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Nutrient uptake, use efficiency and productivity of bread wheat (Triticum aestivum L.) as affected by nitrogen and potassium fertilizer in Keddida Gamela Woreda, Southern Ethiopia
Environmental Systems Research volumeÂ 10, ArticleÂ number:Â 12 (2021)
Abstract
Bread wheat (Triticum aestivum L.) is one of the most important cereal crops in Ethiopia. The productivity of wheat is markedly constrained by nutrient depletion and inadequate fertilizer application. The experiment was conducted to study the effect of nitrogen (N) and potassium (K) fertilizer rates on growth, yield, nutrient uptake and use efficiency during 2019 cropping season on Kedida Gamela Woreda, Kembata Tembaro Zone Southern Ethiopia. Factorial combinations of four rates of N (0, 23, 46 and 69Â kg Nha^{âˆ’1}) and three rates of K_{2}O (0, 30 and 60Â kg Nha^{âˆ’1}) in the form of urea (46â€“00) and murate of potash (KCl) (0060) respectively, were laid out in a randomized complete block design with three replications. The results showed that most parameters viz yield, yield components, N uptake and use efficiency revealed significant differences (Pâ€‰<â€‰0.05) due to interaction effects of N and K. Fertilizer application at the rate of 46Â N and 30Â kgÂ KÂ ha^{âˆ’1} resulted in high grain yield of 4392Â kgÂ ha^{âˆ’ 1} and the lowest 1041 from control. The highest agronomic efficiency of N (52.5) obtained from the application of 46Â kgÂ NÂ ha^{âˆ’1}. Maximum physiological efficiency of N (86.6Â kgÂ kg^{âˆ’1}) and use efficiency of K (58.6%) was recorded from the interaction of 46 and 30Â kgÂ KÂ ha^{âˆ’1}. Hence, it could be concluded that applying 46 and 30Â kgÂ KÂ ha^{âˆ’1}was resulted in high grain yield and economic return to wheat growing farmers of the area. Yet, in order to draw sound conclusion, repeating the experiment in over seasons and locations is recommended.
Background
Wheat is main staple crops in terms of both production and consumption in Ethiopia. It is one of the most important cereals cultivated in Ethiopia (Jemal et al. 2015). Ethiopia is the second largest wheat (Triticum aestivum L.) producer in subSaharan Africa, after South Africa (FAO, 2019). Wheat ranks fourth after tef (Eragrostis teff), maize (Zea mays L.) and sorghum (Sorghum bicolor L. Moench) in area coverage and total production (CSA 2019). Despite the long history of wheat cultivation and its importance to the Ethiopian agriculture its average productivity is still very low 2.76, 2.66 and 2.7 t ha^{âˆ’1} at national, SNNPRS and Kambata Tambaro zone, respectively (CSA 2019). In all cases this is definitely far below the worldâ€™s average yield (3.52 t ha^{âˆ’1}) (USDA 2020) and 6 t ha^{âˆ’1} (at research station) (Assefa et al. 2015). The low yield of wheat may be due to soil erosion, intensive crop cultivation and inadequate application of N and P for a long time without considering soil fertility status and crop requirement were the major constraint in Ethiopia. In addition to this research findings showed acute crop cultivation, complete crop residue removal and high nutrient depletion contribute to low productivity of wheat (Fanuel et al 2018).
Nitrogen (N) and potassium (K) fertilizers play an important role in increasing growth and grain yield of wheat. According to Oosterhuis et al. (2014) N and K are required for plants to complete their life cycles and are the two nutrients acquired in greatest quantities by roots. Yield response to K application depends to a great extent on the level of N nutrition and the interaction is normally positive (Brennan et al., 2009). Different scholars in Ethiopia reported varied rates of N for wheat production. For instance Mesele (2007) in Ofla area obtained grain yield of 2.5 t ha^{âˆ’1} at 69Â kgÂ NÂ ha^{âˆ’1}; Worku (2008) in Densa area achieved grain yield of 3.5 t ha^{âˆ’1} at 138Â kgÂ NÂ ha^{âˆ’1} whereas Solomon and Anjulo (2017) in Doyo Gena area recorded 3.7 t ha^{âˆ’1} at 46Â kgÂ NÂ ha^{âˆ’1}. Additionally, Dereje et al. (2019) reported 46â€“92Â kgÂ NÂ ha^{âˆ’1} for better grain quality and nutrient use efficiency (NUE). Conversely, until recently the application of K fertilizer is not known in Ethiopian agriculture mainly due to the view that K is not a limiting nutrient in the soil. However, research findings indicated the potential of K limitation in soils of southern Ethiopia (Fanuel et al. 2018; Lelago et al. 2016). Correspondingly, positive response and grain yield increment up to 4.27 t ha^{âˆ’1} (Tigist 2017) and 4.33 t ha^{âˆ’1} (Mesele, 2019) to 30Â kgÂ KÂ ha^{âˆ’1} in Wolaita zone, southern Ethiopia was reported.
Most of cultivated soils in Ethiopia were found at low level of N, phosphorous (P), K, Sulphur (S), cupper (Cu), zinc (Zn) and boron (B) (Ethio SIS 2014). The soil survey in Kambata Tembaro Zone, where this study was conducted also indicated the limitation of N, P and K (Lelago et al. 2016). However, Farmers in Ethiopia including the study area for long time apply N and P containing fertilizers through blanket recommendation and application of K has not been practiced. Therefore, site specific information for wheat growers found in N and K deficient soil of Kembata Tembaro zone is lacking. Hence, the study was initiated with the following objectives: to investigate the effect of N and K fertilizer rates on yield and related traits of wheat, to evaluate the nutrient uptake and use efficiency of wheat and to assess the economic feasibility of N and K fertilizer rate in wheat production and to assess the economic feasibility of N and K fertilizer rate in wheat production.
Materials and methods
Description ofÂ theÂ study site
A field experiment was conducted at Zeto shodara Kebele, Kedida Gamela Woreda in SNNPRS Ethiopia. The site is situated at 7Âº14â€² N latitude and 37Âº52â€² E longitude with altitude of 2010 masl. The monthly meteorological data (amount of rainfall and maximum and minimum temperatures) for the growing year of 2019 at the experimental site is presented in (Fig.Â 1). The total rainfall in the cropping season was 1194.91Â mm, and the mean maximum and minimum temperatures were 23.15Â Â°C and 12.05Â Â°C, respectively (HNMA 2019).
Treatments and experimental design
The treatment consisted of four levels of N (0, 23, 46 and 69Â kgÂ ha^{âˆ’1}) and three levels of K_{2}O (0, 30 and 60Â kgÂ ha^{âˆ’1}) arranged in factorial lay out in a randomized complete block design (RCBD) with three replications. Here after K_{2}O in the text represented by elemental form of K. The sources of N and K were urea (46â€“00) and Murate of potash (KCl) (0â€“060) fertilizers, respectively. Here after the form of K_{2}O in the text represented by form of K. The gross plot area was 2â€‰Ã—â€‰3Â m (6Â m ^{2}) and the net plot area 4.16Â m^{2}. Planting was carried in mid July 2019 by drilling seeds in rows at row spacing of 20Â cm. Bread wheat variety Shorma (ETBW 5483) developed and released in 2011 by Kulumsa Agricultural Research Center was used as a test crop.
Data collection and measurements
Soil data
Before planting soil samples were collected from the entire experimental field to a depth of 0â€“20Â cm in a diagonally using soil auger after ploughing. The samples were thoroughly mixed to get a kilogram of composite sample which was air dried; ground to pass through a 2Â mm sieve for analysis of selected soil physicochemical properties such as particle size distribution, soil pH, total N, P, K, Mg and cation exchangeable capacity (CEC).
Crop data
Phenological parameters
Days to heading and 90% physiological maturity.
Growth parameters plant height (cm) and flage leaf area (cm^{2}), at physiological maturity from 10 and 5 respectively plants were randomly selected from central eight rows.
Yield components and yield at physiological maturity from 10 plants were randomly selected from central eight rows of each treatment for measuring total tilers, effective tilers, number of grain per spike and grain weight per spike (g).
Biological (grain and straw yield) were calculated form whole plant of central eight rows of each plot.
The harvest index (%) was calculated at harvest as a ratio of grain to the total biological yield (dry matter) and expressed as a percentage.
Tissue analysis at maturity representative non border wheat plant samples were randomly collected from each plot and partitioned into grain and straw and then kept in paper bags. Grain and straw samples of each treatment were oven dried at temperature of 70 ÂºC to constant weight and thereafter ground using stainless still grinder. The powder was redried at 60 ÂºC and then rubbed in muffle furnace at 550 ÂºC for eight hours. The ash digest in 20% HNO_{3} (Zarcinas et al. 1987). Total N concentrations of the samples were determined using the modified Kjeldahl method (Jackson 1958). Phosphorus concentration of the digests was measured with a spectrophotometer and the K concentration with a flame photometer.
Nutrient use efficiency computed as:
Nutrient use efficiency
The nutrient use efficiencies computed were.

1.
Nutrient uptake It was calculated by multiplying the grain and straw yield (kg ha^{âˆ’1})with the nutrient concentration %) of each treatment as follows

a.
Nutrient (N, P or K)uptake by grain or straw (kg haâˆ’1)â€‰=â€‰[Yield of grain or straw (kg haâˆ’1â€‰Ã—â€‰Nutrient (N, P or K)concentration of grain or straw (%)â€‰Ã—â€‰10â€“2

b.
Total uptake (kg haâˆ’1)â€‰=â€‰Nutrient uptake grainâ€‰+â€‰Nutrient uptake straw

2.
Physiological Efficiency (PE) is the biological production per unit of nutrient absorbed\(\mathrm{PE}\left(\frac{\mathrm{kg}}{\mathrm{ha}}\right)=\frac{\mathrm{Byf}\left(\mathrm{kg}\right)\mathrm{Byu}\left(\mathrm{kg}\right)}{\mathrm{Nf}(\mathrm{kg})\mathrm{Nu}(\mathrm{Kg})}\) where; PE stands for physiological efficiency, Byf is the biological yield (grain plus straw) of fertilized plot (kg), Byu is the biological yield of the unfertilized plot(kg), Nf is the total N uptake of the fertilized plot (kg), Nu is the total N uptake of unfertilized plot (kg).

3.
Recovery Efficiency (RE) the quantity of nutrient absorbed per unit of nutrient applied \(\mathrm{RE\%}= \frac{\mathrm{ Un}\mathrm{Uo}}{\mathrm{n}}\times 100\) Where; Un nutrient uptake by the rate of N and K fertilizer Uo nutrient uptake by unfertilized and n for quantity of fertilizer applied.

4.
Agronomic Efficiency (AE) the economic production obtained per unit of nutrient applied \(\mathrm{AE}\left(\frac{\mathrm{kg}}{\mathrm{kg}}\right)=\frac{\left[\mathrm{Gf}\mathrm{Gu}\right]}{\mathrm{QA}}\) Where; Gf and Gu stand for Grain yield of fertilized plots at â€˜nâ€™ rates of fertilizer and grain yield of unfertilized plots, respectively, and QA stands for nutrient applied.

5.
Nutrient Use Efficiency (NUE) the product of Physiological Efficiency and Recovery Efficiency \(\mathrm{NUE }\left(\mathrm{\%}\right)=\mathrm{Physiological Efficiency}\left(\mathrm{PE}\right)\times \mathrm{Recovery Efficiency}(\mathrm{RE})\)
Partial budget analysis
The mean grain yields of the treatments were used in partial budget analysis as described by CIMMYT (1988). For each pair of ranked treatments, MRR (%) was calculated using the formula \({\text{MRR }}\left( {\text{\% }} \right) = \frac{{{\text{Change in NB }}\left( {{\text{NBb}}  {\text{NBa}}} \right)}}{{{\text{Change in TCV }}\left( {{\text{TCVb}}  {\text{TCVa}}} \right)}} \times 100\) Where, N_{Ba}â€‰=â€‰NB with the immediate lower TCV, NB_{b}â€‰=â€‰NB with the next higher TCV, TCV_{a}â€‰=â€‰the immediate lower TCV and TCV_{b}â€‰=â€‰the next highest TCV.
Results and discussion
Physiochemical properties of the soil of experimental site
The laboratory analysis result of selected physical and chemical properties experimental site is presented in Table 1. The site has clay loam textural class with a particle size distribution of 40% clay, 32% silt and 28% sand. Soil pH was 5.8 which is under moderate acidity (5.5â€“6.5) (Landon 1991) and suitable for wheat (Mengel and Kirkby 2001). The OC content was 1.7% which was low (<â€‰4%) (Landon 1991). The total nitrogen (TN) was 0.14% and categorized under low level (0.1â€“0.2%) (Landon 1991). Olsen available P was 20Â mgÂ kg^{âˆ’1} which is medium (10â€“20Â mgÂ kg^{âˆ’1}) (Landon 1991). The available K was 0.17 Cmolc kg^{âˆ’1}which is under low category (Ethio SIS, 2013). The ratio of K: Mg is 0.06:1 which indicated the potential of Mg induced K deficiency (Loide 2004; Fanuel et al. 2018). The CEC of the experimental soil was 14.4 Cmolc kg^{âˆ’1}which is low (5â€“15 Cmolc kg^{âˆ’1}) (Landon 1991).
In general, the soil analysis result demonstrated the low level of OC and TN, and the potential of Mg induced K deficiency that may be ascribed due to inadequate soil fertility management practices. Thus, application of N and K containing fertilizers are justifiable to satisfy the wheat crop.
Crop phenology
Days to heading
Analysis of variance showed that the main effect of N was significant however; the main effect of K and the interaction effects of N and K were resulted insignificant differences on days to heading. It ranged from 54 to 57.78Â days (Table 2) where the longest day 57.78 was recorded at 69Â kgÂ NÂ ha^{âˆ’1} and the shortest days 54 was seen at 0Â kgÂ ha^{âˆ’1}Â N fertilizer. Days to heading showed an increasing trend with increasing from 0 to 69Â kgÂ NÂ ha^{âˆ’1}. The results agreed with Muhammad et al. (2016) who reported that days to heading increased gradually with increasing levels of nitrogen. This might be due to fact that increasing N rates extended vegetative growth which prolong days to heading.
Days to physiological maturity
Days to physiological maturity was significantly affected due to interaction effect of N and K fertilizer. The longest days were observed at 69Â kgÂ NÂ ha^{âˆ’1} without K fertilizer. However, 69Â kgÂ NÂ ha^{âˆ’1} was statistically at par with 30 and 60Â kgÂ KÂ ha^{âˆ’1}. On the other hand, unfertilized plots followed by K without N fertilizer shortened days to physiological maturity (Table 3). In general, combined application of N and K fertilizers at increasing rates relatively prolonged maturity by two weeks as compared to nonfertilized plots. Comparatively, it is also noted that increasing rates of N at all levels of K rates prolonged days to physiological maturity. This might be suggested that N fertilization is associated with extended vegetative growth. However, the combined application of N and K fertilization regulated the extreme cases maturity of wheat (i.e. too early and too late).
Growth parameters
Plant height
Plant height of wheat was significantly influenced due to interaction effects of N by K rates. The tallest plant height (90.57Â cm) was obtained from the combination of 69Â kgÂ NÂ ha^{âˆ’1} with 30Â kgÂ KÂ ha^{âˆ’1} followed (84.18Â cm) by the same N rate combined with 60Â kgÂ KÂ ha^{âˆ’1}. The shortest plant height (45.71Â cm) was recorded from unfertilized plots. The result further demonstrated that combining N at increasing rate with K up to 30Â kgÂ ha^{âˆ’1} improved plant height of wheat (Table 3). This could probably due to major role of N towards cell division, elongation and enhancing the vegetative growth of plants, and the role of K in promoting vigorous plant growth through efficient photosynthesis. This result is in agreement with Sharma et al. (2005); Amare et al. (2013) and Muhammad et al. (2016) who reported that N and K fertilization significantly increased plant height of wheat. According to Liangwei et al. (2004) and Tiwari (2002) nitrogen enhances vegetative growth of wheat, while K positively affects the root growth. Therefore, when N and K fertilizers applied together enhance vigorous vegetative growth and resulted in increasing plant height of wheat.
Flag leaf area
Data regarding leaf area shows that it was significantly affected by the interactions of N and K fertilizer rates. Leaf area ranged from 12.18 cm2 (nonfertilized plot) to 30.83 cm2 (69Â kgÂ ha^{âˆ’1} and30 kg ha^{âˆ’1}Â K) (Table 3). Wheat plant received 69Â kgÂ NÂ ha^{âˆ’1} and 30Â kgÂ KÂ ha^{âˆ’1} attained 153.12% more leaf area as compared to unfertilized. This result agreed with those obtained by Marschner (2012); Ara et al. (2014) and Tisdale et al. (2000) who reported that N enhanced leaf area and K application enhanced productive life of flag leaves respectively. In general, combined application of N and K consistently increased leaf area with increasing rate of N and K up to 30Â kgÂ ha^{âˆ’1}. This indicated that the higher amount of N seems to have more effect over K.
The interaction effects of N and K indicated that significant relationship with the flag leaf area. The interaction of 46 and 69Â N with 30Â kgÂ ha^{âˆ’1} had justified the maximum of flag leaf areaâ€‰=â€‰0.9903xâ€‰+â€‰17.262 with (R^{2}â€‰=â€‰0.63) (Fig.Â 2). Therefore, combined application of N and K could have synergistic effect to improve the growth and development of wheat and consequently contributed to the higher leaf area of wheat.
Yield components and yield
Total tiller number
Total number of tiller per m2 was significantly affected by the interactions of N and K rates. The highest number of total tillers (595.00) was seen at 69Â kgÂ NÂ ha^{âˆ’1} and 30Â kgÂ KÂ ha^{âˆ’1} followed by (548.33) the same K rate combination with 46Â kgÂ NÂ ha^{âˆ’1}. The lowest number of total tillers (205.33) was obtained from plots without fertilization (Table 3). Total number of tillers tended to increase as N rates increased across all levels of K rate, but peaked up at 30Â kgÂ ha^{âˆ’1} (Table 3). Similarly, Hussain and Shah (2002) and Amare et al. (2013) individual effects of N and K respectively, have positive effects of the nutrients for increased number of tillers per unit area. Furthermore, Bundy and Andraski (2004) also reported that the combined application of N and K increased tiller production. Therefore, strong effects on total number of tillers when both are applied together were observed.
Productive tillers
Number of productive tiller was significantly affected by the interactions of N and K fertilizer rates. The maximum value (453) was achieved from 46Â kgÂ NÂ ha^{âˆ’1} and 30Â kgÂ KÂ ha^{âˆ’1}. The lowest value (121.7) was obtained from nonfertilized plots. It is clear that combined application increased the productive tillers. Overall, combination of K at 30Â kgÂ KÂ ha^{âˆ’1} and N up to 46Â kgÂ ha^{âˆ’1} linearly increased number of productive tiller. Meanwhile, higher doses of N and K beyond 30Â kgÂ KÂ ha^{âˆ’1} and46 kg N ha^{âˆ’1} reduced tiller number as it might be toxic for tiller formation.
The increased number of productive tillers might be ascribed due to combined role of N and K in which N availability initiated cell division and elongation leading to proper vegetative and reproductive growth. The result agreed with Ejaz et al. (2002) who reported that increasing N application increased the number of productive tillers. On the other hand, Tahir et al. (2003) showed that production of productive tillers was promoted with increasing K fertilization. Therefore, combined application of N and K fertilizer may have created synergetic effect and improved the number of productive tillers.
Number of kernel per spike
Analysis of variance indicated that number of kernels per spike was significantly affected by the interactions of N and K fertilizer rates. Number of kernels per spike increased with increasing N rates across all levels of K rate. Similarly, it was also increased with increasing K rates for all levels N rates. Generally, number of kernels per spike ranged from 24.53 to 51.03 (Table 3). The greatest number of kernels per spike (51.03) was achieved from 46Â kgÂ NÂ ha^{âˆ’1} and 30Â kgÂ KÂ ha^{âˆ’1} followed (50.50) by 69Â kgÂ NÂ ha^{âˆ’1} with 30Â kgÂ ha^{âˆ’1}. The least number of kernels per spike (24.53) was observed on nonfertilized plots (Table 3).This result in contrast to Wogane (2017) who reported that increased application of N was associated with proportional increase in the number of kernels per spike.
The utilization of K increased grain spike^{âˆ’1} as material transition in phloem increased cell division and growth (Tabatabaii et al. 2011). Potassium application may have a stimulatory effect on number of kernals per spike (Bahmanyer and Ranjo bar 2008). The findings are in line with the data reported by Mirza et al. (2018) who observed that K application helps to increase the number of grains. As these investigation showed that application of N and K rates promoted the number of kernels per spike. Similarly, Elkholy (2004) and Muhammad et al. (2016) conforming that number of grains per spike increased with the increase in NK levels.
Thousand seed weight
Data regarding thousand seed weight (TSW) was significantly responded to the interactions of N and K fertilizer rates. The highest TSW (50.69Â g) was obtained from combination of 69Â kgÂ NÂ ha^{âˆ’1} with 30Â kgÂ KÂ ha^{âˆ’1} followed (49.83Â g) by the same K rate combined with 46Â kgÂ NÂ ha^{âˆ’1}. The lowest TSW (28.40Â g) was recorded from unfertilized plots. Thousand seed weight significantly and positively correlated with productive tiler (râ€‰=â€‰0.98**), total tillers, (râ€‰=â€‰0.99*) and number of kernel per spike(râ€‰=â€‰0.99*) (Table 3).
Thus, synergistic effects by combining N and K have significant effect on TSW of wheat. In agreement, Bundy and Andraski (2004) and Muhammad et al. (2016) reported significant improvement of TSW with the increasing trend of N and K (Bundy and Andraski 2004 and Muhammad et al., 2016).
Yield
Biomass yield
Analysis of variance revealed that above ground biomass yield was significantly affected by the interactions of N and K fertilizer rates. Above ground biomass yield ranged from 4326 to 9776Â kgÂ ha^{âˆ’1}. The highest biomass yield (9776Â kgÂ ha^{âˆ’1}) was recorded at 69Â kgÂ NÂ ha^{âˆ’1} combined with 30Â kgÂ KÂ ha^{âˆ’1} followed (9494Â kgÂ ha^{âˆ’1}) by 46Â N with the same K rate. The lowest biomass yield (4326Â kgÂ ha^{âˆ’1}) was achieved from unfertilized plots. All fertilized plots out yielded the unfertilized plots (Table 3). Above ground biomass yield of wheat is significantly and positively correlated with number of seeds per kernel (râ€‰=â€‰0.98**), TSW (râ€‰=â€‰0.99**), productive tillers (râ€‰=â€‰0.99**) and total number of tillers (râ€‰=â€‰0.99**) (Table 4). In the present study, increasing N fertilizer rates across all levels of K rates led an increase in biomass yield and it was also higher at the highest N rates for all K fertilizer. The above ground biomass yield peaked at K rate of 30Â kgÂ ha^{âˆ’1} across all levels of N fertilizer rate.
As this investigation clearly indicated that biomass yield of wheat responded differently to variable combination rate of N and K fertilizer. Biomass yield changed with increasing N rates at respective K rates. At K rate 0Â kgÂ ha^{âˆ’1} gain of biomass was for N rate change from 0â€“23 (32.18%), 23â€“46 (36.76%) and 46â€“69 (1.71%). Increasing N rates from 0 to 46Â kgÂ ha^{âˆ’1} across all levels of K associated with increasing dry matter accumulation in relation to N increment. The rate of N increases from 23â€“46Â kgÂ ha^{âˆ’1} resulted 38%, 56% and 38% at all levels of K respectively. Similarly, N increases from 46â€“69Â kgÂ ha^{âˆ’1} resulted in biomass yield gain of 1.71%, 30.1% and 10.3% across all respective K rates. Based on this evidence, N rate above 46Â kgÂ ha^{âˆ’1} at all levels of K rate in increment of biomass yield gain was declined and negligible.
On the other hand, K rate above 30Â kgÂ ha^{âˆ’1} led to decline in dry matter accumulation at all levels of N rates. From a view point of total biomass, K rate 30Â kgÂ ha^{âˆ’1} performed superior at all levels of N rates. Regarding N rates, 46Â kgÂ ha^{âˆ’1}Â N showed superiority of dry matter accumulation gain at all levels of K rates. This strongly suggests that N rate above 46Â kgÂ ha^{âˆ’1} the utilization by plants were very little or nearly negligible and exposed to different losses of N such as leaching, volatilization and immobilization. This illustrated that combination of 46Â kgÂ ha^{âˆ’1}Â N with 30Â kgÂ ha^{âˆ’1}Â K increased the ability of plants for capturing resources which was reflected as evident in their increased dry matter accumulation. Biomass yield had increased with increase in N rate from control to the highest level.
The interaction result shows that highest biological yield was produced when 30Â kgÂ KÂ ha^{âˆ’1} interacted with 46 and 69Â kgÂ ha^{âˆ’1}Â N levels. This result is in line with Allam (2003) and Solomon and Anjulo (2017) who reported that N application enhanced the vegetative growth of wheat crop, which ultimately increased biological yield with increase in straw yield. However, N rate beyond 46Â kgÂ ha^{âˆ’1} showed the tendency of declining dry matter accumulation. With increased level of N and K, increase in number of total tillers m^{âˆ’2} results in biological yield of wheat.
Grain yield
The result showed that grain yield was significantly affected by the interactions of N and K fertilizer. Grain yield as affected by interaction of N by K rates ranged from 1041 to 4392Â kgÂ ha^{âˆ’1}. All fertilized plots had higher grain yield as compared to plots without fertilization. Grain yield tended to increase with increasing N rate up to 46Â kgÂ ha^{âˆ’1} and then declined above that rate of N across all levels of K rates (Table 3). Regarding the effect K rates, higher grain yield was recorded at K rate of 30Â kgÂ ha^{âˆ’1} and then grain yield declined above that rate of K across all levels of N rates. The highest grain yield (4392Â kgÂ ha^{âˆ’1}) was achieved from combination of 46Â kgÂ NÂ ha^{âˆ’1} with 30Â kgÂ KÂ ha^{âˆ’1} followed by the N rate 69Â kgÂ ha^{âˆ’1} with the same K rate with mean grain yield of 4278Â kgÂ ha^{âˆ’1}. The lowest grain yield (1041Â kgÂ ha^{âˆ’1}) was achieved from unfertilized plots. This result was agreed by Inamullah and Muhammad (2014) who reported that on average, the plots where N and K nutrients were applied produced higher grain yields as compared to the plots where no N and K nutrients. Increasing N rates across all levels of K rates led variability in response to N as reflected on grain yield of wheat. In the presence of K, wheat showed greater than 100% yield gain over unfertilized plots of N. With respect of N rate at 46Â kgÂ ha^{âˆ’1} it was recorded a yield gain of 48.77%, 94.77% and 55.56% across all levels of K rates. The grain yield was from 46Â kgÂ NÂ ha^{âˆ’1} with 30Â kgÂ KÂ ha^{âˆ’1} yield gain 322% over unfertilized plots.
The correlation of selected agronomic traits with grain yield, irrespective of N and K fertilizer rates, is presented in Table 4. The association of the parameters with grain yield was highly positively correlated with r (Correlation coefficient) values ranged 0.96 to 0.99 (Table 5). The association of total number of tillers (râ€‰=â€‰0.98) and productive tillers (râ€‰=â€‰0.97) with grain yield was highly positively correlated (Pâ€‰â‰¤â€‰0.01) which suggests that increasing both parameters tended with positive impact on grain yield. Number of kernels per spike (râ€‰=â€‰0.96) and TSW (râ€‰=â€‰0.97)(Table 4) were highly positively associated with grain yield suggesting that both yield components are important traits for selection in improving wheat grain yield. Moreover, straw yield (râ€‰=â€‰0.97), HI (râ€‰=â€‰0.97) and biomass yield (râ€‰=â€‰0.99) were highly positively associated with grain yield probably an indication that increasing the parameters led to an increase in grain yield. This result was supported by the findings by Firehiwot (2014) and Bekalu and Mamo (2016) who reported significantly positive correlation of grain yield with number of seeds per spikes, biological yield and harvest index.
Generally optimization of N and K fertilizer combination resulted in positive effect for the parameters correlated with grain yield suggesting that their increase led to increment in grain yield to a certain optimum level. Moreover, the linear multiple regression Yâ€‰=â€‰1469.98â€‰+â€‰33.90X_{1} (N rates)â€‰+â€‰4.29X_{2} (K rates) with R^{2}â€‰=â€‰0.75 indicated that was very responsive to both N and K fertilization at significant level (Fig.Â 3). As this analysis indicated that grain yield was increased at rate of 33.90 for within N rates of 0 to 69Â kgÂ ha^{âˆ’1} for all levels of from 0 to 60Â kgÂ ha^{âˆ’1}. In line with this, grain yield was increased at rate of 4.29 for K rates from 0 to 60Â kgÂ ha^{âˆ’1} for all levels of N fertilizer rates. As this finding clearly indicated that both fertilizer elements were essential for wheat grain yield. However, grain yield showed greater response to N than K (Table 6).
From the above discussion, it is clear that N at 46Â kgÂ ha^{âˆ’1} and K at 30Â kgÂ ha^{âˆ’1} would be optimal for wheat production. The result in line with Solomon and Anjulo (2017) who reported that increasing N levels to 46Â kgÂ ha^{âˆ’1} increased grain yield. In contrast to Beyenesh et al. (2017) who reported that application of N 69Â kgÂ NÂ ha^{âˆ’1} was produced a high grain yield reported by. Therefore, the increase in grain yield might be attributed due to the collective role of N and K where N is a major constituent of chlorophyll and dry matter accumulation while K aids in water and nutrient uptake, photosynthesis and food formation.
Straw yield
Data on straw yield shows that it was significantly affected by main effect of N but not by K and the interactions of N and K. The highest straw yield (5118.4Â kgÂ ha^{âˆ’1}) was recorded at 69Â kgÂ ha^{âˆ’1}Â N followed (4600Â kgÂ ha^{âˆ’1}) by the 46Â N rate whereas the lowest straw yield (3342.7Â kgÂ ha^{âˆ’1}) was seen on unfertilized plots (Table 5). Similarly, Gul et al. (2011) and Tilahun et al. (2017) reported that N application has more contribution towards production of higher straw yield. Though the straw yield was not statistically significant due to K application, relatively higher straw yield was observed at 30Â kgÂ KÂ ha^{âˆ’1} then declined.
Harvest index
Significant differences on harvest index (HI) by the main effect of N and K fertilizer rates were recorded. The HI ranged from 31 to 44% (Table 5) that was recorded at 0Â kgÂ ha^{âˆ’1} and 46Â kgÂ NÂ ha^{âˆ’1}, respectively. The finding are similar to Solomon and Anjulo (2017) who reported an increasing trend of HI up to 46Â kgÂ NÂ ha^{âˆ’1} and a decreasing HI with further increase in its rate of application.
Nutrient uptake and use efficiency
N, P and K uptake
Nutrient uptake by wheat was significantly influenced by interaction effect of N and K fertilizer (Table 6). Overall, the amount of nutrient uptake was in the order of Nâ€‰>â€‰Kâ€‰>â€‰P. Grain was found to have higher content of N and P than the straw. Conversely, straw of wheat contained higher K than grain yield. The maximum N uptake by the straw (17.2Â kgÂ ha^{âˆ’1}) that was recorded from 69Â kgÂ NÂ ha^{âˆ’1} and 30Â kgÂ KÂ ha^{âˆ’1} had 153% more N than the lowest (6.8Â kgÂ ha^{âˆ’1}) from unfertilized plots.
The result further displayed that combined application of N and K at increasing rates resulted in higher uptake of N, P and K (straw and grain). On the other hand, interaction of 46Â kgÂ ha^{âˆ’1}Â N and 30Â kgÂ ha^{âˆ’1}Â K gave 344.9% and 288.5% higher grain and total N uptake respectively, over unfertilized plot. The results from interaction of N and K indicated that significant effect on grain N up take which is supported with coefficient (R^{2}â€‰=â€‰0.80) (Fig.Â 4). The grain N uptake increased with the interaction of 46Â N and 30Â kgÂ ha^{âˆ’1}Â K, beyond it the grain N uptake decreased. This result was supported by the findings by Hailu et al. (2012) who reported that N uptake by wheat was significantly improved by integrated application of N, P and K. It is observed that N and K fertilizers had positive response on P uptake. It was supported by the findings by Amare et al. (2013) who reported that P uptake by grain of wheat increased by increasing N level and in the presence of K at 50Â kg than in its absence.
Application of 69Â kgÂ NÂ ha^{âˆ’1} with 30Â kgÂ KÂ ha^{âˆ’1} result highest K uptake by straw and total and the lowest K uptake on straw and total were recorded from unfertilized plot (Fig. 4). The maximum K uptake by grain (15.4Â kgÂ ha^{âˆ’1}) was obtained from 46Â kg of N and 30Â kgÂ ha^{âˆ’1} of K. Overall, the maximum K uptake with N and K application resulted 333.3% (straw), 477% (grain) and 283% (total) more K uptake than unfertilized plots. The interaction of 46Â kgÂ NÂ ha^{âˆ’1} and 30Â kgÂ KÂ ha^{âˆ’1} fertilizer rate has significant effect on grain K uptake which was best expressed yâ€‰=â€‰57.1xâ€‰+â€‰211.5 with a significant (R^{2}â€‰=â€‰0.70) (Fig.Â 5). The result was agreed with Mesele (2019) who reported that the highest K uptake of straw, grain and total at 30Â kgÂ KÂ ha^{âˆ’1} and the lowest from the control.
Nutrient use efficiency
Agronomic efficiency
Agronomic efficiency (AE) of N significantly influenced by the main effect of N and K fertilizers. Supplying 46Â kgÂ ha^{âˆ’1}Â N produced maximum AE (52.53Â kg grain per kg N) whereas lowest AE (41.132Â kg grain per kg N) was observed at high N levels i.e. 69Â kgÂ ha^{âˆ’1} (Table 7). This study showed that AE increases up to 46Â kgÂ NÂ ha^{âˆ’1} and beyond this it decreased. This result is in line with those of Solomon and Anjulo (2017) and Fageria and Baligar (2005) who reported that the highest AE of N at the lower rate because of reduced losses. At the lower rate of N, the wheat plant utilized most of the supplied N for grain yield. The AE result in this study was close to the findings of Dereje et al. (2019) who indicated AE of 50.48Â kgÂ kg^{âˆ’1} and Birke et al. (2019) who showed 57.1Â kgÂ kg^{âˆ’1} at 46Â kgÂ ha^{âˆ’1} of N.
The AE of K was significantly affected by the interaction effect of N and K (Appendix Table A6). The maximum AE (29.9 kgkg^{âˆ’1}) was recorded from 46Â kgÂ NÂ ha^{âˆ’1} and 30Â kgÂ KÂ ha^{âˆ’1} and the lowest (10.8Â kgÂ kg^{âˆ’1}) from 60Â kgÂ KÂ ha^{âˆ’1}without N. The result had shown that AE of K decreases with the increasing rate of N and K (Table 8). This result is in line with those of Tariq et al.(2011); Hagos et al. (2017) and Mesele (2019) who reported that AE of K decreased when the rate of K increase. The higher AE from this study was recorded from the lower rates of N and K. As reported by Singh (2004) if a unit of fertilizer does not increase the yield enough to pay for its cost, its application will not be economical.
Physiological efficiency
Physiological efficiency (PE) of N and K was significantly influenced by interaction effects of N and K fertilizers. The maximum PE of N (86.6Â kgÂ kg^{âˆ’1}) was recorded from 46Â N and 30Â kgÂ ha^{âˆ’1} and the lowest (53.294Â kgÂ kg^{âˆ’1}) from 23Â kgÂ NÂ ha^{âˆ’1} and without K (Table 8). Similarly, Brar et al. (2011) who reported application of K greatly influenced PE of N in maize. The highest (83.1Â kgÂ kg^{âˆ’1}) PE of K was recorded from 46 and 60Â kgÂ KÂ ha^{âˆ’1} fertilizer. The lowest (29.31Â kg kg^{1}) PE of K was obtained from 60Â kgÂ K ha^{1} and 0Â kgÂ N ha^{1} level.
On the other hand, K rate above 30Â kgÂ ha^{âˆ’1} led to decline in PE of K at all levels of N rates.. This illustrated that combination of 46Â kgÂ ha^{âˆ’1}Â N with 30Â kgÂ ha^{âˆ’1}Â K increased the ability of wheat to absorb applied nutrients which was reflected as increased PE of N and K. This result is in line with those of Beyenesh et al. (2017) and Mesele (2017) who reported that the highest physiological efficiency recorded with the application of 46Â kgÂ NÂ ha^{âˆ’1} and K up to 30Â kgÂ KÂ ha^{âˆ’1} respectively.
Crop recovery
Data regarding, RE of N was significantly affected by main effect of N and K fertilizer but not by their interaction. The highest RE of N (116.66%) was obtained from 46Â kgÂ NÂ ha^{âˆ’1}and lowest (100.38%) with application of 69Â kgÂ NÂ ha^{âˆ’1}(Table 7). The RE of N increases up to 46Â kgÂ ha^{âˆ’1} and then showed declining trend. The RE of N obtained in this study is comparable with those obtained 89% by (Beyenesh et al. 2017); 68% by (Fresew et al. 2019) and (224.17%) by (Birke et al. 2019), 160% by (Dereje et al. 2019).
The crop recovery of K was significantly affected by the interaction effect of N and K fertilizer rates (Table 7). The highest apparent recovery efficiency of K (56.6%) was obtained with application of 46 and 30Â kgÂ KÂ ha^{âˆ’1} and the lowest (10.51%) with application of K 60Â kgÂ ha^{âˆ’1} and without N fertilizer. Potassium application rates had influenced apparent potassium recovery. The results had shown decreasing trend with increasing N and K rates consistently. Similar result was indicated by Jackson (2018) who reported that RE decreased when the rate of N and K increases. The same result was obtained by Brar et al. (2011) who reported that the recovery efficiencies of K increased with the application of N and K fertilizer in maize.
Nutrient use efficiency
Nitrogen use efficiency (NUE) of wheat was significantly affected by main effect of N and K fertilizer whereas potassium use efficiency (KUE) was significantly influenced by interaction effects of N and K fertilizers. The highest NUE (79.28%) due to N fertilizer was recorded from 46Â kgÂ NÂ ha^{âˆ’1} and the lowest (63.53%) was from 23Â kgÂ NÂ ha^{âˆ’1} (Table 7). Increasing the rate of N from 23 to 46Â kgÂ NÂ ha^{âˆ’1} increased the N use efficiency by 24.78% and increasing N from 46 to 69Â kgÂ NÂ ha^{âˆ’1} decreases the efficiency by 15.2%.
Potassium fertilizer also significantly influenced NUE (Table 8). Application of K up to 30Â kg/ha increased NUE. The maximum KUE (58.6%) was recorded from 46 kg N ha^{âˆ’1} and 30 kg K ha^{âˆ’1} and the lowest (14.9%) was from 0 kg N ha^{âˆ’1} and 60 kg K ha^{âˆ’1}. The result agrees with Amer (2005) who reported that NUE of wheat was increased in the presence of 50 kg K ha^{âˆ’1} than in its absence.
Partial budget analysis
The partial budget analysis was carried out by using the methodology described in CIMMYT (1988) in which considering all variable costs and all benefits of grain yield. Variable cost includes cost of fertilizer during experimental period. All costs and benefits were calculated on ha basis in Birr. The price of potassium was (18.00 birr kg^{âˆ’1}) and urea (13.84birr kg^{âˆ’1}). The average price of wheat grain and straw at local market were 15 and 0.8 ETB kg^{âˆ’1} respectively.
The relationship of interaction of N and K with net benefit was best expressed yâ€‰=â€‰12653xâ€‰+â€‰9742 with a significant (R^{2}â€‰=â€‰0.87) in (Fig.Â 6). The results from interaction effects of 46 and 30Â kgÂ KÂ ha^{âˆ’1} indicated that significant effect on grain yield of wheat correspondingly increases the net benefit. Economic analysis showed that the highest net benefit (57,732.5.5 birr ha^{âˆ’1}) was obtained from wheat plot that received 46 and 30Â kgÂ KÂ ha^{âˆ’1}; whereas the lowest net benefit (16,416 birr ha^{âˆ’1}) was obtained from unfertilized treatment (Table 9).
Conclusion
The results of this experiment indicated increase in grain yield, nutrient uptake, agronomic, recover and use efficiency with mineral N and K fertilization. The amount of nutrient uptake was in the order of Nâ€‰>â€‰Kâ€‰>â€‰P. Nitrogen and P are largely stored on grain than straw whereas, K is more on straw. Based on this study agronomic, economic and nutrient use efficiency application of 46Â N with 30Â kgÂ KÂ ha^{âˆ’1} is suggested for farmers in the study area. Yet, repeating the experiment over seasons and locations is also suggested for strong recommendation.
Availability of data and materials
We declare that the data and materials presented in this manuscript can be made available as per the editorial policy of the journal.
Abbreviations
 AE:

Agronomic Efficiency
 HI:

Harvest Index
 MRR:

Marginal Rate of Return
 NUE:

Nutrient Use Efficiency
 PE:

Physiological Efficiency
 RE:

Recovery Efficiency
 SNNPRS:

Southern Nation Nationality Peoples' Regional State
 TSW:

Thousand Seed Weight
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Acknowledgements
We would like to thank SNNPRS public service Birro for financial support, Kedida Gamela District for their facilitation and providing farm area for the research.
Funding
This research was conducted with the financial support of Southern Nations, Nationality and Peoples' Regional State Public service Biro, Ethiopia (SNNPRS).
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Temesgen contribute to design of the research proposal, field work, data collection, analysis and interpretation of the data using SAS software version 9.20 and writing the manuscript. Dr. Fanuel and Dr. Gobeze assisted in analysis and interpretation of the data and also in writing the manuscript. All authors read and approved the final manuscript.
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Godebo, T., Laekemariam, F. & Loha, G. Nutrient uptake, use efficiency and productivity of bread wheat (Triticum aestivum L.) as affected by nitrogen and potassium fertilizer in Keddida Gamela Woreda, Southern Ethiopia. Environ Syst Res 10, 12 (2021). https://doi.org/10.1186/s40068020002104
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DOI: https://doi.org/10.1186/s40068020002104