Abstract
Soil compaction is a negative factor for plant growth. Subsoiling is a practice that improves the physical, chemical, and biological properties of soil below the conventional tillage depth, leading to increased crop yield and improved water and nutrient use efficiency. The objective of this study was to evaluate the effect of subsoiling on the yield of wheat (irrigated and rainfed), sugar beet, grain maize, and cotton by reviewing domestic studies and applying a meta-analysis approach. To this end, articles, research reports, and student theses published over the past three decades were reviewed using keywords related to subsoiling in the aforementioned crops. After quality screening, 18 studies were included in the meta-analysis using a random-effects model, comprising comparisons of subsoiling versus no subsoiling for wheat (13 studies under irrigated conditions and 5 under rainfed conditions), 8 studies for sugar beet, 4 studies for grain maize, and 5 studies for cotton. The data required for the meta-analysis included the treatment mean (X̄), standard deviation (SDₓ̄), and number of replications or sample size (n) in the experimental design. In data analysis, the standardized mean difference (SMD), one of the key effect size indices, was used to compare yield between the control treatment (no subsoiling) and the experimental treatment (with subsoiling). Based on the effect size values for irrigated wheat (SMD = +0.72), rainfed wheat (SMD = +1.14), sugar beet (SMD = +0.41), grain maize (SMD = +0.86), and cotton (SMD = +1.15), it can be concluded that subsoiling had a positive and significant effect on the yield of wheat (both irrigated and rainfed) and cotton, whereas its effect on sugar beet and grain maize was positive but not statistically significant. Compared with no subsoiling, subsoiling increased the yield of irrigated wheat, rainfed wheat, sugar beet, grain maize, and cotton by 13.27%, 16.02%, 3.74%, 5.84%, and 14.00%, respectively. Therefore, subsoiling is recommended to alleviate soil compaction and improve the yield of the aforementioned crops, particularly wheat and cotton.

EXTENDED ABSTRACT
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.
Material and Methods 
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 and Discussion 
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,