Role of selected macro and micro nutrients on bread wheat (Triticum aestivum L.) productivity under Nitisols of North Western Amhara region, Ethiopia

Application of soil nutrients in the form of synthetic fertilizers is the primary option to enhance crop productivity and feeding over increased population size in Ethiopian context. An on-farm research was conducted in Amhara Region for the objective of identifying major yield-limiting nutrients on bread wheat (Triticum aestivum L.) productivity. The experiment was conducted in 2021 under rainy season on eighteen farm’s fields which are located at three major wheat growing districts (Womberema-Burie, Yilmana Densa-Gonje and Deber Eliyas). It had a total of ten treatments (NPSZnBK, NPSZnK-B, NPSBK-Zn, NPZnBK-S, NPSZnB-K, NSZnBK-P, PSZnBK-N, NP, NPS2 and control). A randomized complete block design (RCBD) with three replications was used. Improved bread wheat variety TAY with 150 kg ha-1 seed rate and Urea, TSP (triple super phosphate), KCl (muriate of potash), MgSO4 (magnesium sulphate), EDTA and Borax was used for the sources of N, P, K, S, Zn and B nutrients, respectively. Except urea, all fertilizer types were applied at planting using basal application. Urea fertilizer was applied at planting, tilering and butting stages of the crop using equal splits. Before planting, one composite soil sample from each experimental site was taken at 0-20 cm depth and analysed the selected soil parameters. Yield components such as plant height, spike length, and biological yields (grain & biomass) showed highly significant differences among treatment means at each individual experimental site as well as from combined analysis in the study districts. The main driving forces of those significant differences among treatment means in the ANOVA were due to omitting nitrogen and phosphorus nutrients. In the result, both yield components (plant height & spike length) and biological yields (grain & biomass) showed quick and automatic responses for nitrogen followed by phosphorus nutrient. Especially yield without nitrogen in all study districts is equivalent with yield attained from the control treatment even if all other nutrients are at optimal levels. However, both grain and biomass yields didn’t show any significant differences either due to adding or omitting of sulphur (S), zinc (Zn), boron (B) and/or potassium (K) nutrients.  This showed that, nitrogen (N) and phosphorus (P) nutrients are still the major bread wheat yield-limiting nutrients at Nitisols of North Western Amhara region, Ethiopia.

Agriculture plays an important role in the Ethiopian economy. It contributes over 35% to the annual GDP, about 80% to the export earnings and it employs over 75% of the population (CSA, 2018). Of the agricultural GDP, the contribution from crop production takes the lion’s share which is about 70% or more. Within the crop production system, the share of cereals in area coverage and production potential is about 80% and 85%, respectively (CSA, 2017). The most important three crops (wheat, maize and tef) have a share of 60% of the fertilizer inputs, 55% of the production area coverage and 60% of the annual production potential (CSA, 2017).
Wheat is the most widely cultivated cereal crop in the world, and provides 20% of the protein and calories consumed by the world population (FAOStat, 2013). It is currently the staple food for more than 35% of the global human population (FAOSTAT, 2013). Continues nutrient depletion, newly emerging diseases and pests and unstable weather conditions deriving from climate change are the major threats for declining wheat productivity globally (CIMMYT, 2016).  Ethiopia is the second-largest wheat producer in Sub-Saharan Africa, following South Africa (White et al., 2001). The crop covers 1.7 million ha area and 4.6 million tons production (CSA, 2018). From the country, Amhara Region accounts 32.7% of area coverage and 30.3% of production volume (CSA, 2018). However, average wheat productivity in the Amhara region is about 2.53 tons ha-1 which is below the national average 2.74 tons ha-1.  
After the introduction of soil fertility map by the Ethiopian Soil Information System (EthioSIS, 2015) and the second growth and transformation plan (GTP II, 2016-2020), the country has increased the fertilizer types used from two to six. For this reason, the annual import and consumption raised to over 100,000 tonns year-1. Currently, Ethiopia imports about 1.4 million tonns of multi nutrient fertilizers and projected to use over 2 million tons at the end of 2025. In targeting the right fertilizers to the right places, the EthioSIS project team has mapped the soil nutrient status of agricultural lands in Ethiopia and identified that a number of essential nutrients are deficient and critically required for enhancing crop productivity in the country. Based on the developed map by the project, N, P, K, S, B, Zn, Fe and Cu are the deficient nutrients identified and recommended for enhancing crop productivity in most of Ethiopian soils. Even though the newly formulated fertilizers needed a validation work, Agricultural Transformation Agency (ATA) and Ministry of agriculture (MoA) in collaboration conducted direct demonstration trials over at 60,000 trial sites within the regions. Due to this, the country has already customized the use of above-mentioned soil nutrients and made available in fertilizer forms without reaching national consciences on the importance of those newly formulated fertilizer types. Therefore, this activity was conducted for the objective of identifying major yield-limiting nutrients for bread wheat productivity in North Western Amhara region, Ethiopia.   

Study area description 
The experiment was conducted at three major bread wheat growing districts (Womberema-Burie, Yilmana Densa-Gonje and Deber Eliyas) in Amhara regional state and located in North West direction from the capital city of Ethiopia (Fig 1).
Experimental materials 
Improved bread wheat variety (TAY) with 150 kg ha-1 seed rate was used. Urea, TSP, KCl, MgSO4, EDTA and Borax was used as a source of N, P, K, S, Zn and B nutrients, respectively. Soil auger and core-sampler was used to collect soil samples.

Figure 1: Map of the study districts

Experimental methods and design 
The experiment was conducted in 2021 at eighteen (18) farmer’s fields.  A randomized complete block design (RCBD) with three replications was used. The test crop was planted in row planting method using 20 cm row spacing. Spacing between plots and blocks were also 1m and 1.5m, respectively. From 12m2 gross plot size, 9.6m2 was used as net harvestable area. The experiment had a total of ten treatments as indicated in Table 1. Except Urea, all fertilizers were applied at planting time using band application method. Urea fertilizer was applied in three equal splits at different crop stages (planting, tilering and butting).

Table 1: Treatment setup used in the experiment

Soil sampling, Preparation and Analysis
From each experimental site, one composite soil sample before planting was taken from five points following X-pattern sampling technique at the depth of 0-20 cm. The sample was air dried and sieved using ≤2 mm sieve for the analysis of the required parameters. Soil pH, organic carbon (SOC), cation exchange capacity (CEC), available phosphorus (AP) and total nitrogen (TN) were analyzed. All the mentioned parameters were analysed at Adet agricultural research centre’s (AARC) soil laboratory. Besides, soil pH was determined using 1:2.5 soil-water suspensions ratios according to Taye et al., 2002. Olsen (1954) was used for AP analysis. TN was analysed following Kjeldahl method (Bremner and Mulvaney, 1982). Soil OC was determined using wet oxidation and CEC determined using ammonium acetate method. As indicated in Table 2, Soil pH values of the experimental sites found from strongly (4.5-5.2) to moderately acidic (5.3-5.9) ranges based on (Tekalign, 1991). Average soil AP values ranged in medium (5-10) based on Olsen (1954) nutrient rating scale. Based on Tekalign (1991) soil OC and TN values found from low to medium nutrient levels. While, CEC reading from medium (15-25) to high (25-40) Cmol(+)kg-1 rating level according to Hazelton and Murphy (2007). 

Table 2: Before planting selected soil properties for experimental sites

Note: PH= Plant height, SL=Spike length, ** = highly significant, * = Significant, NS = non-significant

Grain and Biomass yields 
In Wombrema-Burie district, grain yield of bread wheat showed highly significant difference among treatment means except at one site (Table 4). Most of the observed significant differences among treatment means of the grain yield in the district derived due to control and N omission treatments, respectively. In the other saying, significant difference generated due to the presence of control and N omitted treatments. Almost at all trial sites, the minimum grain yield values recorded at control treatments followed by N omitted treatment. However, the maximum values were observed at any one of the treatments which received N and P nutrients together.   
In the study district, an automatic response on grain yields of bread wheat was observed when either N nutrient was added or omitted. In this district, omitting of phosphorus nutrient also didn’t show significant difference from treatments which received recommended N and P nutrients together. This might indicate us to revise the current P rate to be used in the coming years. This showed that, N still showed its primarily potential on wheat yield-limiting which is in line with the findings of (Tadele et al., 2018) as he stated, the yield-limiting nutrients to produce maize and wheat in major growing areas in Amhara region were N and P, respectively. Exception of N omitted treatment, significant differences didn’t occur among the means of other treatments due to either adding or omitting of other nutrients (S, Zn, B, K and P) in the district.
Table 4: Grain yield values of bread wheat at Wombrema-Burie district

Note: ** = Highly significant, * = Significant, NS = non-significant

Similar to Wombrema-Burie, in the two districts (Yilmana Densa-Gonjie and Deber Eliyas), grain yield of bread wheat also showed highly significant difference among treatment means (Table 5). But unlike Wombrema-Burie, P omission also sourced for the significant differences among treatment means of bread wheat grain yield in addition to control and N omitting treatments (Table 4). This showed that, P is the second yield-limiting nutrient in these study districts which is in line with the findings of (Tadele et al., 2018). Overall trends of the experiment showed, N and P nutrients are still the major yield-limiting nutrients for bread wheat productivity in Nitisols of North Western Amhara region.

Table 5: Grain yield values of bread wheat at Yilmana Densa-Gonjie and Deber Eliyas districts

Note: ** = Highly significant, * = Significant, NS = non-significant

Table 7: Biomass yield values at Yilmana Densa-Gonjie and Deber Eliyas districts

Note: ** = Highly significant, * = Significant, NS = non-significant

Similar to the individual experimental sites, all the biological yields (grain and biomass) showed significant difference among treatment means (Table 8). As discussed for the individual sites, in the combined ANOVA result, P omitting didn’t show significant difference from treatments which received recommended N and P nutrients together at a time. 
Table 8: Combined grain and biomass yield of bread wheat (kg ha-1) in the study districts

Note: ** = Highly significant, * = Significant, NS = non-significant

Except control and N omitted treatments other treatments didn’t show any statistically significant differences with each other on both grain and biomass yields. In this finding,  N showed as a leading yield-limiting nutrient for bread wheat productivity in the Nitisols of North Western Amhara region followed by P which is agreed with (Tadele et al., 2022) finding. In most of the study districts, omitting of each nutrient contributed yield penalty in comparison to the bench mark treatment (NPSZnBK). However, the contribution of each nutrient on the yield penalties didn’t show equal magnitude. Even, omitting of some nutrients showed yield advantages from the bench mark treatment. With these remarks, omitting of K, S, B and Zn nutrients contribute insignificant impact from the bench mark treatment which is agreed with the findings reported by Ayana et al. (2022) and Beamlaku et al. (2022). However, impact of omitting N and P nutrients showed high and significant from the treatment which received all type of nutrients, respectively. Especially, yield penalties due to omitting nutrient N is nearly equivalent to the control (zero input) treatment (Figure 2).

Figure 2: Yield penalty /advantage of bread wheat grain yield due to omitting of nutrients (##: NPSZnBK is a benchmark treatment for this analysis)

Figure 3: Trial performance sample at each study district (2021)

Grain yield of bread wheat showed highly significant differences among treatment means at each individual experimental site as well as at all study district. The study confirmed that, N is the primary bread wheat yield-limiting nutrient in North Western Amhara region Nitisols followed by P. However, S, Zn, B and K nutrients had no significant responses from the bench mark treatment (NPSZnBK) or the former nutrients used (sole NP) on both grain and biomass yields of bread wheat either due to added or omitted them. Therefore, still it is possible to maximize bread wheat yield productivity by using optimal N and P nutrient levels with integrating other improved technologies in the study districts and areas having similar soil type & agro-ecology. However, frequent revision of the soil fertility status is too important for updating nutrient requirements both in types and rates used for enhancing productivity and production of bread wheat in North Western Amhara region, Ethiopia.  

Acknowledgements
Finally, authors would like to acknowledge home grown project which is called All Ethiopian Coordinated Fertilizer Research (AECFR) for its financial support to implement the experiment. We as an author also acknowledge Amhara Regional Agricultural Research Institution (ARARI) and Adet Agricultural research center (AARC) for their administrative and logistic supports.
Statements and Declarations
Funding
This research work was supported financially by home grown national project which is called All Ethiopian Coordinated Fertilizer Research (AECFR) project.
Conflict of interest
We declared that there is no conflict of interest to the best of our knowledge.
Author Contributions
Erkihun Alemu, Tadele Amare, Zerfu Bazie, Tesfaye Feyisa, and Ateneh Abewa were participated on designing, implementing, analyzing the data and writing the manuscript. While, Abere Tenagne, Abreham Awoke, Atakltie Abebe, Zelalem Addis, Bitewlign Kerebeh, Zemie Amibawu, Sefinew Wale and Beamlaku Alemayehu were participated on data collection, executing the field work, and input preparation during experimentation.