Effects of phosphorus and sulfur on yield and nutrient uptake of wheat (Triticum aestivum L.) on Vertisols, North Central, Ethiopia.

Deficiency of phosphorus (P) and sulfur (S) is increasingly being reported in soils of Ethiopia. While some studies have shown significant response of wheat to P and S application, information on the response of wheat to P and S application interactively is conspicuously lacking. In this regard, we evaluated the response of wheat to P and S application interactively in the study area. A field experiment was conducted at two locations to determine the effects of P and S, on yield, uptake and P, S use efficiency of bread wheat. A factorial combination of four levels of P (0,11,22 and 44 kg h-1) and three levels S (0, 15 and 30 kg ha-1) laid out in Randomized Complete Block Design with three replications. Results revealed that interacted application of P and S at 22 and 15 kg ha-1 respectively increased grain yield of wheat by 40.1 % over control. The corresponding increase with straw was 53.4 % over control. Wheat yield obtained with combined application of P and S greater than single application of P or S indicating synergistic between them. The maximum grain N (56.3 kg ha-1), P (12.8 kg ha-1) and S (4.2 kg ha-1) uptakes were obtained due to combined application of P and S at 22 P and 15 S kg ha-1. Agronomic efficiencies of P and S decreased as the rates of P and S application increased. Combined fertilization of S and P is necessary in the study district and 15 kg S combined with 22 kg P ha-1 produced the highest yield. Thus, this treatment is found to be recommended for bread wheat production in Vertisols of the district. While, partial budget analysis result revealed that, combination of 22P and 15S kg ha-1 produced the highest MMR (54.9 %) and thus, this treatment is found to be economically feasible treatment for bread wheat production in study area of the district. We recommend further experiments on different combination of P with S in different agro-ecologies and soil types are required for confirmation of results and the residual effect of P and S on the following crop is needed to study the long-term effect of P and S.

Introduction

Wheat is the most important food security crop in the world with a production of 750 million tons (MT) on about 220 million hectares (Mha) (Tadesse et al., 2018). Worldwide, the demand of wheat by the year 2020 is forecast at around 950 million tons per year. This target can be achieved only, if the global wheat production is increased approximately with 2.5 percent per annum (Bairwa D. et al.). More than 25 million tons of wheat on 10 Mha produced in Africa. Sub-Saharan Africa (SSA) produced a total of 7.5 MT on a total of 2.9 Mha accounting for 40 and 1.4 percent of wheat production in Africa and at global levels, respectively (FAO, 2017). n class="Species">Bread wheat, which accounts for 95 percenpan>t of the wheat production at the global level, is also the dominpan>ant wheat type produced inpan> sub Saharan Africa (Tadesse et al., 2018). Ethiopia, South Africa, Sudan, Kenpan>ya, Tazania, Nigeria, Zimbabwe and Zambia are the most important wheat producinpan>g countries inpan> SSA inpan> descenpan>dinpan>g order. Ethiopia accounts for the largest production area (1.7 Mha) followed by south Africa (0.5 Mha) (Tadesse et al., 2018). In Ethiopia wheat consumption is inpan>creasinpan>g due to population growth and gradual change of life style inpan> urban areas (Abu, 2012). The currenpan>t average productivity of wheat inpan> Ethiopia is 2.76-ton ha−1 (CSA, 2019). The average yield of wheat inpan> the country is lower, compared to potenpan>tial yield which is 5-ton ha−1 (Birhan et al., 2016).

Wheat is a staple food that provides around 20% of protein and calories consumed worldwide (FAOSTAT, 2015) and is a major source of energy and protein for the highland population of Ethiopia. It is consumed in many forms like bread, cakes, biscuits, bakery products and many confectionery products. Its straw is used as animal feed and also for manufacturing paper (Iqtidar et al., 2006).

In Sub Saharan Africa including Ethiopia wheat production constrained by a number of biotic and abiotic stresses at differenpan>t levels of inpan>tenpan>sity across rainpan> fed and irrigated enpan>vironmenpan>ts. The most important abiotic stresses in the rain-fed environment that hinder wheat production are drought, soil acidity, erosion, poor soil fertility, water logging and preharvest sprouting. Such constraints are most common in the East African, highlands of Ethiopia, Eritrea, Kenya, Tanzania, Uganda, Rwanda and Burundi (Tadesse et al., 2018). Comparatively lower productivity in Ethiopia can be explained by several constraints like depletion of soil fertility which is caused by intensive cropping, imbalanced fertilization, limited application of organic manures and soil erosion (Birhan et al., 2016). Nutrient mining due to sub optimal fertilizer use in one hand and unbalanced fertilizer uses on other have favored the emergence of multi nutrient deficiency in Ethiopian soils. For the last five decades, Ethiopian agriculture depended solely on imported fertilizer products, only urea and di-ammonium phosphate (DAP), sources of N and P. However, recently it is perceived that the production of such high protein cereals like wheat can be limited by the deficiency of sulfur (S) and other nutrients.

n class="Chemical">Phosphorus (P) is one of the most limitinpan>g plant nutrienpan>ts inpan> crop production next to N, inpan> most agricultural soils (Akande et al., 2010). It is one of the most limitinpan>g elemenpan>ts inpan> the majority of the soils of Ethiopia. Wheat experimenpan>ts has beenpan> conducted inpan> Ethiopia inpan> major wheat growinpan>g areas and results revealed that inpan>creased yield and yield componenpan>t with successive application of P fertilizer (Minpan>ale et al., 2006; Bereket et al., 2014; Tilahun et al., 2006).

Like P, S deficiency is also beinpan>g reported inpan> soils of Ethiopia. Sulfur is a nutrient most overlooked in Ethiopian agriculture and major prone areas of S deficiency are the central highlands. Assefa Mena (2016) studied the response of wheat to S application and reported that wheat significantly responded to S fertilizer application. Soils in this study had S content below the critical level (11–13 mg kg−1 SO4−2-S) for optimum production of crop.

Sulfur inpan>teracts with P as phosphate ion is more strongly bound than sulphate (Hedge and Murthy, 2005). Application of P fertilizer results in increased of anion adsorption sites by phosphate, which releases sulphate ions into the soil solution (Tiwari and Gupta, 2006). Both synergistic and antagonistic relationship between P and S but their relationship depends on their rate of application and crop species (Sinha et al., 1995). Grain and biological yield of wheat increase significantly with increase both P and S levels and protein content increase by increasing the level of sulfur (Ibrahim et al., 2012). Deficiency of phosphorus (P) and sulphur (S) is increasingly being reported in soils of Ethiopia. While some studies have shown significant response of wheat to P and S application, information on the response of wheat to P and S application interactively is conspicuously lacking in the study district. The present experiment was conducted to study the effects of P and S application on yield, nutrient uptake, nutrients use efficiency and to determine economic feasibility of P and S fertilizer application of wheat grown at Moretina Jiru and Sya debr ena Wayu district.

Materials and methods

Description of the study areas

The field experiment was conducted in 2017/18 cropping season on two locations. The locations are found in Moretina Jiru (Gerba) and Saya deber ena wayu (Deneba) district of north Shewa. Geographically, the study locations lie between 09° 46′ 53.3″ to 09° 52′ 07.5″ N and 039° 10’ 38.1″ to 039° 11′ 28.3″ E. The altitude of the study locations lies between 2600 to 2700 m.a.s.l. The mean annual minimum and maximum temperature ranging from 5.9-11.6 °C and 17.4–31.2 °C respectively. The study locations and the district as a whole are characterized by having a uni-modal rainfall pattern and receives an average annual rainfall of 921.2 mm. Vertisols is the dominant soil type occurring in the districts. Major crops grown in the locations are wheat, tef, lentil, faba bean, chickpea, field pea and grass pea in decreasing orders of area cultivated under these crops. Figure 1 indicated monthly total rainfall, average maximum and minimum temperatures in the growing season and Figure 2 indicated location map of the study districts.

Figure 1

Location map of the study sites.

Figure 2

Monthly total rainfall and average maximum and minimum temperatures.

Ln class="Chemical">ocation map of the study sites.

Description of soil horizon and soil classification of study district

The soil profile pit was dug on representative site. The soil profile description made according to FAO system was recorded on standard form for soil profile description (FAO, 2006). The soil type of the study area was classified as Eutric Vertisol (FAO-WRB 2014 update of 2015). The soil of the study district described as follows.

Profile description

Ap-horizon (0–30 cm depth)- Very dark grayish brown (10YR3/2) dry and very dark brown (10YR2/2) moist, clay; strong coarse prismatic; Very closely spaced cracks; sticky plastic wet, firm moist, very hard dry; common fine interstitial pores; common fine roots; clear and smooth boundary.

A-horizon (30–75 cm depth)- Black (10YR2/1) dry and moist, clay; strong coarse angular bln class="Chemical">ocky; very sticky very plastic wet, firm moist, very hard dry; common finpan>e inpan>terstitial pores; few finpan>e roots; clear and wavy boundary.

B1-horizon (75–150 cm depth)- Very dark brown (10YR2/2) moist, clay; strong coarse angular bln class="Chemical">ocky; very sticky very plastic wet, friable moist; common prominpan>enpan>t slickenpan>sides and pressure faces; few finpan>e inpan>terstitial pores; clear and wavy boundary.

Btc-horizon (150–200 cm depth)- Very dark grayish brown (10YR3/2) moist, clay; strong coarse angular bln class="Chemical">ocky; sticky plastic wet, friable moist; common prominpan>enpan>t slickenpan>sides and pressure faces; few finpan>e inpan>terstitial pores; common CaCO3 concretions.

Soil sampling and analyses

Before planting of the wheat crop, soil samples were collected from each site for the analyses of selected physicn class="Chemical">ochemical properties. The composite soil samples were takenpan> from each experimenpan>tal site from a depth of 0–30 cm usinpan>g augur randomly from 15 spots by walkinpan>g inpan> a zigzag pattern. After carefully mixinpan>g the composite samples, 1 kg of sub-sample was takenpan> and brought to Debre Berhan agricultural research Cenpan>tre soil laboratory. The submitted sample was air dried and grounded to pass 2 mm mesh sized sieve.

The soil texture was analyzed by using Bouyoucous hydrometer method (Bouyoucos, 1962). The soil reaction (pH) was measured using pH-water method by makinpan>g soil to water suspension of 1: 2.5 ratio and was measured using a pH-meter. The soil organic carbon (OC) content was determined by wet digestion method which developed by Walkley and Black (1934). Total nitrogen (TN) was determined by using the modified micro Kjeldhal method (Cottenie, 1980), available P (ava. P) was analysed by using Olsen's calorimetric method (Olsen et al., 1954), and exchangeable potassium (Ex. K) in the soil was extracted with 1 N NH4OAc and the amount was estimated using a flame photometer (Jones, 2001). Soil available sulphur (ava. S) in the soil was determined turbidmetrically using a spectrophotometer (Singh et al., 1999). Cation exchange capacity (CEC) was measured after saturating the soil with 1N NH4OAc and displacing it with 1N NaOAc (Chapman, 1965).

Selected soil physical and chemical properties

The soil analyses data before planting for selected physicochemical properties collected from experimenpan>tal locations at Moretina Jiru (Gerba) and Sayadeber Ena Wayu (Deneba) are summarized in Table 1. The soils of both experimental sites were belonging to clay textural class. Soil reaction (pH) at Gerba and Deneba location is near to neutral and slightly alkaline respectively. The soil organic matter (SOM) and Total nitrogen content of both locations are in low categories (Jones, 2001). The available P content of both locations are slightly higher than 5 mgk−1, which is far lower than the critical soil available P value established for some Ethiopian soils which is 10 mg kg−1 (Tekalign and Haque, 1991). The available S content of both soils were also found below the critical value of S which is 10–13 mg kg−1 SO4−2-S (Tandon, 1991).

Table 1

Pre-planting selected physicochemical properties of experimental soils.

Locations	Physical Properties				Chemical properties
	Sand	Clay	Silt	Textural Class	pH	SOM	TN	Ava.P	Ava.S	Ex. K	CEC
	%					%		Mg kg−1		(Cmol kg−1)
Gerba	6	76	18	Clay	6.8	1.55	0.087	6.95	5.62	0.88	40.0
Deneba	8	74	18	Clay	7.3	1.79	0.088	6.36	4.98	0.82	39.0

pH = soil reaction, SOM = soil organic matter, ava. P = available phosphorus, ava. S = available sulphur, Exch. K = exchangeable potassium, CEC = Cation exchange Capacity. TN = Total nitrogen.

Pre-planting selected physicn class="Chemical">ochemical properties of experimenpan>tal soils.

The exchangeable K content of both soils was very high (Jones, 2001). The Cation exchange Capacity of both soils was also in high category (Landon, 1991). Based on the soil analysis, the result from Table 1, indicated that, the study area soil is deficient in N, P and S which needs N, P and S fertilizer application for optimum crop production.

Treatments, design and experimental procedure

An experiment consisting of four levels of phosphorus (0, 11, 22 and 44 kg ha−1) and three levels of sulfur (0, 15 and 30 kg ha−1) was laid out in Randomized Complete Block Design (RCBD) with three replications. Potassium sulphate (K2SO4) and Triple super phosphate (TSP) was used for S and P fertilizer sources respectively. The wheat variety used for the study was Menze (HAR-3008), which was released by Debre berhan Agricultural Research Centre (DBARC, 2007). The variety is high yielding and disease resistant. The seed was planted in a unit plot size of 3.6 × 3.4m with row spacing of 20 cm apart at a rate of 150 kg ha−1. The whole doses of potassium Sulfate (K2SO4) and Triple super phosphate (TSP) fertilizers were applied at basal in both sides of rows just before planting as per the treatment. To compensate potassium (K) applied along with S fertilizer source in S treatment plots, K was applied in the form of potassium chloride (KCl). The recommended dose of nitrogen (N) fertilizer for wheat production around the study area 167 kg N ha−1 (Adamu, 2018) was applied uniformly to all plots in the form of urea. The Urea-N was split applied in which one half of N was applied at planting and the remaining one half was applied 45 days after planting and first weeding. All agronomic management of the trails was done as per the recommendation for the crop (EIAR, 2007). Data on grain and straw yield, nutrient uptake (NU), Grain Protein content (GPC) and nutrient use efficiency were collected and calculated at appropriate time pertinent for each parameter.

Plant tissue sampling and analysis

Ten (10) randomly selected wheat plants were harvested from six central rows at physiological maturity and partitioned into grain and straw. The grain and straw samples were separately oven-dried at 70 °C for 24 h to a constant weight and ground to pass through 1 mm sieve for analysis of N, P and S in grain and straw. Total nitrogen (N) inpan> grainpan> and straw subsamples were quantitatively determinpan>ed by kjeldahl procedure developed by Bremner and Mulvarey (1982). Phosphorus in grain and straw subsamples were determined by using Meta vanadate method (NSL, 1994). Sulfur was determined by magnesium nitrate dry ashing method (Benjamin, 1982). After the determination of N, P and S concentration, in grain and straw uptake of N, P and S in the grain and straw of wheat was determined by using the following formula as described by Sharma et al. (2012).

Phosphorus and sulfur use efficiency

Based on the results of plant tissue analysis, n class="Chemical">phosphorus and n class="Chemical">Sulfur use efficiency indices were computed (Rakshit et al., 2015).

Agronomic efficiency (AE): is the economic production obtained per unit of nutrient applied and expressed in kg/kg. It can be calculated by using the following equation:Where: Gf is the grain yield of the fertilized plot (kg), GU is the grain yield of the unfertilized plot (kg), and Na is the quantity of P applied (kg).

Apparent Recovery efficiency (ARE): is defined as the quantity of nutrient uptake per unit of nutrient applied. Expressed as percent (%).Where: Nf and Nu are nutrient uptakes (grain plus straw) from fertilized unfertilized plots (kg), respectively, and Na is the quantity of nutrient applied (kg).

Determination of protein content

Data analysis

The collected data were subjected to ANOVA. After verifying the homogeneity of error variances and normal distribution, combined analysis of variance was done using the prn class="Chemical">ocedure of SAS software version 9.3 (SAS, 2011). Mean comparisons were done by Duncan's multiple range tests (Gomez and Gomez, 1984) at the 5% level.

Economic analyses

Partial budget analyses were done to determine economic feasibility of P and S fertilizers as well as their combinations for wheat production around the study areas following prn class="Chemical">ocedures described inpan> CIMMYT (1998). The mean grainpan> and straw yield data of wheat was employed inpan> the analyses. Furthermore, the grainpan> yield and straw yield obtainpan>ed from each treatmenpan>t were adjusted down by 10 % inpan> order to narrow the possible yield gap that may happenpan> due to differenpan>ce inpan> field managemenpan>t by researcher and famers. This is because usually, researcher managed field give higher yield than famer managed field.

Results

The data on response of grain and straw yield, nitrogen uptake, phosphorus uptake, sulfur uptake, phosphorus use efficiency and sulfur use efficiency indices of wheat under phosphorus and sulfur interaction are shown graphically in Figures 3, 4, 5, 6, and 7. The grain protein content of wheat due to main and interaction effect of P and S are presented in Table 2. All the above-mentioned parameters are discussed here after in sub sections. While the response of grain and straw yield, N, P, and S uptake, P and S use efficiency indices under main effect of P and S are presented in Tables 3 and 4 and the correlation coefficient of among each paraments presented in Table 5.

Figure 3

Interaction effects of P and S, on yield of Wheat.

Figure 4

Interaction effects of P and S, on Nitrogen Uptake of Wheat.

Figure 5

Interaction effects of P and S, on Mean Phosphorus Uptake of Wheat.

Figure 6

Interaction effects of P and S, on Mean Sulfur Uptake of Wheat.

Figure 7

Effect of P and S rate on Phosphorus efficiency indices of wheat.

Table 2

Effects of P and S on grain protein contents of wheat.

S-Levels kg ha−1	P-Levels kg ha−1				Mean
	0	11	22	44
0	7.8b	8.6ab	9.3ab	9.4a	8.77b
15	9.1ab	9.1ab	9.3ab	9.4a	9.2a
30	9.1ab	9.4ab	9.8a	9.77a	9.5a
Mean	8.7c	9.0b	9.5a	9.6a
CV (%)	6.25

Table 3

Main effect of different P and S levels on yield and nutrient uptake of wheat.

P Levels (kg ha−1)	GY	STY	NUG	NUS	TNU	GPC	PUG	PUS	TPU	SUG	SUS	TSU
0	2575.7c	3294.6c	37.4c	11.5c	48.9c	8.2c	6.5c	1.2c	7.6c	2.3c	6.7d	9.0d
11	3110.7b	4121.8b	46.9b	14.7b	61.6b	8.6b	10.1b	3.6b	13.7b	3.3b	8.9b	12.2b
22	3299.6a	4589.6a	51.1a	16.8a	67.9a	8.8a	11.5a	5.7a	17.2a	3.6a	12.0b	15.6b
44	3324.1a	4690.6a	0.6a	17.3a	67.9a	8.7a	11.6a	6.3a	17.9a	3.7a	13.3d	17.0a
S Levels (kg ha−1)
0	2779.1b	3456.2b	40.9b	12.6b	53.5b	8.3b	8.5b	3.4b	11.9b	2.6b	7.6b	10.3b
15	3212.5a	4525.5a	49.3a	16.4a	65.7a	8.7a	10.6a	4.7a	15.3a	3.6a	11.3a	14.8a
30	3240.9a	4540.8a	49.3a	16.2a	65.5a	8.7a	10.5a	4.5a	15.1a	3.5a	11.8a	15.2a
CV (%)	5.2	6.1	7.1	11.5	5.7	5.5	7.7	21.8	9.1	10.0	15.49	11.95

BY = Biomass yield, GY = grain yield, STY = Straw yield, NUG = Nitrogen uptake by straw, NUS = Nitrogen uptake by straw, TNU = Total Nitrogen uptake, PUG = Phosphorus uptake by grain, PUS = Phosphorus uptake by straw, TPU = Total Phosphorus uptake, SUG = Sulfur uptake by grain, SUS = Sulfur uptake by straw, TSU = Total Sulfur uptake.

Table 4

Main effect of different P and S levels on P and S use efficiency of wheat.

P-Levels (kg ha−1)	AEP (kg kg−1)	REP (%)	PUE (%)	AES (kg kg−1)	RES (%)	SUE
0	-	-	-	23.8c	16.0c	5.2c
11	94.0a	79.7a	27.8a	36.2b	25.6b	8.5b
22	55.6b	55.9b	18.6b	46.4a	41.4a	12.9a
44	28.3c	29.5c	9.7c	46.3a	42.3a	13.3a
S-Levels (kg ha−1)
0	36.0b	31.9b	12.3	-	-	-
15	47.7a	46.2a	14.9	75.7a	61.8a	19.6a
30	49.7a	45.8a	14.8	38.8b	32.2b	10.4b
CV (%)	22.0	20.7	27.1	17.8	24.2	26.8

CV = Coefficient of Variation, AEP = Agronomic efficiency of phosphorus, REP = Recovery efficiency of phosphorus, PPE, Physiological phosphorus efficiency, PUE = Phosphorus utilization efficiency, AES = Agronomic efficiency of Sulfur, RES = Recovery efficiency of Sulfur, SUE = Sulfur utilization efficiency, GPC = grain protein content.

Table 5

Correlation coefficients of each parameters.

	GY	STY	NUG	NUS	TNU	PC	PUG	PUS	TPU	SUG	SUS	TSU	PAE	PRE	SAE	SRE
GY	1
STY	0.823∗∗	1
NUG	0.948∗∗	0.845∗∗	1
NUS	0.739∗∗	0.927∗∗	0.774∗∗	1
TNU	0.925∗∗	0.920∗∗	0.975∗∗	0.896∗∗	1
PC	0.528∗∗	0.583∗∗	0.766∗∗	0.562∗∗	0.736∗∗	1
PUG	0.952∗∗	0.827∗∗	0.912∗∗	0.767∗∗	0.911∗∗	0.518∗∗	1
PUS	0.742∗∗	0.800∗∗	0.746∗∗	0.748∗∗	0.787∗∗	0.473∗∗	0.84∗∗	1
TPU	0.885∗∗	0.848∗∗	0.866∗∗	0.79∗∗	0.887∗∗	0.517∗∗	0.961∗∗	0.958∗∗	1
SUG	0.835∗∗	0.873∗∗	0.861∗∗	0.836∗∗	0.899∗∗	0.579∗∗	0.84∗∗	0.757∗∗	0.833∗∗	1
SUS	0.657∗∗	0.869∗∗	0.724∗∗	0.875∗∗	0.817∗∗	0.559∗∗	0.704∗∗	0.737∗∗	0.751∗∗	0.857∗∗	1
TSU	0.698∗∗	0.887∗∗	0.760∗∗	0.886∗∗	0.846∗∗	0.573∗∗	0.739∗∗	0.754∗∗	0.778∗∗	0.896∗∗	0.997∗∗	1
PAE	0.489∗∗	0.33∗	0.456∗∗	0.253∗	0.409∗∗	0.244∗	0.492∗∗	0.311∗	0.42∗∗	0.402∗∗	0.137ns	0.182ns	1
PRE	0.530∗∗	0.449∗∗	0.510∗∗	0.380∗∗	0.492∗∗	0.294∗	0.584∗∗	0.495∗∗	0.564∗∗	0.497∗∗	0.284∗	0.323∗	0.948∗∗	1
SAE	0.613∗∗	0.595∗∗	0.596∗∗	0.465∗∗	0.582∗∗	0.333∗	0.550∗∗	0.418∗∗	0.506∗∗	0.570∗∗	0.403∗∗	0.437∗∗	0.256∗	0.304∗∗	1
SRE	0.600∗∗	0.749∗∗	0.638∗∗	0.661∗∗	0.681	0.436∗∗	0.604∗∗	0.545∗∗	0.599∗∗	0.700∗∗	0.699∗∗	0.713∗∗	0.189ns	0.29∗	0.875∗∗	1

Interaction effects of P and S, on n class="Chemical">Nitrogen Uptake of Wheat.

Interaction effects of P and S, on Mean n class="Chemical">Phosphorus Uptake of Wheat.

Interaction effects of P and S, on Mean n class="Chemical">Sulfur Uptake of Wheat.

Effect of P and S rate on n class="Chemical">Phosphorus efficienpan>cy inpan>dices of wheat.

CV = Coefficient of Variation, AEP = Agronomic efficiency of phosphorus, REP = Recovery efficienpan>cy of phosphorus, PPE, Physiological phosphorus efficiency, PUE = Phosphorus utilization efficiency, AES = Agronomic efficiency of Sulfur, RES = Recovery efficiency of Sulfur, SUE = Sulfur utilization efficiency, GPC = grain protein content.

The effects of P and S on grain and straw yield of wheat

Grain and Straw yield were significantly influenced by the main effects of P, S, as well as their interaction rate (Table 6). The grain yield as influenced by different levels of P and S are presented in Figure 3. The addition of 22 kg P ha-1 with 15 kg S ha−1 produced maximum grainpan>s yield (3526.8 kg ha−1) which was significantly differed from other treatmenpan>t combinpan>ation except, 22P+30S, 44P+15S and 44P+30 S kg ha-1. This treatment has a yield advantage of 40.1% over control treatment. However, grain yield become decreased numerically as interactive application rate of P and S beyond 22P and 15S kg ha−1. The minimum grains yield (2076.7 kg ha−1) was recorded in control plot (untreated with P and S) shown in Figure 3. The straw yield of wheat was also influenced significantly (P ≤ 0.01) by different levels of P and S application interactively. The higher straw yield (5120.0 kg ha−1) was produced where treatment receiving 22 kg P ha−1 and 15 kg S ha-1 interactively (Figure 3) which have straw yield advantage of 53.4 % over control treatment. Similar trends were also observed like grain yield due to the influence of different P and S application rates. The minimum straw yield (2385.6 kg ha−1) was recorded in control plot.

Table 6

Mean squares of combined analysis of variance for the effects of P and S level on different parameters of bread wheat.

Parameters	Mean squares for sources of variation with respective degrees of freedom in parenthesis
	L (1)	Rep in L (2)	P (3)	S (2)	P∗S (6)	L∗P (3)	L∗S (2)	L∗P∗S (6)	Error (46)
GY	316325∗∗	11538	2178333∗∗	1607309∗∗	151955∗∗	28159ns	57317ns	29746ns	21512
STY	9452024∗∗	100113	7293954∗∗	9279709∗∗	268525∗∗	216326∗	639151∗∗	160578∗	58930
NUG	292.1∗∗	17.9	725.8∗∗	571.3∗∗	66.8∗∗	2.2ns	0.3ns	9.1ns	10
NUS	318.9∗∗	0.4	124.7∗∗	110.3∗∗	6.8ns	6.1ns	19.7∗∗	3.8ns	3.0
TNU	1221.3∗∗	23.4	1442.7∗∗	1182.9∗∗	113.3∗∗	15.3ns	18.4ns	15.8ns	12.2
GPC	2.2∗∗	0.3	1.2∗∗	1.2∗∗	0.4ns	0.3ns	0.3ns	0.2ns	0.2
PUG	17.2∗∗	0.4	104.4∗∗	34.4∗∗	2.9∗∗	0.01ns	0.5ns	0.4ns	0.6
PUS	20.3∗∗	1.8	97.5∗∗	11.4∗∗	4.7∗∗	1.3ns	0.9ns	0.2ns	0.8
TPU	74.8∗∗	1.6	398.3∗∗	85.5∗∗	8.8∗∗	1.6ns	1.1ns	0.8ns	1.6
Parameters	L (1)	Rep in L (2)	P (3)	S (2)	P∗S (6)	L∗P (3)	L∗S (2)	L∗P∗S (6)	Error (46)
SUG	17.9∗∗	0.2	7.2∗∗	6.3∗∗	0.2ns	0.01ns	0.012ns	0.01ns	0.1
SUS	727.4∗∗	2.1	161∗∗	122.9∗∗	12.9∗∗	15.3∗∗	15.7∗∗	2.4ns	2.5
TSU	973.6∗∗	3.5	231.5∗∗	183.8∗∗	16.1∗∗	16.4∗∗	16.1∗∗	2.2ns	2.6
GPC	22.012	0.942	3.628∗∗	2.352∗∗	1.946∗∗	5.513∗∗	0.019ns	0.433ns	0.331
AES	54.7ns	721.5	2066.2∗∗	34406.7∗∗	1065∗∗	86.7ns	17.4ns	22.5ns	46
RES	4448.8∗∗	151.9	2941∗∗	22944∗∗	1635∗∗	305.3∗∗	1537.9∗∗	177.2∗∗	57.4
SUE	283.8∗∗	45.3	271.1∗∗	2316.2∗∗	167.5∗∗	17.9ns	103.1∗∗	9.6ns	7.7
GPC	22.012	0.942	3.628∗∗	2.352∗∗	1.946∗∗	5.513∗∗	0.019ns	0.433ns	0.331

BY = Biomass yield, GY = grain yield, STY = Straw yield, NUG = Nitrogen uptake by straw, NUS = Nitrogen uptake by, straw TNU = Total Nitrogen uptake, PUG = Phosphorus uptake by grain, PUS = Phosphorus uptake, by straw TPU = Total Phosphorus uptake, SUG = Sulfur uptake by grain, SUS = Sulfur uptake by straw, TSU = Total Sulfur uptake, AEP = agronomic efficiency of Phosphorus, REP = recovery efficiency of phosphorus, PUE = phosphorus utilization efficiency, AES = agronomic efficiency of Sulfur, RES = recovery efficiency of sulfur, SUE = sulfur utilization efficiency = GPC = grain protein.

Mean squares of combined analysis of variance for the effects of P and S level on different parameters of n class="Species">bread wheat.

The effects of P and S on nutrient uptake

Nitrogen uptake

n class="Chemical">Nitrogen uptake due to mainpan> effect of P and S as well as inpan>teraction of P and S rate inpan> grainpan>, straw was significantly (P < 0.01) differenpan>t (Table 6). Interaction effect of P with S significantly inpan>creased the uptake of N inpan> grainpan>, Straw and total (Figure 4). Combinpan>ed application of 22 kg P with 15 S kg ha−1 gave the highest N uptake (56.3 kg ha−1) inpan> grainpan> over lower rates and control, more uptake of N was observed inpan> grainpan> compared to straw. It might be due to higher concenpan>tration of N inpan> grainpan> compared to straw. Similarly, N uptake by straw inpan>creased with inpan>creasinpan>g combinpan>ed application P and S. However, the uptake N was not significantly differenpan>t beyond the application of 22 kg P with 15 kg S kg ha−1. The maximum total N uptake was observed with inpan>tegrated application of P and S at 22 kg P with 15 kg S ha−1. This might be due to the fact that favorable effect of P and S, on N absorption and biomass production of wheat.

Phosphorus uptake

n class="Chemical">Phosphorus uptake inpan> grainpan>, straw anpan>d total were significanpan>tly (p < 0.01) inpan>fluenpan>ced by mainpan> effects of various level of P anpan>d S anpan>d their inpan>teractionpan> (Table 6). pan> class="Chemical">Phosphorus uptake due to different levels of P and S application interactively in grain, straw and total P uptake presented in Figure 5. Combined application of P and S at 22P and 15S kg ha−1 gave the highest P uptake of 12.8 & 6.9 kg ha−1 in grain and straw respectively. However, it is statistically similar as the combined application of P and S increased above 22P with 15S kg ha−1. More uptake of P was noted in grain compared to straw. It also might be due to higher concentration of P in grain compared to P concentration in straw. In case of total P uptake, there was also maximum P uptake noted with interaction application of P and S at of 22 kg P and 15 kg S ha-1.

Sulfur uptake

n class="Chemical">Sulphur uptake was significantly (P < 0.01) inpan>fluenpan>ced by mainpan> effect of P and S inpan> wheat grainpan> and straw. Similarly, their inpan>teraction effect was significantly inpan>fluenpan>ced S uptake by grainpan> and straw and, total at P < 0.05, and P < 0.01 respectively (Table 6). Data regardinpan>g to S uptake by grainpan>, straw and total presenpan>ted inpan> Figure 6. The S uptake varied from 1.8 to 4.2 and 3.8–14.8 kg ha−1 inpan>, grainpan> and straw respectively due to inpan>teracted application of P and S. The highest S uptake (4.2 kg ha−1) inpan> grainpan> was obtainpan>ed with combinpan>ed application of P and S at 22 kg P and 15 kg S ha−1 followed by 44 kg P and 15 kg S ha−1 (4.1 kg ha−1), while, the lowest one (1.8 kg ha−1) was recorded inpan> treatmenpan>t that don't received any P and S fertilizers (control). The maximum S uptake (14.8 kg ha−1) inpan> straw was produced by combinpan>ed application of P and S, at 44 kg P and 15 kg S ha−1 and followed by 22 kg P and 15 kg S ha−1 (17.1 kg ha−1). Based on total S uptake, inpan>teracted application of P and S at 44 kg P and 15 kg S ha−1 gave the highest S uptake; but it was statistically similar with S uptake obtainpan>ed by application of P and S at 22 kg P and 15 kg S ha−1. Genpan>erally, inpan>teraction of differenpan>t P and S levels inpan>creased the uptake of S significantly over control.

The effects of P and S on P and S use efficiencies

Phosphorus use efficiency

Agronomic of Efficiency (AE) is an economic term which indicated that the direct production impact of an applied fertilizer on the production of crops. The AE of P on wheat was statistically significant (P < 0.01) due to different levels of P and S application (Table 6). Agronomic efficiency decreased as the rates of P application increased in all observed treatments. Data regarding to phosphorus agronomic efficienpan>cy presenpan>ted on Figure 7. The results revealed that lower AE of P was seenpan> at higher P rates and vice versa. The maximum AE of P (110.8 kg kg−1) was observed at 11 kg P combinpan>ed with 30 kg S ha−1 and it decreased significantly at higher P rates. This revealed that combinpan>ed application of phosphorus with sulfur enhanced agronomic efficiency of wheat over control and main effect of P. However, the least AE of P value (22.9 kg kg−1) was noted at maximum dosage of P (44 kg ha−1) without sulfur.

Apparent recovery efficiency (ARE) is another nutrient use efficiency index and commonly defined as the difference in nutrient uptake in above-ground parts of the plant between the fertilized and unfertilized crop relative to the quantity of nutrient applied. The present study revealed that ARE of P significantly (P < 0.01) varied in response to P, S and their interaction (Table 6). Data regarding to apparent recovery efficiency of P presented on Figure 7. The highest apparent recovery efficiency of 97.2% was obtained with application of 11 kg P with 30 kg S ha−1 and lowest one (28.0%) with application of 44 kg P ha−1 without S. Apparent recovery efficiency of P was considerably affected by S levels at all P levels. Increasing levels of P was decrease the ARE of P at given S level. With keeping P at 11 kg ha−1, ARE of P was significantly increased with increasing levels of S. Apparent recovery efficiency of P highly enhanced with the application of S because of the synergistic effect in between P and S at low levels of P.

Data on n class="Chemical">phosphorus utilization efficienpan>cy (PUE) of wheat are givenpan> inpan> Figure 7. The results revealed that lower PUE was seenpan> at higher P and S rates. The maximum PUE of 28.1 % was observed at 11 kg P ha−1 combinpan>ed with 15 kg ha−1 S and it decreased at higher P and S rates.

Sulfur use efficiency

Different indices of Sulfur Use Efficienpan>cy of the conducted experimenpan>t are givenpan> inpan> Figure 8. Sulfur Agronomic Efficiency (AE) was considerably affected by S levels at all P levels (Table 6). Increasing levels of S was decrease the S agronomic efficiency at given P level. As application levels of P increase up to 22 kg ha−1, significantly increased the agronomic Use Efficiency of S with keeping S at 15 kg ha−1. The maximum agronomic efficiency of S (96.7 kg kg−1)) was obtained in the plots receiving 22 kg P ha−1 in combination with 15 kg S ha− 1 followed by 95.3 (kg kg−1) in the plots receiving 44 kg P ha−1 in combination with 30 kg S ha−1. While minimum (28.4 (kg kg−1) was noted in the plots receiving only 30 kg S ha−1. Application of P and S at a rate of 22 P kg ha−1 with 15 S kg ha−1 and 44 P kg with 30 S kg ha−1 has statistically similar results. AE of S declined significantly when the level of S was raised from 15 to 30 kg S ha−1.

Figure 8

Effects of P and S, on Sulfur efficiency indices of wheat.

Effects of P and S, on n class="Chemical">Sulfur efficienpan>cy inpan>dices of wheat.

Apparent recovery efficiency (ARE) of S provides the quantity of nutrient uptake per unit of nutrient applied. The highest recovery efficiency (89.0%) of S was noted due to combined application of P and S at 44 kg P ha−1 with 15 kg S ha−1. However, the lowest recovery efficiency (20.9 %) was noted due to application of S at 30 kg ha−1 with 0 kg P ha−1.

n class="Chemical">Sulfur utilization efficienpan>cy (SUE) of wheat significantly affected by combinpan>ed application of P and S (Table 6). Data regardinpan>g to SUE of wheat presenpan>ted on Graph 8. The result revealed that inpan>creasinpan>g levels of P, inpan>creased SUE at a givenpan> level of S. But with inpan>creasinpan>g application rate of S from 15 to 30 kg ha−1 SUE become declinpan>ed. Maximum SUE (28.0 %) was noted inpan> the plots receivinpan>g 44 kg P ha−1 inpan> combinpan>ation with 15 kg S ha−1 followed by 27.7 % inpan> the plots receivinpan>g 22 kg P ha−1 inpan> combinpan>ation with 15 kg S ha−1 while minpan>imum (7.4 %) was noted inpan> the plots receivinpan>g 30 kg S ha−1 without P.

Effects of P and S on grain protein content (GPC) of wheat

Protein content due to various single P, S and their interaction showed highly significant (P < 0.01) difference (Table 6). The protein content affected by single P, S and their interaction as presented in Table 2. Application of S at 15 and 30 kg ha−1 increased the grain protein content of wheat by 4.3 and 7.4% over control respectively. This indicates that application of different S levels increased grain protein content of wheat over control. Similarly, grain protein content was varied with P levels. The highest protein content (9.6%) obtained from treatment that received P at 44 kg ha−1. The second highest protein content (9.5%) was recorded at 22 kg P ha−1 but statistically similar with 44 kg P ha−1. This implies that application of P beyond 22 kg ha−1 not significantly increased grain protein content. In contrast the lowest protein grain content obtained in control.

The data on grain protein content influenced by P and S interaction are presented in Table 2. The combined application of 22 kg P ha-1 with 15 kg S ha−1 produced maximum grainpan> proteinpan> contenpan>t (9.8%). This treatmenpan>t has a grainpan> proteinpan> contenpan>t inpan>cremenpan>t of 20.4% over control treatmenpan>t. The minpan>imum grainpan> proteinpan> contenpan>t (7.8%) was recorded inpan> control plot. This implies that combinpan>ed application of P with S improves grainpan> proteinpan> contenpan>t of bread wheat than individual application of P and S, this showed synergistic effect of P and S on each other as both the nutrients mutually helps in their absorption and utilization to give better grain protein content of wheat.

Partial budget analysis

Partial budget analysis allows assessing the impact of a change in the production system on a farmer's net income without knowing all his costs of production. The data related to partial budget analysis is given in Table 7. The maximum net benefit (45351.7 ET Birr) was obtained from combined application of P and S at the rate of 22 kg P and 15 kg S ha−1 and followed by rate of 44 kg P and 15 kg S ha−1 (44128.3 ET Birr.). Minimum net benefit (25697.4 ET Birr.) was recorded in control (treatment that did not receive any S and P fertilizers).

Table 7

Partial budget Analysis for Phosphorus and Sulfur fertilizers Studied area.

Treatment	Adj. GY	Adj. STY	GBGY	GBSTY	TVC	TGB	NB	MRR
S0P0	2076.7	2385.6	21805.6	5558.5	1666.6	27364.0	25697.4	D
S0P11	2941.5	3554.9	30885.9	8282.8	2120.2	39168.7	37048.5	25.0
S0P22	3016.0	3754.7	31667.7	8748.5	2573.7	40416.1	37842.4	1.8
S15P0	2722.9	3666.7	28590.3	8543.5	2706.4	37133.8	34427.5	D
S15P11	3094.8	4233.5	32495.0	9864.1	3148.0	42359.1	39211.1	10.8
S0P44	3082.3	4129.7	32363.8	9622.3	3484.7	41986.1	38501.4	D
S15P22	3526.8	5120.0	37031.6	11929.5	3609.4	48961.2	45351.7	54.9
S30P0	2927.4	3831.6	30737.7	8927.7	3748.6	39665.4	35916.7	D
S30P11	3295.7	4576.9	34604.9	10664.2	4190.3	45269.1	41078.8	D
S15P44	3505.6	5081.6	36808.5	11840.2	4520.4	48648.7	44128.3	D
S30P22	3356.0	4894.2	35237.7	11403.5	4651.7	46641.2	41989.5	D
S30P44	3384.4	4860.6	35535.8	11325.1	5558.9	46860.9	41302.0	D

Adj.GY = Adjusted grain yield (kg ha−1), Adj. STY = Adjusted Straw yield (kg ha−1) TGB = Total growth benefit, TVC = Total variable cost, NB = Net benefit, MC = marginal cost, MB = marginal benefit, MRR = marginal rate of return, P0 = 0 kg ha−1P, P11 = kg ha-111P, P22 = 22 kg ha−1P, P44 = 44 kg ha−1 phosphorus, S0 = 0 kg S ha-1, S15 = 15 kg S ha-1, S30 = 30 kg S ha-1.

Partial budget Analysis for n class="Chemical">Phosphorus and n class="Chemical">Sulfur fertilizers Studied area.

Depend on dominancy analysis, treatments of S and P at 15 kg S without P, 44 kg P without S, 30 kg S without P, 30 kg S with 22 kg P, 15 kg S with 44 kg P, 30 kg S with 44 kg P, 30 kg S with 11 kg P ha−1 and control were dominated by the rest four treatments and they were also excluded from further economic analysis. The data regarding the marginal rate of return (MRR) revealed that maximum MRR (5490%) was obtained when P and S was applied at the combined rate of 15 kg S and 22 kg P ha−1 followed by rate of 11 kg P ha−1 without S (2502%). Minimum MRR was (175%) recorded in treatment where application of P at 22 kg ha−1 without S. Data from Table 7 clearly revealed that the non-dominated treatments assn class="Chemical">ociated with MRR are greater than 100%. This implies that the four non-dominpan>ated treatmenpan>ts are economically feasible alternative to the other dominpan>ated treatmenpan>ts. The marginpan>al rate of return, 5490 % means the producer obtainpan>ed an additional inpan>come of 54.9 Ethiopian birr per a unite cost they have inpan>vested. Genpan>erally, treatmenpan>t combinpan>ation of S and P at 15 kg S and 22 kg P ha−1gave better MRR value relative to the other four non-dominpan>ated treatmenpan>ts and profitability can be optimized by usinpan>g this treatmenpan>t.

Discussions

Nutrient interaction in crop is probably one of the most important factors affecting yields of annual crops (Fageria, 2014). Soil nutrients interaction affects their availability to crops as on overabundance one may result in deficiency of another nutrients (Karimizarchi et al., 2014). Application of P fertilizers results in increased of anion adsorption sites by phosphate which releases sulfate ions into the soil solution and phosphate ion is more strongly bound than sulfate (Tiwari and Gupta, 2006). Different combination of P and S revealed significant influence on grain and straw yield of wheat. In the present study the synergism and antagonism relationship between P and S fertilizer levels on wheat yield was observed. Synergism relationship was observed where treatment receiving up to 22 kg P with 15 kg S ha−1. Interacted application of P and S at 22 P and 15 S kg ha−1 increased grain and straw yield of wheat by 40.1 and 53.4 % respectively over control. The beneficial synergism relationship reported by Randhawa and Arora (2000) and Phogat et al. (2018) where P and S interaction significantly improved grain and straw yield of wheat. It was also observed from another studies that significantly the maximum grain yield was produced when wheat crop was fertilized with 90 kg P2O5 ha−1 with 40 kg S ha−1 (Bairwa et al., 2018). The authors (Hussain and Leitch, 2005) who stated that grain yield improved with phosphorus use and those plots receiving 90 kg P ha−1 gave maximum grain yield as compared to lower dose. Furthermore, the authors (Khan et al., 2006) reported that 43% raise in grain yield with the addition of 90 kg P and 60 kg ha−1 S. In addition, combined application of P and S at 50 kg P2O5 and 20 kg S ha−1 improve yield of wheat over control (Sandeep et al., 2017). Similarly, the interaction of P and S levels increased their beneficial synergistic effect and applying of the individual S level with P level produced increases in the grain yield of wheat (Ibrahim et al., 2012). The variation of P and S levels may affect yields in different field trials in the diver's location of the soils. Antagonism relationship was also observed in the present study at higher rates of P and S fertilizer. Findings from this study shows that application of P and S on vertisols beyond 22 kg P with 15 kg S ha−1 result in reduction of wheat yield. Interactive application of P and S at higher rate (44P + 30 S kg ha−1) showed reduction in grain and straw yield of wheat. Similarly, Islam (2006) who reported that application of P and S at higher rate (35.2 kg P and 40 kg S ha−1) showed reduction in wheat yields.

Interaction among plant nutrients can yield antagonistic or synergistic outcomes that influence nutrient uptake and use efficiencies (Rietra et al., 2017). Results from the present study revealed that, interaction of P with S significantly increased the uptake of N in grain and Straw. Combined application of 22 kg P with 15 S kg ha−1 gave the highest N uptake by grain over lower rates and control. Previous study has showed that the application of P and S improved the N uptake (Singh et al., 1999). While, the uptake of N showed a decreasing at higher rates of P and S (44P and 30 S kg ha−1). This implies that, beyond 22 kg P and 15 kg S ha−1 failed to increase in total N uptake and it was declined which might be due to inhibition effect of higher doses of S on N uptake (Dwivedi and Bapat, 1998).

In the present study, the P uptake in grain and straw significantly improved with combined application of P and S fertilizer. The uptake of P lower at higher P and S application and vice versa. Application of P and S cause significant increase in total P uptake up to 22 kg P and 15 kg S ha−1. The interaction of both P and S produced significant steady increases in P uptakes by both straw and grains (Ezzat, 2016; Teotia et al., 2000). In the current finding, the uptake of P showed a decreasing trend at higher rates of P and S (44P and 30 S kg ha−1). This probably due to competition between these two anions for adsorption sites on soil colloids at higher levels of S application (Bapat et al., 1986). The interaction between P and S on wheat yield has also reported by Randhawa and Arora (2000) who revealed that highly significant positive interaction was obtained between P and S in terms of total P uptake at lower rates of S application and higher rates of S resulted in a decreased total P uptake. The synergistic effect of P and S on each other as both the nutrients mutually helps in their absorption and utilization probably due to balanced nutrition (Mamta et al., 2017). Similarly, Mandai et al. reported that P and S interaction is synergistic at low to medium level of P and antagonistic only at higher levels. Significantly, positive interaction was obtained between P and S in terms of P uptake at lower rates of application and at higher combinations reduced P uptake (Islam et al., 2006; Randhawa and Arora, 2000; Deo and Khaldelwal, 2009).

The data on the application of P and S levels revealed a significant effect on S uptake in grain and straw of wheat (Table 6). Interaction of different P and S levels increased the uptake of S and more S accumulation noted on in straw than grain (Figure 6). In the current finding, total S uptake improved with application of P and S at 44 kg P and 15 kg S ha−1; but it was statistically similar with S uptake obtained by application of 22 kg P with 15 kg S ha-1. Similarly, previous study by Dash et al. (2015) reported that, accumulation of S, inpan> straw was higher than grainpan>. In addition, other finpan>dinpan>g reported by Venpan>katesh et al. (2002) who found that P application with respective S inpan>creased the S uptake by which seemed to have inpan>duced better root developmenpan>t as well as inpan>creased activity of S-oxidizinpan>g bacteria. The inpan>creasinpan>g does of P and S to the higher does (44 kg P and 30 kg S ha−1) did not differ statistically with that of 22 kg P and 15 kg S ha−1 and but showed decreasinpan>g trenpan>d numerically at higher rates of P and S. Similarly, inpan> other finpan>dinpan>g reported by Randhawa and Arora (2000) showed that the uptake of P or S inpan>creased at lower levels of counter ion but decreased at higher rates. Results are inpan> conformity with those of Rahim et al. (2010) and Sandana and Pinpan>ochet (2014) who found that at higher P application rates decreased P or S utilization efficiency. In addition, Yashbir et al. (2014) who found that the maximum agronomic efficiency noticed on lower rate (15 kg ha−1) of sulfur and vis versa and crop recovery efficiency declined as the level of S application was increased from 15 to 45 kg S ha−1; it declined from 41.3% at 15 kg ha−1 to 29.8% at 45 kg S ha−1. In the present experiment agronomic efficiency of S declined significantly when the level of S was raised from 15 to 30 kg S ha−1.

Data regarding on grain protein content of wheat as affected by various P and S levels are shown in Table 2. Application of P with S improve grain protein content of bread wheat. Jarvan et al. (2008) who reported that inpan>crease inpan> proteinpan> contenpan>t of wheat due to maximum supply of S. In currenpan>t experimenpan>t, combinpan>ed application of P and S improve grainpan> proteinpan> contenpan>t of bread wheat than individual application of P and S, this showed synergistic effect of P and S on each other as both the nutrients mutually helps in their absorption and utilization to give better grain protein content of wheat. The combined application of 22 kg P ha-1 with 15 kg S ha−1 produced maximum grain protein content and has an advantage of 20.4% over control treatment. While, in other study reported by Ibrahim et al. (2012) interaction of P and S showed no significant effect on grain protein content of wheat.

Conclusions

Nutrient mining due to sub optimal fertilizer use in one hand and unbalanced fertilizer uses on other have favored the emergence of multi nutrient deficiency in Ethiopian soils. For the last five decades, Ethiopian agriculture depended solely on imported fertilizer products, only urea and di-ammonium phosphate (DAP), sources of N and P. However, recently it is perceived that the production of such high protein cereals like wheat can be limited by the deficiency of S and other nutrients. Sulfur is a nutrient most overlooked in Ethiopian agriculture and major prone areas of S deficiency are the central highlands. The results of this experiment revealed that application of P and S fertilizer has significantly increased yield, nutrient uptake and nutrient use efficiencies of wheat compared to unfertilized control plot, indicating insufficient soil P and S content for optimum production of wheat. Combined application of P and S produced significantly higher yield of wheat than that obtained with single application of S or P indicating synergistic interaction between these nutrients. But this was true for treatments involving combined applications low level of S and increasing levels of P or vice versa. Otherwise, there was decreasing trend of yield gains with treatments involving at higher levels of both nutrients. In all cases, optimum grain yield of wheat was obtained with treatment involving at 22 P + 15 S kg ha−1. The maximum grain N, P and S uptakes were obtained due to P and S application at rate of 22 P and 15 S kg ha−1. Phosphorus and Sulphur use efficiency indices were also improved significantly by combined application of P and S. While, partial budget analysis result revealed that, combination of 22P and 15S kg ha−1 produced the highest MMR (54.9 %) and thus, this treatment is found to be economically feasible treatment for bread wheat production in study area of the district. Based on the results obtained, therefore, P and S fertilization is necessary in the study district and 22 kg P combined with 15 kg S ha−1 produced the highest yield of wheat and thus, this treatment is found to be recommended for bread wheat production in Vertisols of the district. We recommend further experiments on different combination of P with S in different agro-ecologies and soil types are required for confirmation of results and the residual effect of P and S on the following crop is needed to study the long-term effect of P and S.

Declarations

Author contribution statement

Shawl Assefa: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper.

Wassie Haile; Wondwosen Tena: Conceived and designed the experiments; Analyzed and interpreted the data; Wrote the paper.

Funding statement

This work was supported by the Debre Birhan Agricultural Research Center (DBARC), Ethiopia.

Data availability statement

Data will be made available on request.

Declaration of interests statement

The authors declare no conflict of interest.

Additional information

No additional information is available for this paper.