Response of Maize (Zea mays L.) to Blend (Nitrogen, Phosphorus, Sulfur, Boron) and Potassium Fertilizers Rates at Boloso Sore Woreda, Ethiopia

In the study area, maize accounts for about 30.5% of the area from cereal crops and about 7,368.1 ton of maize grain yield is produced on 2,532 ha with a productivity of 2.91 t ha^-1 (BSWAO, 2020). Maize (Zea mays L.) is also the most widely cultivated cereal crop, covering 16.08% of the area and 25.81% of the production  (CSA, 2016). However, the national and average results of maize for smallholder farmers is 3 t ha^-1 (CSA, 2016) which is less than from world’s average yield  (5.6 t ha^-1) (FAOSTAT, 2017).
       
The above data showed that the average maize yield is less than the potential yield (15.8 t ha^-1) of maize (Van et al., 2013). Among several production constraints, low soil fertility, mainly nutrient deficiency, inadequate soil management practices (Tolera, 2016) and the application of a low rate of organic fertilizers (Sanchez, 2002) are responsible for this low productivity of maize.
       
 
According to the agricultural transformation agency of Ethiopia (ATA)  (2016), elements like N, P, K, S, Zn, B and Cu are deficient in major Ethiopia soils. Besides this, recent studies conducted by Mesfin et al., (2021) indicated that OC, TN and available P and K were low in cropland in different areas of the Wolaita Zone. Blended fertilizers contain balanced nutrients with recommended amounts of macro and micronutrients to produce significantly higher yields in cereal crops than the blanket recommendation of DA .

 Different studies showed that the application of newly introduced blended fertilizers had a significant difference in maize crops in various regions of Ethiopia. Mulisa (2019) reported that using fertilizers at a specific rate could improve maize production. However, in the wolaita zone, farmers are currently using 100 kg ha^-1 NPSB and 100 kg ha^-1 urea without K for maize production as a blanket recommendation, which may or may not be sufficient to meet the crop requirement. There is no information found on site-specific recommended rates of blended (Nitrogen, Phosphorus, Sulfur, Boron) and Potassium for maize. Therefore, this experiment was conducted to evaluate the response of maize in growth, yield and yield components to blended  fertlizers.

The current research was conducted at Achura Kebele Farmers‘ Training Center (FTC), Boloso Sore Woreda, Wolaita Zone and Southern Ethiopia (Fig 1) from December  2020 to June 2021. Random surface soil samples were collected at a depth of 0-30 cm using an auger sample  before sowing. The collected soil samples were air-dried and grounded.
 
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Soil texture was analyzed using the hydrometer method (Sahlemedhin and Taye, 2000). The soil pH value was measured potentiometrically using a pH meter with a combined glass electrode in a 1: 2.5 soil: water ratio (Carter 1993). The soil organic carbon was determined following the wet combustion method of Walkley and Black as outlined by (Sahlemedhin and Taye 2000). The total soil nitrogen was analyzed using the Kjeldahl digestion and distillation method (Jackson, 2003). The available phosphorous was determined by Bray and Kurz (1945). Available boron: was determined by using a Spectrophotometer method. The cation exchange capacity (CEC) of the soil was determined by the 1M ammonium acetate pH-7 method (Van Reeuwijk, 1993). The soil’s exchangeable bases (K) were determined from the leachate of 1 molar ammonium acetate solution at pH 7 and were read using a flame photometer (Rowell, 1994).
       
Chemical fertilizers and urea were used as experimental materials, whereas KCl and urea were used as a source of Potassium and nitrogen, respectively. Hybrid (BH540) maize was used. The treatments consisted of a factorial combination of five rates of NPSB, (N,18.9%, P₂O₅ 37.7%, S, 6.95%, B,0.1%), fertilizer (0, 50, 100, 150 and 200 kg ha^-1), with four rates of KCl, (0-0-60 ) (0, 47.76, 95.5 and 143.25) kg ha^-1  which, contained totally 20 treatments.
       
The treatments were laid out in a randomized complete block design (RCBD) with three replications, 3 x 20 = 60 treatments and they were randomly assigned to each experimental plot (Table 1). The gross size of each field was 3 m x 3 m (9 m²), consisting of four rows and with a space of 0.75 m x 0. 3 m between row and maize plants,  respectively.
       

 
To measure ear diameter (cm), about five ears were taken randomly from the net plot area and then their diameters were measured at the middle of the ear with a caliper ruler. The mean value was recorded as ear diameter. A thousand kernel weight (g) was measured by taking about 1,000 kernels randomly and counting by using a kernel counter from the bulk of threshed kernels in each net plot. The area was then weighed using a sensitive balance level. Harvest Index (%) HI was recorded as the ratio of grain yield to total above-ground biomass yield and multiplied by 100% at harvest in each plot.
   
Where;
TAGBY = Total above-ground biomass yield.
       
The mean separation was carried out using the least significant difference (LSD) test at a 5% level of significance when the variance analysis indicated substantial differences busing SAS  software 9.2 versions (SAS, 2008).

Pre-sowing soil physio-chemical properties

The soil of the study site has a proportion of 43% sand, 28% silt and 29% clay (Table 2). Thus, according to Bouyoucos’s (1962) classification, its texture was sandy clay loam. Soil reaction was moderately acidic soil with a pH value of 5.84 (Table 2) at the surface and suitable for maize production (EthioSIS, 2013). This result is in line with, Shimeles (2006), who suggested that the lower pH values at the surface soil could be due to the seasonal soil.



The available phosphorus content of the soil in the study area was low (Bray and Kurz 1945) (8.42 mg/kg) and needs phosphorous fertilizer. This may be due to the high P fixation of the soil, complete removal of crop residues and low input of organic amendments to the cultivated land in the study area. Thus, the soil indicates a crop response to P fertilizers to obtain optimum maize production. The result is in line with (Mesfin et al., 2021), who indicated the most cultivated land in the region showed low available soil phosphorus.

Soil organic carbon content was 1.14% having an organic matter content of 1.97% (with a conversion factor of 1.724. According to Tekalign, (1991), the composite sample could be rated as low. Soil organic matter content of the composite surface soil sample from the experimental area was found to be 1.97% (Table 2) and the rating is classified as low. This is may be due to the complete removal of crop residues and low input of organic amendments like compost to the cultivated land. The result is in line with (Tuma 2007), who indicated that intensive cultivation aggravates OM oxidation and reduces OC content. Regarding this fact, proper soil management practices such as adding crop residuals and compost were one the crucial practices to solve the problem.

The total nitrogen content of the experiment area before sowing was  0.19% (Table 2). According to EthioSIS (2013), TN content was low. The low total N contents may be associated with factors that cause a deficiency in organic matter. This indicates that the study area’s soils are deficient.

The CEC of the site was 11.72 cmol kg^-1 (Table 2). According to the result obtained from the soil laboratory, the value of CEC was in the low range (Landon, 1991). This can be associated with the low OM content and the role of clay minerals which attract or adsorb many cations from the soil. This finding agrees with Kedir, (2015), who reported the variation in CEC is due to variation in OM.

The content of available potassium contents of the composite sample had 112.8  ppm (Table 2), indicating the soil is deficient in available potassium content (Horneck et al., 2011). The result is similar to (Mesfin et al., 2021), who indicated in their study, that cultivated land in Wolaita Zone was low in available potassium content. Additionally, the result agrees with (EthioSIS, 2013), which recommended potash fertilizer for the study area and contradicted with (Murphy 1968) which stated that Ethiopian soils are rich in K and hence no need for K application.

The available sulfur content of the experimental area was 18.09 (ppm) (Table 2) and the soil has been rated as low. So the addition of fertilizer that contains S is relevant. This low content of S may be due to loss of OM and continuous cultivation, which results in intensive mining of S from the soil. Available Boron in the soil was 0.50 mg kg^-1 (Table 2). According to Horneck et al., (2011), soil availability B content was rated as low, suggesting the application of fertilizer containing B. Intensive cultivation in the area was responsible for the low B content of the soil.

Plant height

As shown in Table 3, the interaction of NPSB and K fertilizer significantly (p£0.05) affected the maize height. The maximum plant height (314.10 cm) was obtained from the application of 150 kg NPSB + 50 kg K kg ha^-1, while the minimum plant height (178.40 cm) was recorded from control treatments. According to this study, the combined application of NPSB fertilizer with K has significantly increased the plant height compared to the control. This indicated that as early discussed; the soil laboratory result showed that the soil of the study area was low in the amount of available P and K nutrients.



Leaf area and leaf area index

Maize leaf area and leaf area index was significantly (P<0.05) affected by the main effect and their interaction of NPSB and K fertilizer application. However, the maximum (8783.8 cm²) leaf area and (3.90) leaf index were recorded from 200 kg NPSB + 75 kg K ha. In comparison, the minimum (4650.1 cm²) leaf area and (2.07) leaf area index were recorded from the control. Moreover, the application of NPSB with K had a statistical difference compared to the same used rate of NPSB without K application. For example, the application of NPSB, 200 kg with K, 70 kg/ha was, increased leaf area by (33.21%) and leaf area index by (33.10%) as compared to the same used rate of NPSB at 200 kg ha without K (Table 3). This could be related to applying adequate blended NPSB with K fertilizers as proven macro and micronutrients to the plant, which encouraged cell elongation and more vegetative growth of maize.

Ear length and diameter

The result of this study indicated that there was a significant (P<0.05) variation in main and interaction effects on ear length and diameter in response to applied blended NPSB and K fertilizer in the study area (Table 3). Thus, the longest (24.20 cm) and (6.4 cm) ear length and ear diameter were recorded from the application of (200 kg NPSB + 75 kg K ha^-1),  respectively (Table 3). The variation in ear length and diameter might be related to the applied balanced nutrient content and the role of Potassium in the utilization of this nutrient.

Number of kernels per ear

There was a significant variation at (P<0.05) in the Number of kernels per ear among treatments. Thus, the highest average number of kernels per ear (469.00) was recorded from the combined application of NPSB with K at a rate of (200 and 50 kg ha^-1) followed by (200 and 75 kg ha^-1) and (150 and 75 kg ha^-1) while, the lowest (155.90) was obtained from control (Table 4). However, the result implied that the application of balanced fertilizer NPSB with K had significant deference from NPSB without K application treatments. For instance, NPSB 200 kg ha with K 75 kg ha application increased maize number of kernels per ear by 54% compared to the same rates of NPSB 200 kg ha^-1 without K (Table 4). This could be related to the effect of applied nutrient content in (NPSB and K) compared to without K. Moreover, Potassium plays multiple functions in plant growth and development (Sultana et al., 2015).



Thousand-grain weight

The output of the field experiment indicated that there were significant differences (P<0.05) in thousand-grain weight in response to the combined application of NPSB with K fertilizer application (Table 4). The maximum (539.80 gm) grain weight was obtained from the application of (NPSB, 200 kg ha^-1 with K, 50 kg ha^-1), while the minimum value was recorded from the control (Table 4). This may be related to the role of used potassium fertilizer in balanced NPSB. It ensures the utilization of applied nutrients and improves seed quality by activating the work of enzymes.

This output is similar to the findings of Mulisa (2019), the highest (425.31 gm) thousand-grain weights was recorded from the application of 250 NPS + 150 urea+ 100 KCl kg ha^-1 while the lower (300.78 gm) thousand-grain weights were recorded from the control treatment.

Above-ground dry biomass yield

The analysis of variance in this study revealed that there were highly significant (P<0.0170) differences among fertilizer rates on biological yield). The maximum amount of biological yield (17838 kg ha^-1) was obtained under the application of 200 NPSB + 100 kg urea + 50 K kg ha^-1 and the minimum biological yield (8415 kg ha^-1) was obtained from the control (Table 4). Besides, balanced fertilizer NPSB and K-applied treatments had statistically significant differences from only NPSB without K-application treatments. For example, NPSB 200 kg ha^-1 with K 75 kg ha^-1 increased maize biological yield by 15.47% compared to the same used rates of NPSB at 200 kg ha^-1 without K applied treatment (Table 4).

Grain yield

There were significant differences at (P<0.05) in grain yield in response to the combined application of NPSB and fertilizer application in the study area (Table 4). The highest grain yield (7200.9 kg ha^-1) was obtained in the application of 200 kg NPSB ha^-1 with 50 kg K ha^-1, whereas the lowest (2111.0 kg ha^-1) grain yield was obtained from the control treatment (Table 4). Therefore, this response could be due to an adequate supply of N, P, K, S and B application in the form of NPSB and KCl and the role of potassium fertilizer, which might have played an essential role in the overall function of plant growth. Supporting this, before sowing soil surface sample analysis, the study area was low in soil fertility status and available K had found at a low level. In conformity with this result, (Mulisa, 2019) reported that the maximum amount of biological yield (18521.50 kg ha^-1) was obtained under the application of 250 NPSB + 100 kg urea + 100 KCl kg ha^-1 and the minimum biological yield (7675.4 kg ha^-1) was obtained from the control treatment. Similarly, Dagne (2016) confirmed that blended fertilizer application significantly increased maize biological yield compared to control and recommended NP.

This result is supported by Mulisa (2019), who reported that the highest harvest index (47%) was recorded from the application of 250 NPSB + 100 urea + 100 KCl kg ha^-1 and the lowest (38%) was received from the control treatment.

Harvest Index

Harvest index was highly significantly (P<0.0170) influenced by the combined application of blended NPSB with K (Table 4). According to the interaction effect, the highest value of HI (42.97%) was obtained due to 200 kg blended NPSB ha^-1 combined with 50 kg K ha^-1, whereas the lowest (25.13%) was obtained from the control treatment (Table 4).

Therefore, this study concluded that most soil’s physical and chemical properties were found at low levels at the surface and maize growth, yield and yield components were significantly affected by the combined application of blended NPSB and KCl fertilizers at the study site. Thus, treating the soil with organic amendments such as compost and bio-fertilizers and adding residuals improve the soil fertility for better crop production.  

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