Reclamation of salt-affected soils by leaching is commonly used, and considered to be one of the most feasible and economical choices. Leaching efficiency is a function of:
1. Soil texture;
2. Water application modes (intermittent ponding or sprinkling or continuous ponding);
3. Types and amount of salt present, etc. Leaching efficiency has been comparatively studied in both field and laboratory conditions. In the field study, the traditional leaching method was considered; ponding water continuously on the soil surface was tested; whereas, in the laboratory, both continuous ponding and intermittent ponding were examined in terms of leaching efficiency.
The abandoned field of 2.5 ha that was previously investigated was selected as the site for soil reclamation study for the farm. Four subplots, each 10 m wide and 150 m long, were prepared in the experimental site for a leaching study in 1997. Irrigation water was conveyed into each plot and ponded on the soil surface continuously, at water depths of 372, 677, 937, and 1,248 mm, respectively. Soil was sampled in each plot at three locations before and after leaching, at the depths of 5, l0, 20, 40, 60, 80, 100, 120, 140, and 160 cm, respectively. The air-dried soil samples were analyzed for EC of extracts at soil to water ratio of 1:5 (EC1:5), and for chemical composition.
In the laboratory experiment, soil samples from the A horizon of the experimental site (sandy loam) were used for soil columns (50 mm in diameter and 100 mm long). Irrigation water simulating the composition of the irrigation water on the site was applied to the columns from the top by two methods— intermittent ponding and continuous ponding. In intermittent ponding, water was ponded at 0.1 pore volume (PV) per day. One pore volume was 90.7 ml for the soil (46.2 mm in depth). Under continuous ponding, a constant water head was kept at 10 mm and a fraction collector was used to collect the effluents. All the effluents were measured for volume and EC. The experiment was terminated after 1.8 PV of the leaching water passed through the soil columns. Then, the soil columns were sliced at 20 mm intervals and the sections were oven dried at 105°C. EC and soluble salts were measured in the saturated soil solution.
Fig. 9.1 shows the relationship between the soil EC change after leaching and the amount of leaching water.1 EC1:5 and ECe were used as the soil EC in the field and laboratory experiments, respectively. The ordinate axis was expressed by the fraction (ECf - ECw)/(ECi -ECw), where ECw is the EC of irrigation water, ECi and ECf are the soil EC (dS/m) before and after leaching, respectively. The abscissa axis was expressed by the amount of water leaching through the profile per unit depth of soil, Dw/Ds. The fitting equation, showing a power relationship, is given in Fig. 9.1(a). In the field experiment, when Dw/Ds= 1, the reduction in soil EC1:5 was 50%, which means that only half of the initial salt in the soil was removed. Over the whole profile, a high reduction in soil EC was obtained within 50 cm, and around 60% of the initial salinity still remained in the deeper layers irrespective of the amount of leaching applied. The leaching efficiency apparently was lower than the value of 70% or more shown by Hoffman.3
In the laboratory experiment, there was no observed difference in leaching efficiency with continuous ponding (CP) and intermittent ponding (IP). This was in agreement with Hoffman's result that leaching efficiency was less affected by leaching methods on the sandy loam soils. 90% of the initial salt was removed when Dw/Ds = 1, which is significantly higher compared with the field result. The different leaching efficiencies of the field and column experiments possibly resulted from the different pore-size distribution; soil was homogeneous in the column, but heterogeneous in the field. In both cases, around 10% of the initial soil salinity was remained even under high water application. Since the soil contained considerable amounts of gypsum, gypsum dissolution may be the main reason for the decline in leaching efficiency.
Desalinization of soil and leaching efficiency depend on salt transport processes which are affected by water application modes (saturated or unsaturated flow), soil texture, initial water content etc. Intermittently ponding water on the soil surface usually causes unsaturated water flow in the soil. On the other hand, continuous ponding creates saturated water flow. Salt leaching is efficient where unsaturated water flow prevails, and vice versa. In soils where more aggregates exist, soil pore size distribution is more diverse, and consequently large differences exist in water flow caused by the various water application methods. In the column study with sandy loam, because of the low clay content, 14%, there are few aggregates and pore size distribution is more uniform. Therefore, unsaturated water flow occurred under both intermittent ponding and continuous ponding. As a result, leaching efficiency was similar under both intermittent ponding and continuous ponding.
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