Recovery of Degraded Urochloa Pasture: Effects of Polyhalite on Dry Mass Accumulation and Macronutrient Dynamics

Abstract

Grasslands cover more than 25% of the Earth’s surface and play essential ecological roles, such as forage production, supporting pollinators, and carbon sequestration. This study aimed to evaluate the recovery of a degraded pasture of Urochloa decumbens cv. Basilisk through aerial dry mass production, plant height, and foliar macronutrients concentration and uptake after fertilization with polyhalite. The experiment was carried out at the Teaching, Research, and Extension Farm of the School of Agrarian and Technological Sciences, UNESP—Dracena Campus, in a dystrophic red–yellow latosol soil. A randomized block design with four replications was used. The treatments included the following: (T1) control, (T2) N + P + liming, (T3) T2 + 30 kg ha⁻¹ K₂O (polyhalite), (T4) T2 + 60 kg ha⁻¹ K₂O (polyhalite), (T5) T2 + 60 (30 + 30) kg ha⁻¹ K₂O (polyhalite), and (T6) 60 kg ha⁻¹ K₂O (polyhalite). The treatment with N + P + liming + 60 kg ha⁻¹ K₂O (polyhalite) resulted in 93% more dry mass production when compared with the control treatment. This treatment was most effective for grassland recovery, whereas polyhalite alone was ineffective. Tissue N and S concentrations increased as a result of the addition of N + P + liming + 60 kg ha⁻¹ K₂O (polyhalite).

1. Introduction

Grasslands are among the world’s most vital ecosystems, covering more than 25% of the Earth’s surface. They play a key role in forage production, support pollinators, and contribute to carbon sequestration [1]. In 2022, Brazil had more than 177 million hectares of pastures, which is equivalent to 20% of the Brazilian territory [2]. A survey carried out by the MapBiomas institute found that, in 2020, 53% of Brazilian pastures showed some degree of degradation, with 14% being severely degraded [3]. However, increasing pressure from consumers, public institutions, and private stakeholders demands that production systems improve cattle productivity while reducing environmental impacts [4]. Pasture degradation is characterized by a continuous decline in productivity over time and is a common problem throughout Brazil. This process causes agronomic, economic, and environmental losses and is linked to multiple causes, both natural, such as adverse weather, and anthropogenic, such as inadequate management and overgrazing. The consequences of pasture degradation include soil compaction, invasion by undesirable plant species, and a reduction in the carrying capacity of the pasture. Additionally, degradation varies by region and time period, making it difficult to define precise indicators, as the ideal productivity is relative to each specific context [5,6].

Well-managed tropical grassland plays a key role in mitigating climate change through carbon sequestration in soil organic matter. In integrated systems, deep-rooted forage grasses can promote substantial accumulation of belowground biomass, improving soil structure and enhancing long-term carbon storage [7,8]. Practices such as proper fertilization, rotational grazing, and recovery of degraded areas can maximize carbon inputs and reduce greenhouse gas emissions from livestock systems [9]. Thus, grassland rehabilitation using nutrient management strategies not only restores productivity but also contributes to climate mitigation, particularly in tropical regions with high potential for carbon sequestration under improved soil conditions [10,11].

N fertilization is among the most extensively studied agronomic practices due to its widespread application and ability to increase biomass production, primarily by stimulating leaf expansion and tillering. However, excessive N application may accelerate senescence unless it is carefully managed. Additionally, increased N supply often increases the uptake of K, necessitating complementary K fertilization to maintain productivity in N-fertilized systems [12]. The use of N fertilization also contributes to increased plant production and stimulates the proliferation of fungi at the expense of bacteria. However, this increase can be hindered if excessive doses of N are applied [13]. These dynamics underscore the importance of long-term pasture sustainability in balanced nutrient management programs to optimize crop yield and longevity. Balanced nutrient strategies may also mitigate N leaching risks.

K is essential for key physiological processes in plants, including photosynthesis, enzyme activation, protein synthesis, and the regulation of water balance [14]. However, despite its importance, K leaching occurs in diverse soil types, particularly in those with low charge potential, and under the application of highly soluble fertilizers. Soluble K⁺ ions, derived from fertilizers, soil amendments, or organic matter decomposition, are susceptible to leaching, especially under heavy rainfall or excessive irrigation. The primary factors governing K⁺ leaching include the soil texture, cation exchange capacity (CEC), water regime, and fertilizer solubility. When present in the soil solution, K⁺ can migrate with water flow, becoming displaced outside of the root zone [15]. Thus, optimizing K fertilization efficiency requires careful selection of appropriate fertilizer sources to minimize losses and enhance nutrient retention. Strategies such as split applications or precision irrigation could further reduce losses.

Polyhalite is a naturally occurring hydrated sulfate mineral with the chemical formula K₂Ca₂Mg(SO₄)₄·2H₂O. Its composition is as follows (by percentages): 14% K₂O, 48% SO₃, 17% CaO, and 6% MgO [16]. Polyhalite contains four essential macronutrients: K, Ca, Mg, and S. Unlike blended fertilizers, polyhalite exists as a homogeneous crystalline compound rather than a mechanical mixture of its constituent elements [17]. This mineral offers several agronomic advantages, including (1) high nutrient solubility and plant availability; (2) multiple commercially available particle size fractions; (3) minimal environmental impact; (4) a comparatively low carbon footprint relative to that of conventional fertilizers; and (5) a lower salt index than similar fertilizers such as potassium chloride (MOP) and potassium sulfate (SOP) although higher than sulfate of potash magnesia (SOPM) [18,19,20]. Furthermore, to ensure a continuous nutrient supply throughout the crop-growing season, polyhalite demonstrates a prolonged nutrient release while reducing environmental losses [21,22].

Grassland degradation is one of the main constraints to livestock productivity in tropical regions, often linked to declining soil fertility, particularly with respect to macronutrients such as K, Ca, magnesium Mg, and S [7,8]. Among the recommended strategies for grassland rehabilitation, the use of correct fertilization plays an important role in reestablishing productivity [23]. In soils with low K availability, doses of 60 kg ha⁻¹ of K₂O have been used to restore nutrient balance [24]. Therefore, evaluating the agronomic efficiency of polyhalite in degraded pastures is of both scientific and practical relevance, especially under tropical conditions where nutrient limitations are widespread.

Given the extensive area of degraded pasturelands worldwide, particularly in Brazil, and increasing institutional demands for more efficient natural resource utilization, there is an urgent need to develop sustainable pasture recovery strategies. Considering the vast area of degraded grasslands in Brazil, the use of polyhalite emerges as a particularly attractive restoration strategy, as it provides four of the six essential macronutrients. This may reduce the logistical effort and input costs associated with applying individual fertilizers separately. This study aimed to evaluate the effects of polyhalite fertilization on the nutritional status, growth, and development of forage grass in a degraded dystrophic red–yellow latosol soil. This research seeks to establish optimized polyhalite management practices that can contribute to the technological advancement of sustainable pasture restoration.

2. Materials and Methods

2.1. Experimental Site

The study was conducted between 5 December 2023 and 11 June 2024 at the UNESP Teaching, Research and Extension Farm (Fazenda de Ensino, Pesquisa e Extensão) of the Faculty of Agricultural and Technological Sciences, located at the Dracena Campus in São Paulo State, Brazil (21°27′42.3″ S, 51°33′04.0″ W); the plots were designed as 4 m long by 3 m wide, totaling 12 square meters (Figure 1).

2.2. Meteorological Data

The experiment was carried out in a region with an Aw climate, and daily meteorological data were recorded from a weather station located approximately 300 m from the experiment site (Figure 2). Throughout the study period, the cumulative precipitation reached 54% of the historical average for the corresponding timeframe.

2.3. Treatment and Experimental Design

The experiment employed a completely randomized block design with four replicates and repeated measures across four growth cycles. Liming was necessary to achieve a soil exchange capacity (CEC) of 50%; for this purpose, dolomitic limestone was used. Treatments were applied 35 days after plot liming and consisted of the following:

• T1—Control: No fertilization;
• T2—NP + liming: 75 kg ha⁻¹ N + 60 kg ha⁻¹ P₂O₅ + liming;
• T3—T2 + 30 kg ha⁻¹ K₂O (polyhalite) + liming;
• T4—T2 + 60 kg ha⁻¹ K₂O (polyhalite) + liming;
• T5—T2 + split application of (30 + 30) kg ha⁻¹ K₂O (polyhalite) + liming;
• T6—K-only: 60 kg ha⁻¹ K₂O (polyhalite).

The initial fertilizer application for the experiment occurred on 5 December 2023. In the fertilized plots, the same doses of N and P₂O₅ were applied: 75 kg ha⁻¹ and 60 kg ha⁻¹, respectively. After the second and third cuts, two more doses of 25 kg ha⁻¹ of N, via urea, were applied in treatments 2, 3, 4, and 5 to reach the specified 75 kg. Phosphorus in the form of monoammonium phosphate was applied only at the beginning of the experiment. The second dose of K₂O (30 kg ha⁻¹) via polyhalite in T5 was applied after the second cut of the forage.

The polyhalite product used in this study was POLY4^®, a branded granulated polyhalite product of Anglo American crop nutrients, London, England (www.poly4.com) (accessed on 5 September 2025). Throughout the study, the POLY4^® treatment is referred to as polyhalite.

2.4. Soil Collection and Analysis

Soil samples were collected from the 0–20 cm depth layer of a red–yellow latosol [25] and analyzed following the methodology described previously by [26], the soil samples were collected from each of the 24 plots, and then a representative average of the experimental area was calculated. Initial chemical characterization revealed the following soil composition properties: 4.3 mg dm⁻³ Phosphorus (P-resin), 1.7 mmolc dm⁻³ potassium (K⁺), 2.3 mg dm⁻³ sulfur (S), 8.5 mmolc dm⁻³ calcium (Ca²⁺), 1.7 mmolc dm⁻³ magnesium (Mg²⁺), 1.0 mmolc dm⁻³ aluminum (Al³⁺), and 9.6 g dm⁻³ soil organic matter (SOM); pH (CaCl₂) of 4.79; sum of bases (SB), 12 mmolc dm⁻³; cation exchange capacity (CEC), 33 mmolc dm⁻³; and base saturation (V%), 36%.

2.5. Sample Harvest

Forage samples were collected by placing a 0.5 m⁻² iron quadrat (1 m × 0.5 m) at least 50 cm away from the plot borders. The plants within the quadrat were then cut at a height of 15 cm, collected, and weighed before and after drying. Subsequently, the dried samples were ground in a Willey mill and analyzed according to the methodology described by Malavolta [27]. Macronutrient accumulation was determined as a function of the nutrient concentration and the dry mass production of the aerial part.

2.6. Statistical Analysis

The data were tested for normality (Shapiro–Wilk test) and homoscedasticity (Levene’s test) prior to analysis. The accumulated DM production data were analyzed using ANOVA in a randomized complete block design with a split-plot arrangement. The treatment means were compared using the Scott–Knott test (p < 0.05) [28]. All analyses were performed using Sisvar statistical software version 5.6 [29].

The remaining datasets were analyzed using analysis of variance (ANOVA) in a randomized complete block design, with treatment means compared via the least squares means test at the α = 0.05 significance level [30]. Analyses were performed using SAS statistical software (version 3.81, Enterprise Edition) [31] with the GLIMMIX procedure.

3. Results

ANOVA revealed the significant effects (p < 0.05) of treatments, cutting cycles, and their interaction on the forage DM yield.

First cutting cycle: Treatments 3, 4, and 5 resulted in significantly higher yields than the other treatments did (

Table 1. Forage DM production of Urochloa decumbens cv. Basilisk as affected by polyhalite application in combination with N and P fertilization.

Treatments	January (1st Cut)	February (2nd Cut)	March (3rd Cut)	June (4th Cut)
	kg ha−1
T1—Control	1174 Da	572 Cc	579 Dc	892 Db
T2—N + P + Liming	1679 Ba	892 Ac	755 Bd	1046 Cb
T3—T2 + Liming + 30 kg ha−1 K2O via polyhalite	1837 Aa	819 Bc	782 Bd	1291 Bb
T4—T2 + 60 kg ha−1 K2O via polyhalite	2079 Aa	748 Cc	1387 Ab	2024 Aa
T5—T2 + Liming + 60 (30 + 30) kg ha −1 K2O via polyhalite	1851 Aa	691 Cc	1371 Ab	1366 Bb
T6—60 kg/ha −1 K2O via polyhalite	1182 Ca	687 Bb	666 Cb	1138 Ca

Means with the same uppercase letters in the same column and the same lowercase letters in the row are not statistically different according to the Scott–Knott test. (p ≤ 0.05).

), resulting in 56.5, 77.1, and 57.6% higher DM production, respectively, when compared to the control.

Second cutting cycle: Treatment 2 resulted in the highest biomass (Table 1) with 55.9% more DM production when compared to the control.

Third cutting cycle: Treatments 4 and 5 outperformed the other treatments (Table 1), producing 139.5 and 136.7% more DM than the control.

Fourth cutting cycle: Treatment 4 resulted in superior performance, with 126.8% greater DM production (p < 0.05) than the N + P + liming control (Table 1).

Compared with all other treatments, the combined application of treatment 4 significantly (p < 0.05) increased forage productivity. Conversely, both the unfertilized control and treatment 6 presented the lowest DM yields (Figure 3).

3.1. Nutrient Concentration and Uptake

3.1.1. Macronutrient Concentrations

Figure 4 presents the macronutrient concentrations in Urochloa decumbens during the rainy season.

During the rainy season, macronutrient concentrations varied significantly. N ranged from 7.57 to 10.98 g kg⁻¹, with the highest concentrations observed in treatment 5 with a concentration yield 34.1% higher than the control. This treatment also showed the highest P concentrations, which ranged from 1.84 to 2.53 g kg⁻¹ which represents 36% higher yields when compared to the control.

The K (20.07–25.07 g kg⁻¹), Ca (1.83–2.43 g kg⁻¹), Mg (1.81–2.02 g kg⁻¹), and S (0.93–2.46 g kg⁻¹) concentrations were also measured. However, there were no significant differences in K, Ca, or Mg concentrations among the different treatments.

Compared with the other treatments, treatment 4 resulted in a significantly greater S concentration (p < 0.05) with a concentration yield of 105.5% higher than the control, whereas both the control and treatment 2 presented the lowest values. Figure 5 presents the ANOVA results for the macronutrient concentrations during the dry season.

The N concentration was significantly greater (p < 0.05) in treatment 5 than in the other treatments; treatment 5 presented 89.1% higher concentration yields than the control. No significant differences (p > 0.05) in P, K, Ca, or Mg concentrations were detected among the treatments. The S concentration was elevated in three treatments: (1) treatment 4, (2) treatment 5, and (3) treatment 6, presenting yields 42.3, 36,0, and 38.7 higher than the control, respectively. While these three treatments showed similar S concentrations (p > 0.05), they all resulted in significantly higher S levels than did the other treatments (p < 0.05).

3.1.2. Macronutrient Uptake

Significant differences (p < 0.05) in macronutrient uptake were observed across all of the treatments during the rainy season (Figure 6).

Nitrogen (N): Treatment 2 resulted in the highest N uptake (4.72–9.01 kg ha⁻¹), resulting in 90.9% higher yield when compared to the control, significantly outperforming the other treatments (Figure 6a).

Phosphorus (P): All the completed fertilized treatments (receiving N, P, and K) presented greater P uptake (1.06–1.89 kg ha⁻¹) than the incomplete fertilization regimes did (Figure 6b).

Potassium (K): The treatments 2 and 3 resulted in the highest K uptake (12.20–20.32 kg ha⁻¹) (Figure 6c), where both treatments resulted in 60.6% higher uptake in comparison to the control.

Calcium (Ca) and magnesium (Mg): Treatment 2 yielded maximal uptake of both nutrients (Ca: 5.77–13.12 kg ha⁻¹; Mg: 2.17–5.34 kg ha⁻¹) (Figure 6d,e).

Sulfur (S): Compared with the other treatments, treatment 4 resulted in significantly greater S uptake (0.96–3.21 kg ha⁻¹) (Figure 6f); analysis revealed that treatment 4 had an uptake 170.6% higher than the control.

During the dry season, macronutrient uptake values were recorded within the following ranges: N (3.18−9.73 kg ha⁻¹; Figure 7a), P (2.07−3.56 kg ha⁻¹; Figure 7b), K (21.47−42.54 kg ha⁻¹; Figure 7c), Ca (5.76−13.12 kg ha⁻¹; Figure 7d), Mg (2.17−5.33 kg ha⁻¹; Figure 7e), and S (0.96−3.21 kg ha⁻¹; Figure 7f).

Compared with that in the other treatments, N uptake was significantly greater (p < 0.05) in the N + P + liming + polyhalite treatments (both treatments 4 and 5). Those treatments also presented the highest percentage change when compared to the control (202.3 and 187.5%, respectively). Among all macronutrients analyzed (P, K, Ca, Mg, and S), only treatment 4 resulted in significantly greater (p < 0.05) uptake than the control and other fertilization regimes did. The uptake measured for treatment 4 presented higher percentages of the other five macronutrients aside from N when compared to the control (119.8, 98.1, 127.4, 146.1, and 227.6, respectively) (Figure 7).

4. Discussion

The limited precipitation during the experimental period reduced polyhalite solubilization via mass flow, which aligns with previous findings [9] reporting its lower solubility than that of conventional fertilizers, with residual material persisting for over four weeks post-application a pattern consistently observed in our study. This behavior is related to the mechanisms of polyhalite dissolution, governed by surface reactions and diffusion through secondary mineral layers, where leaching and evaporation can contribute to the dissolution of the fertilizer granules [21]. The release of nutrients like K, Ca, Mg, and S from polyhalite tends to be more gradual, particularly under dry conditions, which reduces their availability when water is scarce [32].

Significantly higher (p < 0.05) DM yields were observed in treatments 4 and 5 than in the other treatments (Table 1), particularly during the first (due to maximized nutrient availability from recent application) and fourth (likely due to sufficient time for nutrient uptake and utilization) cutting cycles. This enhanced performance in treatment 4 is attributable to complete macronutrient fertilization at optimal rates, promoting robust plant development, improving stress resilience, and maintaining productivity [33], which is consistent with previous findings [34]. The superior performance of treatment 4 can also be explained by the interaction between N and S. Sulfur showed positive effects on sugarcane productivity when applied together with N, which contributed to a more balanced nutrient uptake and greater nitrogen use efficiency, especially in tropical soils poor in nutrients [35], demonstrating that a balanced nutrient supply at appropriate concentrations maximizes growth potential. In contrast, polyhalite application alone (treatment 6) had no significant effect, highlighting the necessity of combined N and P fertilization and polyhalite supplies K, Ca, Mg, and S; the lack of readily available N and P may limit its standalone effectiveness in highly weathered tropical soils. This reinforces the concept that while polyhalite is agronomically valuable, its use should be integrated into a comprehensive fertilization strategy, particularly in pastures under recovery.

Interestingly, in treatment 4, significantly higher concentrations were observed for the macronutrients Ca and S. This effect is likely due to the “dilution effect”, where accentuated plant growth results in lower concentrations of some nutrients in treatments with higher productivity [36]. While a previous study [17] reported increased DM production in wheat (Triticum aestivum L.) with polyhalite application, another study [37] reported higher forage DM yields in Urochloa decumbens when N rates were equal to that of ammonium nitrate with calcium sulfate, demonstrating that synergistic N–S interactions enhance leaf production. A recent study also demonstrated that wheat fertilized with polyhalite achieved higher dry matter yield, plant height, and nutrient use efficiency compared to the KCl control [38]. An additional study [39] investigated the effects of cattle manure-derived organic biofertilizer on Urochloa hybrid cv. Mulato II, and showed significantly increased (p < 0.05) aerial biomass compared with that of unfertilized controls.

During the rainy season, treatment 5 resulted in significantly higher (p < 0.05) N and P concentrations in plant tissue; however, no significant differences (p > 0.05) were observed for K, Ca, or Mg, whereas the S concentration was highest (p < 0.05) in treatment 4. During the dry season, treatment 5 resulted in significantly greater (p < 0.05) N concentrations, with no significant differences (p > 0.05) for the other macronutrients except for significantly elevated (p < 0.05) S concentrations in treatments 4, 5, and 6. These results reinforce polyhalite’s sustained nutrient release behavior, which is capable of progressively releasing S throughout the growing period. Since approximately 50% of the S is immediately available, while the remainder is released over time, this can be advantageous in situations that demand a constant supply of nutrients [32].

Macronutrient analysis revealed variations between treatments and seasons: greater N, P, and K extraction during the rainy season occurred in the treatments with complete fertilization (Figure 5), and increased macronutrient uptake in the dry season was associated with greater biomass production, with treatment 4 resulting in the highest uptake values for all macronutrients. The polyhalite treatments resulted in the highest Mg and S levels, whereas the polyhalite-only treatment resulted in lower N, P, K, and Ca uptake, highlighting the importance of complete fertilization. Similar effects were reported in a previous study on barley (Hordeum vulgare) and corn (Zea mays) [14], which revealed increased S availability and a positive correlation between DM production and S extraction, despite K uptake showing significant differences, while its concentration remained unaffected. Although K concentrations increased with polyhalite application up to 1500 kg ha⁻¹, additional applications did not increase absorption [17]. The decreased Ca and Mg concentrations in aerial plant parts, coupled with increased nutrient uptake, suggests that Ca and Mg availability did not limit biomass production. This supports the hypothesis that the dilution effect, rather than nutrient deficiency, accounts for these reduced concentrations [36]. The significant differences in S concentrations occurred only at very high rates, with an overall negative plant response to S. The increased uptake without a corresponding concentration increase suggests that, while the fertilizer provides nutrients, plants may not fully utilize them due to physiological limits or growth partitioning dynamics [17].

Overall, the positive response observed in treatment 4 reflects the advantage of a complete and balanced nutrient supply. While polyhalite enhances the availability of K, Ca, Mg, and S in a single application, its agronomic efficiency is maximized when combined with N and P fertilization. The consistent S accumulation across seasons reinforces polyhalite’s role as a slow-release fertilizer, particularly relevant under tropical conditions where nutrient leaching and erratic rainfall are common challenges. Additionally, the physiological response of the plants, such as improved N efficiency in the presence of S and higher root growth in response to adequate K, further justifies integrated fertilization as a key strategy in degraded pasture recovery.

5. Conclusions

The balanced crop nutrition along with liming improved the yield and nutrient uptake of U. decumbens cv. Basilisk pasture in Brazilian soils. The application of potassium through polyhalite at the 60 kg rate, along with N and P fertilization, effectively restored the degraded U. decumbens cv. Basilisk pasture. This treatment resulted in increased dry matter production and greater potassium uptake while simultaneously increasing the nitrogen and sulfur concentrations. However, the polyhalite application alone, without the nitrogen and phosphorus application, was unable to restore U. decumbens. The results demonstrate that the use of polyhalite along with N and P supply, whether in a single application or as a split application, improved productivity and enhanced the nutrient uptake of the critical nutrients while providing a sustainable and promising solution to restoring the degraded pastures in the Brazilian agricultural systems.