Evaluation of chemical extractants and determination of the potassium critical level in soils under the bean cultivation

Soil tests play a very important role in management of nutrients in the field. The efficiency of a soil test result depends on extractant suitability for determination of available nutrient. Also, for calibration of each nutrient in the soils of a specific region and plant, a critical limit should be determined so that the soil test for each element can be the basis of fertilizer recommendation. Bean (Phaseolus vulgaris L.) is important agricultural product due to high protein. Due to the lack of relevant information in this field, this study was conducted on bean soils in Markazi province. 30 samples of surface soil (0-30cm) with a wide range in terms of available potassium, as well as physical and chemical properties were selected from the bean cultivation soils of the Markazi province and after preparing soil samples in the greenhouse, beans were planted in them.In order to evaluate the response of bean to potassium fertilizer application, two levels of 0 and 100 mgKkg-1 were used as potassium sulfate in this experiment. The greenhouse experiment was performed in a completely randomized design with three replications. At the end of the growing season, harvested plants and plant responses including dry matter weight,  potassium concentration and uptake, and relative growth were determined. Six extractants including distilled water, hydrochloric acid 0.13M, nitric acid 0.5M, calcium chloride 0.025M, Olsen and ammonium acetate 1M were evaluated to extraction of available potassium. The results of analysis of variance showed that the main effects of soil and potassium fertilizer were significant (p<0.01) on dry matter weight, potassium concentration and uptake. But the effect of soil and fertilizer interaction was significant only on plant potassium concentration.Comparison of the mean showed that the effect of potassium fertilizer application were significant on plant responses.The results of correlation coefficient of different extractants with plant responses showed that ammonium acetate (1M) had a positive and significant correlation with plant potassium concentration (0.67**) and uptake (0.63**) while other extractants did not show a significant correlation.Using the Cate and Nelson visual method, the potassium critical level was obtained 150 mgkg-1 soil by extracting with ammonium acetate. Dry matter weight had a positive and significant correlation with available potassium concentration, cation exchange capacity, clay and sand. Plant potassium concentration and uptake had a positive and significant correlation with available potassium concentration (r=0.555**) and soil organic carbon (r = 0.620*).