Nutrient Content and Uptake of Wheat (Triticum durum L.) Fertilized with Potassium Sulfate under Semi-arid Conditions

Cereal crops are the main staple food worldwide, comprising over half of human calorie consumption. Wheat constitutes about 30% of cereal crops production and is the major food source for the ever-growing world population (Abd El-Mageed et al., 2023).
       
In Jordan, wheat is a major field crop mainly grown under rain-fed conditions. However, the country’s wheat productivity of 2.39 t ha^-1 (FAOSTAT, 2021) is relatively lower than the world’s average (3.52 t ha^-1) (Godebo et al., 2021). Low rainfall, poor distribution, drought and unbalanced fertilization are the main limiting factors responsible for low cereal productivity (Laekemariam et al., 2018).
       
Nutrient content and uptake of crops are highly dependent on soil, crop types and genotypes, climatic factors, fertilizer forms and application rates (Oosterhuis et al., 2013; Luo et al., 2023; Kaur et al., 2024). Nutrient uptake is also strongly linked to rainfall amount and soil moisture content (Ragályi et al., 2023; Mazouz et al., 2024). Besides that, the nutritional status of crops is vital in their adaptation to environmental stress. In this context, K is leading in improving crop performance, photosynthetic efficiency, yield and quality under drought situations (Marschner, 2012; Sedri et al., 2022).
               
The current research is warranted as studies concerning vital issues linked to the best K nutrient management under rainfed conditions are inadequate in Jordan. Therefore, the effect of different application rates of soil-applied mineral K fertilizer on the wheat crop performance in terms of nutrient content and uptake of local wheat varieties in some rain-fed areas in Jordan was investigated. The effect on selected soil chemical properties was also studied. 

Study areas and experimental sites
 
Two field trials were conducted at two Agricultural Research Stations of the National Agricultural Research Center (NARC) in rain-fed locations of Jordan for two successive growing seasons (2019/2020 and 2020/2021). The stations were Mushaqar (796.3 m above sea level with a mean annual rainfall of 305 mm) and Maru (589 m above sea level with a long-term average rainfall of 400 mm). The soils at both locations were classified as vertisols (Alkhader et al., 2023). 
 
Plant material
 
Two local wheat (Triticum durum L.) varieties, Cham 1 and Um Qais were tested at the Mushaqar and Maru sites, respectively. According to common cultural practices, the seeding rates were 120 and 130 kg ha^-1, respectively.
 
Cultural practices
 
The soil was cultivated and well-prepared at the two experimental sites. In the first season, sowing was carried out at the Maru and Mushaqar stations on the 3^rd and 4^th of December 2019, respectively. Sowing in the second season was carried out on the 8^th and 9^th of  December 2020, respectively. Diammonium phosphate (DAP) and urea were applied at 100 and 50 kg ha^-1, respectively. The total amount of DAP, half the quantity of urea and micronutrients in a chelating form at 10.0 kg ha^-1 rate were broadcasted and incorporated into the soil as a basal dose. The second half of urea was added at the tillering stage. Potassium (as sulfate of potash, SOP (K₂SO₄) was applied to the soil at the time of sowing as per treatments. Plant harvesting was performed after complete plant maturity.
 
Treatments
 
Five levels of K: T₁ = 0 kg K₂O ha^-1 (Control); T₂ = 20 kg K₂O ha^-1; T₃ = 40 kg K₂O ha^-1; T₄ = 60 kg K₂O ha^-1 and T₅ = 80 kg K₂O ha^-1.
 
Experimental design and statistical analysis
 
A randomized complete block design (RCBD) with four replications was followed and analysis of variance (ANOVA) was conducted using Statistical Analysis Software (SAS) version 9.0 for Windows (SAS Institute Inc. 2002). Whenever treatment effects were significant, means separation was made using the least significant difference (LSD) test at the 5% probability level.
 
Soil chemical and physical analysis
 
Before sowing, a composite surface soil sample (0-30 cm depth) was collected from the experimental site for physical and chemical analyses. Soil sampling was also made after plant harvesting, from three replicates. The samples were air-dried, crushed and passed through a 2 mm sieve for analysis. The following analyses were conducted: soil pH and salinity, as electrical conductivity (EC), (Bower and Wilcox, 1965), organic matter (Allison, 1965), calcium carbonate (Allison and Moodie, 1965), total N (Bremner, 1965), available P (Olsen and Dean, 1965), available K (Pratt, 1965), micronutrients (Chapman and Pratt, 1982) and soil texture (Day, 1965).
       
The soil of the Mushaqar Station was calcareous and clay in texture, exhibiting an alkaline reaction with an average pH of 7.7±0.1 and salinity of 0.891±0.182 dS m^-1. Meanwhile, the soil of the Maru Station was slightly calcareous and clay in texture, with an alkaline reaction, average pH and salinity of 7.7±0.1 and 0.651±0.066 dS m^-1, respectively.
 
Plant chemical analysis
 
Collected plant samples (grains and straws) were oven-dried (72^o C to constant weight), ground and stored in clean dry plastic bags. Chemical analysis of representative samples for nutrients (N, P, K, Mg, Fe, Cu, Zn and Mn) from three replicates was also performed according to Chapman and Pratt (1982). Plant nutrient content was converted into nutrient uptake by multiplying with plant dry matter (Saudy et al., 2023). 

Effect of K on the nutrient content
 
Cham1 variety
 
The K application rate showed no significant effect on the nutrient content in wheat grain and straw of Cham1 variety at the Mushagar site in both seasons. The macronutrient (N, P, K and Mg) content in the wheat grain during the first season averaged at 1.51, 0.41, 0.51 and 0.221%, respectively for all the treaments. Meanwhile, the mean micronutrient (Cu, Fe, Mn and Zn) content in the grains was 6.07, 750.10, 65.32 and 35.39 mg kg^-1, respectively. On the other hand, the macronutrient content in the wheat straw averaged as 0.57, 0.043, 1.43 and 0.0231%, respectively. The mean of micronutrient content in the straw was 0.833, 757.20, 65.97 and 30.55 mg kg^-1, respectively in T₁ to T₅ treatments.
       
In the second season, the macronutrient content in the grains’ macronutrient content averaged at 2.33, 0.33, 0.54 and 0.23%, respectively. Meanwhile, the mean of the grains’ micronutrient content was 5.62, 146.00, 73.96 and 43.43 mg/kg, respectively. And, the straws’ macronutrient content averaged at 1.00, 0.0627, 0.228 and 0.414 %, respectively. Meanwhile, the mean of micronutrient content  in straw was 0.430, 111.90, 40.45 and 2.91 mg/kg, respectively.
 
Um qais variety
 
K application rate, also, exhibited no significant effect on the plant grains’ and straws’ nutrient content of the Um Qais variety at the Maru site in both seasons. The average of grains’ macronutrient (N, P, K and Mg) content in grain in the first season was 1.45, 0.58, 0.64 and 0.264%, respectively. Meanwhile, the grains’ micronutrient (Cu, Fe, Mn and Zn) content averaged at 4.79, 240.55, 125.72 and 61.45 mg/kg, respectively. On the other hand, the average of macronutrient content was 0.54, 0.059, 0.785 and 0.310%, respectively, whereas the micronutrient content in straw averaged at 1.78, 124.48, 39.66 and 14.76 mg/kg, respectively.
       
Macronutrient content mean in grain, in the second season, was 1.91, 0.358, 0.456 and 0.206%, respectively. Meanwhile, the grains’ micronutrient content average was  4.50, 173.65, 81.68 and 41.55 mg/kg, respectively. While, the straws’ macronutrient content averaged at 1.959, 0.0203, 1.475 and 0.301%, respectively, the mean of micronutrient content in straw was 0.847, 431.50, 75.43 and 16.79 mg/kg, respectively. 
 
Effect of K on nutrient uptake
 
Cham 1 variety
 
The macro and micro nutrient uptake in the wheat grains and straws, respectively of the Cham 1 variety at the Mushaqar site during the first season tended to increase as the K application rate increased (Table 1). However, K-treated plots showed significantly higher Mg uptake in the straw than the control.


       
On the other hand, during the second season, the K application rate has caused significant effects on the macro and micronutrient uptake in the grains (Table 2). The treatments, T₃, T₄ and T₅ presented significantly higher grain N, Cu and Mn uptake than the control. Meanwhile, T₄ and T₅ treatments showed significantly higher P and Zn uptake by grains than the control. However, there was an increasing trend in the grain uptake of K, Mg and Fe as the K application rate increased. The increments in the grain uptake of N, P, K and Mg in T₃ were at the rate of 49.26, 49.33, 45.23 and 40.52% , respectively relative to the control. Meanwhile, the respective increment in the uptake of micronutrients (Cu, Fe, Mn and Zn) in the grain in T₃ was in the tune of 51.47, 53.56, 51.13 and 48.87%, respectively relative to the control. The macro and micronutrient uptake in the wheat straws on the other side had shown an increasing trend with the K application rate (Table 2).


 
Um Qais variety
 
K application rate illustrated a significant effect on the uptake of K and Mg in the grains of the Um Qais variety during the first season with the highest values reported in T₅ with an increment of 42.0 and 38.7%, respectively, compared to the control (Table 3). However, an increasing trend in the grain uptake of N and P was seen as K rate increased. Similarly, K application rate significantly affected the grain uptake of micronutrients with the highest values reported in T₅.  Increasing K rate has increased the N, P and K uptake in the wheat straw with the lowest and highest values in control (T₁) and T₅, respectively. However, K rate  has a significant effect on the Mg uptake by straw with the highest value in T₅. K Potassium rate also has significantly affected the uptake of Cu, Fe and Mn by the straw, with the highest value in T₅. The Zn uptake by straw as well increased with the increasing K rate.


       
The macronutrient uptake by the grains of Um Qais variety displayed an increasing trend during the second season with the increase in the K application rate (Table 4). The Fe and Zn uptake in grain was found significantly higher in T₃, T₄ and T₅ than in the control. The uptake of Cu and Mn by the grains and all the macro and micronutrients by the straw increased as K application rate increased.


       
The noticeable response of the wheat nutrient uptake to K application could be attributed to the fact that the soils of the experimental sites were vertisols characterized by a high fixation capacity for K (Misskire et al., 2019). Hence, the indigenous K supply by such soils to support wheat crops might be insufficient. Irregular rainfall distribution might also contribute to instabilities in water and nutrient uptake (Gaj and Gorski, 2014).
       
Several researchers supported the above results (Dar et al., 2021; Bhattacharjee et al., 2022; Khatun et al., 2022; Lotfi et al., 2022; Ragályi et al., 2023; Nawaz et al., 2024) and mentioned that K fertilization could improve the wheat crop’s nutrient uptake, yield, water use efficiency, chlorophyll content and photosynthetic rate.  The increased nutrient uptake of the wheat crop could be attributed to the enhancement in the crop dry matter yield due to the external addition of K to the soil which can make K more readily available (Jat et al., 2014). Similar results were reported by Dakshina et al., (2015) in rice.
       
The present findings are linked to the investigated wheat crop varieties grown under the prevailing environ-mental conditions. And, adopting cultural practices different from the present study might produce different results.
 
Effect of K application on soil chemical properties
 
Mushagar site
 
The K application rate showed no significant effect on the studied soil chemical properties at the Mushaqar site in the first season. However, soil salinity increased as K application rate increased, from 0.442 dS m^-1 in T₁ (control) to 0.5132 dS m^-1 in T₄ (+16%). The average values of the soil pH, EC and organic matter for all treatments were 7.68, 0.472 dS m^-1 and 0.229%, respectively. The mean values of soil nutrients viz., N, P, K, Fe, Mn, Zn and Cu were noted as 0.075%, 17.24 mg kg^-1, 506.10 mg kg^-1, 0.756 mg kg^-1, 7.88 mg kg^-1, 0.61 mg kg^-1 and 1.33 mg kg^-1, respectively.
       
On the other hand, K application rate significantly affected soil salinity in the second season. The increments in soil salinity in T₃ (0.468 dS m^-1), T₄ (0.421 dS m^-1) and T₅ (0.555 dS m^-1) treatments were 40, 33 and 49%, respectively, compared to the control (0.281 dS m^-1). The soil pH, EC and organic matter content averaged as 8.14, 0.408 dS m^-1 and 0.94%, respectively, whereas the average of soil nutrients (N, P, K, Fe, Mn, Zn and Cu) was 0.066%, 8.12, 521.7, 0.877, 3.71, 0.531 and 1.132 mg/kg, respectively.

Maru site
 
No significant effect of for the K application was noticed on the soil chemical properties at the Maru site in both  seasons. However, soil salinity increased with increasing K rates in the first season, where it increased from 0.297 dS m^-1 in T₁ to 0.380 dS m^-1 in T₄ (+28%). The mean of soil pH, EC and organic matter average values for all treatments were was 8.12, 0.347 dS/m and 1.30%, respectively. Meanwhile, the soil nutrients (N, P, K, Fe, Mn, Zn and Cu)  averaged at 0.130%, 20.16 mg/kg, 334.40 mg/kg, 0.743 mg/kg, 5.23 mg/kg, 1.14 mg/kg and 3.27 mg/kg, respectively.
       
Similarly, during the second season, soil salinity also showed an increasing tendency as K application rate increased from 0.325 dS/m^-1 at T₁ to 0.453 dS/m^-1 at T₅ (+39%). The average of soil pH, EC and organic matter content averages was 8.10, 0.395 dS/m^-1 and 1.72%, respectively. Meanwhile, the soil nutrients (N, P, K, Fe, Mn, Zn and Cu) averaged at 0.108%, 14.68 mg/kg, 355.84 mg/kg, 1.78 mg/kg, 0.702 mg/kg, 2.13 mg/kg and 1.48 mg/kg, respectively.
       
The shortage in seasonal rainfall during the second growing season might be responsible for the insufficient leaching of soluble salts that usually accompany the application of chemical fertilizers. This could eventually increase the salinity of the soil. However, the possible reason for the non-significant effect of the chemical fertilizers on soil pH could be attributed to the high buffering capacity of the calcareous soil (Alkhader and Abu Rayyan, 2015). The above results were in conformity with those of Amsalu and Beyene (2020) and (Govada et al., 2024) who reported that chemical fertilizers had increased soil EC but had no significant effect on soil pH. However, the nonsignificant variations in the soil’s available nutrients after harvest may be attributed to the balance between the addition of nutrients through fertilization and the withdrawal of nutrients by the crop. The results follow the findings of Sharma and Jain (2014).

The results revealed that K application rate positively affected the nutrient uptake in wheat and increased soil salinity. K application rate at 40 to 60 kg K₂O ha^-1 (as SOP) can be recommended at both Mushaqar and Maru sites. Judicious nutrient management, particularly K, should be practiced in semi-arid regions to help improve the nutrient uptake for higher grain yield and quality in wheat. Further research is highly recommended to investigate the  other wheat varieties’ response to K application under similar environmental conditions. 

The National Agricultural Research Center (NARC) is highly appreciated for its financial and technical support. Gratefulness is also extended to the Directorate of Laboratories of NARC (Soil and Plant) for sample preparation, analysis and reporting assistance.

The author declares that there are no conflicts of interest regarding the publication of this article.