Redefining the Soil available Water Indicator to assess the soil Physical Quality in the rice paddies

Soil physical quality plays a major role in soil quality studies and it is considered necessary for sustainable economic production, conservation of the environment and prevention of soil degradation. To determine the soil physical quality indicators and their optimal range, the specific conditions of land use and type of cultivated plant should also be considered. Unlike other land uses in rice paddies, many soil physical, chemical and biological behavior and properties are altered by adding water to the soil and puddling. Hence, it is expected that its limitations will vary with other soils. Therefore, this study was conducted with the aim of reviewing the definition of soil available water index in assessing soil physical quality based on specific conditions of rice plant and land preparation operations.

40 soil samples were selected from rice paddies of Guilan province, and soil physical and chemical properties were measured. In addition to Dexter's S index, the soil available water index for plant (PAW) was calculated in three different ways including 1) by assumption of filed capacity at soil suction of 100 cm as upper limit of soil water availability (PAW100), 2) by assumption of filed capacity at soil suction of 330 cm (PAW330), and 3) Redefining of PAW using soil saturation moisture and soil moisture at a suction of 2000 cm as the upper and lower limits of soil available water indicator for rice, respectively (PAWrice).

The results showed that assuming the moisture content of the field capacity at the soil suctions of 100 and 330 cm, 65 and 55 percent of the soil samples had a good physical quality, respectively. While redefining of the PAW indicator of rice paddies (PAWrice) confirmed a good to great soil physical quality in 57.5% of the studied samples. In the studied samples, the use of PAW100 overestimated the amount of soil available water in all soil texture classes. While the use of the PAW330 concept has led to an overestimation of the soil available water value in medium soil textural classes (i.e. silty loam and silty clay loam). However, as the soil texture becomes clayey, the estimated soil available water using PAW330 was less than actual value. The variation of the average values of Dexter S and PAWrice indicators in different soil texture classes was observed by the same trend and as silty loam > silty clay > silty clay loam > clay loam > clay. It should be noted that despite the redefining of the soil available water indicator in rice paddies, the value of Dexter's S index in its optimal range for studied soils was more than 0.035.

Difference in the range of soil water availability in rice paddy fields and its effect on rice yields confirmed the need to redefine soil available water indicator paddy fields. In addition, despite the redefining the soil available water indicator, the results indicated that the Threshold value of 0.035 for S index is probably suitable to evaluate soil good physical quality based on the soil water availability for plant in rice paddies.