 40, 37, and 33%, compared to the control treatment and 27, 25, and 22% compared to Si alone.

4. Discussion

Increasing soil salinity through irrigation with saline water is a common challenge in semiarid regions around the globe. Biosaline agriculture, in conjunction with saline stress attenuators, proved to be efficient in improving the productivity of crops and enabling agricultural production under salty stress conditions. In our study, silicon combined with Trichoderma harzianum and organic matter was more effective in mitigating the effects of saline irrigation on forage sorghum and mainly favoring its growth and yield.

4.1. Salinity Parameters

The decrease in EC over time can be explained by the precipitation observed during the experiment period, which was 176.5 and 252 mm during the second and third sorghum cuts, respectively, totaling 428.5 mm. Soil salinity is seasonal and follows climate change, so salinity is higher in dry periods and tends to decrease in rainy periods, as water promotes the dilution of the salts present, which are leached out of the soil profile, thus promoting its washing.

In addition to this natural decrease caused by rain, it was observed that treatments with Si and other salt stress attenuators also reduced EC compared to the control treatment, emphasizing the application of isolated Si and Si + T. Ref. [42] stated that silicon interacts with soil moisture and can regulate EC. As all salt stress attenuators were applied with silicon, alone or combined, our results are in agreement with previous studies such as [43,44], which had also observed that the application of silicon reduced the EC of the soil and favored plant growth. A similar result was found by [45], who, studying different sources of Si in a saline soil cultivated with corn, also found a decrease in EC with the application of potassium silicate.

In saline and sodic soils, silicon interacts with Na⁺ present in the soil and forms sodium silicate (Na₂SiO₃), which increases the mobility and absorption of Ca²⁺, Mg²⁺, and K⁺ and reduces the transport of Na⁺ and Cl⁻ to the roots of plants, favoring their leaching [30]. Furthermore, Ref. [42] stated that Si’s ability to bind to other minerals and its adsorption on clay particles makes it difficult for more Na or Cl ions to bind to essential mineral nutrients or to the surface of soil particles, which can reduce the availability of these salts and their absorption by plant roots in the soil, and consequently promote their leaching.

The predominance of Cl⁻ and Na⁺ in the soil solution mainly influenced the increase in EC in the control treatment, reflecting the low quality of the water used in this experiment, which contains a high concentration of these ions in its composition (Table 2). According to [6,9], the predominance of Na⁺ and Cl⁻ in the soil solution in irrigated areas is expected due to the low groundwater quality in the region. The predominance of ions in the soil solution was in the following order: Na⁺ > Cl⁻ > Ca²⁺ >Mg²⁺ > K⁺. The high concentration of these elements in the soil solution contributes to increasing the ionic strength of the soil solution and, consequently, its electrical conductivity [46].

Soil pH showed high variability between treatments in all soil layers. It ranged from 6.68 to 7.75, values above the ideal range for sorghum cultivation, which is 5.5 to 6.5 [47], but which are in line with expectations for salt-affected soils in semiarid regions [48]. This increase may be due to the imbalance in ionic composition resulting from excess Na⁺ and the presence of bicarbonates (5.70 mmol_c L⁻¹), which are also present in the water used for irrigation (Table 2) [49,50].

In general, applying Si alone or in combination promoted an increase in pH in some soil layers. Previous studies have demonstrated positive correlations between silicon and soil pH [51,52], as well as increases in soil pH in response to the application of silicon sources in the soil [31,53]. It occurs because OH⁻ ions are released during the dissolution of silicate fertilizers [54], contributing to an increase in soil pH. Ref. [55] further stated that the mineral Si has an alkaline nature, and its surface can easily transport exchangeable cations in large quantities and exchangeable H⁺ in small quantities, resulting in solid hydrolysis of exchangeable cations in the soil, which produces a high proportion of NaOH in the soil solution and increases soil pH.

Compared to the initial soil sample (Table 1), ESP increased between 200 and 400% in the soil layers across the three sorghum cuts. The soil was classified as saline-sodic [37], with an EC > 4 dS m⁻¹ and a PST > 15%. Increased sodicity promotes physical deterioration of the soil, mainly affecting water infiltration into the soil and root penetration [56,57]. The soil used in this study is especially susceptible to salinization and sodification due to its finer texture (Table 1) [58,59]. However, despite the increase, the application of Si alone or combined allowed this increase in ESP to be smaller than the control treatment, mainly related to the decrease in Na⁺ in the soil sorptive complex and the increase in K⁺, Ca²⁺, and Mg²⁺.

4.2. Soluble and Sorptive Complex

Compared to the initial soil characterization (Table 1), there was a reduction in soluble K⁺, Ca²⁺, and Mg²⁺, while soluble Na⁺ increased by approximately 10 times in the soil layers along the sorghum cuts. This increase in Na⁺ levels is explained by its high concentration in the irrigation water used in this work. Thus, the components of the soluble fraction of the soil are quickly altered in areas using irrigation with saline water [60]. Furthermore, it is normal for nutrient ions to decrease over time due to the absorption of these nutrients by plants, especially K⁺, which sorghum absorbed in more significant quantities in this experiment.

Si applied alone was ineffective in increasing nutrient concentrations in the soil, since, in its composition, the only nutrient provided in addition to Si is K⁺. Still, it reduced the concentrations of Na⁺ and Cl⁻ in the soil, with the treatment which proved to be more effective in mitigating soil salinity when applied in combination with Trichoderma. Although there was a reduction in the levels of Ca²⁺, Mg²⁺, and K⁺ compared to the initial characterization of the soil, in general, the applied silicon combined with organic matter and Trichoderma (Si + OM, Si + T, and Si + T + OM) presented higher concentration values of K⁺, Ca²⁺, and Mg²⁺ in the solution as well the soil sorption complex, compared to the control.

Adding organic matter can mitigate the effect of salts and improve plant growth, as it is a source of nutrients, increasing their availability, while also increasing water retention, improving soil structure, and increasing the bioactivity of microorganisms [61,62]. The action of Trichoderma may be associated with the fungus’ ability to increase the availability of nutrients in the soil through the secretion of organic acids into the root environment, as well as acting as a decomposer and mineralizer of OM, releasing the nutrients present in it more quickly [63,64,65,66].

In the soil exchange complex, the abundance of cations was in the following order: Ca²⁺ > Mg²⁺ > Na⁺ > K⁺. Ref. [9] observed the same sequence of cation abundance in cultivated soils in semiarid Brazil. Some cations are adsorbed more strongly to soil colloids, with this binding force being more significant the greater the charge on the ion and the smaller its hydrated radius [67]. Ca²⁺ and Mg²⁺ (present in high levels in irrigation water), because they have a greater charge (+2) and a smaller hydrated radius than Na and K (+1), are more easily retained in the soil exchange complex. Regarding Na⁺ and K⁺, although K⁺ has a smaller ionic radius, the high concentration of soluble Na⁺ in the soil solution causes it to become more adsorbed [68].

Concerning available P, treatments applying organic matter (S + OM and Si + T + OM) contributed most to its increase. Ref. [69] stated that the organic P present in OM can contribute to the long-term supply of P to crops through mineralization processes, so that organic P constitutes, on average, 25% of the total P in agricultural soils. It has also been demonstrated that the long-term application of organic fertilizer can improve soil P availability, reducing P sorption by competition or altered pH, accelerating the dissolution of soil metal oxides by organic acids, and promoting microbial P mineralization [70,71,72]. Additionally, Ref. [73] found that organic amendments can improve crop yield and phosphate fertilizer use efficiency by altering soil P fractions and increasing phosphatase activity. Similarly, Ref. [74] showed that combining organic and chemical fertilizers increased vegetable production and decreased total P leaching losses by >20%. In addition to increasing P availability and soil organic carbon (C), applying organic matter improves soil aggregation and environmental conditions for the soil microbial community, consequently improving SOM mineralization and nutrient cycling [75,76].

4.3. Ions Content at Plant Tissue Level

In sorghum plants, a higher content of K⁺ without Na⁺ was observed with the application of Si alone or in combination. In other crops, the isolated efficiency of Si, OM, and T in improving the absorption of K⁺ at the expense of Na⁺ has been proven, such as in spinach [77], fava beans [78], wheat [79], and in sorghum itself [18,80,81]. It may have occurred due to Si’s ability to increase the absorption of K⁺ at the expense of Na⁺, thus balancing the Na⁺/K⁺ ratio [19,20,82]. Furthermore, Si forms a silica barrier in the roots that reduces the passage of Na⁺ to the shoot [20,83]. This is supported by the results of this study, since Na⁺ was the cation absorbed the least by sorghum, despite its higher concentration in the soil solution. The accumulation of ions in the sorghum shoot followed the following order: K⁺ > Cl⁻ > Ca²⁺ > Mg²⁺ > Na⁺ > P.

Despite the low absorption of Na⁺ by plants, this behavior was not observed for the Cl⁻ content, which was high, second only to K⁺. Other authors observed the opposite effect, such that the application of Si reduced the absorption of Cl⁻ in different cultures. Si decreased Cl⁻ absorption in rice, while Cl⁻ concentration in roots was not changed [84]. Similar results were found in okra [85] and grapevine [86].

The greater absorption of Cl⁻ with the application of attenuators (Si + OM, Si + T, and Si + T + OM) did not negatively affect sorghum production, since these were the same treatments that obtained the highest fresh and dry mass production. The high K⁺ content in the plant may justify not affecting production, as potassium is an activator of many cytoplasmic enzymes necessary for photosynthesis and respiration [87]. Therefore, a deficiency in K⁺ suppresses photosynthesis and even reduces plant growth, while an increase in its absorption can alleviate the harmful effects of salinity [88]. Furthermore, K⁺ plays a vital role in maintaining and creating turgor pressure and adjusting plant water balance [87].

Applying Si + OM, Si + T, and Si + T + OM increased the K⁺, Ca²⁺, Mg²⁺, and P content in the aerial part of sorghum plants. It can be explained by the ability of OM to improve soil fertility. As already reported, applying organic matter enhances soil aggregation, improving environmental conditions for the soil microbial community and consequently improving SOM mineralization and nutrient cycling [75,76]. Trichoderma, in addition to exuding organic acids, can solubilize nutrients, especially phosphates, assists in the mineralization of organic matter, improves soil fertility, and improves the efficiency of nutrient use by plants, consequently contributing to their growth [25,63,89]. Trichoderma spp. is also related to the production of phytohormones, such as auxins, that help plant growth [90]. Such substances favor cell elongation in higher plants and increase the surface of the root system, enabling greater access to soil nutrients [91,92].

4.4. Sorghum Yield and Water Use Efficiency

Isolated Si increased sorghum dry matter productivity by around 10%, and this gain may be mainly associated with the reduction in the EC of the saturation extract in this treatment. However, the EC of all treatments did not exceed the threshold salinity for forage sorghum (10 dS m⁻¹) [93]. However, Si’s ability to reduce Na⁺ and Cl⁻ concentrations in the soil changes the ionic composition of the EC and reduces the toxicity caused by these ions [30,42]. The gain in productivity with combined Si (Si + OM, Si + T, and Si + T + OM) is associated with improvements in chemical attributes, soil fertility, and plant nutrition promoted by saline stress attenuators [19,94,95,96]. This study evidenced this by a reduction in toxic ions (Na⁺ and Cl⁻) in the soil and an increase in nutrient ions (K⁺, Ca²⁺, Mg²⁺, and P) in the soil and plant tissue using combined Si.

Trichoderma are recognized as synthesizers of phytohormones, such as auxin, and hormone-like compounds called harzianolide, substances that, together with auxin, act to expand the cell wall and increase biomass production by plants [97,98]. OM acts as a source of nutrients, soil conditioner, and food for the soil microbiota, and these characteristics improve fertility, soil aggregation, and nutrient cycling, thus promoting better nutrition and more significant biomass accumulation by sorghum [99,100]. The productivity obtained in this study is compatible with that found even when forage sorghum is irrigated with good-quality water [101,102,103].

Regarding WUE, in the bibliography, we found positive results on the efficiency of Si [104,105], OM [106,107], and T [108,109] in improving WUE. It is known that T increases root growth [110,111], and as a result, a greater area of soil is explored by the roots, resulting in better absorption and efficiency in the use of water by sorghum. Ref. [112] reported that Trichoderma increased sorghum root biomass by 50% under salinity. T also acts on stomatal regulation, thus reducing water loss and improving WUE [108]. Meanwhile, OM promotes greater water retention in the soil (irrigation + rain), increasing available water and its absorption by sorghum [113]. Si mainly contributes by adjusting nutritional imbalances, the photosynthetic system, and antioxidant mechanisms, factors associated with WUE [104].

5. Conclusions

Due to the soil in this study being, in particular, more susceptible to salinization and sodification due to its finer texture, the chemical quality of the soil deteriorated over time (>EC, ESP, Na⁺ and Cl⁻), making this type of soil unsuitable for irrigation with saline water. However, the applications of salt stress attenuators used in this study caused this increase in salinity parameters to be reduced in comparison with the control treatment. This way, the risk of degradation and loss of the soil’s productive capacity using mitigating agents is lower. The association of silicon with organic matter and Trichoderma harzianum increases the concentration of soluble K⁺ in the soil and the content of this element in plants. In this sense, plants’ tolerance to salinity is increased, since K⁺ participates in several critical metabolic processes in plants, alleviating the harmful effects of salinity. In general, the tested combinations improve plant nutrition by increasing the absorption of K⁺, P, Ca²⁺, and Mg²⁺ and, consequently, increasing the tolerance of forage sorghum to saline stress. In general, plants subjected to combined Si were better nourished and exposed to greater soil water retention and lower concentrations of toxic ions (Na⁺ and Cl⁻). Thus, productive capacity and WUE were improved. Therefore, agricultural practices with combined Si management (Si + T, Si + OM, and Si + T + OM) are essential for semiarid regions, as they favor the development of forage sorghum and allow the use of saline waters common to these regions.