A Quick Dielectric Method to Determine Insitu Soil Water Content for Precision Water Use under Sustainable Agricultural Practices

Precise water use systems under sustainable agriculture may only be achieved if in-situ soil water content and availability of the plant can be measured quickly.  Soil water content can be determined directly using the gravimetrical method by calculating the loss of water when the soil dries out, and indirectly by measuring other variables from which soil water can be calculated.  The indirect methods have some advantages when compared to the direct gravimetrical method because measured; non-destructive soil water data are available instantly.  Soil dielectric properties have strong relationships with soil water content and have been used widely as indirect methods of soil water determination, but the cost of the available instrument is too expensive for small-scale farmers.  Two studies were conducted in 2011 and 2015 to develop a quick, cheap method to determine in-situ soil water content and precise water use by crops.  The method applied a non-linear relation between soil water content (θ, in gg-1) and soil electrical impedance (Z, in kΩ) as follows: θ = a.Zb where a and b are constants.  Parameter Z showed a good predictor for soil water content (R ≥ 0.90) therefore can be used to quickly determine soil water content in the field.  The dielectric method has been successfully used to determine the water balance in the vegetated soils, in wich changes in soil water content caused by daily rainfall as low as 10 mm was detected up to the depth of 60 cm.  The method was also successfully capable of calculating the amount of water used by palm oil nursery grown in media with different levels of soil organic matter.