نشریه پژوهشهای خاک (علوم خاک و آب)
سال بیست و چهارم شماره 3 (زمستان 1389)

The geographical distribution and salinity levels of the soil resources of Iran are discussed and quantified information presented at national and regional scales. At national level, salinity information was extracted from 1:1,000,000 soil resources and potentiality map of Iran on a total-land basis. The information on salinity levels at regional scale was obtained through manipulation of the measured soil EC values, which are accumulated in the archives of Soil and Water Research Institute of Iran over the last 50 years. From a total of 6.8 million hectares of salt-affected agricultural lands, about 4.3 million hectares have only salinity limitation without posing any other limitations for sustainable crop production. The other 2.5 million hectares have other limitations related to soil, topography/erosion, and depth to the groundwater table as well as salinity limitation. In most salt-affected soils in the agricultural lands of the country, the depth to the groundwater table is lower than the root zone. The land areas in which the depth to the water table falls within the root zone cover only 8.4 percent of the total salt-affected agricultural lands. This suggests that reclamation of salt-affected lands is technically practical and economically viable.

In the arid and semi arid regions, accumulation of excess soluble salts in plants root zone is a limiting factor for crop production. The level and distribution of salinity in cropped lands vary in different times during the growth season as a result of interaction of rainfall, irrigation, evapotranspiration, leaching, and drainage. These relationships and interactions are complicated and not fully understood. Therefore, computer models are useful for predicting the response of the system to such complicated conditions and final evaluation of irrigation water quality. In the present study, three steady state models including WATSUIT, Hoffman and Van Genuchten models (exponential, trapezoid and 40-30-20-10 water uptake pattern) and crop water production function (CWPF) were evaluated. The required input data for running the models and the measured soils salinity were collected from experimental plots as well as farmer's cropped fields in different years in Fars Province. The results showed that soil salinities predicted by WATSUIT model and CWPF model were, respectively, lower and higher than that by the other models. The models were evaluated by 2 statistical methods including root mean square error (RMSE) and coefficient of determination (D). Statistical index showed that the least error belonged to Hoffman and Van Genuchten models (40-30-2-10 water uptake pattern). Therefore, the capability of this model to predict soil salinity is more than the other models. Both statistical indexes showed that CWPF model predicted soil salinity with considerable error. Thus, this model was not suitable for predicting soil salinity in the present experimental conditions.

Soil structure and aggregate stability affect soil erodibility and increasing aggregate stability against erosive factors such as wind and water is a necessity. This study was conducted on surface soil samples (0-20cm) collected from agricultural lands susceptible to erosion located in Chahoo Plain, southeast of Fars Province. The experimental design was CRD with 3 replicates and10 treatments as follows: control (without addition of soil amendments), pure gypsum (1% w/w), chopped wheat straw (1% w/w), animal manure (1% w/w), wheat straw + gypsum, animal manure + gypsum, cement (0.3% w/w), cement (0.6% w/w), cement (0.9% w/w) and gypsum (1% w/w) + cement (0.9% w/w). After one, four, seven and ten months, the amounts of soil aggregates in 53-4000 µm sizes were determined by wet sieving and MWD were calculated. Application of wheat straw and wheat straw with gypsum increased MWD compared to the control and other treatments. These increases were significant at p<0.05. The mentioned treatments increased the MWD as much as 2.8 and 2.9 times compared to control, respectively. Generally, for increasing the stability of the soils similar to the soil understudy, application of wheat straw alone or wheat straw with gypsum seems recommendable.

Salt tolerance threshold is determined by considering the yield reduction rate in saline media compared to non-saline conditions. This value can be used for comparing salt tolerance among crop varieties. To determine salt tolerance threshold of wheat, a field experiment arranged as spilt plot in completely randomized block design was conducted with three replications. Experimental treatments were irrigation with saline water (2, 5, 8, 11, 14 and 20 dS.m-1) and wheat cultivars, namely, Kavir, Roshan, Mahoti and Marvdasht. Results showed that salt tolerance threshold values for Kavir, Roshan, Mahoti and Marvdasht were 5.0, 4.6, 3.3, and 5.7dS.m-1, respectively. With each increase in salinity level above the salt tolerance threshold, grain yield of Kavir, Roshan, Mahoti and Marvdasht was reduced by 4.5, 4.1, 4.5, and 5.5 percent, respectively. The salinity at which the yield is reduced by 50% were 15.1, 16.0, 13.0 and 12.9 for Kavir, Roshan, Mahoti and Marvdasht, respectively. Based on the U. S. Salinity Laboratory rating, these cultivars are moderately salt tolerant.

Electromagnetic induction tools are extensively applied worldwide for rapid evaluation of spatial variability in soil salinity. This study was conducted in pistachio orchards of Chah Afzal area of Yazd Province for assessment of soil salinity using electromagnetic induction (EM38) device in two plots of 45 and 40 ha with clay loam soils and average moisture contents of 25 and 35 percents, respectively. A systematic sampling grid with the size of 20×80m was established. At each grid point two EM38 readings were taken in horizontal (EMh) and vertical (EMv) orientations. Different calibration equations were employed for conversion of the readings to electrical conductivity of the saturation paste and the best method was selected. Results of this study showed that the best calibration method was multiple linear regression equations fitted to the field data. It was also found that more reliable regression equations could be derived at 35% moisture content in comparison with 25% moisture content between soil salinity at different depths of soil and horizontal/vertical readings, with correlation coefficients (R2) of 0.67 to 0.85, respectively. Consequently, soil salinity maps of different depth intervals and also soil salinity profile maps at upstream and downstream of the field borders were generated and interpreted.

The purpose of this experiment was to study the effect of salinity on absorption of some nutrition elements and the symbiosis of Sinorhizobium meliloti with different clones of alfalfa. Strains of Sinorhizobium meliloti were isolated from alfalfa fields in Tehran Province. After the isolation and purification processes, alfalfa seeds were inoculated with the efficient strains of symbiotic Sinorhizobium meliloti. Finally, 2 strains were selected, one of which was resistant and the other was sensitive to salinity. The effect of inoculation with these strains were investigated on the growth and the yield of 3 medicago sativa Genotypes (Hamadani, Gharayonjeh and Gharghalogh) at 3 levels of salinity (EC= 0, 6, 12 dS/m), using a completely randomized block designed in factorial form with 3 replications. Results showed that with increase in salinity, shoot and root dry weight, nodule number, and the plants concentration of nitrogen, phosphorus, and potassium decreased significantly, whereas the concentration of sodium increased. In addition, according to the results, inoculation with the Sinorhizobium meliloti strain resistant to salinity caused significant increase in shoot and root dry weight, nodule numbers, and the concentration of nitrogen, potassium, and phosphorus in comparison to the control. There were no significant differences between the alfalfa genotypes in yield or other growth indices.

Determination of drainage water salinity can be a useful tool for precise soil salinity control. On the other hand, leaching fraction under steady state conditions can be estimated simply by knowing drainage water salinity. But, when drainage facilities are not installed, drainage water is not available conveniently. A device called "Wetting Front Detector" has provided this possibility using no electrical or mechanical device. In this research, actual drainage water salinity and leaching fraction was directly measured by installing this device in irrigated pistachio orchards of north Ardakan, in southern parts of the Iranian Central Plateau. The measured data were used as entries for simulation models to estimate mean root zone salinity. Estimates of the models were compared with the measured mean root zone salinity. Leaching fraction was estimated to be about 40% using this approach and measured mean soil salinity was generally between approximated values using two models. There was also a fair correlation between the actual measured values and the estimated factors using the two models.

Total soluble salt is the most important index for assessing irrigation water quality. High concentrations of anions or cations affect quality of irrigation water either directly or indirectly. In order to assess water chemistry and type, sixty groundwater samples were collected from different wells in Fars Province. Anionic and cationic concentrations of water samples were measured and saturation indices were calculated. The chemical composition of water samples was plotted in piper diagram. This diagram revealed that Cl- and Na+ were the dominant ions in the water samples studied. Seven out of 60 water samples fell in the nonrestrictive class. The higher value of SAR coincided with the high values of EC, therefore, 95 percent of the samples could be used for irrigation with little danger of infiltration problems. Sixty three percent of the groundwater samples had magnesium ratios more than 50%, showing an imbalanced concentration of Mg compared to Ca. Saturation indices of the studied waters showed that water samples were saturated with respect to calcite, aragonite and dolomite, while a small number of water samples (1.6 %) contained high concentrations of SO42-, which, upon further concentration, could precipitate in the soil.

To determine various stability indices related to grain yield of wheat genotypes in different saline conditions, an experiment was conducted at the Salinity Research Farm, National Salinity Research Center. Treatments consisted of 14 wheat genotypes that were evaluated under saline and non-saline conditions. Electrical conductivities of the irrigation water used for saline and non-saline conditions were 10.0 and 2.5 dS/m, respectively. Results showed that grain yield and kernel per spike under non-saline conditions were significantly higher than those under saline conditions, for all genotypes. There were no significant differences between saline and non-saline conditions with regard to thousand kernel weight. Grain yields of the genotypes were significantly different in both conditions. Genotype No. 5 (PF70354/BOW) produced the highest grain yield under saline and non-saline conditions by producing 723.44 and 976.56 g.m-2, respectively. The correlations among grain yield in saline and non-saline conditions, mean productivity (MP), and Salinity Tolerance Index (STI) were significant. Thus, STI could be considered as an effective index for selecting wheat genotypes under different saline conditions.