Nondestructive Evaluation of

Hardpan Characteristics of Paddy Field

Using X-Ray Computed Tomography (Ct) Scanner

Tineke Mandang*, Kengo Watanebe**, and Seishu Tojo***

*) Associate professor at Bogor Agricultural University, Indonesia

**) Professor at Tokyo University of Agriculture and Technology, Japan

***) Associate professor at Tokyo University of Agriculture and Technology, Japan

Abstract

In paddy field, hardpan is important to prevent leakage of water and support the weight of field machinery during the operation. Density of soil is considered the most important physical properties of agricultural soils especially for hardpan of paddy field.

Conventional method for measuring density is generally invasive in that probes must be placed in the region of interest. In response to a desire to quantify soil physical properties nondestructively, X-ray computed tomography (X-ray CT) is receiving increasing attention. Modern CT scanner with CT mode was used in this study and the output of measurement was CT number.

The objective of this study was to evaluate the hardpan characteristics of paddy field in order to find the proper machinery operating condition.

A Kanto loam soil in Tokyo was selected. The samples were collected from University Farm (Paddy Field Experiment station). The analysis was emphasized on 1) CT number changes, 2) Bulk density of soil and moisture content, 3) Penetration resistance and 4) Soil permeability. Sampling block consists of 1) T: field where tillage operation is performed every planting season, 2) NT1 : field where tillage operation was not performed in the last one year, and 3) NT2 : field where tillage operation was not performed in the last two years.

Soil samples for CT scanning was undisturbed sample taken from paddy field at the depth of 0-30 cm depth and diameter of sample 10 cm. CT analysis was based on image processing (histogram, profile, tomogram image) and CT number (shown by region of interest, ROI or profile) of each 2 mm slice. It was clearly shown by CT image that the layer in bottom part of all samples (around 20 cm depth) have higher density than the top layers. The results are also agreed to the CT number analysis which showed higher value at the bottom part of the samples, up to 171 (NT2), 154 (NT1) and 125 (T). As shown by histogram analysis, CT number of top layer varies in wide range because there is much pore space especially in the root zone, while CT number of the bottom layers does not vary so much as the area occupied almost by solid materials.

Bulk density (BD) distribution was evaluated as it is one of the conventional method to describe the physical characteristics of soil. Samples for bulk density analysis was undisturbed samples taken from the depth of 0-10, 10-20, 20-30, 30-40 cm at some sampling points. In comparison with other methods, this method is not accurately explain the micro and internal condition of the samples From the analysis of soil bulk density, it was found that there is no significant difference on bulk density of top and bottom layers. The minimum bulk density was found in the field which has tillage treatment every planting season (T), 0.99 g/cm³ and the maximum bulk density was found in the field which has no tillage treatment for the last two years (NT2), 1.33 g/cm³.

Penetration resistance (PR) pattern was evaluated using penetrometer SR-2. The measurement by SR-2 was done at the depth of 0,5,10, 15, 20, 25, 30, 40 cm at some measurement points. .Penetration resistance shows more interesting and clear pattern of soil profile up to the depth of hardpan. In tilled field (T) the PR shows low value up to the depth of 20 cm, less than 10 kg/cm2. The PR start to increase from the 25 cm depth and keep the high value up to 40 cm depth, greater than 15 kg/cm2. In the 1 year non-tillage field (NT1) the PR is gradually increase from the 5 cm depth and reach the maximum at 20 cm depth. For the 2-years untilled field, the PR start to increase from 5 cm depth and keep constant up to the 15 cm depth, and increase to the maximum level at 25 cm.

Measurement of water permeability of soil was done at the laboratory using sample taken from the surface layer (root zone), hardpan layer, and under the hardpan layer.

More data are necessary to be collected form many types of paddy field. In the traditional paddy field, the characteristics of soil hardpan are very different. The soil types also give much contribution on these differences. To find the suitable machinery operating condition, proper depth of hardpan is necessary.