1. Introduction
Durum wheat (
Triticum durum) is an essential strategic crop for food security because it is a main source of protein, which provides gluten for bread making [
1]. In Algeria, durum wheat is consumed in various forms: paste, bread, and couscous [
2]. At the national level, it is the most cultivated cereal, and its current yield is around 1800 kg ha
−1 [
3]. Previous studies have shown that a yield of 4000 kg ha
−1 would be necessary to meet the needs of the population. However, several factors, such as water deficit, seed quality, sowing date, and inadequate use of fertilizers, influence wheat crop productivity [
4,
5]. Among these factors, drought stress is a very important abiotic stress that constricts wheat production [
6,
7]. According to a report published by the United States Department of Agriculture (USDA), the water deficit has caused a decline in wheat production, which has led to imports of about 8 million tons for the year 2021 [
8]. Wherefore, rain-fed cropping systems should be considered as one of the major ways for increasing and stabilizing cereals production, especially in sub-humid Algerian areas [
9]. The cultivated area by durum wheat covered an average of 22,496 ha, in which durum wheat is generally cropped in rotation with either legumes or farmed through fallow practice [
10,
11,
12].
Potassium is the most abundant cation in plants [
13]. In the soil, potassium is found in four forms: exchangeable, constitutive, retrograde, and soluble [
14]. The exchangeable and soluble forms constitute the compartment available to plants. The potassium-supplying capacity of soil depends on its total potassium content and the releasing characteristics of different forms, which are influenced by the physico-chemical properties of the soil [
15]. Its uptake by the plant decreases with decreasing soil water content [
16]. Indeed, it has multiple functions that affect the physiological functioning of plants, including improving plant quality, the photosynthetic process, osmotic regulation, enzyme activity, assimilate stimulation and transport, protein synthesis, and stress tolerance [
17,
18]. Nitrogen and phosphate fertilizers are the main fertilizers used in wheat production, while potassium fertilizer is rarely applied [
19], and its deficiency reduces the qualitative and quantitative yield of wheat crops [
20]. Additionally, in the absence of adequate potassium fertilization, significant soil K deficiency occurs [
15]. Halillat [
21] and Belaid [
22] have shown that potassium fertilization has a favorable effect on agronomic behavior, including certain characteristics of durum wheat, such as yield and thousand-grain weight.
Despite the importance of the role of potassium in the formation of yield, the practice of potassium fertilization on the wheat crop remains very limited in Algeria [
23]. The main objective of the present study is to identify the optimal K fertilizer dose that could stabilize and improve growth and yield components of durum wheat crop, particularly under organic cropping management in sub-humid climate. We hypothesize that both low and moderate K application will maintain simultaneously yield and K acquisition by durum wheat. The specific objectives of this study were (i) to evaluate the influence of different K fertilizer application rates on wheat yield, (ii) to understand the response of the crop to variation in K use on morphological traits with different rates of potash fertilizer, and (iii) to determine the influence of potassium rates on the quality of wheat grain. The field experiment was conducted within the same conditions of crop and soil management that are commonly practiced by the local farmers under rain-fed conditions.
3. Results
3.1. Morphological Characteristics of Durum Wheat
Table 2 shows different morphological parameters that are involved in the vegetative growth of durum wheat. These parameters include plant height, spike length, spike neck, grain number per spike, spike number per m
2, and spikelet number per spike. Thus, plant height was significantly affected by the potassium application rate and K-level*season interaction (
p ≤ 0.05). Comparing the two years of experimentation, it was found that the highest plant height was recorded with the 200 kg ha
−1 k
2O rate (85.79 cm for the first year and 83.65 cm for the second year), while the lowest plant height was observed with the 400 kg k
2O ha
−1 application rate (81.73 cm for the first year and 80.93 cm for the second year).
Spike length was significantly affected by potassium fertilizer levels (p ≤ 0.05). The highest spike lengths were observed with the 100 kg k2O ha−1 and 200 kg k2O ha−1 application rates (7.57 cm and 7.58 cm for the first year and 7.04 cm and 6.76 cm for the second year), while the lowest value of spike length was noted with the control plots (7.08 cm for the first year and 6.57 cm for the second year). Therefore, compared to the control, 100 kg k2O ha−1 resulted in an increase in spike length of 6.47% and 6.60% for the first year and 7.76% for the second year.
Potassium fertilizer levels and K-level*season significantly influenced the spike neck (p ≤ 0.05). The 100 kg k2O ha−1 rate produced the highest values (18.48 cm for the first year and 17.51 cm for the second year), representing increases of 13.74% and 9.03%, respectively, compared to the control who recorded the lowest values of 15.94 cm and 16.06 cm.
Increasing levels of potassium fertilizer, cropping season, and K-level*season had a significant effect on the spike number per m2 (p ≤ 0.05). However, the 100 kg k2O ha−1 rate produced the highest values for this parameter (226.77 spikes m−2 in the first year and 264.25 spikes m−2 in the second year). In contrast, the lowest rates were obtained with the 300 kg k2O ha−1 rate for the first year (213 spikes m−2) and with the control for the second year (228.25 spikes m−2).
Finally, morphological parameters of vegetative growth results showed that increasing K fertilizer application had no significant effect on the grain number per spike and the spikelet number per spike (p > 0.05). For these two parameters, there was no significant difference between the different levels of K fertilizers tested. These results suggest that other factors, such as irrigation, temperature, and plant variety, may have a greater influence on grain and spikelet production.
3.2. Grain Yield and Yield Attributes
3.2.1. Grain Yield
According to the results presented in
Table 3, wheat grain yield was significantly affected by the potassium fertilizer application levels (
p ≤ 0.05), the cropping season (
p ≤ 0.001), and the K-level*season (
p ≤ 0.01). The results indicate that the 100 kg k
2O ha
−1 application rate produced the highest grain yields of 2.58 t ha
−1 in the first year and 2.09 t ha
−1 in the second year. These numbers represent a significant increase of 23.27% over the control test. However, when the potassium fertilizer rate was increased to 200 kg k
2O ha
−1, a 3.97% decrease in grain yield was observed. Interestingly, the lowest grain yield was obtained with the highest potassium rate (400 kg k
2O ha
−1), i.e., 1.83 t ha
−1 in the first year and 1.53 t ha
−1 in the second year. In contrast, using the 100 kg k
2O ha
−1 rate resulted in an increase of 0.54 t ha
−1 over the control plots.
3.2.2. Dry Matter
The results in
Table 3 describe the effect of potassium fertilizer levels on wheat crop dry matter. The results indicate that the potassium fertilizer levels, the cropping season, and the K-level*season had a significant impact on dry matter content (
p ≤ 0.05). The use of 200 kg k
2O ha
−1 of potassium fertilizer resulted in the highest dry matter contents, with 20.01% in the first year and 16.85% in the second year. A dose of 100 kg k
2O ha
−1 also resulted in a significant increase, with values of 18.34% and 16.5% for the two years, respectively. These results show an increase of 15.09% and 7.36% over the control when both potassium fertilization rates are used. Nevertheless, the use of a 400 kg k
2O ha
−1 dose generated the least amount of dry matter, with a value of 14.46% for the first year. For the second year, the plots had the lowest amount of dry matter, only 16.35%.
3.2.3. Thousand Grain Weight
The data presented in
Table 3 highlight the significant effect of potassium fertilizer levels and the K-level*season on the thousand-grain weight of wheat (
p ≤ 0.05;
p ≤ 0.01). The results indicate that application rates of 100 kg k
2O ha
−1 and 200 kg k
2O ha
−1 caused a significant increase in this parameter (
Table 3). In other words, the use of these two levels of potassium fertilizer may be beneficial in increasing wheat grain weight under the study conditions. The results obtained in the two years of experimentation indicated that the plots that received fertilization at a rate of 100 kg k
2O ha
−1 produced the maximum thousand-grain weight. Specifically, the thousand grain weight was 53.35 g for the first year and 49.7 g for the second year. In contrast, for plots fertilized with 200 kg k
2O ha
−1, the thousand-grain weight was 51.2 g in the first year and 48.5 g in the second year. This represents increases of 7.7% and 3.83% over the control plots with the two fertilization rates used. In other words, the application of 100 kg k
2O ha
−1 appears to be more effective in increasing grain weight than the application of 200 kg k
2O ha
−1. However, there is a 3.87% decrease in yield when the potassium fertilizer rate is increased from 100 to 200 kg k
2O ha
−1. Application rates of 300 kg k
2O ha
−1 and 400 kg k
2O ha
−1 led to the lowest thousand-grain weights in both years of the experiment (
Table 3). In fact, the lowest thousand-grain weight was recorded for these two rates. For the first year, the lowest weights were 48.13 g and 48.14 g for the 300 kg k
2O ha
−1 and 400 kg k
2O ha
−1 rates, respectively. For the second year, the lowest weights were 45.54 g and 46.74 g for the same respective doses. These results suggest that the use of high rates of potassium fertilizer may have a negative effect on grain weight.
3.3. Protein Content of Wheat Grains
The data presented in
Table 4 clearly show that applied K fertilizer levels and K-level*season had a significant effect on the protein amount of wheat grains (
p ≤ 0.05). In general, these results indicate that wheat grain protein content is closely related to potassium fertilizer levels, with an increase in potassium fertilizer rate resulting in an increase in grain protein content.
In the first year, protein content fluctuated between 14.82% and 16.51%, while, in the second year, it ranged from 14.88% to 16.29%. The 100 kg k2O ha−1 rate resulted in the highest protein content, 16.51% in the first year and 16.29% in the second year. In contrast, the 200 kg k2O ha−1 rate yielded the second-highest protein content, with 15.82% in the first year and 15.6% in the second year, respectively. On the other hand, the results of the study revealed that the application rate of 400 kg k2O ha−1 led to the lowest protein content of 14.82% in the first year and 14.88% in the second year. These numbers indicate that excess k2O can have a negative impact on protein production in the crop. Therefore, finding the right balance in terms of k2O application rate is essential to maximize protein production. These results can help farmers adjust their fertilizer use to optimize crop quality and yield.
Based on these results, we observed a significant increase in protein content with potassium fertilizer application compared to control plots. Specifically, we found increases of 7.33%, 3.29%, and 1.35% at 100 kg k
2O ha
−1, 200 kg k
2O ha
−1, and 300 kg k
2O ha
−1, respectively. However, it should be noted that potassium fertilizer application resulted in a decrease in crop protein content. Indeed, in the second year, we observed an increase of 7.43% compared to the control plots with a rate of 100 kg k
2O ha
−1 (
Table 4).
3.4. Nutritional Status of Plants
The data presented in
Table 5 indicate that the application of potassium fertilizer levels of 300 kg k
2O ha
−1 and 400 kg k
2O ha
−1 resulted in an increase in the plant potassium content, reaching 2.03 g kg
−1 for both doses in the first year, as well as 1.98 g kg
−1 with the 300 kg k
2O ha
−1 dose and 2.01 g kg
−1 with the 400 kg k
2O ha
−1 dose for the second year, respectively. However, it should be emphasized that, for both years of experimentation, the results did not show a significant effect for the cropping season factor studied (
p > 0.05). On the other hand, for the other two factors, namely, the K-level and K-level*season interaction, the differences observed in plant potassium content were statistically significant (
p ≤ 0.05). The results also revealed that the control plots had the lowest leaf potassium levels, with minimum values of 1.84 g kg
−1 for the first year and 1.67 g kg
−1 for the second year, respectively.
In addition, analyses were conducted to assess the nitrogen content of the leaves of the wheat crop, and the results are summarized in
Table 5. This assessment provides a better understanding of the nutritional status of the plant in terms of nitrogen, which is a crucial element for the growth and development of the crop. It was found that the plots that received 300 kg k
2O ha
−1 fertilization had the highest nitrogen contents, with 2.98% in the first year and 2.96% in the second year. However, an application of 100 kg k
2O ha
−1 led to the lowest N contents, at 2.81% in the first year and 2.84% in the second year.
In general, the analyses showed that crop leaf N contents were in the range of 2.81% to 2.96% for both years of experimentation and that there was no significant correlation with the levels of potash fertilizer applied. In other words, the application of potassium fertilizer at different levels did not have a significant impact on leaf nitrogen levels in the study crop (p > 0.05). It is important to note that leaf nitrogen content is a key indicator of plant nitrogen nutrition, which can have a significant impact on growth, yield, and crop quality. The results of this analysis suggest that, in this particular context, leaf nitrogen content does not appear to be influenced by the levels of potassium fertilizer applied.
3.5. Residual Soil Potassium
The data in
Table 6 indicate that the average amount of potassium available in the soil varied with the cropping season, the rates of potassium fertilizer applied, as well as the K-level*season interaction. The results also show that the amount of potassium available in the soil after harvest was significantly influenced by potassium fertilizer application levels in both years of experimentation (
p ≤ 0.05). This trend is also observed for the cropping season and K-level*season interaction, but, in this case, the differences are highly and very highly significant, respectively (
p ≤ 0.01;
p ≤ 0.001). Potassium fertilizer rates influenced the levels of available potassium in the soil. The highest available potassium levels were obtained with the 400 kg k
2O ha
−1 application level, with values of 144.3 mg kg
−1 for the first year and 109.22 mg kg
−1 for the second year. In contrast, the lowest levels were recorded in the control plots, reaching 81.9 mg kg
−1 for the first year and 74.1 mg kg
−1 for the second year. It is important to emphasize that the application of 300 kg k
2O ha
−1 also resulted in a concentration equivalent to that measured in the control plots, but only during the first year of the experiment. These results clearly indicate the importance of applying potassium fertilizer to increase the availability of potassium in the soil, which can have positive effects on the growth and production of the crops grown there.
3.6. Yield and its Correlation Analyses with Spike Length and Thousand Grain Weight
In this study, data were collected on two agronomic characteristics of durum wheat, namely, spike length and thousand-grain weight. Then, these data were analyzed to assess their correlation with grain yield. For this purpose, simple linear regression was used, a statistical method commonly used to evaluate the relationship between two variables.
Figure 3 shows the results of this analysis, indicating the relationship between two previously mentioned agronomic characteristics and grain yield. The results showed that grain yield was strongly correlated with thousand-grain weight (r
2 = 0.91), as well as with spike length (r
2 = 0.90 ***). Thus, this study determined whether ear length and thousand-grain weight were reliable indicators of grain yield, which may be useful for selecting the best-performing wheat varieties and optimizing farming practices to increase grain yield.
4. Discussion
From the parameters studied in this study, it was found that the application of a potassium dose of 100 kg k
2O ha
−1 results in the highest grain yield and protein content (
Table 3 and
Table 4). Indeed, this content led to a yield increase of 600 kg ha
−1, which corresponds to a productivity index of 6 kg of grain per kg of k
2O. In Mediterranean climate regions, the practice of potassium fertilization is essential to maximize the grain yield of a crop. However, its effectiveness depends on plant water conditions.
It is also important to remember that the soil of the experimental plot lacks available potassium and that, even at harvest, its available potassium content does not meet Mutcher’s standard, which recommends an optimal level of 0.55 meq/100 g of soil [
25]. Potassium fertilizer levels used are insufficient to achieve optimum potassium fertility, mainly due to crop removal and the potassium retrogradation phenomenon in the soil [
26].
Moreover, it should also be noted that the climatic conditions of the region during the two years of experimentation were favorable and that the total amount of rainfall for the wheat vegetative cycle was only 330.7 mm in the first year and 320.8 mm in the second year, which is significantly insufficient compared to the typical requirements of a cereal crop, estimated at around 450 mm throughout its growth cycle [
26]. The lack of water during the growth phase resulted in an unbalanced absorption of vital nutrients, which prevented the plant from growing and developing normally, thus resulting in a limited height. The negative effect of K fertilizers on plant height was confirmed by this result, which is consistent with the finding of Hefni [
30]. The study results show that grain number per spike, and spikelet number per spike, did not show any significant difference under the different potassium fertilizer rates used. However, spike length, plant height, spike neck, and spike number per m
2 displayed statistically significant differences with the influence of the three doses of potassium. It is important to note that these results diverge significantly from those obtained in the previous study conducted by Maurya [
31] for parameters that did not show significant differences. At the same time, the results are consistent for parameters that showed significant differences.
The research conducted by Lateef and Altamimi [
32] has shown that potassium fertilization can improve agronomic parameters, despite water constraints. According to these studies, the application of potassium fertilizer at doses of 100 kg k
2O ha
−1 and 200 kg k
2O ha
−1 can optimize certain agronomic parameters, such as spike length (200 kg k
2O ha
−1) and spike neck (100 kg k
2O ha
−1). These results are encouraging for farmers who have to face difficult weather conditions while seeking to improve their production.
According to the works of Samar Raza et al. [
33], as well as Faisal Mihbes Al-Taher and Howraa Hussein AL-Naser [
34], the application of potassium fertilization can also improve agricultural yields. However, the optimal amount of fertilization varies depending on yield parameters. For dry matter content, the highest value is obtained with a dose of 200 kg k
2O ha
−1, while, for other characteristics, such as 1000 grain weight, grain yield, and protein content, the optimal dose is 100 kg ha
−1 k
2O.
The research results have demonstrated that potassium plays an important role in water use efficiency by plants, improving their growth and cell division, as well as the production of proteins and hydrocarbons that are rapidly transferred to the grains [
1]. Potassium fertilizer levels also had a significant effect on thousand-grain weight (
p < 0.05), which is consistent with the findings of Ghulam et al. [
35]. However, the highest grain yield was obtained with a rate of 100 kg k
2O ha
−1 during the two campaigns, which seems to be relatively low compared to years when weather conditions are favorable [
36]. The crop yield was increased by 0.6 t ha
−1 compared to the control trial in the first year and by 0.54 t ha
−1 in the second year. These results are in agreement with the findings of Mirza et al. [
37], who confirm that potassium is directly or indirectly responsible for this increase in yield by regulating the biosynthesis, conversion, and allocation of metabolites. Moreover, these results are consistent with the findings of Ghulam et al. [
35], as well as Mohamad Javad Raisi and Enayatollah Tohidi-Nejad [
38].
The application of various levels of potassium fertilizer had a significant impact on grain protein content (
p ≤ 0.05), which is a key quality parameter of the studied wheat. This result is in agreement with the finding reported by Alam et al. [
39].
For optimal nutrition, it seems that the application of 100 kg K
2O ha
−1 is sufficient to reach adequate potassium levels with foliar K
2O content ≥2.30% [
38]. Similarly, this supply also appears to be sufficient to achieve adequate nitrogen levels in the leaves with K
2O ≥ 2.50% [
40]. According to the results of this present study, we suggest that the potassium content in the soil would be sufficient to meet the nutritional needs of the plant, which would promote healthy and optimal growth. However, it should be noted that potassium requirements can vary depending on various factors, such as crop type, environmental conditions, and soil management practices. It is, therefore, important to assess the potassium requirements of each specific crop before determining the optimal application rate.
The regression analysis results have shown a positive correlation between grain yield, spike length, and thousand-grain weight. These plant characteristics are, therefore, considered key factors for improving yields and selecting wheat genotypes better adapted to low potassium and water deficit conditions. In conclusion, it can be said that grain yield has improved throughout wheat development. The results obtained by Qiang Lu et al. [
41] showed similar positive correlations for wheat, indicating that the increase in grain yield following K fertilizer treatments was mainly due to an increase in thousand-grain weight. On the other hand, numerous studies have also noted a positive correlation between grain yield and ear length [
42,
43,
44], which is consistent with our own results.
The key results of this study suggest that the reasoned application of potassium fertilizer on durum wheat cultivars can improve the growth of morphological traits and yield quality. However, this improvement depends on plant water and seasonal weather conditions. It was observed that a potassium fertilizer rate of 100 kg k2O ha−1 is sufficient to obtain satisfactory grain yield under the study conditions. It should be noted that the optimum amount of potassium fertilizer may vary depending on environmental conditions and soil properties. Thus, a higher rate than recommended may not lead to a significant increase in yield and may even result in nutrient loss due to excessive leaching. In addition, the plant may not assimilate nutrients properly, which could hinder its growth and development, resulting in low yields. Therefore, it is important to adjust the amount of potassium fertilizer, according to the needs of each plot of land, to optimize crop production. Furthermore, the application of potassium fertilizer should not be considered the only solution to improve yields. Other cultural practices, such as water management, crop rotation, plant protection, and the use of other types of fertilizers, can also have a significant impact on durum wheat production. Therefore, it is important to consider these factors in a comprehensive approach to maximize yields and maintain soil health.