Introduction:
Water scarcity and drought are among the most common agricultural problems in Iran. Therefore, finding solutions to enhance the resistance of agricultural crops such as button mushrooms resistant to drought stress is of particular importance. Increasing soil water retention and irrigation management are vital to preventing water scarcity and physiological stress in agricultural crops. Therefor the amount of water available to plants and its measurement has been of interest to farmers and researchers. One of the solutions considered in recent years for optimal management of soil water status is the addition of organic amendments to the soil, including biochar. Biochar can change the physical properties, improving access to water, air, and nutrients for the plant, and as a result, affect the growth and yield of agricultural crops. Therefore, irrigation of casing soil under button mushroom cultivation should also be managed according to these changes to prevent inefficient irrigation.
Methods:
This study was conducted in a completely randomized factorial design with three factors: type of casing soil at four levels, amount of biochar at four levels, and size of biochar particles at two levels. The experiments had three replications. The control treatment consisted of casing soil used in the factory with a combination of North peat, South peat, and spent mushroom compost (SMC) in equal weight ratios. The experiments involved a total number of 29 treatments across 87 experimental plots at the Negin Fasl compost and mushroom production factory, located in Shahrekord city, Iran, in 2023. In this regard, the initial casing soil compositions including North peat, peat vermicompost, cow manure vermicompost, and mushroom compost vermicompost were prepared. To prepare different treatments, biochar derived from pruning waste of plum and pomegranate trees, with particle sizes of 0–1 mm and 1–2 mm, was added at 5%, 10%, and 15% by weight to the aforementioned casing soil compositions with a 20-day incubation period. Then, 15 days after the compost loading operation in the button of mushroom cultivation hall, when 70% of the compost bed was covered with mushroom mycelium, the casing stage began. For each square meter of the cultivation bed, 32 kg of casing soil was added as described in the experimental treatments in the form of caking. After the casing stage, sampling the casing soil was carried out from each experimental plot using a metal cylinder with a diameter and height of 4.5 and 5 cm, respectively. Finally, the soil moisture characteristic curve obtained by measuring moisture at suctions of 0, 10, 20, 50, 60, 80, and 100 hectopascals. The indices of air volume percentage after irrigation (AIR), available water (EAW), water buffering capacity (WBC), water holding capacity (WHC), and their integral energy were calculated. Then, the obtained data were analyzed using SPSS software.
Results: 

 The results showed that the application of biochar led to a decrease in AIR in the studied soils which could be due to the high frequency of fine pores that were affected after irrigation. The highest volumetric percentage of air after irrigation was observed in the S₀ treatment (0.17) and the lowest in the S₄l₂z₁ treatment (0.02), which had no significant difference with the S₃l₃z₂ treatment (0.02). The application of biochar in different soils except for the S₁ treatment did not cause any changes in the EAW value compared to the commercial casing soil and the EAW value of all treatments was at the optimal level. The application of biochar to the casing soil resulted in an increase in the WBC value of water and this increase led to enhance in water retention in the casing soil. However, WHC value in the S₁l_o, S₂l_o, S₃l_o and S₄l_o treatments decreased by 34, 29, 5 and 19 percent compared with the S₀ treatment, respectively. The results showed that the variation of EI_AIR among different casing soils and also commercial casing soil was small; also in all four studied soil types EI_AIR increased after irrigation with increasing biochar application. Soil water availability for plants was almost constant in the moisture range of 10 to 50 hPa. The application of biochar to casing soil increased this energy requirement but this increase was not significant. Also, the amount of energy required for water absorption in the moisture range between 50 and 100 hPa was almost the same among different casing soils and also commercial casing soil. There was no significant difference between the energy requirements of these soils, but the application of biochar -except for soil S₁- led to an increase in EI_WBC, although this increase was not significant.
Conclusion: 
Overall, the results showed that adding biochar to casing soil did not cause any problems in terms of soil aeration and kept the amount of accessible soil water at an optimal level. However despite the increase in the amount of water buffering capacity (WBC) and water holding capacity (WHC) in the soil, the amount of integral energy (EI) increased due to the increase in the specific surface area of the soil, making water extraction more difficult. Therefore, it seems that using biochar as a percentage replacement for casing soil does not cause a significant change in soil water availability compared to commercial and conventional casing soil.