Soil salinity/sodicity occupieva prominent place amongst the soil problems that threaten the sustainability of agriculture in Pakistan. Rafiq (1990) reported that there were 5.8 mha of salt affected land in Pakistan. Out of which 3.16 mha were within the canal commands (CCA) and 2.64 mha out of CCA. Of the area within CCA 2.93 mha are canal under cultivation and thus has real importance of practical significance. Problems faced by these type of soils consist of: dense layer, high salt contents, intermittent flooding etc.
Bhatti et al. (1998) observed that variable rates of gypsum application, depending on soil characteristics, were more effective in reclaiming the soil than a single uniform rate. Gypsum requirement was significantly lower in the variable management strategy than uniform rate. Similarly the seed, fertilizer, irrigation water are not only wasted but there is also a great loss in the form of reduced farm productivity (Khan, 1988).
The choice of method for planting crops depends on its permeability. Under poorly drained soils water can stand on the soil surface for a long time causing waterlogging. In this situation land should be prepared in the form of raised beds (Qureshi and Barrettt-Lennard, 1998). Wheat grown with conventional cultivation yielded 0.6 tons ha1. However, 15 cm high and 30,45 and 60 cm wide beds had yielded 1.07, 1.06 and 1.20 tons ha1, respectively (Qureshi and Aslam, 1988). Abrol et al. (1988) reported that with double beds most of the salts accumulated in the middle of the bed, leaving the edges relatively free of salts. Sloping beds may be slightly better on highly saline soils because seed can be planted on the slope below the zone of salt accumulation.
Understanding spatial variability within individual fields may offer unique insight leading to more precise and therefore successful management. In order to plan remedial measures for the areas and judicious use of costly inputs it is necessary to assess the areas affected and the rate at which salinization and sodication has taken place. Mapping of soil salinity is an essential component of a reclamation scheme. Considering the importance of problem, present piece of work was carried out to evaluate the spatial variability of soil salinity/sodicity within field and profile and its effect on maize crop under two planting methods.
Materials and Methods
A field study was conducted at Mardan. The study site is located on 34-12° E latitude and 72-03° N longitude. The soil of the site belongs to Mardan series classified as fine Ustertic Camborthid developed in filled basin and river beds, grayish brown, non to slightly calcareous material of Holocene age. Study site measured 272 m × 61 m. The experiment was laidout adopting two planting techniques (Bed-furrow and flat). The maize crop in flat was sown by bullock drawn drill and with bed-former cum planter under bed-furrow system. Seedling emergence, crop stand establishment and yield data were recorded.
A grid wise (20 m × 20 m) salinity/sodicity survey of the site was carried out. Soil samples from profiles of the grid points were collected for four depths (0-15 cm, 15-30 cm, 30-60 cm and 60-90 cm). Salt concentration and pH of the soil samples were determined through a 1:1 suspension method (Richards, 1954) where equal weight of dried and grinded soil and distilled water were mixed and shook for one hour. The saturation percentage of soil was 50. So EC values can be doubled to convert on saturated soil paste basis. The EC and pH for different depths were plotted separately. Salinity and sodicity classes were developed with an appropriate class intervals. The data were digitized and site was classified into different classes for each depth using Arc/Info and Arc View computer software. Three locations representing slight, moderate and severe soil salinity/sodicity were selected for measurement of infiltration characteristics in triplicate. Double ring infiltrometer was used for infiltration measurements (Michael, 1978).
The data thus generated for soil salinity/sodicity, and crop characteristics like seedling emergence, crop-stand, plant height, yield etc were analysed statistically for different parameters (mean, standard deviation, coefficient of variation and regression equation etc). Treatments were evaluated applying T-test and LSD-test (Steel and Torrie, 1980).
Results and Discussion
Spatial Analysis for Soil Salinity and Sodicity Soil pH: Table 1 indicates the distribution of soil pH in different layers of soil profile. The difference in maximum and minimum values of pH in different layers ranged from 2.09 to 2.45 units. Mean values of pH ranged from 8.04 to 8.60. Soil pH increased with depth. Of the 36 grid points 75% showed a gradual increasing pH trend with depth. Whereas 25% points showed mixed trend. The difference in pH of 0-15 cm and 6090 cm depth ranged from 0.04 to 1.36 units with an average value of 0.56 unit and Coefficient of Variation (CV) 60 percent.
|Table 1:||Statistical analysis of soil pH and EC of the experimental area, Mardan, Pakistan
|Table 2:||Distribution of soil pH and EC in the experimental area, Mardan, Pakistan
|Table 3:||Correlation between pH and EC in the experimental area, Mardan, Pakistan
The variation in pH within different layers of soil profile was low with CV of 8.5 to 8.4%.
The site was classified into different pH classes (Table 2). Considering pH of different layers it was observed that in the surface 0-15 cm 81.7% area possessed pH less than critical. Whereas 67.2, 58.5 and 68.0% areas of 15-30, 30-60 and 80-90 cms layers have pH below this critical level. So the subsurface layers pose greater sodicity problem than the surface layer.
|Fig. 1:||Cumulative intake as affected by soil salinity/sodicity at ACIAR project site, Mardan
|Fig. 2:||Infiltration rate as affected by soil salinity/sodicity at Elapsed Time (minutes) ACIAR project site, Mardan
Total Soluble: Salts (US): As regards with soil salinity, of the 36 grid points 19% showed gradual increasing trend in EC with depth, 19% showed gradual decreasing trend and remaining 62% showed mixed trend. The average values of EC for different layers ranged from 0.69 to 0.73 dSm1 with CV of 76.4 to 84.5% (Table 1). The higher values of CV indicate that soil salinity in different layers varied greatly than soil sodicity. This variation may be attributed to different factors. A common reason is un-even soil surface. Another reason for the occurrence of these anomalies could be the differences in soil permeability caused by textural/structural and sodicity variations which resulted in differential leaching of salts.
Soil salinity (EC) is not of much significance. Only 5.8% area of surface layer (0-15 cm) contains EC greater than critical level. Whereas, 3.1, 2.9 and 8.4% areas of 15-30, 30-60 and 60-90 cms layers have EC below this critical level (Table 2). Regression analysis between soil pH and EC for different layers was carried out and models are given in Table 3. The R2 values increased with the increase in soil depth. For 0-15 cm and 15-30 cm layers it is quite poor. However, for lower layers values of R2 are reasonable. It may be due to the fact that subsurface layers contain sodium salts which have leached down from surface layers.
Intake Rats and Depth: Soil intake characteristics were affected by soil sodicity. Cumulative intakes and intake rates for locations having with slight (pH<7.5), moderate (pH = 8.5) and severe (pH > 9.0) soil sodicity are given in Fig. 1 and 2.
|Table 4:|| Statistical parameters of plant growth and yield components at experimental area, Mardan, Pakistan
|Table 5:|| Correlation between pH, EC and different plant characteristics under two methods of planting maize at experimental area, Mardan, Pakistan
|Subscript: indicates 0-15 cm soil depth
|Table 6:|| Correlation between pH, EC and plant height at harvest under two planting methods at experimental area, Mardan, Pakistan
|Subscript 1 = 0-15 cm depth; 2 = 15-30 cm; 3 = 30-60 cm and 4 = 60-90 cm.|
* and ** = indicated that R2 values are significant at 1 and 5% level
|Table 7:|| Correlation between pH, EC and total biomass, grain yield of maize under two planting methods at experimental area, Mardan, Pakistan
|Subscript 1 = 0-15 cm depth; 2 = 15-30 cm; 3 = 30-60 cm and 4 = 60-90 cm.|
* and ** = indicated that R2 values are significant at 1 and 5% level, respectively
There was very little difference in cumulative intake and intake rate of moderately and strongly saline sodic spots. Cumulative intake after 3 hr was 6 and 8 times lower than slightly saline-sodic spots. Whereas, intake rate initially was 3 and 2.5 times lower which turned to 7.7 and 11.5 times lower after about 3 hours.
Spatial Variability in Yield: Maize yield data were collected from selected grid points and were analysed (Table 4). There was quite variations in yield and yield components which may be attributed to spatial distribution of soil salinity/sodicity in the experimental field. In this study Kharif maize showed reduction in dry matter accumulation and yield at elevated levels of soil salinity and sodicity. Soil salinity/sodicity has a two fold effect on plants. Salts in soil solution decrease the availability of water to roots and salts taken up by the plants can accumulate to toxic levels in certain tissues. Both factors affect growth and finally the yield.
Highest CV (71%) was observed for grain yield which is due to cumulative effect of other yield components like plant population, plant height, cob size etc. The average grain yield of spots with less than 8.5 pH was 0.336 kg m2 whereas, mean yield of spots with pH higher than 8.5 pH was 0.066 kg m2. Considering the percentage areas with pH less than 8.5 and greater than 8.5 and their respective yield levels it can be concluded that patchy sodicity problem reduced productivity of the study site by about 15%.
Table 5 and 6 present regression models for plant population, plant heights (41 days after planting and at harvest) and soil pH, EC under two planting methods. Considering the R2 values for different regression models it can be inferred that greater amount of plant population and plant height variations were attributed to soil pH than EC. Similarly the models for raised-beds possess higher R2 values than respective flat planting method and have a better fit of regression line to data. Table 7 presents correlation between pH, EC with total biomass, and grain yield for two planting methods. Like plant population and plant heights total biomass and grain yield regression models gave better R2 values under raised beds than flat planting method. The magnitude of R2 for models show that relationship between total biomass and grain yields and pH was more than their relation with EC. The R2 values improved when pH of all the four layers were considered instead of pH of surface layer. The ill effects induced by pH were offset by raised beds. Soil sodicity reduces its permeability. Under poorly drained soils irrigation/rainfall water can stand on the soil surface for a long time causing temporary water logging. Water stagnation in standing crops results in problems of aeration and nutrient supply (Abrol et al., 1988). In this situation raised-beds provide better root environment and improves crop-stand and yields especially for water sensitive crops like maize (Qureshi and Aslam, 1988; Qureshi and Barrettt-Lennard, 1998). Results of this study indicated that there is great spatial variability (horizontally and vertically) of soil salinity and sodicity in cropped field. About 75% grid points showed gradual increasing trend in pH with depth, whereas, EC should mixed trend, Maize crop yield and yield components were significantly reduced with elevated levels of soil salinity and sodicity. Raised bed planting method improved crop yield over the flat planting method. Thus under sodic soils raised bed planting technique can be employed to offset the soil permeability problems caused by soil sodicity and enhance crop productivities.
This piece of research work was carried out under the ACIAR funded project at LWR2/1998/131 Permanent Raised Beds to Improve Productivity and Central Salinity in Pakistan the financial and technical support is highly acknowledged.