عنوان مقاله [English]
Introduction Dryland farming is a major agricultural practice in northwest of Iran. Knowledge of soil fertility status in this areas is one of the basic needs of dryland agricultural system. Soil chemical properties play important role for the soil fertility and determined after soil testing. The measurement of the fertility of soils is usually done by chemical analysis for plant nutrients. Assessing soil fertility is essential to help identify strategies with less environmental impact in order to achieve more sustainable agricultural systems. Unfortunately, many dry farming areas in Iran do not have adequate levels of all the necessary plant nutrients, or conditions in the soil are unfavorable for plant uptake of certain nutrients. Soil scientists focus on using commercial fertilizers and manures (rotation system and conservation tillage) to add nutrients and organic matter to the soil. Soil fertility can be further improved by incorporating cover crops that add organic matter to the soil, which leads to improved soil macro-nutrients and micronutrients, structure and promotes a healthy. Therefore, soil fertility evaluation of Iran dryland regions is most basic decision making tool for the sustainable soil nutrient management in this areas and estimation of capacity of soil to maintain a continuous supply of plant nutrients for a crop production. Evaluation of soil fertility in drylands of the northwest Iran have two objectives 1) Assess nutrient status of soil-crop system 2) Diagnose suspected nutrient imbalances.
Materials and Methods This study was carried out in northwest of Iran drylands included: west Azarbayjan, east Azarbayjan, Kordistan and Kermanshah provinces. A total of 674 soil samples were collected from farmer’s fields in east Azarbayjan, weast Azarbayjan, Kurdistan and Kermanshah 414, 97, 90 and 73 samples respectively. The surface soil samples were taken from 0-25 cm depth in each field before the sowing of the rainfed plants in autumn by composite sampling method. Immediately after collection soil samples were dried, grounded, screened through 2 mm sieve, labelled and stored in plastic container. The samples were analyzed for 12 chemical and physical parameters include: soil texture (hydrometer method), pH (saturation paste) and EC (saturated extract), organic carbon (Walkley and Black, 1934), Total N (Kejeltak), calcium carbonate equivalence (acid-neutralizing value), phosphorus (Olsen), potassium (sodium bicarbonate extracted) and iron, zinc, Mn and copper (DTPA extracted). Soil samples were categorized as low, medium and high on the basis of their availability in soils by two Gomes (1985) (equation 1) and common (nutrient classification by critical level method for dryland wheat) methods.
Where, , and SD are soil property, average of soil property in all area and standard deviation of soil property, respectively. In order to compare the levels of soil fertility of one province with those of another it is necessary to obtain a single value for each nutrient. Nutrient index value (NIV) was calculated by Parker et al., (1951) method (equation 2) for soil samples of each province or district from the proportion of soils under low, medium and high categories using following equation:
Where, , and are number of samples testing low, medium and high category in each province, respectively. If the NIV is less than 1.67, the soil fertility status is low (nutrients or other property) and the value is 1.67-2.33 the fertility status is optimum (sufficiency) nutrients. The value greater than 2.33, the fertility status is high nutrients.
Results and Discussion The results showed that, the Gomes (1985) method could not classify the soil properties in all region (population) correctly, due to the tends towards central limit theorem (optimal condition). In the calculation of NIV, the conventional method (critical levels) for classification of soil properties was better than Gomes (1985) method because it was more compatible with the field conditions. Soil salinity and calcium carbonate are not problems in dryland areas seriously. But with increasing amount of calcium carbonate, soil phosphorus, potassium, Fe, Mn, Zn and Cu decreased significantly. But soil phosphorus and Zn deficiencies were more sensitive to increase soil calcium carbonate. Assessment of soil fertility status by NIV showed that, soil organic matter were low (deficient) in west and east Azerbaijan with 92 and 69 percent of those areas. But total nitrogen were optimum (sufficient) in all areas with 98 percent averagely (except east Azarbayjan). This is mainly due to the application of nitrogen fertilizers for dryland wheat production and apply conservation tillage in some areas. Soil phosphorus were evaluated low in two west Azerbaijan (81%) and Kermanshah (67%) provinces, but in east Azerbaijan (68 %) and Kurdistan (85%) were sufficiency or high for wheat production. Potassium was more than sufficiency (high) in 90 percent of all areas averagely. Micronutrients deficiency were observed in some provinces. Deficiency of Fe with 100 and 69 percent in west Azarbayjan and Kurdistan respectively, Mn with 89 percent of west Azarbayjan, Zn 84 percent in east Azarbayjan and Cu with 100 and 87 percent in west and east Azarbayjan respectively. These results suggest that, in addition, nitrogen and phosphorus fertilizer applications should also be important for micronutrient management in dryland areas.
Conclusions It can be concluded that, the capability of critical level method is better than Gomes (1985) method in the classification of soil properties. Nutrient index value (NIV) method can be evaluated soil fertility status in Iran dryland conditions. According to this, there is deficiency of Fe, Zn and Cu elements in addition to the P and N nutrients in Iran dryland areas.