..Investigation of Nutritional Status of Potato (Solanum tuberosum L.) in Khorasan Razavi Province by "Deviation from Optimum Percentage" (DOP) Method

Because leaf is the main and most important organ of plant metabolism, therefore, its analysis and interpretation of the results using standard methods, can help to provide an optimal fertilizer program. There are different methods for interpreting the results of decomposition of plant leaves, including critical concentration values and sufficiency ranges. The main drawback of these methods is that only the concentrations of each nutrients are considered and the balance of these nutrients is not considered. The DOP is one of the most up-to-date methods for interpreting the results of leaf decomposition. In this method, the relationship between the concentration of the nutrient and the reference concentration is used instead of the mere concentration of nutrients. Due to insufficient information about the optimal level of nutrients and nutritional status of potato in Khorasan Razavi province, the present study was conducted by using the DOP index to assess the balance of nutrients and to determine the order of nutrient requirement for this plant. In order to have a database, 30 potato fields were selected from the major cultivation areas of this crop in Fariman, Torbat-e Heydarieh, and Quchan counties. Among these fields - based on questionnaires completed by farmers - those with good management and high yield (above average) were selected to determine the reference concentration. Accordingly, out of 30 surveyed fields, 16 fields were selected as reference fields and 14 fields as non-reference fields. In each field, before flowering and at the beginning of tuber formation, a composite sample of leaves (from fully developed leaves) was prepared, transferred to the laboratory, rinsed with distilled water and 20 mM EDTA solution, oven dried at 50 °C, ground, digested and analyzed for determining the macro and micro nutrients concentrations by atomic absorption spectrometry (AAS) - (Perkin Elmer, 2380). According to leaf analysis results, some statistical indices, including the maximum, minimum, mean, standard deviation, and coefficient of variation of data in both groups of reference and non-reference fields, were calculated. Furthermore, the reference concentration (Cref) and DOP indices were calculated to evaluate nutritional status of potato and priority nutrients requirement in low-yielding fields. The results of this study showed that the coefficient of variation of micronutrients was higher than that of macronutrients, stating that fertilizers containing macronutrients have been used more homogeneously by farmers, compared to those containing micronutrients. Using nutrients concentration in high yielding fields, Cref was obtained for N, P and K nutrients 4.65, 0.32 and 4.28 percent respectively. In addition, Fe, Mn, Zn and Cu Cref values were 190.13, 123.88, 35.19 and 12.44 mg/kg respectively. The results also showed that absolute value of DOP index for all nutrients was greater than zero, indicating imbalance of absorbed nutrients by potato. Due to the fact that the concentration of nutrients in the leaves is a function of the amount of nutrients in the soil, so these data indicate the lack of proper management and imbalance fertilizer application in the studied fields. Among macronutrients, potassium had negative indices in 36% of the studied fields while nitrogen had positive indices in 80% of the studied fields. In other words, most cultivated potatoes of Khorasan Razavi province suffer from K deficiency and excessive N. Due to consecutive cultivation, excessive use of nitrogen fertilizers such as urea and low consumption of potassium fertilizers, higher amount of potassium harvested from the soil as compared with that released from the soil, potassium deficiency is observed in many potato fields. Among micronutrients, zinc had highest negative indices in 57% of the studied fields. After Zn, Fe had the most negative index (highest deficiency) in 35% of fields. Excessive consumption of phosphate fertilizers and lack of sufficient organic matter in soils are the main reasons of iron and zinc deficiency in soils and agricultural products in Razavi Khorasan province. Overall, the results showed an imbalance of all absorbed nutrients by potato and imbalance application of fertilizer in the studied fields. Due to the severe deficiency of K and Zn, fertilization of these two nutrients should be a priority in nutrition management programs in the province's potato fields.