Saiful Adhar *)
Gelombang tsunami yang menimpa lahan sawah menyebabkan terjadinya peningkatan salinitas tanah. Pengukuran pertama pasca tsunami dilakukan pada Maret 2005, yang merupakan waktu musim kemarau. Diduga curah hujan dapat menurunkan salinitas tanah melalui proses leaching yang terjadi secara alami. Penelitian ini bertujuan mengamati penurunan salinitas tanah akibat pencucian secara alami oleh air hujan. Hasilnya menunjukkan bahwa selama delapan bulan sejak Maret sampai Desember 2005 dengan total curah hujan selama kurun waktu tersebut sebesar 845,2 mm.telah menurunkan salinitas tanah antara 21,51 sampai 96,33% Dari total luas lahan yang dianalisa, sebesar 45 Ha masih menunjukkan beresiko untuk penanaman padi (ECe > 4 mS/cm), dan 128 Ha berpotensial beresiko untuk penanaman padi (nilai ECe bekisar antara 3 sampai 4 mS/cm).
Earthquake followed by tsunami in Aceh has caused significant damage and disturbance to the soil and agricultural activities. The damage may influence and reduce the agriculture productivity. This is due to the salt intruded to the top soils has increased the salinity level higher than acceptable level. The most efficient way to overcome the problem is by leaching taking advantage from the rainy season.
FAO SPFS Emergency Indonesia has done a thorough survey immediately after the tsunami in three districts: Pidie, Bireuen and Aceh Utara. The survey was done in the dry season. At present, the tsunami affected area has entered to rainy season. Therefore, it is important to observe the impact of rainy season on the natural leaching process.
Based on the above mentioned consideration, conducted the research in Aceh Utara district to evaluate the natural leaching process driven/stimulated by the rain water. The result of the survey hopefully can be used as reference for land/agriculture rehabilitation and recovery as well as reference for further research activities.
The objective of this research was to observe to what extent the soil salinity and pH is improved as the advantage of the rainwater in the tsunami affected area in Aceh Utara. The salinity value used to determine the level of soil salinity.
Equipments and Materials
The equipments used in this research are EC meter for electric conductivity measurement, GPS and pH meter. In addition, it used supporting equipments such as scoop, measuring glass, ruler, balance, etc.
The Target Site
The target site of the research is Aceh Utara District, consists of five (5) Sub-Districts what its paddy fields affected by tsunami, they are Syamtalira Bayu, Baktiya Barat, Tanah Pasir, Samudera, and Seunuddon.
a. Determination of sampling point
The research is carried out by taking soil samples in the paddy field of the affected villages. In each village three sample points were taken (the closest from the sea, the middle of affected area, and the most end of the affected area). Samples were taken under a transect line. Coordinate of each sampling points the determined using GPS. Information of the coordinate will be used for development of Geographical Information System.
Figure 1. Determination of sample point
b. Soil sampling
At each sampling point, soil sample was taken by making small pit with depth 20 cm. Within the pit, it was observed layers of sediment that brought by the tsunami. In general the sediment consists of three layers: clay, sands, and original soil. The thickness of the sediment was measured.
From each layer a 30 g sediment samples are taken. The sediment then mixed with pure water to have 150 ml solution. In this case, it was used solution ratio 1:5. In order to have a well solution, the suspension was stirred for about 15 minutes. EC then is measured using portable EC meter directly in the field. The EC that measured by this method is perceived as EC soil water (ECw). Beside the EC, pH of solution was also measured to check its acidity although the tsunami does not significantly affect the soil acidity.
Figure 2. Soil sampling at a mini pit
c. Procedure for Data Analysis
EC that measured from the sampling is categorized as Soil Water Salinity (ECw). Meanwhile, the standard for crop tolerance that widely used as international reference is Soil Extract Salinity (ECe). Therefore it needs conversion from ECw to ECe. The salinity is measured in mS/cm, therefore before conversion average salinity from each layers is calculated. The detail analysis data of soil sampling is described as follows.
1. Measurement of ECw for each layer to have ECw clay (A), ECw sand (B) and ECw original soil (C).
2. Measure of depth or thickness of each layer. Total depth is 20 cm. Therefore the thickness of original soil (D) will be 20 minus thickness of clay (E) minus thickness of sand (F).
3. Calculate the average EC at sample point by weighted average that is ((A x D) + (B x E) + (C x F))/20. The average ECw will be perceived as ECw at sample point. ECw1 is ECw at sample point no 1.
4. Within the village three sample points were observed. Therefore, there are ECw values; ECw1, ECw2, and ECw3.
5. Calculate average ECw from three sample points. The average ECw is perceived as ECw at the village.
6. Convert the ECw into ECe by multiplying with 8, because the conversion rate (according to IRRI) ECe = 8 ECw for typical paddy field soil.
7. The ECe will be used to land evaluation and making conclusion of the research.
Besides the soil salinity test, the research also recorded general situation on the village after tsunami, including recorded reconstruction and rehabilitation that has been made in the tsunami affected area. The information was taken from interview with the farmers (villagers).
In the district of Aceh Utara, most of paddy fields were inundated due to high rainfall. In some areas, farmers have started their farming by cultivating their land, while in some others area, farmers have not cultivated yet their land. Farmers have also done some rehabilitation works before they started their cultivation. There were 9 villages, where farmers have not done any rehabilitation intervention. It has been found that the salinity of these areas was relatively high.
Measurement that conducted on March 2005 indicates that soil salinity ranged from 2.48 to 38.51 mS/cm. After 8 months, measurement on December indicated that the soil salinity from 1.16 to 14.49 mS/cm. It shown the soluble salt content in soil has been reduced by the rainwater during March until December 2005. Total rainfall on the range time is 845.2 mm. Amount of the soluble salt content reduced by the natural leaching process is from 21,51 to 96,33%.
The classification on the land damages due to salinity in paddy field reveals that 18 villages were classified into class A (ready and safe for cultivation, ECe <4 mS/cm), while only 9 villages have ECe >4 mS/cm. Among the 9 villages, 7 villages are estimated would become class A during the planting season on March 2006. This is due to the estimate total rainfall December-January, February will adequate to leach the salinity dropping to less than 4 mS/cm.
Based on the result, it can be classified as follows (1) Area that still high risk for cropping due to salinity (ECe > 4 mS/cm) in Aceh Utara are 45 ha in village Baroh Blang Rimung, 72 ha in Sawang, 100 ha in Blang Nibong, 80 ha in Matang Tunong, 120 ha in Matang Baroh, 40 ha in Kuala kerto timur, 21 ha in Lhok Euncien, 150 ha in Ulee Matang, 200 ha in Matang Lada. Total is 828 ha. (2) Area that has potential risk for cropping due to salinity (3 < ECe < 4) mS/cm are 25 ha in Dayah Baro, 103 ha in Kuala Cangkoi and 100 ha in Ulee Reubek Barat. In total 128 ha.
1. The soil salinity in the tsunami affected paddy field that has been affected has decreased naturally by rainwater. However, the natural leaching process was not been able recovered optimally.
2. In Aceh Utara from about 2732 ha of the affected area, 828 ha now still has high salinity that hampered cultivation, and 128 ha is has potential risk of failure for cropping.
3. Interview with farmers victim indicated that reconstruction and rehabilitation program was not well directed in an integrative manner. All interested stake holder assist the farmer with their own way.
It is urgently to rehabilitate the tsunami affected paddy field particularly those EC index above 4 mS/cm. This is important to facilitate farmers to cultivate their land immediately to find their farming and to recover livelihood. For effective rehabilitation, some activities must be done including repair of drainage, tillage of paddy field topsoil, leaching of the paddy field.
Considering rainy season will end immediately, hence the effort has to be conducted immediately while the rain water is still available. Alternatively, irrigation system has to be to speed up land rehabilitation.