Introduction
Agriculture is facing new challenges due to climate change, and food shortages are imminent. As a result, new sources of food and water will be needed. In agricultural fields, the subsoil (the layer below the plowed soil) can store approximately 50% of the total nitrogen and 25-70% of the total phosphorus. It can also retain water even in dry conditions. However, the availability of these resources varies between crops. Soil compaction is a negative factor for plant growth. To remove the dense soil layer, subsoiling has been introduced, which reduces soil resistance and facilitates deeper root penetration. Thus, plants have access to subsoil resources. Subsoiling improves the physical properties of the soil (reducing soil resistance, reducing soil bulk density, and increasing water infiltration into the soil) without turning the soil over. Subsoiling is a field measure to improve the physical, chemical, and biological properties of the soil below the common plowing depth to increase crop yields, water, and nutrient use efficiency. Subsoiling requires a significant amount of fuel and energy, as well as the use of powerful tractors. Therefore, before doing so, it is necessary to ensure that a hard layer (cone index greater than 2 MPa) is present, which reduces performance. Otherwise, the only result of subsoiling will be increased fuel consumption, wear and tear on the tractor and equipment, and destruction of the soil structure.
Method
The purpose of this research is to investigate the effect of subsoiling on the yield of wheat, sugar beet, corn, and cotton, using internal studies and research, and applying the meta-analysis method. For this purpose, articles, research reports, and student theses from the last thirty years were reviewed using keywords related to subsoiling in the mentioned products. Finally, after quality control, 18 studies in wheat, 8 studies in sugar beet, 4 studies in grain corn, 5 studies in cotton were entered into the meta-analysis process to compare subsoiling with using a random model. The data required for meta-analysis are the treatment mean((X)) ̅, standard deviation (SD_X ̅ ), and the number of replicates or sample size (n) in the experimental design. In data analysis, the standardized mean difference parameter, which is an important factor of effect size, was used to compare the performance between the control treatment (without subsoiling) and the experimental treatment (with subsoiling).

Results
Considering the total SMD value (0.84+), it can be concluded that subsoiling had a positive and significant effect on wheat yield. Subsoiling significantly increased wheat yield in both irrigated conditions (SMD = 0.72+) and dryland conditions (SMD = 1.14+). The average total yield of wheat in the no-subsoiling and subsoiling methods was 4191.6 and 4707.4 kg/ha, respectively. After weighing each yield, the average weighted yield of the no-subsoiling and subsoiling methods was 207.1 and 234.6, respectively. Subsoiling in irrigated conditions increased wheat yield by 13.27% compared to no subsoiling. Subsoiling in dryland conditions also increased wheat yield by 16.02% compared to no subsoiling. Considering the effect sizes of sugar beet (SMD = +0.41), grain corn (SMD = +0.86), and cotton (SMD = +1.15), it can be concluded that subsoiling had a positive and insignificant effect on sugar beet yield, a positive and insignificant effect on grain corn yield, and a positive and significant effect on cotton yield. Subsoiling increased sugar beet yield by 3.74% compared to the no-subsoiling method. The average total sugar beet yield in the no-subsoiling and subsoiling methods was 55868 and 59067 kg/ha, respectively. After weighing each yield, the average weighted yield of the no-subsoiling and subsoiling methods was 7126 and 7393, respectively. Also, the subsoiling in the cotton crop increased its yield by 14%. The average cotton yields for the no-subsoiling and subsoiling methods are 2457.1 and 2806.8 kg/ha, respectively. After weighing the yields, the average weighted yields of the no-subsoiling and subsoiling methods were 497.9 and 567.6, respectively. Subsoiling increased the yield of grain corn by 5.84% compared to the no-subsoiling method. The average total yield of grain corn in the no subsoiling and subsoiling methods was 9850.5 and 10392.3 kg/ha, respectively. After weighing each yield, the average weighted yields for the no-subsoiling and subsoiling methods were 2474.9 and 2619.5, respectively. The probability level values in the Begg and Egger methods (for estimating publication bias) for wheat studies were 0.76 and 0.41, respectively. This suggests that publication bias does not exist in wheat crop research. Also, the probability level values in the Begg and Egger methods in sugar beet studies were 0.09 and 0.17, in corn studies 1 and 0.81, and in cotton studies 0.14 and 0.42, respectively, which can be concluded that publication bias (except in sugar beet) does not exist in the research on the mentioned crops.

Conclusions
The effect of subsoiling on wheat and cotton yield was positive and significant, whereas on sugar beet and grain corn yield, it was positive but insignificant. The positive effect of subsoiling on wheat, cotton, sugar beet and corn yields can be attributed to improving soil physical properties such as reducing soil resistance, reducing soil bulk density, increasing water penetration in the soil, increasing soil moisture (in dryland conditions), and expanding the depth of root penetration in the soil, which provides conditions for better plant growth. The greatest effect of subsoiling on wheat yield was in soils with silty clay loam texture. Subsoiling increased the yields of irrigated wheat, rainfed wheat, sugar beet, grain corn, and cotton by 13.27%, 16.02%, 3.74%, 5.84%, and 14.00%, respectively, compared to no subsoiling. Therefore, it is recommended to subsoil in the presence of soil compaction, especially in crops such as wheat and cotton. It is also recommended that further studies be conducted on the impact of subsoiling on various crops.

Author Contributions
Heidari Ahmad extracted and analyzed the data and wrote the manuscript.

Data Availability Statement
This section details where supporting data can be found, typically including links to publicly archived datasets. If no data is reported, "Not applicable" should be stated.

Acknowledgements
We would like to thank the Hamedan Province Agricultural and Natural Resources Research and Education Center for providing the necessary facilities to conduct this research.
Ethical Considerations
This section states ethical approval details (e.g., Ethics Committee, ethical code) and confirms adherence to ethical standards, including avoidance of data fabrication, falsification, plagiarism, and misconduct.
Conflict of Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
Funding Statement
The author(s) received no specific funding for this research