JWSS - Journal of Water and Soil Science

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Methods of seedbed preparation affect establishment, growth and yield of crops via manipulating physical and chemical properties of soil. These effects on wheat-mungbean double-cropping have not been studied in Isfahan. Therefore, the influences of various seedbed preparation methods on soil physical properties and nutrients content and on establishment of mungbean (experimental line 1-61-16) were studied in a wheat-mungbean double-cropping system during 1996 at the Agricultural Research Station, Isfahan University of Technology. Two residue management treatments (burned and unburned) along with four tillage systems (moldboard plow, disk, khishchee and no-till) were laid out in a strip plot design within a randomized complete block design with four replications. Bulk density and penetrability of soil at various depths were not affected by residue management at emergence. However, at the time of flowering, bulk density at 0-15 cm soil depth was lower and soil penetrability at 3-15 cm depth was higher in unburned treatment due to mineralization. Rate and coefficient of emergence of seeds were lower in unburned residue treatment. Soil bulk density at 0-30 cm depths was lower and penetrability at 3-21 cm depth was higher with moldboard plowing at both emergence and flowering time. In addition, lower soil N, P and K content at 0-15 cm depth at emergence and flowering time was obtained with moldboard plowing. Rate and coefficient of emergence were strongly lower in no-till treatment. Based on these results and considering sustainable agricultural goals, incorporating residue with disk tillage might be advantageous under conditions similar to this experiment.

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High-yielding rice varieties are prevailing in Guilan Province which is one of the most important rice producing regions in Iran. However, little is known about K status in this region and no suitable extraction method can yet be introduced to the farmers. This experiment was carried out to determine the available K by 15 chemical extractants and also to examine the critical K levels in paddy soils of the region. The treatments included 23 soils and 2 K levels (0 and 300 mg/kg as KCl) in a factorial experiment in a randomized complete block design with three replications.

 Results indicated that K application increased grain and straw yields, K concentration in straw as well as K uptake by rice. High correlation coefficients were observed between MgOAc, H₂SO₄, Morgan and 0.01 M CaC_l2 methods with K concentration in straw (0, 0.83, 0.78, 0.8 and 0.78, respectively) and with K concentration in plant tops (0.79, 0.76, 0.75 and 0.73, respectively). Similarly, high correlation coefficients were observed between the same extraction methods with K absorbed by straw (0.83, 0.82, 0.78 and 0.76, respectively) and with total plant K uptake (0.79, 0.83, 0.74 and 0.71, respectively). Potassium critical levels by different extracting solutions were determined by Cate-Nelson procedure and were 38, 74, 160, 111, 36, 112 and 100 mg kg^-1 for MgOAc, Morgan and Kelowana methods, Texas method H₂SO₄ 0.01 M CaCl₂ and NH₄OAc without and with submergence of soil samples, respectively. In this study, available K was below the critical level in almost 75% of the soils.

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Quantity-intensity (Q/I) curves and their derived parameters provide general information about soil K availability. This investigation was conducted to obtain Q/I parameters and to study the relationships between the Potassium Q/I parameters and the properties of 15 soils from central and northern Iran. Soil samples were equilibrated with solutions containing different potassium activity ratios (AR^k). Changes in potassium concentrations in the equilibrium solution (ΔK) were plotted against activity ratio of K, and Q/I parameters were determined. Correlations between Q/I Parameters and some properties of soils were studied.

The Q/I plots showed the common shapes described in the literature whereas high variations were observed in the soils with respect to AR°, ΔR°, BPC^k and K_x. The potassium activity ratio at equilibrium ranged from 0.005 to 0.532 with an average of 0.02 (mmol L^-1)^0.5. The potassium potential buffering capacity (PBCk) ranged from 4.40 to 76.3 with an average of 16.2 (mmol kg-1) / (mmol L-1)0.5. The readily exchangeable potassium (ΔK°) ranged from 0.007 to 0.737 with an average of 0.25 cmol kg^-1 and slowly exchangeable potassium (K^x) ranged from 0.023 to 0.55 with an average of 0.3 cmol kg^-1. Highly significant linear correlations were found (α=0.01) between PBC^k and CEC (r=0.66), AR° and exchangeable potassium percentage (r=0.76), AR° and solution potassium (r=0.89), and between ΔK° and exchangeable potassium (r=0.79). Because of high variations in physicochemical properties of soils, correlation coefficients between Q/I parameters and soil properties were not very high.

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Evaluation of nutrient status in soil is important from nutritional, environmental and economical aspects. Potassium is a very important plant nutrient not only because of its large demand, but also because of its important physiological and biochemical functions in plant. In order to evaluate K availability and common K extractants in the central region of Isfahan Province, surface soil samples from 26 important soil series were taken, out of which 16 soil samples were selected for further study based on their ammonium acetate extractable K and texture. Available K was extracted from the soil samples by NH₄OAC (4 methods), CaCl₂ (2 methods), NaCl (2 methods), NaOAC, AB-DTPA, Mehlich I, Mehlich III, Morgan-Wolf and HNO₃. A green house experiment using corn (Zea mays L.) variety Single-cross 704 as the test plant with 2 treatments (zero and 150 mg/kg K) and three replications was performed with the 16 selected soils to correlate K uptake with K extracted by different extractants.

 From XRD analysis, the dominant clay mineral was found to be Illite. Based on the amount of potassium extracted by these extractants from the soils the extractants used in this research may be divided into three groups: relatively weak extractants including CaCl₂, Mehlich I and Morgan-Wolf relatively strong extractants including NH₄OAC, NaCl (2N), Mehlich III, AB-DTPA, NaOAC and NaCl (1N), the strongest and weakest extractants in this group were NH₄OAC and NaCl (1N), respectively and finally, a very strong extractant which includes only HNO₃. In each group, the quantities of extracted K showed high correlation with each other. A high and significant correlation was observed between the quantities of K extracted by NH₄OAC, AB-DTPA and HNO₃. The amount of K extracted by 2N NaCl, 1N NaOAC, 1N NH₄OAC, and AB-DTPA was significantly correlated with K taken up by plant and its K concentration. 2N NaCl and 1N NaOAC were selected as the most suitable extractants for K based on their high correlation coefficients with plant uptake (0.79** and 0.72**, respectively) and K concentration in plant (0.86** and 0.85**, respectively). Yield, relative yield and plant response were not significantly correlated with K extracted by different extractants indicating that available K of the selected soils was not a limiting factor for corn growth therefore, it was impossible to determine the critical level of K for corn by the selected extractant.

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Potassium fixation and release by phillosilicate clay minerals in soils are very important processes influencing the availability of K to plants. This investigation was conducted to determine the potassium fixation capacity and charge characteristics of soil clays of 15 surface soils (0-30 cm) from central and northern Iran. After clay particle separation, both total and tetrahedral cation exchange capacity of soil clays were determined. Tetrahedral CEC was measured after saturation with Li and heating at 300 C to reduce octahedral charge to near zero. Potassium fixation was obtained in both wet (1:10 soil:solution, 16 h on a shaker) and dry conditions (after drying for 24 h at 70°C) using three different levels of added K

The total CEC in soil clays of Isfahan, Char-Mahal and Gilan provinces ranged from 22.1-36.0, 33.0-55.8 and 31.3-47.9 cmol kg^-1, respectively. Tetrahedral CEC in soil clays of Isfahan, Char-Mahal and Gilan provinces ranged from 17.9-4504, 26.2-32.5 and 8.3-23.8 cmol kg^-1, respectively, which consisted of 81.0-98.4, 58.5-95.8 and 24.7-72.5% of their total charge, respectively. The amount of K fixation increased with drying and the level of k added. Mean potassium fixation in soil clays of Isfahan, Char-Mahal and Gilan ranged from 5.42-9.13, 6.63-14.67 and 8.87-10.36, respectively. Mean potassium fixation by soil clays (except for soil clays of Gilan) best correlated with total CEC. In the soil clays of Isfahan, mean potassium fixation correlated with tetrahedral CEC, whereas no correlation was observed in soil clays from other places. The average amount of potassium fixation in clay fractions was in the order: Gilan clays > Char-Mahal clays > Isfahan clays.

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The soil potassium status and effect of soil physical and chemical properties on it were investigated using 109 surface soils (0-30cm) from four regions in Guilan province. Neutral molar ammonium acetate extractable K (K-NH₄OAc), boiling molar nitric acid extractable K (K-HNO₃) and water soluble K were determined. Results showed that CEC and clay were normally frequency distributed while distribution frequency of other variables such as silt organic carbon and pH were not normal and have significant skew and kurtosis. In addition sand, K-NH₄OAc and K-HNO₃ has logarithmic normal distribution. Water-soluble potassium was affected by sand, pH, O.C and K-NH₄OAc (R²_a=0.73**). Ammonium extractable potassium was affected by soluble potassium, non-exchangeable potassium and CEC (R²_a=0.72**). Boiling nitric acid was affected by exchangeable potassium and silt (R²_a=0.55**). Geographic distribution of K-NH₄OAc data shows amount of it in central parts of Guilan, particularly Sepidrood river watershed is more than other regions. Fomanat region soils have the less clay, CEC, K-NH₄OAc and K-HNO₃. According this study about 68.8 percent of Guilan soils was classified in the range of medium to very low potassium status.

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Potassium is one of nutritional element that has an important effect on the quantity and quality of rice plant. Knowledge about the K critical level and plant response to different application rate of K, can effectively help with the proper use of this element. Therefore, in this study potassium (K) critical level for rice (Khazar variety) and prediction equation for grain yield were determined in 21 rice fields of Guilan provience during 2001-2002. After land preparation, 6 experimental plots (4×5m) were constructed in each selected field. Nitrogen and phosphorus were applied before transplanting, based on soil test from urea and triple superphosphate, respectively. Potassium was applied in different rates including 0, 100, 200, 300, 400, and 500 kg ha^-1 of K₂O as muriate potassium. Khazar variety was transplanted in a space of 25×25cm, just after fertilizing of plots. Plots were harvested in 5 m² at the end of maturity stage from the first node above the ground, and grain and straw were measured separately. Statistic analysis was done on grain weight based on 14% moisture. The results showed, that effect of different K fertilizer application in 5% and the effect of soil type in 1% were statistically significant on the grain yield. The mean increase of grain yield in responsive fields to K application was 982 kg ha^-1. The results also showed that the responses of rice fields with available K less than 100 mg kg^-1, 100-160, and more than 160 mg kg^-1 to KCl application were high, moderate and low, respectively. Potassium critical level based on 90% relative yield was determined as 110 mg kg^-1 of soil . In final relative yield, prediction equation, available potassium, potassium content of irrigation water, and rate of KCl fertilizer application entered positively in the equation, while available phosphorus showed negative effect on relative yield.

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Sugarcane (Saccharum officinarum L.)‌ and rotational crops have been cultivated without potassium fertilizers for many years in southwestern Iran. Although potassium was removed from this soil, no response has been reported to K fertilizers by crops. This study was conducted to evaluate the effects of wetting and drying, and cultivation systems on potassium fixation in some Khouzestan soils. The results showed that expandable clay mineral was observed in cultivated soils but not in the non-cultivated soils. This may be related to irrigation, and cultivation practices in this soils. By adding K and sequential wetting and drying the amount of K-available increased in sugarcane, rotational cropping and uncultivated soils from 132.6, 226.2 and, 171.6 mgkg^-1 to 266, 447 and 628, respectively. These results showed that more K can be fixed after cultivating soils, especially by sugarcane, but available K increased by adding K. All surface soils had higher K fixation capacity than subsurface ones. Also, by application K to these soils, the amount of K fixation decreased with increasing wetting and drying times. The K fixation increased by the increase of cation exchangeable capacity. Significant difference was observed between cultivation system and depth of sampling in 1% levels. High K fixation can be attributed to illite minerals depleted from K. Mica-like minerals formed after adding K, and wetting and drying cycles. Amount of K fixation by clay particle samples was more than the same soil samples but in both the same trend was observed for K fixation. Also, drying and wetting decreased K fixation in the last periods. It may be due to trapping K in the interlayer positions.

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  Sugarcane is one of the most important sugar crops in the world. Because of semi-arid climate and salinity of its cultivation area in our country, increasing salt tolerance of sugarcane is signifying. To achieve this goal determining salt tolerant cultivars and understanding salinity mechanisms in sugarcane are very important. This study was conducted to evaluate 8 commercial and promising sugarcane cultivars at early stage of growth. A complete randomized design with three replicates and four salinity treatments (0, 0.25, 0.5, 0.75 % NaCl) was used in a hydroponics system. The effect of salinity on absorption, transport and accumulation of Na⁺, Cl^- , K⁺ and Ca²⁺ ions in shoot and root was determined. At high level salt concentration, Cl^- content in shoot and root increased. Result showed that sodium accumulation in sugarcane plants was more than potassium. By increasing salinity level, sodium uptake and its translocation to shoots increased reducing growth and dry matter yield of plants. With rising salt concentration from medium (0.5%) to high (0.75%), content of chloride in shoot and root of NCO-310 was constant showed that this cultivar had genetic ability to avoid Cl^- uptake. CP82-1592 with lowest ratio of shoot / root chloride had minimum transport of Cl^- to shoots. Also this cultivar had high content of Ca²⁺ in shoot and low Na⁺/Ca²⁺ ratio at all salinity levels. CP48-103 had low sodium in shoot and relatively low sodium in root. Thus it probably has genetic potential to avoid sodium uptake. At last, exclusion of Na⁺ and Cl^- to older leaves and tillers was seen in CP82-1592 and CP72-2086 cultivars. According to results, to avoid once of absorption and transport, and exclusion of harmful Na⁺ and Cl^- ions were mechanisms that could be used in salinity tolerance of sugarcane.

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Successive cropping of potassium-demanding crops like sunflower leads to depletion of soil potassium (K). This study was conducted to investigate the different forms of K and quantity–intensity (Q/I) relationship for sunflower growing soils in Khoy region, West Azarbaijan province. Twenty composite soil samples were collected from different soil series. Different forms of K, including solution K (Kso), exchangeable K (Kex) and non-exchangeable K (Knex) were determined. The results showed that Kso values varied from 0.061 to 0.54 (mmol L-1), with an average of 0.28 (mmol L-1), Kav values ranged from 55 to 699 (mg kg-1), 265 mg Kg-1 on average, and Kex values ranged from 54 to 694 (mg Kg-1) with an average of 261 (mg kg-1). Furthermore, Knex values varied from 160 to 612 (mg kg-1), 261 (mg kg-1) on average. Available K (NH4OAc- extraction) was less than 250 mg Kg-1 in half of the soils indicating the depletion of potassium from these soils. The Q/I curves were predominantly located in the adsorption regions. The Q/I curves were linear and lacked the curvature part relating to loss and gain of potassium present in the specific sites. The PBCK values varied from 11 to 108 (cmolc kg-1)/ (mol L-1) 0.5 and an average of 38 (cmolc kg-1)/(mol L-1)0.5. There was a linear significant relationship between PBCK and CEC (r2=0.82***). The AReK values ranged from 0.0014 to 0.027 (mol L-1) 0.5 and 0.0076 (mol L-1)0.5 on average. There was a significant relationship between the values of AReK and those of the soil solution K (r=0.68**). The Ko values varied from 0.0050 to 0.49 cmolc kg-1and an average of 0.21cmolc kg-1. Additionally, there was a high significant relationship between the values of Ko and ARoK (r= 0.95***) in the soils under study.

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Compost and sewage sludge contain high concentration of plant nutrients and, thus, have been used extensively as an inexpensive fertilizer. The objective of this study was to evaluate cumulative and residual effects of compost, sewage sludge and cow manure on nitrogen, phosphorus and potassium in soil and wheat. The experiment included compost, sewage sludge and cow manure, each applied at 3 rates (25, 50 and 100 Mg ha-1), a chemical fertilizer (250 kg ha-1 amonium phosphate + urea) and a control plot with 3 replications. The experimental design consisted in completely randomized blocks with treatments arranged in split plots. To study the cumulative and residual effects of the organic amendments, application was repeated on four fifths of each plot in the second year. Wheat was grown in the plots. The results showed that one application (residual effect) of organic amendments had not significant effect on total N in soil and wheat leaves and stem, but it led to significant increase of available phosphorus and potassium in soil and wheat leaves and stem. Cumulative effects of organic amendments significantly (P≤ 0.05) increased the total N (in 50 and 100 Mg ha-1 Cow manure and 100 Mg ha-1 Compost treatments), available P in all organic treatments and K (in all Compost and Cow manure treatments) in soil. Also, cumulative effects of organic amendments significantly (P≤ 0.05) increased the N (in 100 Mg ha-1 sewage sludge), P (in 100 Mg ha-1 compost) and K (in all organic treatments with the exception of 25 Mg ha-1 Compost) concentrations in leaves and stem.

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Correlation between components of Q/I and wheat (Triticum aestivum L.) growth indices was studied in a greenhouse experiment during 2003-04 growing season. Eighty soil samples (0 – 30 cm depth) were collected randomly in farms around the Abyak region, Ghazvin province. Twenty-one samples were then selected based on soil texture and NH4OAC-K. The research included treatments of 21 soils and two potassium (K) levels (0, 100 mg/kg) and was conducted in a factorial manner in a randomized complete block design with three replications for studying correlations between Q/I and treatments. Effect of potassium application on the wheat growth was found to be significant at 5% level. The effect of soil properties on wheat growth was also found to be significant at 1% level. However, the interaction of K and soil was not significant. There were no significant differences among the different equilibrium time intervals. The soil potassium buffering capacity (PBCK) values were strongly correlated with CEC (r =0.996**), clay content (r = 0.921**) and moisture saturation percentage (r = 0.811**). Final equation derived from stepwise regression for prediction of PBCK was as follows: PBCK = 7.419 CEC –19.743 R2adj = 0.782** The equilibrium potassium activity ratios (ARke) were strongly correlated with soluble potassium (r =0.846**), NH4OAC-K (r =0.730**), SP (r = 0/794*), OC (r = -0.477ns) and clay content (r=-0.602*). Similarly, readily exchangeable potassium (ΔK0) was strongly correlated with ammonium acetate extractable potassium (r = 0.871**), soluble potassium (r = 0.778**), saturation percentage (r = 0.551*), organic carbon percentage (r=-0.045ns) and clay content (r=-0.206ns). There was no significant correlation between ΔK0 and ARKe with potassium uptake values by wheat. In contrast, there was a strong correlation between PBCK values and potassium uptake by wheat (r = 0.729**), and relative wheat yield (r = 0.735**). There was no significant correlation between Q/I parameters when KCl and K2SO4 were used, and the soil physicochemical properties. However, Q/I parameters obtained from KCl showed a higher correlation with wheat plant's growth indices.

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Gray mold, caused by Botrytis cinerea, is a serious disease of cut rose flowers (Rosa hybrida L.) in Iran. In order to elucidate the effects of different potassium and calcium levels in nutrient solution on susceptibility of cut rose flowers to gray mold, this experiment was carried out as factorial design in a randomized complete block with four replications at Safi Abad Agricultural Research center in 2008 for one year. In this experiment, rose plants were grown and subjected to three levels of potassium (1.0, 5.0 and 10.0 mM) in combination with two levels of calcium (1.6 and 4.8 mM) under hydroponic condition. Rose flowers from two consecutive harvesting periods were sprayed with the conidial suspension (104 spore/ml) of B. cinerea isolate. At the end of experiment the disease severity was recorded and analyzed. The results indicated that application of 10.0 mM K in the nutrient solution led to increasing rose disease severity to gray mold (30.4 % day-1) compared to 1.0 mM (24.8 % day-1) and 5.0 mM (26.2 % day-1) of K levels (P< 0.01). The increased susceptibility was associated with a decreased concentration of Ca in the rose petals. Correlation analysis revealed that susceptibility of rose flowers to gray mold significantly increased with K to sum cations ratio in the nutrient solution (r = 0.94*). The increase of Ca supply from 1.6 to 4.8 mM resulted in decline of disease severity from 29.6 to 24.6 % day-1 (P< 0.01). Therefore, balanced application of potassium and calcium (5.0 and 4.8 mM, respectively) is recommendable for preventing antagonistic effects between them and reducing of rose gray mold intensity under hydroponic conditions.

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Potassium (K) fixation by phyllosilicates clay minerals is a very important process influencing the availability of K for plants. Information about K fixation is limited in Hamadan soils. The objectives of this research were to determine potassium fixation capacity (PFC) and potassium fixation index (PFI) of 10 surface soils of Hamadan province and the correlation of these parameters with soil charge characteristics. Potassium fixation capacity was obtained using six different levels of added K as KCl and three wetting and drying cycles. Also charge characteristics of soils were determined. Tetrahedral CEC of soils was determined after saturation of samples with LiCl and heating at 300 oC to reduce octahedral charge near zero. The results showed that tetrahedral and octahedral CEC ranged from 4.6 to 16.0 and 0.8 to 4.9 cmolckg-1 respectively. Mineral and organic portion CEC ranged from 6.1 to 20.9 and 0.97 to 9.7 cmolckg-1 respectively. The amount of PFC increased with increasing added K. Mean amount of PFC ranged from 58.2 to 175.03 mgkg-1. Potassium fixation index ranged from 0.23 to 0.67. The results of correlation studies indicated that PFI were significantly correlated with octahedral CEC, mineral and organic portion CEC, total CEC, exchangeable and nonexchangeable K. The results of this research showed that PFI very different in soils. So that this parameter should be consider in K soil testing.

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An experiment was conducted in order to evaluate the effects of increased concentration of potassium and calcium in hydroponic nutrient solution on the response of safflower to salinity. The experiment was carried out in a glasshouse using a completely randomized design. Four saline treatments (Johnson solution containing 100 mM of NaCl, Johnson solution containing 100 mM of NaCl + 10 mM potassium, Johnson solution containing 100 mM of NaCl + 5 mM calcium, Johnson solution containing 100 mM of NaCl + 10 mM potassium + 5 mM calcium) and Johnson solution without any addition of salt as control. Leaf area per plant, shoot and root dry matter was decreased in saline treatments. The concentration of potassium and calcium in the plants were decreased but those of sodium were increased at salinity. The extent of shoot and root dry mater reduction with salinity was less in saline treatment with additional calcium alone. The addition of potassium into the nutrient solution could not mitigate the negative effects of salt stress on the plants. Increasing the concentration of both potassium and calcium in saline nutrient solution resulted in a greater reduction of shoot dry matter. The results showed that the negative effects of salinity may be alleviated by increasing the concentration of calcium in nutrient solution.

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Kinetical study on non-exchangeable potassium (NEK) release is necessary for management, optimum use of soil resources, availability and supplying power of potassium. Information about NEK release rate in Golestan soils is limited. The aim of this study was to investigate the NEK release of saturated soil sample by calcium using acid citric and CaCl2 extractants from 2 to 1844 h duration in 12 dominant soil series in Golestan province. Different soils indicated various responses to continuous extraction. The rate of K released in different soils was lower in CaCl2 than that of Citric acid. Potassium release was faster in earlier periods of the extraction in all soils followed by a lower release rate. Due to high coefficient of determination and low value of the standard error of the estimate, the NEK release kinetic by citric acid and CaCl2 were well described by Elovich and Parabolic diffusion equations and Power, First order and Elovich equations, respectively. Therefore, NEK release rate was controlled by K diffusion from weathered surface of soil minerals.

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Many studies have been carried out on the effect of organic matter on soil physical, chemical, biological, and nutritional properties, including the effect of organic matter on the availability of such elements as P, N and heavy metals. There is, however, no information on the effect of organic matter on potassium uptake from micaceous minerals. The objective of this study was to investigate the effect of organic matter on potassium uptake from micaceous minerals released by alfalfa. An experiment was laid out in a completely randomized design with factorial combination and three replicates. Growth medium was a mixture of quartz sand, micaceous mineral (muscovite or phlogopite) and organic matter (0, 0.5 and 1 %). Rehnani cultivar of alfalfa was used in the experiment. During 120 days of cultivation, plants were irrigated with either complete or K-free nutrient solution and distilled water as needed. At the end of cultivation, plant shoots and roots were separately harvested and their K concentration was measured by flame photometer following dry ash extraction. Under the K-free nutrient solution, a significant increase in biomass occurred in pots containing phlogopite and organic matter as compared to those with no organic matter amendment. Also, under K-free condition, potassium concentration in shoot was above the threshold value only in phlogopite amended pots. There was no significant difference in K concentration among different levels of organic matter in control treatment as well as in muscovite added treatment. Under both nutrient solutions treatments, significant increase of K uptake occurred in pots containing phlogopite and organic matter, as compared to those without it. In contrast, under K free nutrient solution, organic matter amendment could not enhance the K uptake in pots containing dioctahedral mica (muscovite). Root activities and organic matter decomposition appear to have increased rhizosphere acidity which, in turn, facilitate the K release from trioctahedral mica (phlogopite) in K deficient medium. Thus, the effect of organic matter on K release greatly depends on the type of micaceous mineral.

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Soil characteristics are affected by climate. Available potassium is one of the most important soil fertility indices. This study was conducted to determine the availability of potassium using Quantity- Intensity (Q/I) relationships in Vertisols of Isfahan and Chaharmahal-Va-Bakhtiari provinces with aridic and xeric moisture regimes, respectively. Soil mineralogy showed that smectite was the dominant clay in Chaharmahal-Va-Bakhtiari soil. The results showed that the activity ratio of K (AReK) in the soil solution of the surface soil in Isfahan and Chaharmahal -Va -Bakhtiari soils, ranged from 0.019 to 0.11 and 0.0037 to 0.0078 mmol.L-1 respectively. The labile K (∆K0) in Isfahan and Chaharmahal Va Bakhtiari soils ranged from 0.23 to 3.8 and 0.72 to 1.6 mmolkg-1, respectively. Potassium on specific sites (KX) in Isfahan and Chaharmahal-Va-Bakhtiari soils ranged from 2.8 to 7.1 and 2.6 to 3.7 mmolkg-1 respectively. The potential buffering capacity (PBCK) in Isfahan and Chaharmahal-Va-Bakhtiari soils ranged from 12 to 36 and 191 to 201 mmolkg-1/(mmolL-1)0.5 respectively. The results suggested that the Q/I parameters were affected by soil depth. In all of the soils studied, PBCK increased with soil depth.

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In order to assess the effect of drought stress and various levels of potassium on solutes accumulation and chlorophyll of canola and Indian mustard, a field experiment was conducted in a factorial design based on randomized complete block design with three replications including three irrigation regimes (I1=irrigation after 50% depletion of soil water(control),I2 =irrigation after 70% water depletion and I3 =irrigation after 90% water depletion), two species (Hyola 401 hybrid of canola and landrace cultivar of mustard) and three levels of potassium fertilizer (K1=0 ,K2=150 and K3= 250 kg.ha-1 K2SO4 ) at Agricultural and Natural Resources Research Center of Sistan in 2008-2009 cropping season. Water stress increased proline and soluble carbohydrate accumulation in the leaves of Brassica sp. In non stressed condition (control) in different growth stages, proline was lower than water-stressed plants and Leaf proline content decreased significantly after irrigation. Mustard landrace showed higher capability for accumulating assimilates such as proline, soluble carbohydrates and potassium than hybrid Hyola 401. Water stress decreased the amount of chlorophyll a, b and total leaf chlorophyll, but Potassium application caused an increase in the mentioned parameters. The highest content of chlorophyll pigments was observed at flowering stage. Potassium application caused a decrease in proline and an increase in soluble carbohydrates concentration in the leaf under water stress condition. There was a negative correlation between grain yield and proline content and soluble carbohydrates, but grain yield was positively correlated with chlorophyll, a, b. It was concluded that osmotic adjustment can be an important mechanism for Brassica species under water stress conditions and that organic and inorganic compounds such as proline, soluble carbohydrates and potassium play key roles in this regard.

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Non-exchangeable potassium (NEK) release is necessary to supply potassium for plant in different soils. A few studies on the NEK release have been done in soils and particularly on its components. This study was intended to investigate the NEK release from soil and the components of clay and silt through successive extractions with oxalic acid (0.01 M) over a period of 1844 (h) in 4 soil series of Golestan province. The amount of NEK release from whole soil, silt and clay fractions after 1844 (h) were 242 to 450, 380 to 550 and 105 to 199 mg kg-1, respectively. The cumulative rates of NEK release after 1844 (h) were well described by simplified Ellovich, parabolic diffusion and power function equations. The rate of K release was high initially, and then decreased gradually in all soils and in their silt and clay fractions. The amount of released NEK (with high rate) consisted of lower percentage of total NEK, but in spite of less amount, this K form plays a vital role in K dynamic and soil fertility.