تاثیر اسیدهای آلی و ورمی کمپوست بر تغییر شکل های مختلف پتاسیم در خاک های آهکی جنوب ایران

مواد و اسیدهای آلی، نقش مهمی در افزایش فراهمی زیستی عناصر غذایی خاک به ویژه پتاسیم بازی می نمایند. در این مطالعه 9 خاک با تیپهای متنوع از مناطق مختلف استان فارس انتخاب گردید. آزمایش های مربوطه به صورت فاکتوریل در قالب طرح کاملاً تصادفی در 3 زمان خوابانیدن (5، 15 و 60 روز) با 4 تیمار ترکیبات آلی (شامل ورمی کمپوست 2 درصد، 3 نوع اسید آلی شامل اسیدهای سیتریک، مالیک و اگزالیک با غلظت 250 میلی مول برکیلوگرم و یک تیمار شاهد) در سه تکرار انجام گرفت. پتاسیم تبادلی و غیرتبادلی خاک ها به ترتیب در دامنه 166 تا 378 و 282 تا 1694 میلی گرم در کیلوگرم خاک بوده اند. کانی های غالب در خاک ها شامل ایلیت، پالیگورسکیت، کلریت و اسمکتیت بودند. نتایج نشان داد که تیمار اسیدهای آلی و ورمی کمپوست همگی موجب افزایش معنی دار پتاسیم محلول (از 25 به 78، 58، 90 و 75 میلی گرم در کیلوگرم به ترتیب برای اسیدهای سیتریک، مالیک و اگزالیک و ورمی کمپوست) در زمان 60 روز نسبت به تیمار شاهد شدند. همچنین این تیمارها موجب افزایش معنی دار پتاسیم تبادلی نسبت به تیمار شاهد (از 235 به 400، 430، 445 و 425 میلی گرم در کیلوگرم) شده اند. تفاوت معنی داری بین پتاسیم تبادلی در تیمارهای اسیدهای آلی و ورمی کمپوست در زمان های 5 و 15 مشاهده نشد، اما بین تیمارهای اسید مالیک و اسید اگزالیک در زمان 60 روز تفاوت معنی داری مشاهده شد. میزان پتاسیم غیرتبادلی در تیمارهای اسید سیتریک، مالیک و اگزالیک و ورمی کمپوست افزایش معنی داری در زمان 60 روز (به ترتیب از 712 به 1140، 992، 1210 و 1035 میلی گرم در کیلوگرم) نسبت به تیمار شاهد نشان داد.

Effect of Organic Acids and Vermicompost on Potassium Transformations in Calcareous Soils of Southern Iran

Introduction: Potassium is an essential element for plant growth and exists as four forms in soils: soluble, exchangeable, nonexchangeable, and mineral. Soluble and exchangeable K are considered as readily available and nonexchangeable K as slowly available. Organic matters and acids play an important role in increasing the bioavailability of nutrients especially potassium in the soils. Organic acids are lowmolecular weight CHO containing compounds which are found in all organisms and which are characterized by the possession of one or more carboxyl groups. Depending on the dissociation properties and number of these carboxylic groups, organic acids can carry varying negative charge, thereby allowing the complexation of metal cations in solution and the displacement of anions from the soil matrix.The ability of an organic acid to release K from soils depends on some factors such as: diffusion rate of the organic acid in soil, the diffusion capability of organic acidelement complexes, the contact time of the organic acid on a mineral surface, the ionization of the organic acid, the functional group of the organic acid and its position, and the chemical affinity between the organic acid and the mineral elements. This study was conducted in order to evaluate the effect of organic acids and vermicompost on transformation of K in some selected soils of Fars Province, southern Iran. Materials and Methods: In this study, nine soils with enough diversity were selected from different parts of Fars Province. The experiment was done as a completely randomized design with three replications, consisting of three incubation times (5, 15 and 60 days) and four organic compounds (including 2% vermicompost, three acids of citric, malic and oxalic acid eachat a concentration of 250 mmolkg1and one control). The samples were incubated at 50% of saturation moisture at 22°C. Routine physicochemical analyses and clay mineralogy were performed on soil samples. Soil reaction, texture, electrical conductivity, calcium carbonate, and gypsum were identified. Soluble, exchangeable, nonexchangeable and mineral potassium were measured. The amounts of K forms in each sample were determined. Total K was determined following digestion of soil (110°C) with 48 % HF and 6 M HCl. Water soluble K was measured in the saturated extract. Exchangeable K was extracted with 20 ml 1.0 M NH4OAc (pH 7.0) for 5 min. Nitric acidextractable K was measured by extraction of a soil sample with boiling 1.0 M HNO3 for 1 h. Potassium was measured on all filtrated extracts by flame photometer. The content of clay minerals was determined semiquantitatively, using peak areas on the diffractograms of ethylene glycol solvated specimens. Statistical analysis was accomplished using the SPSS 16.0 software and the comparison of mean values was done using the Duncan test at the 5% level of significance. Results and Discussion: The amount of different forms of K including water soluble, exchangeable, HNO3extractable, and mineral K are relatively high in the studied soils. Mineralogical analysis indicated that smectite, illite, palygorskite and chlorite were the major minerals in the clay fractions. The results also showed that exchangeable, nonexchangeable and total potassium were in the range of 166 to 378, 282 to 1694, and 2312 to 8437 mg/kg1, respectively.Organic acids and vermicompostwere led toa significant increase in soluble K at all times compared to control and vermicompost treatment exhibited greater effect. These treatments also significantly increased exchangeable potassium compared to control. Significant differences between exchangeable potassium of organic acids and vermicompost treatments were not observed at 5 and 15 days, but significant differences were observed between treatments of mallic and oxalic acids at 60 days. Compared to the control, the non exchancheable K showed significant increase in all three organic acid treatments and vermicompost at 15 and 60 days. Conclusion: Based on the results, while exchangeable and nonexchangeable (NEK) potassium showed a clear trend in treatments, solution potassium was first increased and then showed a decreasing trend due to the rapid changes in liquid phase compared to the solid phase. All treatments significantly increased soluble potassium in each 3 times. The greatest potassium increase associated with vermicompost. In general, oxalic acid > malic acid >vermicompost > citric acid, were increased exchangeable potassium, while the trend for NEK was in the order of oxalic acid > malic acid > citric acid >vermicompost, respectively. All treatments at all times (except for treatment 5 days of NEK), showed a significant increase in the exchange and NEK potassium compared to the control. The results also reflect the effect of the dominant soil clay mineral on transformation of exchangeable and NEK, so that the highest and lowest rate of increase was related to the soils with dominant palygorskite and illite, respectively. In general, it seems that the use of organic acids and organic matter leads to a rapid increase of potassium, which must be properly managed in the soils with high leaching. Due to the complexity of soil environment in terms of soil physical, chemical and biological aspects and the role of these factors on potassium transformation, repeating of this experiment in other soils is recommended.