Morphological traits

The results of the study indicate that the choice of potassium sources and the application of biofertilizer had a significant impact on the morphology of potato plants (Table 4). During the first and second growing seasons, the combination of 25% filter cake and 75% fertilizer (25FD + 75FC) resulted in the tallest plants (75.00 and 75.54 cm), with no significant differences compared to other treatments except the control (64.78 and 65.32 cm), respectively. The application of biofertilizer led to the highest plant height (75.69 and 76.23 cm) during the first and second growing seasons, respectively. Considering different potassium sources and biofertilizer cumulatively, the plants treated with 75% filter cake and 25% fertilizer along with the biofertilizer exhibited the maximum plant height (80.22 and 80.76 cm) during the first and second growing seasons, respectively.

In terms of the number of stems, plants treated with 100% fertilizer (100FD) had the highest number (3.89) during the first growing season. The biofertilizer significantly influenced this trait, resulting in the highest average number of stems (3.52 and 3.75) in the plants treated with the biofertilizer during the first and second growing seasons, respectively. The interaction between potassium sources and biofertilizer did not have a notable impact on the number of stems in both growing seasons.

Plants treated with 100% filter cake (100% FC) had the highest dry matter of tuber (14.14 and 16.88 tons ha− 1) in both growing seasons, respectively. The biofertilizer had a significant impact on the dry matter of tuber only in the first growing season, but not in the second growing season. The highest dry matter of tuber was observed in the plants treated with biofertilizer (12.57 and 13.56tons ha− 1) during the first and second seasons, respectively. The interactive effect of potassium sources and biofertilizer resulted in substantial variations in the dry matter of tuber, with the highest dry matter recorded in the plants treated with 75% filter cake and 25% fertilizer along with the biofertilizer (19.51 and 20.50 tons ha− 1) during the first and second growing seasons, respectively.

Table 4 The integrated effect of feldspar (FD), Filter cake (FC) treatments and their combinations with biofertilizer on growth variables of potato plants after two successive seasons (2021/2022–2022/2023) Macro nutrients Uptake

The results indicate that the choice of potassium sources and bio-fertilizer had a significant impact on the uptake of macro nutrients by potato tubers, as shown in Table 5. During the first and second growing seasons, potato plants treated with 100% FC exhibited the highest nitrogen uptake (427.43 and 428.29 kg ha− 1), comparable to the treatment 25% FD + 75% FC (326.67 and 327.53 kg ha− 1). The bio-fertilizer treatment MBc + also led to high nitrogen uptake (291.53 and 291.39 kg ha− 1), without significant differences compared to MBc. Cumulatively, the highest nitrogen uptake occurred in plants treated with 100% FC without the bio-fertilizer (518.86 and 519.72 kg ha− 1) during the first and second growing seasons, respectively.

Phosphorus uptake was highest in plants treated with 100% FC during both growing seasons (107.38 and 108.30 kg ha− 1). The bio-fertilizer significantly increased phosphorus uptake, with the highest values observed in bio-fertilizer-treated plants (81.86 and 82.79 kg ha− 1). The combination of potassium sources and bio-fertilizer had substantial effects on phosphorus uptake, with the highest uptake occurring when using 75% filter cake and 25% feldspar along with the bio-fertilizer (130.11 and 131.04 kg ha− 1) during the first and second growing seasons, respectively.

Potassium uptake was highest in plants treated with 75% feldspar and 25% filter cake in both growing seasons (1037.60 and 1038.30 kg ha− 1). The bio-fertilizer also significantly impacted potassium uptake, with the highest uptake observed in plants treated with MBc+ (860.38 and 861.05 kg ha− 1). The interactive effect of potassium sources and bio-fertilizer resulted in substantial variations in potassium uptake, with the highest uptake occurring when using 75% FC and 25% FD along with the bio-fertilizer (1563.80 and 1564.50 kg ha− 1) during the first and second growing seasons, respectively.

Table 5 The integrated effect of feldspar (FD), Filter cake (FC) treatments and their combinations with biofertilizer on the uptake (kg ha− 1) of nitrogen (N), phosphorous (P), and potassium (K) of potato tubers after two successive seasons (2021/2022–2022/2023) Yield traits

The results indicate that the choice of potassium sources and bio-fertilizer had a significant impact on the yield traits of potato plants, as shown in Table 6. During the first and second growing seasons, plants treated with 100FC exhibited the highest weight of large tubers (26.04 and 26.46 tons ha− 1), respectively. Regarding the bio-fertilizer, it resulted in the highest weight of large tubers (21.76 and 22.19 tons ha− 1) during the first and second growing seasons, respectively. Cumulatively, when considering different potassium sources and bio-fertilizer, plants treated with 100FC along with the bio-fertilizer exhibited the maximum weight of large tubers (28.00 and 28.43 tons ha− 1) during the first and second growing seasons, respectively.

For the weight of medium tubers, plants treated with 100FD showed the highest weight (11.52 and 11.75 tons ha− 1) with potassium sources during the first and second growing seasons, respectively. The bio-fertilizer significantly influenced this trait, resulting in the highest weight of medium tubers (9.42 and 9.65 tons ha− 1) in plants treated with the bio-fertilizer during the first and second growing seasons, respectively, without significant differences compared to MBc-. The interaction between potassium sources and bio-fertilizer led to substantial variations in the weight of medium tubers, with the highest weight observed in plants treated with 100FC without bio-fertilizer (11.69 and 11.92 tons ha− 1) during the first and second growing seasons, respectively.

Based on the results in Table 6, the highest weight of small tubers (6.20 and 6.61 tons ha− 1) was recorded in plants treated with 50% feldspar and 50% filter cake in both growing seasons, respectively, with no significant differences among the other treatments. The bio-fertilizer also significantly impacted the weight of small tubers in both growing seasons, with the highest weight (6.19 and 6.60 tons ha− 1) observed in plants treated with MBc + in both growing seasons, respectively. The interactive effect of potassium sources and bio-fertilizer resulted in substantial variations in small tuber weight, with the highest weight observed in plants treated with 50FC with 50FD along with the bio-fertilizer (6.83 and 7.24 tons ha− 1) during the first and second growing seasons, respectively, with no significant differences among the other treatments.

Table 6 The integrated effect of feldspar (FD), Filter cake (FC) treatments and their combinations with biofertilizer on the graded weight (ton ha− 1) of potato tubers after two successive seasons (2021/2022–2022/2023)

The results presented in Fig. 2 indicate that the choice of potassium sources and bio-fertilizer had a significant impact on the yield traits of potato plants. Considering the cumulative effect of different potassium sources and bio-fertilizer, the plants treated with 100FC with the bio-fertilizer exhibited the maximum total tuber weight (44.68 and 46.54 ton ha− 1) during the first and second growing seasons (Fig. 2A and B), respectively. However, there were no significant differences observed between the treatments of 100FC alone and 25FD + 75FC with MBc in both seasons. On the other hand, the plants under the control treatment without the bio-fertilizer (MBc) recorded the minimum total tuber weight (16.77 and 17.84 ton ha− 1) during the first and second growing seasons, respectively.

Figure 2. The integrated effect of feldspar (FD), Filter cake (FC) treatments and their combinations with biofertilizer on total tuber weight (ton ha− 1) of potato in season 2021/2022 (A), total tuber weight (ton ha− 1) of potato in season 2022/2023 (B). Where MBc(-) = Without biofertilizer (Bacillus cereus); MBc(+) = With biofertilizer (Bacillus cereus); Control = Non-fertilizer; 100FD = 100% Feldspar; 100FC = 100% Filer Cake; 75FD + 25FC = 75% Feldspar + 25% Filer Cake; 25FD + 75FC = 25% Feldspar + 75% Filer Cake; 50FD + 50FC = 50% Feldspar + 50% Filer Cake. The values shown in table are means three replicates. Means followed by the same letters are non-significantly different (p ≤ 0.05).

Post-harvest fertility status of soil

The results presented in Figs. 3 and 4 demonstrate that the application of different potassium sources and bio-fertilizer had a significant impact on the chemical properties of the soil. Regarding soil pH, the use of 100FC resulted in a reduction in soil pH compared to the control treatment in both growing seasons. The bio-fertilizer did not play a significant role in reducing pH during either growing season (Fig. 3A, and 4 A).

The electrical conductivity (EC) of the soil was influenced by the different treatments. The maximum EC was recorded when 100FC was applied along with the bio-fertilizer (MBc) in the first season (Fig. 3B), while the second season, the highest EC was observed when 100FC was applied alone (Fig. 4B).

Figure 3. The integrated effect of feldspar (FD), Filter cake (FC) treatments and their combinations with biofertilizer on soil reaction pH (A), Electrical conductivity (B), organic matter (OM) (C), available nitrogen (D), available phosphorus (E), and available potassium (F) (mg kg− 1) after successive season 2021/2022. Where MBc(-) = Without biofertilizer (Bacillus cereus); MBc(+) = With biofertilizer (Bacillus cereus); Control = Non-fertilizer; 100FD = 100% Feldspar; 100FC = 100% Filer Cake; 75FD + 25FC = 75% Feldspar + 25% Filer Cake; 25FD + 75FC = 25% Feldspar + 75% Filer Cake; 50FD + 50FC = 50% Feldspar + 50% Filer Cake. The values shown in figure are means three replicates. Means followed by the same letters are non-significantly different (p ≤ 0.05).

The application of 100FC, with or without the bio-fertilizer, increased organic matter (OM) content. The highest OM content (24.88 and 26.64 g kg− 1) was recorded in the plants treated with 100FC along with MBc during the 1st and 2nd growing seasons, respectively (Figs. 3C and 4C).

In terms of nutrient availability, the application of potassium sources and bio-fertilizer, either individually or in combination, increased the levels of available nitrogen (N), phosphorus (P), and potassium (K) in the soil (Figs. 3D-F and 4D-F). Considering the interaction effect, the highest available N (95.43 and 113.86 mg kg− 1) was recorded when 100FC was applied along with the bio-fertilizer during the 1st and 2nd growing seasons (Fig. 3D, and 4D), respectively. For available P, the maximum values (19.42 and 21.36 mg kg− 1) were observed when 100FC was applied along with MBc during the 1st and 2nd growing seasons (Fig. 3E, and 4E), respectively. Similarly, the highest available K (364.00 and 536.67 mg kg− 1) was recorded when 100FC was applied along with MBc during the 1st and 2nd growing seasons (Fig. 3F, and 4 F), respectively.

Figure 4: The integrated effect of feldspar (FD), Filter cake (FC) treatments and their combinations with biofertilizer on soil reaction pH (A), Electrical conductivity EC (B), organic matter (OM) (C), available nitrogen (D), available phosphorus (E), and available potassium (F) (mg kg− 1) after successive season 2022/2023. Where MBc(-) = Without biofertilizer (Bacillus cereus); MBc(+) = With biofertilizer (Bacillus cereus); Control = Non-fertilizer; 100FD = 100% Feldspar; 100FC = 100% Filer Cake; 75FD + 25FC = 75% Feldspar + 25% Filer Cake; 25FD + 75FC = 25% Feldspar + 75% Filer Cake; 50FD + 50FC = 50% Feldspar + 50% Filer Cake. The values shown in figure are means three replicates. Means followed by the same letters are non-significantly different (p ≤ 0.05).

Correlation between soil properties and plant characteristics

The results of the Principal Component Analysis (PCA) in Fig. 5 reveal important insights: The first two principal components (PCs) collectively explained a substantial portion (76.10%) of the variation in soil and potato traits. PC1 displayed a significant positive correlation with various soil and plant characteristics. It was positively associated with soil electrical conductivity, organic matter content, available nitrogen, available phosphorus, nitrogen uptake, phosphorus uptake, potassium uptake, medium tuber weight, and total tuber weight. This suggests that these variables tend to vary together and contribute to a common underlying factor. PC2 exhibited positive correlations with available potassium, plant height, number of stems, weight of large tubers, and weight of small tubers. It also showed a significant negative correlation with pH value. This implies that these variables are interrelated and contribute to a separate factor that is distinct from PC1. Different treatments, such as 100% Filter Cake, 25% Feldspar + 75% Filter Cake, 100% Filter Cake with Bacillus cereus, 75% Feldspar + 25% Filter Cake with Bacillus cereus, 25% Feldspar + 75% Filter Cake with Bacillus cereus, and 50% Feldspar + 50% Filter Cake with Bacillus cereus, had a positive impact on nutrient availability and plant growth indicators. These treatments contributed to the observed correlations and positively influenced the measured soil and plant characteristics. In summary, the PCA results highlight the interrelationships among various soil and potato traits, with PC1 and PC2 capturing different aspects of these associations. Additionally, the mentioned treatments positively affected nutrient availability and plant growth indicators, influencing the observed correlations.

Figure 5. Principal component analysis (PCA) between soil properties and potato traits. Where MBc(-) = Without biofertilizer (Bacillus cereus); MBc(+) = With biofertilizer (Bacillus cereus); Control = Non-fertilizer; 100FD = 100% Feldspar; 100FC = 100% Filer Cake; 75FD + 25FC = 75% Feldspar + 25% Filer Cake; 25FD + 75FC = 25% Feldspar + 75% Filer Cake; 50FD + 50FC = 50% Feldspar + 50% Filer Cake; pH = pH value; Ec.= Soil electrical conductivity; OM = organic matter; Available-N = Available Nitrogen; Available-P = Available Phosphor; Available-K = Available Potassium; Uptake-N = Nitrogen uptake;Uptake-P = Phosphor uptake; Uptake-K = Potassium uptake; PH = Plant height; NS = No of stem; DM = Dry matter of tuber; WLT = Large Tuber weight; WMT = Medium Tuber weight; WST = Small Tuber weight; TWT = on total tuber weight.