To Evaluate the Effect of Soil Physical and Chemical Characteristics on the Growth Characteristics of Saffron (Crocus sativus L.) Corms in Tornbat-e Heydariyeh Area

Saffron is one of the most economically important plants across Iran and all over the world. The most important cultivated areas of saffron are in Khorasan-e Razavi and Southern Khorasan provinces (Jihad Keshavarzi Khorasan Razavi, 2013). The corm is the reservoir of photosynthetic materials and plays an important role in the saffron life cycle. Corm size and physicochemical characteristics of soil determine the growth and yield of saffron (Aytekin et al., 2008). It has been advised to use corms with diameter more than 2.5 cm (Kafi et al., 2002). Despite the importance of this plant, few studies have been conducted on the effects of soil characteristics on the growth of the plant in natural field conditions. Therefore, the objective which we will try to achieve is: to evaluate the effect of soil physical and chemical characteristics of the growth of saffron corm and determine the most important effect characteristics.
In September 2012, 30 samples of 3 to 5 years old saffron corms were taken from the fields with similar management in Torbat Heydariyeh. The surrounding soil corms (depth of 0 to 30 cm) were sampled, too. Dry weight of corms was measured in the laboratory. Air dried soil samples were passed through a 2 mm sieve and used for physical-chemical analyses. Soil texture was determined by using pipet method. Total nitrogen, available phosphorous, available potassium, soil organic carbon and calcium carbonate equivalent were measured in bulk soil samples. Calcium, magnesium, sodium, EC and pH were measured in the saturated soil paste. Fe, Cu and Zn were extracted by DTPA and measured by atomic adsorption spectroscopy. Correlation, regression and neural network technique were used to analyze the data and to identify the most important soil characteristics on the corms characteristics.
Diameter and dry weight of corms with mean values of 34.04 mm and 3.72 g, ranged from 22.8 to 51.7 mm and 1.5 to 6.98 g, respectively. Wet weight of the corms with an average of 11.4 g varied between 5.4 to 19.86 g that categorized in medium to very coarse classes. Soil characteristics showed wide range variability; therefore, the studied soils indicated suitable range of characteristic variables in this study. Calcium carbonate equivalent of soils was less than 17% with mean values of 12.75%. Mean values of EC and pH were 2.6 dS m-1 7.75, respectively. The average of soil organic carbon was 0.59% which reached up to 1.91% due to the application of manure. Silt with mean value of 43.55% was the dominant fraction of the soils, while the highest content of clay and sand were 27.65 and 47.6%. The Average concentration of Fe, Cu and Zn were 4.3, 1.56 and 0.42 mg.kg-1 respectively.
Weak correlation of soil characteristics with corms diameter and weight and the result of ANOVA regression models indicated that there is a non-linear relationship between growth characteristics of corm and soil characteristics. In such cases, regression analyses cannot explain the relationship between growth parameters and soil characteristics. Artificial Neural Network (ANN) has the ability to model the non-linear relationships. Therefore, the relationship between soil parameters and saffron corm were determined by ANN to find the relationship between soil characteristics and corm growth. Perceptron Multi-Layers Neural Network with arrangement of 1-21-18, explained the relationship of between corm diameter (R2=0.94, ME=0.01 and RMSE=0.028) and dry weight of saffron corm (R2=0.92, ME=0.008 and RMSE=0.047) with soil properties. The proposed neural network explained 94% relationship of the saffron corm diameter and 92% relationship of the dry weight of saffron corm with soil characteristics. Sensitivity analysis indicated that electrical conductivity, Zn, N, pH, P, K and gravel percentage are the most effective characteristics on dry weight and sand, Cu, silt, clay, electrical conductivity, SAR, Zn and P, are the most important effective characteristics on the diameter of saffron corm.
The results of this study revealed that there is no significant correlation between diameter and weight of saffron corm and soil characteristics. Therefore, the ANN technique was used to determine the effective soil characteristics of corm growth parameters. Electrical conductivity, Cu, Zn, N, pH, P, K, SAR, gravel, sand, silt and clay are the most effective characteristics on corm growth. More researches should be conducted to understand, comprehensive relationship between soil characteristics and corm growth