Effect of Different Levels of Zinc and Compost on Yield and Yield Components of Wheat

Abstract

Management of organic matter and micronutrients is very important for the sustainable improvement of soil health. Poor soil organic matter usually results in lower availability of zinc (Zn) micronutrients in plants. Such deficiency in Zn causes a significant decrease in the growth and yield of crops. The need at the current time is to balance the application of organic amendments with Zn micronutrients to achieve optimum crop yields. Thus, the current study was conducted to investigate wheat, using compost as organic matter and Zn as a micronutrient. There were three levels of compost (i.e., control (0C), 5 t/ha (5C) and 10 t/ha (10C)) and four levels of Zn (control (0Zn), 2.5 kg Zn/ha (2.5Zn), 5.0 kg Zn/ha (5.0Zn) and 10.0 kg Zn/ha (10.0Zn)) applied with three replicates. The addition of 10C under 10.0Zn produced significantly better results for the maximum enhancement in plant height (8.08%), tillers/m² (21.61%), spikes/m² (22.33%) and spike length (40.50%) compared to 0C. Significant enhancements in 1000-grain weight, biological yield and grain yield also validated the effectiveness of 10C under 10.0Zn compared to 0C. In conclusion, application of 10C with 10.0Zn showed the potential to improve wheat growth and yield attributes. The addition of 10C with 10.0Zn also regulated soil mineral N, total soil N and extractable soil P. Further investigation is recommended with different soil textures to verify 10C with 10.0Zn as the best amendment for the enhancement of wheat yield in poor organic matter and Zn-deficient soils.

1. Introduction

In developing areas of the world, micronutrient deficiency is creating a situation of malnutrition. Approximately two billon people around the globe suffer from zinc (Zn) deficiency [1,2,3]. Severe Zn deficiency in humans adversely affects the development of bones, immunity, skin, the brain and reproduction [1,2,3]. This deficiency in humans is directly associated with the minimum uptake of Zn in the plants that are consumed as food by humans [4]. Plants generally require Zn in small quantities. In different soils, it can range from the lowest values of 10 ppm to a maximum of 1000 ppm [5]. Zn is a major requirement for optimum plant development and maturity because of its role in the formation of structural components and growth hormones [6]. Furthermore, it plays a critical role in enzyme activation [7]. Zinc is indirectly and directly involved in protein and starch formation, nucleic acid metabolism and pollination [8,9,10]. It additionally regulates photosynthesis, sugar transformation, flowering and grain formation [11]. Zn deficiency affects fertilization in plants. It causes alterations in the functioning of stigma and pollen grains, thus affecting pollen viability [12]. A balanced Zn uptake provides resistance for plants, helping them survive under adverse conditions [13]. However, poor soil organic matter can minimize the uptake of zinc in plants [14]. Similarly, calcareousness, high pH, excessive amounts of salts and unbalanced application of fertilizers in soil also act as associated factors causing Zn deficiency in plants [15].

To overcome these issues, most scientists suggest utilizing inorganic fertilizers in combination with organic amendments [16,17,18]. Compost is one such organic amendment [19]. It is prepared from soil organic material, such as plant residues, household waste, industrial waste and sewage sludge [20]. When compost is applied as an organic fertilizer in soil, it plays an imperative role in the improvement of soil fertility [21]. Compost is also a rich source of nutrition for plants and has high organic matter content [22]. The characteristics of compost depend on the material from which it has been produced. However, most compost is rich in nutrients and organic carbon compared to inorganic fertilizers [23,24,25]. It has also been observed that the stability of biological compounds is enhanced when a material is subjected to the composting process. In microbial decomposition, these biological compounds provide a shared contribution towards soil humic content accumulation [26].

As cereals are considered an important source to address Zn deficiency in humans, wheat (Triticum aestivum L.) is widely consumed as a staple food in many areas of the world [27]. It serves as basic nutrition for approximately one third of the global population. As a nutrition source, wheat provides carbohydrates, protein and other essential nutrients for human beings. However, limited uptake of micronutrients, especially zinc (Zn), is a major yield-restricting factor for wheat [27]. It has been observed that Zn concentration always remains lower than 10–15 mg/kg in wheat cultivated in Zn-deficient areas; i.e., India, Australia, Turkey and some parts of China [9].

Hence, the current study was conducted with the aim of exploring the effect of the combined application of inorganic Zn fertilizer with variable levels of compost. The experiment was undertaken to fill the knowledge gap regarding compost application amount synchronization with different levels of Zn. The novel aspect of the study is the selection of the best application amount combination for Zn and compost to ensure better wheat production. This research is also novel in terms of the suggested improvement to current practices regarding the application amounts of Zn and compost. It is hypothesized that the combined use of Zn with compost is a more effective approach to achieving better wheat growth and yield than their separate addition to the soil as amendments.

2. Materials and Methods

2.1. Experimental Site and Design

A field trial entitled “Effect of different levels of zinc and compost on yield components of wheat” was carried out at the Swabi Agricultural Research Station (ARS) during the Rabi season in 2015 and 2016. The experimental study was designed in randomized complete blocks (RCBs) and replicated three times.

2.2. Plot Dimensions and Wheat Variety

A plot size of 3 × 2.5 m was used. The wheat variety Shahkar-2013 was planted. The seed rate was 120 kg ha⁻¹ and sowing was undertaken in 3 m long rows 30 cm apart. A total of eight rows were present in the subplots.

2.3. Fertilizer and Compost

Recommended amounts of nitrogen, phosphorous and potassium (NPK) (120:90:60 kg ha⁻¹) were applied to each plot, with some modification to the P as per the cropping area [28]. Urea, diammonium phosphate (DAP), compost and sulphate of potash (SOP) were used as sources of nitrogen, phosphorous, potassium, and compost, respectively. Compost was purchased from a certified local company. The application of compost was undertaken at the time of field preparation as per the treatment plan application amounts; i.e., 0 (0C), 5 (5C) and 10 t/ha (10C).

2.4. Irrigation

The plants were irrigated (a total of 16 acre-inches) with the following schedule, as described by Zafar-ul-Hye et al. [29]:

• First = crown root initiation (4 acre-inches);
• Second = tillering stage (4 acre-inches);
• Third = heading stage (4 acre-inches);
• Fourth = milky stage/soft dough (4 acre-inches).

2.5. Treatment Plan

There were two factors in the treatment plan; i.e., different zinc (0 (0Zn), 2.5 (2.5Zn), 5.0 (5.0Zn) and 10.0 kg/ha (10.0Zn)) and compost (0 (0C), 5 (5C) and 10 t/ha (10C)) application amounts. The treatments (

Table 1. Treatment plan.

Treatment	T	Abbreviation
Control (no compost + no zinc)	T1	0C + 0Zn
5 t/ha compost	T2	5C
10 t/ha compost	T3	10C
2.5 kg/ha zinc	T4	2.5Zn
2.5 kg/ha zinc + 5 t/ha compost	T5	2.5Zn + 5C
2.5 kg/ha zinc + 10 t/ha compost	T6	2.5Zn + 10C
5.0 kg/ha zinc	T7	5.0Zn
5.0 kg/ha zinc + 5 t/ha compost	T8	5.0Zn + 5C
5.0 kg/ha zinc + 10 t/ha compost	T9	5.0Zn + 10C
10.0 kg/ha zinc	T10	10.0Zn
10.0 kg/ha zinc + 5 t/ha compost	T11	10.0Zn + 5C
10.0 kg/ha zinc + 10 t/ha compost	T12	10.0Zn + 10C

) included combinations.

2.6. Soil Characterization

Before sowing the seeds, four composite soil samples were taken from various parts of the field at 0 to 10 cm depths and categorized into ten levels. The samples were then passed through a sieve to keep them free of litter and roots [30]. Chemical and physical properties of the soil are provided in

Table 2. Characterization of soil and compost.

Pre-Experimental Soil				Compost
Properties	Unit	Values	References	Properties	Unit	Values	References
Sand	%	24.5	[31]	N	%	2	[32]
Silt	%	10.5		P	ppm	0.8	[33,34]
Clay	%	65		K	ppm	2	[33,35]
Soil texture	-	Silt loam		OM	%	25	[36]
pH	-	7.66	[37]	CEC	60	cmolc/kg	[38]
EC	dS m−1	0.0375	[39]	C:N	20:1	-	[32,40]
Organic matter	%	0.1957	[36]
Total nitrogen	mg kg−1	1949.09	[32]
Mineral nitrogen	mg kg−1	18.17	[32]
Phosphorus	mg kg−1	4.05	[41] [42]
Zinc	mg kg−1	0.25

.

2.7. Harvesting and Data Collection

Plants were harvested manually at the time of maturity. Plant height was determined at the maturity stage with 10 randomly selected plants. Data were collected by measuring with measuring tape from the soil surface to the tip of the spike. Tillers per meter square were determined with help of the below formula:

$$\mathrm{Number} \mathrm{of} \mathrm{tillers} ({\mathrm{m}}^{-2})=\frac{\mathrm{Productive} \mathrm{tillers} \mathrm{counted} \mathrm{in} \mathrm{two} \mathrm{central} \mathrm{rows}}{\mathrm{Distance} \mathrm{within} \mathrm{row} \left(\mathrm{m}\right) \times \mathrm{Row} \mathrm{length} \left(\mathrm{m}\right)\times \mathrm{No}. \mathrm{of} \mathrm{rows} }$$

The number of spikes was noted by totaling the number of spikes in two rows in each plot:

$$\mathrm{Number} \mathrm{of} \mathrm{spikes} ({\mathrm{m}}^{-2})=\frac{\mathrm{No}.\mathrm{of} \mathrm{spikes}\hspace{0.17em}\mathrm{counted} \mathrm{in} \mathrm{two} \mathrm{central} \mathrm{rows}}{\mathrm{Distance} \mathrm{within} \mathrm{row} \left(\mathrm{m}\right)\times \mathrm{Row} \mathrm{length} \left(\mathrm{m}\right)\times \mathrm{No}. \mathrm{of} \mathrm{rows} }$$

The spike length (cm) was measured with the help of measuring tape from 10 randomly selected spikes. The 1000-grain weight was calculated using electric scales from threshed, clean grains from each plant. The four rows were harvested, and the grains were weighed and the value converted into kg ha⁻¹:

$$\mathrm{Grain} \mathrm{yield} \left({\mathrm{kg} \mathrm{ha}}^{-1}\right)=\frac{\mathrm{Grain} \mathrm{yield} \mathrm{in} \mathrm{four} \mathrm{central} \mathrm{rows}}{\mathrm{Distance} \mathrm{within} \mathrm{row} \left(\mathrm{m}\right) \times \mathrm{Row} \mathrm{length} \left(\mathrm{m}\right)\times \mathrm{No}. \mathrm{of} \mathrm{rows} }\times \hspace{0.17em}\mathrm{10,000}$$

The four main rows of the wheat plant were reaped and the biological yield was taken from these rows. The sun-dried and weighed yield was calculated as follows:

$$\mathrm{Biological} \mathrm{yield} \left({\mathrm{kg} \mathrm{ha}}^{-1}\right)=\frac{\mathrm{Biological} \mathrm{yield} \mathrm{in} \mathrm{four} \mathrm{central} \mathrm{rows}}{\mathrm{Distance} \mathrm{within} \mathrm{row} \left(\mathrm{m}\right) \times \mathrm{Row} \mathrm{length} \left(\mathrm{m}\right)\times \mathrm{No}. \mathrm{of} \mathrm{rows} }\times \hspace{0.17em}\mathrm{10,000}$$

Then, 1000 grains were manually placed on an analytical balance to determine the 1000-grain weight. For the determination of the soil EC, deionized water was mixed with soil (10:1 ratio of water: soil). After that, extract was collected as per the standard protocol. Finally, the EC was determined with a pre-calibrated EC meter [39]. For the analysis of soil organic matter potassium, the dichromate method was used. Ferrous ammonium sulphate was used for titration purposes [43].

2.8. Statistical Analysis

The standard statistical procedure was followed for the statistical analysis of the data [44]. Pairwise comparison in OriginPro 2021 (OriginLab Corporation, Northampton, MA, USA) was used for the comparison of treatments. Pearson correlation and principal component analysis graphs were also produced using the software OriginPro 2021 [45]. Fisher’s LSD test was applied for the determination of significant differences among the treatments. Parallel plots were also made using OriginPro2021 to assess the dominant and recessive combinations resulting from the different application amounts of compost and Zn.

3. Results

3.1. Plant Height

Application of 5C and 10C caused significantly enhanced plant height compared to 0C with 0Zn, 5.0Zn and 10.0Zn. Treatment 10C showed significantly better improvements in plant height compared to 5C with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn. No significant change was observed for plant height between 5C and 0C with 5.0Zn. It was also noted that plant height was significantly higher with 5.0Zn and 10.0Zn compared to 0Zn when 10C was applied. However, the 10C with 2.5Zn and 0Zn treatments remained statistically similar to each other in terms of plant height (Figure 1A). The maximum increases in plant height for 10C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 6.25, 6.64, 7.37 and 8.08%, respectively. Similarly, the maximum increases in plant height for 5C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 2.87, 2.63, 2.25 and 3.83%, respectively.

3.2. Tillers/m²

For tillers/m², the 5C and 10C treatments performed significantly better compared to the 0C treatment with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn. A significant improvement in tillers/m² was noted for 10C compared to 5C when applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn. It was observed that there were significantly more tillers/m² with 5.0Zn and 10.0Zn than 0Zn (Figure 1B). The maximum increases in tillers/m² for 10C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 31.87, 28.83, 26.33 and 21.61%, respectively. Similarly, the maximum increases in tillers/m² for 5C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 21.05, 20.53, 19.37 and 16.02%, respectively.

3.3. Spikes/m²

In the case of spikes/m², the 5C and 10C treatments performed significantly better with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn than 0C. Application of 10C with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn caused significant enhancements in spikes/m² compared to 5C with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn (Figure 1C). The maximum increases in spikes/m² for 10C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 29.48, 26.72, 24.17 and 22.33%, respectively. Similarly, the maximum increases in spikes/m² for 5C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 19.14, 18.96, 17.08 and 12.88%, respectively.

3.4. Spikes Length

Regarding spike length, a significant enhancement compared to 0C was noted when 5C and 10C were applied as the treatments with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn. Results showed that, with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn, the performance of 10C was significantly better than that of 5C in terms of the improvement in spike length. Furthermore, spike length was significantly greater for 5C and 10C with 10.0Zn compared to 0Zn and 2.5Zn (Figure 1D). The maximum increases in spike length for 10C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 30.56, 29.92, 31.11 and 40.50%, respectively. Similarly, the maximum increases in spike length for 5C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 11.11, 9.85, 14.81 and 12.90%, respectively.

Figure 1. Impact of various zinc application amounts (0, 2.5, 5.0 and 10.0 kg/ha) with different levels of compost (0, 5 and 10 t/ha) on plant height (A), tillers/m² (B), spikes/m² (C) and spike length (D) of wheat. Different letters over bars represent significant changes at p ≤ 0.05 using Fisher’s LSD test.

Figure 1. Impact of various zinc application amounts (0, 2.5, 5.0 and 10.0 kg/ha) with different levels of compost (0, 5 and 10 t/ha) on plant height (A), tillers/m² (B), spikes/m² (C) and spike length (D) of wheat. Different letters over bars represent significant changes at p ≤ 0.05 using Fisher’s LSD test.

3.5. 1000-Grain Weight

Results showed that 10.0Zn and 5.0Zn in combination with 5C and 10C caused significant increases in 1000-grain weight compared to 0Zn with 5C and 10C. Treatments 5C and 10C did not bring any significant change in the 1000-grain weight when applied with 0Zn and 2.5Zn. However, 10C performed significantly better than 5C under 10.0Zn for the 1000-grain weight (Figure 2A). The maximum increases in the 1000-grain weight for 10C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 21.28, 19.39, 20.66 and 19.35%, respectively. Similarly, the maximum increases in the 1000-grain weight for 5C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 11.72, 11.35, 12.96 and 9.63%, respectively.

3.6. Biological Yield

For the biological yield, treatments 5C and 10C differed significantly from 0C with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn. Application of 10C was significantly better with 0Zn and 10.0Zn compared to 5C for enhancement of biological yield. However, at 2.5Zn and 5.0Zn, 5C and 10C did not differ significantly for biological yield. Biological yield at 10C was statistically similar with 0Zn, 2.5Zn and 5.0Zn but was significantly higher with 10.0Zn compared to 0Zn, 2.5Zn and 5.0Zn (Figure 2B). The maximum increases in biological yield for 10C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 7.42, 7.16, 7.50 and 10.89%, respectively. Similarly, the maximum increases in biological yield for 5C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 3.43, 4.16, 4.43 and 4.08%, respectively.

3.7. Grain Yield

In the case of the grain yield, 10C performed significantly better with 10.0Zn compared to 10C with 0Zn, 2.5Zn and 5.0Zn. Application of 10C and 5C with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn resulted in significant enhancements in grain yield compared to 0C. The addition of 10.0Zn resulted in significantly higher grain yields at 0C than 0Zn, 2.5Zn and 5.0Zn (Figure 2C). The maximum increases in grain yield for 10C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 19.79, 16.62, 16.44 and 18.37%, respectively. Similarly, the maximum increases in grain yield for 5C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 13.12, 10.48, 10.28 and 7.28%, respectively.

Figure 2. Impacts of various zinc application amounts (0, 2.5, 5.0 and 10.0 kg/ha) with different levels of compost (0, 5 and 10 t/ha) on 1000-grain weight (A), biological yield (B) and grain yield (C) of wheat. Different letters above bars represent significant changes at p ≤ 0.05 using Fisher’s LSD test.

Figure 2. Impacts of various zinc application amounts (0, 2.5, 5.0 and 10.0 kg/ha) with different levels of compost (0, 5 and 10 t/ha) on 1000-grain weight (A), biological yield (B) and grain yield (C) of wheat. Different letters above bars represent significant changes at p ≤ 0.05 using Fisher’s LSD test.

3.8. Soil EC and Organic Matter

The addition of 10C with 0Zn caused a significant increase in soil EC compared to 0C. Treatments 5C and 0C were statistically similar to each other for soil EC with 0Zn. No significant change was noted for soil EC between 5C and 10C with 2.5Zn. However, both 5C and 10C differed significantly from 0C with 2.5Zn for soil EC. With 5.0Zn and 10.0Zn, 5C showed significantly higher soil EC compared to 10C and 0C. Furthermore, 10C also showed a significant increase in soil EC compared to 0C with 5.0Zn and 10.0Zn (Figure 3A). For soil organic matter, 5C and 10C were significantly different compared to 0C with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn. The addition of different levels of Zn—i.e., 0, 2.5, 5.0 and 10.0—did not bring any significant changes in soil organic matter (Figure 3B).

3.9. Soil Mineral Nitrogen

For soil mineral N, 10C differed significantly compared to 5C and 0C with 0Zn. Treatments 5C and 0C did not differ significantly from each other with 0Zn for soil mineral N. It was noted that 10C and 5C caused significant increases in soil mineral N compared to 0C with 2.5Zn, 5.0Zn and 10.0Zn. The addition of 10.0Zn and 5.0Zn showed significantly better improvements for soil mineral N compared to 0Zn (Figure 4A). The maximum increases in soil mineral N for 10C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 66.29, 66.51, 69.12 and 75.56%, respectively. Similarly, the maximum increases in soil mineral N for 5C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 12.39, 21.41, 33.60 and 35.07%, respectively.

3.10. Soil Total Nitrogen

Similar results were noted for total soil N. Application of the 10C and 5C treatments showed significantly better enhancements in soil N with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn. On average, soil N was significantly higher with 5.0Zn and 10.0Zn compared to 0Zn (Figure 4B). The maximum increases in total soil N for 5C with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 58.01, 65.22, 69.30 and 84.57%, respectively. However, the maximum increases in total soil N for 10C with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 129.48, 122.81, 114.61 and 119.72%, respectively.

3.11. Soil Extractable Phosphorus

In the case of soil extractable P, with 0Zn and 2.5Zn, addition of 5C and 10C caused significant improvements compared to 0C. It was noted that 10C remained significantly better than 0C when applied with 5.0Zn and 10.0Zn. However, no significant changes compared to 0C were noted in soil extractable P when 5C was applied with 5.0Zn and 10.0Zn. A higher level of Zn (i.e., 10.0Zn) caused a significant enhancement in soil extractable P compared to 0Zn (Figure 4C). The maximum increases in soil extractable P for 5C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 26.56, 38.71, 17.71 and 9.12%, respectively. The maximum increases in soil extractable P for 10C applied with 0Zn, 2.5Zn, 5.0Zn and 10.0Zn compared to 0C were 37.99, 71.63, 48.43 and 45.18%, respectively.

3.12. Pearson Correlation, Principal Component Analysis and Sankey Diagram

Pearson correlation showed that all the growth attributes were significantly positively correlated with the different compost application amounts. However, varying Zn application amounts also caused significant positive changes in the biological yield and grain yield of wheat in the current study. Improvement in the 1000-grain weight, soil mineral N, total soil N and extractable soil P were also significantly positively correlated with the biological and grain yield of wheat (Figure 5). Principal component analysis showed that the majority of the growth attributes were more closely associated with the highest compost application amount; i.e., 10C. However, 5C was more closely linked with the change in soil EC. The principal component analysis variables described 91.5% of differences in the first two axes (Figure 6); i.e., 85.9% and 5.6% of variations were accounted for by the first and second principal components, respectively. The compost principal component (PC1) caught a higher number of traits than the second (PC2). The parallel plots also validated the dominance of 10C and 10.0Zn (blue lines) as compared to all other combinations. All the studied plant attributes (i.e., plant height, tillers/m², spikes/m², spike length, 1000-grain weight, biological yield and grain yield) showed the maximum enhancement when the 10C and 10.0 Zn combination was provided as a treatment. (Figure 7A). The 0C and 0Zn treatment (red lines) were the weakest combination for plant height, tillers/m², spikes/m², spike length, 1000-grain weight, biological yield and grain yield. In the case of soil attributes, the 10C and 10.0Zn combination was dominant for the increases in soil organic matter, soil mineral N, total soil N, soil extractable Zn and soil extractable P. However, for soil EC, the 5C and 10.0Zn combination was the most prominent. It was also observed that the 0C and 0Zn combination was the most recessive combination for the studied soil attributes (Figure 4B).

4. Discussion

The results of the current study showed that the impact of 10C with 10.0Zn was more prominent for improvements in plant height, tillers/m², spikes/m² and spike length compared to 0C with 0Zn. Parallel plots also indicated the dominance of 10C with 10.0Zn in the improvement of the studied crop attributes compared to the most recessive combination, 0C with 0Zn (Figure 7A). These improvements in the growth attributes were due to improvements in soil mineral and total nitrogen, extractable phosphorus and organic matter. Enhancement of soil organic matter increases the cation exchange capacity of soil. This increase in the CEC plays a vital role in the retention of soil nutrients, thus improving their uptake in plants [46]. Organic matter also produces organic acid when decomposed in soil [47,48]. These organic acids regulate the soil pH [48]. A decrease in the pH of alkaline soils caused by organic acids has been found to result in the mobilization of fixed phosphorus [49]. The results of the current study support the above arguments. It was noted here that a higher compost application amount played a vital role in the improvement of soil phosphorus and nitrogen compared to control (0C). Application of compost in soil also demonstrated significantly positive effects on the 1000-grain weight, biological yield and grain yield. Such improvements in yield attributes are also associated with improvements in soil fertility [50]. Better N availability in the soil promotes the vegetative growth of plants. Improvements in photosynthesis due to the balanced uptake of nitrogen play a significant role in the enhancement of crop yields. Better N availability also helps efficiently control plant metabolic enzyme activities [51,52]. It further facilitates sucrose synthase (SS) and sucrose phosphate synthase (SPS), which play important roles in conversion for the synthesis of starch. Such improvements in plants ultimately result in better crop yields [53]. Additionally, the role of phosphorus in the reproductive stage is vital [54]. Ample uptake of P in plants promotes better yield attributes through enhancement of root elongation [55]. Healthy roots facilitate the optimum uptake of water and nutrients in plants, thus causing a significant enhancement in the grain weight [56]. They also regulate physiological, phonological and morphological processes in plants, causing significant improvements in quality attributes. Most rhizobacteria also need a balanced amount of carbon in soil [18,57]. In the current study, the improvement in soil microbial proliferation due to a higher compost application amount (i.e., 10C) might have been a major cause of the improvement in the growth attributes. Furthermore, improvement in the soil attributes due to the addition of organic fertilizer to the soil might have been another related factor responsible for the significant enhancement of yield attributes. The application of organic fertilizers facilitates the proliferation of beneficial microbes that efficiently regulate the nutrient cycle in the soil [58]. These microbes also secrete growth hormones (i.e., auxins) that efficiently improve the development of plant roots and shoots [59,60,61,62,63]. In addition, better uptake of Zn also plays a prominent role in the enhancement of the growth and yield of crops. It has been observed that Zn has an irreplaceable role in many physiological processes in plants; i.e., metabolic processes, synthesis of many enzymes, biosynthesis of chlorophyll and protein synthesis [64]. Auxin and carbohydrate metabolisms are also regulated by the balanced uptake of Zn in the plants. It further plays a key role in the activation of the carbonic enzyme in chlorophyll and photosynthetic tissues [65,66]. Furthermore, improvements in chlorophyll synthesis and photosynthesis are directly associated with enhancement of crop yields [67]. According to Faran et al. [68], sufficient Zn uptake in plants improves the yield and dry biomass of grains. The findings of the current study are in agreement with the above arguments. A significant improvement in the yield attributes was noted when increasing Zn concentrations (i.e., 0Zn to 10.0Zn) were applied to wheat in combination with the 10C treatment.

5. Conclusions

In conclusion, improvements in soil organic matter can enhance growth and yield attributes. Application of the 10C treatment demonstrated the potential to promote wheat production when applied to the soil in combination with Zn. The addition of 10.0Zn significantly facilitated the 1000-grain weight and the grain and biological yields of wheat when applied in combination with compost. Treatment 10C with 10.0Zn also showed the potential to improve soil mineral nitrogen, total nitrogen and extractable phosphorus concentrations. However, the significant increase in soil EC at 10C + 10.0Zn and 5C + 10.0Zn needs further scientific study. Growers are recommended to apply the 10C + 10.0Zn treatment to achieve maximum wheat growth and production in poor organic matter soils. More investigations are suggested at the field level with various soil organic matter and Zn concertation statuses to verify 10.0Zn and 10C as the best application amounts for optimization of wheat growth.