The content and soil moisture capacity depends upon various factors such as soil structure (arrangement of soil). One of the effective factors on soil structure is soil compaction. Based on the change of soil compaction during the year and in variousyears, the content and soil moisture capacity are varied in terms of time. On the other hand, moisture hysteresis has great effect on moisture behavior of soil. In the current study, the effect of hysteresis and soil compaction on calibration curve of gypsum block was investigated. To do this, calibration curve of a gypsum block in one heavy soil (Silty Clay) in three bulk density (natural in the farm and plus 10% and minus 10%) for two phases of wetting and drying were provided. The results showed thatblock calibration curve was different in two phases of wetting and drying in different densities. The effect of moisture hysteresis on calibration curve of gypsum block is increased by reducing soil compaction (reducing of bulk density). This is due tothe fact that by reducing soil compaction and because of heavy texture of soil, the fine pores are reduced and distribution of soil pores will be more non-uniform. Thus, considering the effect of ink bottle, hysteresis is increased. The effect of soil compaction on gypsum block calibration curve is reverse during drying and wetting such that for specific soil moisture during drying phase, with increasing soil compaction, the electric resistance of the block is reduced. While, during wetting phase by increase of soil compaction, the electrical resistance of the block is increased.
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