Evaluation of Soil Fertility Status in Northwest of Iran Drylands by Nutrient Index Value (NIV)

Dryland farming is a major agricultural practice in northwest of Iran. Accurate understanding of soil fertility status is one of the basic needs of dryland agricultural system. Soil chemical properties play an important role in soil fertility. Assessing soil fertility is an essential need to identify environmental-friendly strategies leading to more sustainability in agricultural systems. Unfortunately, plant nutrients are insufficient in many dry farming areas of Iran, or soil conditions do not allow plant to uptake certain nutrients. Therefore, soil scientists focus on using commercial fertilizers and manures (rotation system and conservation tillage) to add nutrients and organic matter to soil. The fertility of soils can be further improved by cultivation of cover crops through adding organic matter to the soil which leads to a healthy soil with more macro-nutrients and micro-nutrients content and better structure. Therefore, evaluation of soil fertility is a basic tool for decision management in drylands 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.

This study was carried out in northwest of Iran drylands including: west Azarbayjan, east Azarbayjan, Kurdistan and Kermanshah provinces. A total of 674 soil samples were collected from farmer’s fields in east Azarbayjan, west 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. After collection, soil samples were immediately 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, 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. Low Medium Equation (1) High 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: Equation (2) 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, the value of 1.67-2.33 reveals optimum fertility (sufficient nutrients). The values greater than 2.33 denote high fertility status.

The results showed that, the Gomes (1985) method could not classify the soil properties in all studied regions (population) correctly, due to the tends towards central limit theorem (optimal condition). Calculation of NIV showed that using conventional method (critical levels) for classification of soil properties was better than Gomes (1985) method because it was more compatible with the field conditions. The results revealed that soil salinity and calcium carbonate did not seriously make problems in dryland areas. However, increasing the amount of calcium carbonate decreased soil phosphorus, potassium, Fe, Mn, Zn and Cu, significantly. Soil phosphorus and Zn were more influenced by increasing calcium carbonate. Assessment of soil fertility status by NIV showed that, soil organic matter was low (deficient) in west and east Azerbaijan with 92 and 69 percent of those areas. But total nitrogen was optimum (sufficient) in all areas with 98 percent averagely (except east Azarbayjan). This is mainly due to the application of nitrogen fertilizers in wheat drylands and conservation tillage system in some areas. Soil phosphorus was 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. The results showed the deficiency of Fe with 100 and 69 percent in west Azarbayjan and Kurdistan, respectively. Deficiency of Mn with 89 percent of west Azarbayjan, Zn with 84 percent in east Azarbayjan and Cu with 100 and 87 percent in west and east Azarbayjan were also the other obtained results of this study, respectively. The results of present study suggested that nitrogen and phosphorus fertilizer applications can also be important in micronutrient management in dryland areas.

It can be concluded that the capability of critical level method is better than Gomes (1985) method in classification of soil properties. Nutrient index value (NIV) method can efficiently evaluate soil fertility status in Iran drylands. According to this research, Fe, Zn and Cu nutrient deficiencies are just as important as P and N deficiencies in Iran dryland areas.