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Soil salinity is one of the most important limiting issues in agriculture that affects plant yield. Considering the transfer of Chah-nime water to 46,000 hectares of Sistan plain lands and high soil salinity in some areas, it was necessary to conduct studies to determine the need for leaching and desalination of the soils of this region. Therefore, this study was carried out in order to know the potential of modifying saline and alkaline lands and to estimate the water needed for leaching to reduce soil salt from a certain depth of the soil for the desired plant and the need or not to use ameliorating materials in sodium lands. . For this purpose, it was necessary to select study bases for leaching and desalination in construction units. According to the aim of these studies, which was to generalize the results to other similar areas, the selection of leaching and desalination sites was very important. For this purpose, first, the leaching points in the construction units were determined using the soil salinity and alkalinity map of the studied area in the soil series. Among the soil series, those with the highest percentage of high salinity and high alkalinity classes were determined and suitable starting points for leaching were suggested. The initial selection points were visited by experts and farmers were interviewed and general information was collected. Finally, after expert discussion and field visit, the final number of 12 points were selected to conduct leaching and soil improvement studies, which included four leaching bases, four bases for soil improvement with plaster, and four bases for simultaneous leaching and improvement operations. These 12 points were located in the soil series of Arbab, Charak, Hamoun, Khajedad, Kol, Zabol, Hamoun 2, Akbarabad, Nimroz 1, Balakhane and Zahk Chahar, which cover more than 75% of the same water units. . Finally, leaching and improving the soil was done with one meter of water and alternately in four stages (in each stage with 25 cm of water).
After leaching and analyzing the results of leaching and improving the soil, the most suitable model for estimating the need for leaching in each soil series was determined. Then, based on initial soil salinity and final salinity for four dominant crops in the region, including barley (soil salinity resistant group, with a tolerance threshold of 8 decisiemens/m), wheat (relatively resistant to soil salinity, with a tolerance threshold to a salinity of 6 decisiemens/meter), melon and cumin (relatively sensitive group to soil salinity, with a threshold of tolerance to salinity of 4 decisiemens/meter) and watermelon and melon (a group sensitive to soil salinity, with a threshold of tolerance to salinity of 2 decisiemens/meter) the amount of pure water needed to wash 30 cm of the surface of the soil and bring it to salinities of 8, 6, 4 and 2 decisiemens/meter at any point based on the model obtained in each The soil type was estimated. Finally, maps of the amount of pure water for leaching the soil of the same water units were prepared.
Based on the obtained results, the level of the same water units without the need for leaching varies with the change in the desired plant for planting. So that by changing plants from salinity-resistant plants to sensitive plants, this level of land was reduced and the area of the hydrological units that needed more than one meter of water was increased. In plants sensitive to salinity, the surface area of the land without the need for leaching has been greatly reduced, so that in some areas such as Nimroz construction unit, this amount has reached zero and in Hamun construction unit, it has reached 0.7 hectares. In other words, planting plants sensitive to salinity such as watermelon in Nimroz construction unit has no economic justification at all. Because it requires a lot of water for soil correction, and considering the amount of water allocated in the 46,000-hectare Sistan plain plan (8,700 cubic meters per hectare), this is not possible. On the other hand, with the change of cultivation pattern, from resistant plants to sensitive plants, in addition to increasing the area of land that needs to be washed, the amount of water required for corrective leaching has also increased. So that in planting resistant plants, the maximum pure water required for leaching was about 112 cm. While this amount reached 130, 215 and 270 cm in relatively resistant, relatively sensitive and sensitive plants respectively. Therefore, due to the lack of water in the Sistan plain, it is necessary to observe the cultivation pattern for each region so that the amount of water allocated to each region meets the needs of leaching and evaporation-transpiration of plants. Amounts of pure leaching water estimated based on models resulting from leaching and soil improvement operations, in high salinity (salinity class S4) and medium (salinity class S3) are highly correlated with Rio (1957) and Dillman models, respectively. (1963) had. The results of field studies of desalination and desalination showed that most of the areas that need large amounts of water for leaching are located in areas with heavy texture along the entire soil profile and slow and very slow permeability. In these areas, due to the presence of heavy layers with a lot of silt and clay, the leaching process went very slowly. The existence of such a layer greatly limits the movement of water and the transfer of solutes in the soil and makes leaching and modification operations difficult. In addition, due to the long leaching time, in a region like Sistan where there is a lot of evaporation from the soil surface, the depth of pure water for leaching is reduced and as a result, the leaching efficiency is reduced. The results of soil leaching and soil improvement showed that from the four bases where soil leaching and soil improvement operations were carried out using agricultural plaster; Only one base whose soil did not contain gypsum reacted to the increase of gypsum, and the increase of gypsum in the other three cases did not show a significant difference in the increase of permeability, improvement of salinity, and reduction of exchangeable sodium percentage of the soil compared to the treatment without gypsum. Also, this case was observed in all four bases where only leaching operations were carried out without adding any ameliorating substances; So that only with the leaching operation, the values of exchangeable sodium percentage and sodium absorption ratio, especially in the 25 cm surface of the soil, had a downward trend along with the decrease in salinity. Therefore, it can be said that in most of the soils of Sistan Plain, the use of the right amount of water, the method of intermittent leaching and the correct time of leaching can be effective in reducing salinity and exchangeable sodium of the soil at the same time. It is clear that the amount of water presented in this report is based on the pure water used for leaching the soil, and in practice, the amount of evaporation from the soil surface, as well as the lack of soil moisture up to the capacity of the farm, should be added to this water and gross water or the total water used. Get it for leaching. It is suggested to add about 10 cm of water to the estimated values for leaching each unit of water. Another important point in leaching is the time to start leaching. Considering the cultivation season in the Sistan plain, which is almost the beginning of November; According to the meteorological statistics of Sistan and Baluchistan province, it is necessary to wait about 10 days before starting agricultural operations in Zabul, Zahak and South Hirmand areas (due to the lighter texture of the soil and greater permeability), and about 15 days before starting cultivation in Nimroz, Hamoun areas. and the north of Hirmand and areas of the center and east of Hamoun should be washed so that the land will be in a good state when cultivation begins.
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