Evaluation of Soil Physico-Chemical Properties in Pre and Post Flood Conditions at Thatta Sindh Pakistan

Heavy rainfall on a scale unprecedented to 100 years caused massive floods in Pakistan during 2010. Due to climate change, heavy rainfall brought the biggest ever natural rain flood to River Indus. Agricultural area on the lower Indus plane is frequently flooded. Floods damage soil properties, lower agricultural productivity, and worsen food shortages. This study observed changes in soil physico-chemical properties and nutrient status under pre and post flood conditions. In pre-and post-flood conditions, 13 locations were sampled (0-15 cm and 15-30 cm). Clay was dominant in pre-flood conditions with 69.24%, while sandy clay was higher with 53.80% in post-flood conditions. Soil reaction was moderately (pH 8-8.5) and strongly alkaline (pH >8.5-9.0) in pre-flood and post-flood conditions, respectively. The EC was low to moderate (2-8.0 dSm-1) and severely saline (8.00-16.00 dSm-1) in pre and post-flood conditions. SAR hazards increased from 11.15 to 17.09 in post-flood conditions. Organic matter was poor (<0.5 %) and lime content was in moderate category (10-15%) in post flood conditions. N, P, Ca and Mg content showed decreased trend of 20, 42, 19 and 23%, respectively in post flood conditions, whereas K content was increased by 19 %. In post flood conditions, the pH and SAR were highly positively and significantly correlated (r=0.53) with each other indicated the dispersion and deterioration of soil with decreasing nutrients availability to plants. The information obtained from this study will be helpful to develop flood qualification strategies for proper soil management.


Introduction
Heavy rainfall persuaded the flooding disasters that occur repeatedly in many areas of world and often cause extraordinary harm to crop production. The alteration of soil physical and chemical properties during flood could considerably decrease crop stand, growth and yield of crops. Under present climate conditions, the decline in crop yields are unsettled to overload rainfall actions and have been extensively, harmfully influencing the grain supply and creating food insecurity (Xi et al., 2022). The heavy rain (above normal annual rainfall 87.5%) in the northern parts of Pakistan, caused the biggest ever flood in River Indus and estimated that 4.5 million acres of crop were destroyed (Rehman et al., 2021, Bhatti et al., 2010. The chemical changes may alter the soil nutrients availability and exerts negative impact on food sustainability (Nancy et al., 2011). The effect of flood for farmers consisted of changes in soil physicochemical properties, decrease availability of nutrient to crop and harvest losses (Visser and Voesenek 2004). Floods bring some unfavorable or beneficial effects on soil physico-chemical properties (Abubakar et al., 2012, Kalshetty et al., 2012, Saint-Laurent et al., 2010. The soil properties differed significantly with each other according to area in flood water and fine silt is a common feature of flood deposits, along with a reduction in organic carbon, while increase in pH that may lead to harvest losses from soil (Saint-Laurent et al., 2010 andBaker, 2006). Lida et al. (2012) reported non-significant differences for soil SAR before and after flood conditions, while a decrease in electrical conductivity (EC) after flood conditions. Whereas, Xi et al., 2022, Abubakar et al., 2012, Tavasoli, 2001Kowsar 1992, found that when floodwater spreading on plains caused to an increase in pH, EC and sodium adsorption ratio (SAR). Flood can reduce the productivity of soils by causing nutrient loss due to erosion (Abubakar et al., 2012). The change in soil properties in conjunction with an increase in Na + and pH, decrease the yields considerably (Wahid 2004, Ashraf andSarwar 2002). There is a growing need for information on soil quality following rapid land use changes taking place after flood. The different studies are available regarding the impact of different management practices of soil and crop, however very limited work has been reported regarding the effect of pre and post flood on soil physic-chemicals properties. Hence, the study was conducted to determine the impact of flood on soil properties and fertility status of the flooded soils in order to work out the immediate estimation of flood effects on the soil conditions for agricultural production in flooded area of Thatta, Sindh.

Material and Methods:
Study area:The historic city of Sindh province "Thatta" (23° 43' to 25° 26' N and 67° 05' to 68° 45'E) is the part of lower Indus plane was hit by flood. The sub-district Thatta is situated on right bank of River Indus, hit by flood waters due to breach on Bijora dike during the year 2010. The sub-district's soil are diversified by the mountains in its west north and plain agricultural land on east south (District Govt. Thatta 2006). The soils, heavy textured, alkaline-calcareous and contain high amounts of soluble salts; with low organic matter, N, P and adequate in K (Qureshi, 1996). The crop like rice, sugarcane, oil crops, wheat and vegetables extensively growing in the area. Rainfall periods remains increasing or decreasing and becoming more erratic in recent years, suggest the evidence of climate change. Climatic conditions are semi-arid and sub-tropical continental with average minimum and maximum temperature in ranges of 10.41-39.00ºC. However, humidity and wind velocity are in ranges of 41.04-76.13 % and 13.57-30.23 kmh -1 with a mean of 59.06% and 21.63 km h -1, respectively. The rainfall has an erratic nature and monsoon starts in the July and ends in October with in ranges of 152.0 to 383 mm. Soil sampling and analysis methods: Before flood, soil samples collected in just before flood at 0-15 and 15-30 cm depth for analysis of soil during August 2010. It was also expectations that flood water could breach the dykes near Thatta city. It was decided that flooded soil should be sampled. After flood the soil sampling carried out at same 13 sorted spots where the sampling had been done before in pre flood conditions shown in figure 1. Samples were air dried under shadow, ground, passed through 2 mm sieve and stored in the plastic bags. Physico-chemical properties like particle size distribution (soil texture) were carried out according to Bouyoucos Hydrometer method (Bouyoucos, 1936), EC and pH in 1:2 (soil water extract) as suggested by McLean (1982). Organic matter and CaCO3 (lime content) were determined by Walkley-Black method (Jackson, 1969) and acid neutralization method (Soltanpour and Workman, 1981). The analysis for fertility status comprised of total Nitrogen (%) by Kjeldahl's apparatus (Jackson 1969) and available Phosphorus (mgkg -1 ) by Olsen method (Olsen et al., 1954). Potassium and Sodium extracted with AB-DTPA and determined by the method of Soltanpour and Schawab (1977). Exchangeable bases calcium and magnesium determined by EDTA titration method (Mclean, 1965). Sodium adsorption ratio (SAR) was determined by the formula SAR = Na + √ + )/2 of (USSL 1954).

Statistical analysis:
The categorization of soil for pH, EC, OM and lime content carried out according to NMSU (2000) and soil textural classes identified as suggested by Foth (1982). The soil data so collected was subjected to descriptive mean minimum, maximum, standard deviation, and coefficient of correlation (r), by using Microsoft statistix 8.1 program. and clay loam (7.71 %). The soil clay particle was found predominant in pre flood conditions as compared to sand and silt. In the post flood conditions the clay was decreased significantly (p>0.05) and sand was increased indicated the deposition of sand during the flood water. Flood conditions comparatively changed texture from clay to sandy clay and developed soil became coarser that may improve infiltration rate of soils. The improvement of texture in post-flood soil was reported previously by Zolfahgeri et al., (2013) and Lida et al., (2012).  Table 1  Soil pH results indicated that majority of soil samples remained moderately alkaline in pre-flood conditions and strongly alkaline (pH >8.5-9.0) in postflood conditions. The soil EC and SAR generally indicated for the salt concentration in soils and found that most soils were low to moderately saline (2-8.00 dSm -1 ) in pre-flood conditions and in post-flood conditions EC was found to be highly to severe saline (>8.00-16.00 dSm -1 ) categories. Similarly SAR hazard was increased from 11.15 in non-flood to 17.09 in postflood conditions. Organic matter was fall in poor (<0.5 %) and lime was found in the moderate category (10-15%) category in post flood.

Results and Discussion
The values of soil pH, and SAR were increased 5 and 53 % in post-flood as compared to pre-flood at surface 0-15 cm depth and 4 and 57% in sub-surface 15-30 cm depth indicated increasing soil alkalinity with high deposition of sodium. It might be due to evaporation of water leaving behind salts and Na + . The results were with the conformity of Papadopoulos (1986) who revealed that when irrigation water containing SO4, Na and Cl applied to soil, it increased the soil EC levels. It may later on increase SAR of soil solution. Whereas, increased trend of SAR was also observed by Naderi et al. (2000), stated that SAR was increased by 73% under flooded conditions. Soil nutrients status: Fertility status of study area in Table1 indicated that significant differences were observed for N, P, K, status at (p>0.05) in pre and post flood conditions. The macronutrients N, P, K, mean±standard deviation values were 0.05±0.02 %, 10.73±8.80 and 162.±26.97 mg kg -1 respectively at 0-15 cm depth in pre-flood. Whereas, in post flood conditions the mean values were 0.04±0.02 %, 4.65±2.34 and 201±69.65 mg kg -1 . Similarly at subsurface depth 15-30 cm the NPK values were 0.03 ±0.02%, 7.21±6.61 and 211 ±59.40 mg kg -1 in pre flood and in post flood th values were 0.03±0.01, 6.37 ±4.82 and 228±74.70 mg kg -1 . Secondary macro nutrients i.e. Ca and Mg mean±standard values were 7.62± 2.58 and 7.08±2.55 mg kg -1 at 0-15 cm depth and in post flood the values were 6.12 ±1.86 and 5.41±2.83 mg kg -1 . At depth 15-30 cm Ca and Mg mean values were significantly increased and found to be 7.44± 1.48 and 7.98±1.75 mg kg -1 in pre flood and in post flood the values were 4.44 ±1.68 and 4.88±1.57 mg kg -1 . It was estimated that N, P Ca and Mg, showed decreased trend of 20, 42, 19 and 23% respectively in post flood conditions, whereas K was increased by 19 % at surface soil. Whereas at sub-surface soil there were no significant difference were observed for mean values of NP and K. The results also indicated a positive increase in K while N, P, Ca and Mg were declined in post-flood conditions. It can be assumed that the depletion of N, P Ca and Mg might be due to leaching or run off of nutrients by the heavy load of water. The nitrogen lower values in post flood conditions could be as a result of losses of N through different sources. N is very mobile element is prone to be loose easily through percolation and leaching and volatilization once when the flood water recede. The increase in K content might be possible due to saturation of soil may have effect on smectitic minerals thus release of previously fixed K or deposition of K occurred from river flood water contained considerable amount of K. The results were also in conformity with (Ahmad et al., 2012, Naderi et al., 2000and Eulenstien et al., 1998.

Relationship of physico-chemical Properties:
The relationship in soil properties between pre and postflood indicated that sand, silt, clay, OM and N showed a slight negative correlation with each other in both conditions (Table1). The pH and SAR were negatively correlated (-0.4). EC, lime and Ca had slight positive correlation, P highly significantly positive (0.45) and Mg least significantly negative relationship (-0.30) in both conditions.

Conclusion:
It is concluded that flood had negative effects on soil physico-chemical properties. In the post flood conditions the clay was decreased significantly and sand was increased indicated the deposition of more sand. Flood conditions comparatively changed texture from clay to sandy clay and developed soil became coarser that may improve infiltration rate of soils. The PH and SAR of post flood soils were increased and showed significant positive relationship with each other indicated the disbursement of soil. The high pH affects the nutrients availability to plant. The nutrients i.e. N, P, Ca and Mg values of post flood soils showed decreased trend. Over all it can be implicit that flood conditions increase the soil pH and SAR and depleted the essential nutrients considerably, making soil unproductive for cultivation. To improve crop production on crop term basis the mineral green manuring practices, application of organic matter and fertilizers, are may be recommended