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Experiments were conducted for thirty representative surface and subsurface soils collected from different physiographic units and moisture regimes in southern Iran to assess factors affecting K pools distribution and potassium release rate, using two organic and inorganic extractants and their association to mineralogy and some physical and chemical properties in calcareous soils. Mineralogical analysis indicated that illite, smectite, chlorite and palygorskite were the main clay minerals in the studied soils. The highest content of illite and smectite was observed in the piedmont plain (P.P) units, whereas that of smectite in Lowland (L.L) unit was higher than in soils of alluvial fans (A.F). It seems that the P.P unit which is in more stable landform position contains larger amounts of all K compounds due to higher clay and illite contents. The exchangeable, non-exchangeable (NEK), structural, and total potassium were found to be significantly and positively correlated to the clay content, cation exchange capacity, and the contents of illite, smectite, and vermiculite minerals, while a negative correlation as observed between the mentioned K forms and the calcium carbonate (CaCO3) content. The cumulative rate of K release to 0.01 M CaCl2 and oxalic acid was examined for a period of 1200 h, ranging within 104–286 and 87–233 mg kg–1 for the surface soil and 98–241 and 82–211 mg kg–1 for the subsurface soil samples, respectively. The percentage of non-exchangeable K released varied greatly between the different units and moisture regimes. Higher values of NEK release were observed for P.P followed by A.F, and L.L units. Considering the obtained values of the highest amount of potassium compounds as well as the release values in the piedmont plain physiographic unit and the xeric moisture regime as well as the Alfisols, the results confirm that in addition to physicochemical and mineralogical properties, physiographic units and soil moisture regimes also play a key role in nature and in spatial distribution of different forms of potassium. [ABSTRACT FROM AUTHOR]

The potassium supplying capacity of the waste mica powder (WMP) was evaluated through chemical and biological means. Four different size fractions of the WMP were studied in laboratory by different chemical extractants. At the same time, experiment of the plant growth was conducted in pot using K-deficient soil. The K release from the WMP by different extractants followed an increasing trend with an increase in the fineness of the particles. The highest K release was recorded by boiling 1 M HNO3 (376.4–1726.4 mg kg−1) followed by 1 M ammonium acetate (33.5–226.5 mg kg−1), 0.01 M citric acid (31.6–158.3 mg kg−1) and water (12.0–112.0 mg kg−1) irrespective of the various size fractions. Significantly, higher cumulative K release from the WMP in successive extraction was recorded with 1 M boiling HNO3 that predicts its slow-release property. The K release by different extractants was correlated significantly and positively with the biomass yield and K uptake by plants. 1 M boiling HNO3 extractable K showed higher correlation values with the biomass yield and K uptake. Therefore, it suggests that the extractant might be a better indicator of K supplying capacity of the WMP. The plant intervention was able to extract 32.8% of total K from the WMP simultaneously improving the exchangeable and non-exchangeable K pool in soil. These above-mentioned results conclude that plants can get access to a significant amount of K from the WMP and it could be an effective source of K in highly weathered tropical soils under K deficiency. [ABSTRACT FROM AUTHOR]

Abstract Studies to evaluate the release threshold level of potassium in Vertisols (shrink-swell soils) in different agro-ecological regions of India and how that may help in K fertility interpretations are rare. Thus the objectives of this study were (i) to evaluate the K status of Vertisols through release threshold levels and (ii) to relate the findings with soil K release and fertilizer management. Three established soil series samples were collected from Vertisols of different agro-ecological regions of India. Release threshold level (RTL) was assessed after equilibrating soil with 0.01 M CaCl 2 solution having series of soil: solution ratio, after which the remaining amount of exchangeable K was extracted with 1 N NH 4 OAc (pH 7). Total amount of K extracted (K T = K desorbed in CaCl 2 solution plus K extracted with 1 N NH 4 OAc) remained more or less constant with decreasing K-level up to a certain value (say threshold value) below which K T increased sharply indicating K release from non-labile form. The threshold value in terms of K concentrations (Release Threshold Concentration) of Panjari, Nagpur, Maharashtra (Typic Haplusterts, Sub-humid dry), Teligi, Bellary, Karnataka (Sodic Haplusterts, Semi-arid dry) and Kheri, Jabalpur, Madhya Pradesh (Typic Haplusterts, Sub-humid moist) soils were 0.044–0.049, 0.034–0.062 and 0.043–0.11 mm, respectively. The high release threshold K levels in terms of concentration and activity ratio (AR) were observed in soils of Teligi and Kheri series as compared to soils of Panjari series due to high content of trioctahedral mica in the former two soils. The relationships between clay minerals, release threshold level, exchangeable potassium and non-exchangeable potassium determined by 1 N HNO 3 and sodium tetra-phenyl borate (NaBPh 4) have been discussed. Highlights • Vertisols represent fertile lands and have vast geographical distribution in India. • Optimization of the efficiency of K requirement is a significant contribution to management of K fertilizer. • Knowledge of critical threshold value of K release in Vertisols for imminent loss of K due to fixation from added K. [ABSTRACT FROM AUTHOR]

Long term field experiment provides a vehicle for scientific investigations on use of manures and fertilizers on soil health and crop productivity under a specified cropping system. The present study was undertaken to know the potassium release characteristics under continuous cropping and fertilizer application on rice based cropping system. The AICRP on Long Term Fertilizer Experiment was initiated at the Zonal Agriculture Research Station, VC Farm, Mandya at "I" Block during 1989-90. The long term fertilizer experiment consists of permanently laid out plots in randomized block design with sixteen treatments replicated two times. The K release characteristics of soils were determined by employing successive extraction of soil with boiling 1N HNO3 and the potassium fixation was studied by taking representative samples from surface and subsurface soil. The results prevailed that the highest rate of potassium fixation was observed in the treatment 50% NPK+25%N-GM+25%N-FYM (0.58 meq. lit-1) and lowest in the control plot (0.39 meq.lit-1) in the surface soil. But in subsurface soil recorded highest fixation capacity compare to surface soil. The cumulative K release from different treatments ranged from 253 to 857 meq lit-1. The soils with higher contents of NH4OAc-K recorded relatively higher cumulative release of K than the soils with lower amounts of available K, the highest amount was total K was recorded in 50% NPK+25%NGM+ 25%N-FYM and lowest was recorded in the control plot, which was not supplied with any organic and inorganic fertilizers. The constant rate potassium (CR-K) content of soils as influenced by long term soil fertility management were ranged from 2 to 7 meq lit-1 in the surface layer of soils. [ABSTRACT FROM AUTHOR]

A produção de biomassa, a manutenção dos resíduos vegetais sobre o solo e sua posterior decomposição são fatores de grande importância no estudo da ciclagem de nutrientes. Este estudo foi desenvolvido na área experimental do CEFET-Uberaba-MG, onde foram avaliados oito tipos de coberturas vegetais: milheto (Pennisetum americanum sin. tiphoydes), braquiária (Brachiaria brizantha), sorgo-forrageiro (Sorghum bicolor L. Moench), guandu (Cajanus cajan (L.) Mill sp.), crotalária (Crotalarea juncea), aveia-preta (Avena strigosa Schreb), pousio e área em preparo convencional de solo (testemunha) em área de Cerrado, na região do Triângulo Mineiro. Avaliaram-se a fitomassa seca (FS), a decomposição dos resíduos em bolsas de decomposição, e a liberação de K. Utilizou-se um modelo matemático para descrever a decomposição dos resíduos e a liberação de K, e calcularam-se a constante de decomposição (k) e o tempo de meia-vida (T½). O milheto, o sorgo e a crotalária foram as coberturas que apresentaram maiores produções de matéria seca. O maior acúmulo de K ocorreu em gramíneas e a maior liberação de K ocorreu no milheto, aveia, braquiária e crotalária nos primeiros 42 dias após manejo, nos dois períodos avaliados. A braquiária apresentou o menor T½ vida e a maior taxa de liberação de K.Crop residue production, plant residue maintenance and their decomposition are important factors in the understanding of nutrient recycling process. To evaluate K accumulation and release a study with eight cover crops types was developed: pearl millet (Pennisetum americanum sin. tiphoydes), brachiaria grass (Brachiaria brizantha), sorghum (Sorghum bicolor L. Moench), pigeonpea (Cajanus cajan (L.) Millsp), sunn hemp (Crotalarea juncea) and black oats (Avena strigosa Schreb), fallow land and conventional culture (control) in the experimental area of CEFET-Uberaba-MG, in a Cerrado area. The dry mass production, crop residue decomposition in litter bags and K release were evaluated. A mathematical model was used to describe residue decomposition and K release, which calculates the decomposition constant (k) and half-time life. Pearl millet, sorghum and the sunn hemp were the cover crops that produced most dry matter while K accumulation was highest in the grasses. K release was highest in pearl millet, oats, brachiaria grass and sunn hemp in the first 42 days after handling. Brachiaria grass had the shortest half-time life and the highest K release rate.

The plant minimal exchangeable K ( EPl,min) defines the lower accessible limit of the most available pool of soil K to plants. It is also an index of long-term K reserve in soils. However, its estimation by the classical method of exhaustion cropping is laborious. This study aimed at comparing EPl,min values obtained by the exhaustion cropping method with EPl,min values estimated by an alternative approach based on the cationic exchange capacity (CEC) of the infinitely high selective sites for K ( i.e., always saturated with K) in the K-Ca exchange ( EK-Ca,min). A set of 45 soil samples, corresponding to the various fertilization K treatments of 15 long-term K fertilization trials, was used in this study. The selected soil samples presented a wide range of texture, CEC, and exchangeable K. The plant minimal exchangeable K was found more or less independent of the K treatment, whereas EK-Ca,min increased when the soil exchangeable K content increased. The plant minimal exchangeable K was systematically lower than EK-Ca,min, showing that EK-Ca,min is at least partially available to the plant. Hence, EK-Ca,min is not a surrogate of EPl,min. Conversely, the plant minimal exchangeable K was strongly, positively correlated to soil CEC (measured at soil pH; r2 = 0.90***). This soil property can consequently be used as a proxy of EPl,min. [ABSTRACT FROM AUTHOR]

Excess of exchangeable sodium (Na) in salt-affected soils causes ion toxicity and decrease in nutrient uptake by plants, particularly potassium (K). A number of studies have been conducted to investigate the effect of K-fertilization on plant growth under sodic and saline-sodic conditions but the results are much diverse to process for concrete recommendations. To explore the possible reasons, it was hypothesized that Na applied as NaCl to produce salinity/sodicity in the soil may release non-exchangeable K, minimizing the effect of K-fertilization. Incubation studies were conducted for 2, 4 and 6 days in the light (sandy loam) and heavy (clay loam) textured soils producing two saline/sodic levels, i.e. 20 and 30 sodium adsorption ratio (SAR) along with control (SAR 3). Potassium fertilizer applied was calculated according to 40 (general recommendations based on soil-nutrient status), 80 and 160 kg K ha−1. Interestingly, it was observed that addition of NaCl possibly released non-exchangeable K from the soil minerals and increased the K concentration in soil solution. Total K release was more in heavy textured soil but initial release was more in light textured soil. This release may eliminate the effect of K-fertilization applied under salt stress induced by NaCl. Therefore, it is suggested that while studying Na–K interaction in salt-affected soils, NaCl should be avoided to produce salinity, and naturally occurring saline-sodic soils may be used. Soil Na–K interaction studies including ameliorating effect of K under sodic or saline-sodic conditions should be conducted carefully considering the above-stated argument. [ABSTRACT FROM PUBLISHER]

Aims: In Alfisols, potassium (K) deficiency limits productivity, as these soils are poor in K-bearing minerals such as mica. As nutrient management practices greatly influence K nutrition of crops especially in the longer term, we evaluated the effects of 27 (1978-2004) years of cropping fingermillet ( Eleusine coracana G.) under different manure and mineral fertilizer treatments on K release, balance and yield sustainability on K deficient Alfisols in the semi-arid tropical region of southern India. Methods: Fingermillet (variety: PR-202) was grown each year under rainfed conditions with 5 different nutrient management treatments: control (no amendment), 10 Mg ha farm yard manure (FYM), 10 Mg ha FYM +50 % NPK, 10 Mg ha FYM +100 % NPK and 100 % NPK. Potassium release characteristics in the soil profile were determined using 1 N boiling HNO (strong extracting solution), 0.01 M HCl (medium extracting solution) and 0.01 M CaCl (mild extracting solution). Results: Continuous cropping of Alfisols for 27 years resulted in a decrease in K supplying capacity due to soil K depletion through crop K uptake. In soils without K addition, inherent soil supply could not meet the K requirement of fingermillet; thus, a negative K balance following 27 years of cropping affected K nutrition of the crop in all the treatments. As a result, the highest sustainable yield index (SYI) was observed using an integrated nutrient supply (combined application of nutrients from organic and inorganic sources), and the lowest index was obtained without K additions. Conclusion: For balanced nutrient management in cereal production systems, K nutrition needs urgent attention in the K deficient Alfisol region of southern India. Addition of any amount of organic manures available at field level offers an alternative strategy for maintaining soil K fertility to improve and sustain crop productivity. [ABSTRACT FROM AUTHOR]

A balance sheet of potassium (K) was prepared in a long-term experiment that was started in 1985 on a loamy sand Udic Haplustept at the Potash Research Institute of India, Gurgaon, Haryana, India, involving a sorghum (fodder)–wheat cropping system. During the 20 cycles of sorghum and wheat, as much as 2,404 kg K ha−1was released from the structural form of soil K that contributed 77.3% K to the total K removed by growing sorghum and wheat. However, this K released from the structural form could no longer sustain the growth of sorghum and wheat. Both water-soluble and exchangeable soil K forms contributed 3% K and nonexchangeable K contributed 6.6% K. Groundwater used in irrigation contributed 13.1% K. Potassium fertilizer is needed for the growth of sorghum and wheat on a loamy sand Udic Haplustept as in this long-term experiment and other representative soils both to sustain yield and maintain soil K fertility. [ABSTRACT FROM AUTHOR]

Though the potassium (K) status of the soil often relies only on its exchangeable-K content, release from nonexchangeable K also contributes to plant nutrition. This process, as well as the opposite one, K fixation, are diffusion controlled and thus depend on time. This study was conducted to (i) assess the influence of time (2 h, 1 d, and 8 d) on soil K dynamics; and (ii) compare the soil ability for K release and fixation (β) estimated from sorption–desorption experiments with the soil fixation capacity (FC) estimated from fixation experiments at two different K doses. The studied soil samples came from a long-term fertilization experiment established in 1976 by the Swiss Research Station Agroscope ACW in a Gleyic Cambisol (FAO classification system). They correspond to two extreme K treatments: (i) no K fertilizer applied but with P fertilizer applied in quantities equivalent to the uptake by the crops and (ii) K and phosphorus (P) fertilizers applied in quantities equivalent to the uptake by the crops with an additional 166 kg K ha−1y−1and 26.2 kg P ha−1y−1. Estimated equilibrium K concentrations decreased and, conversely, the soils’ K buffer power (i.e., the soil capacity to resist a change in soil solution K concentration following removal or input of K to the soil–plant system), β, andFCincreased when the soil K content decreased. Time significantly influenced only the sorption–desorption curve and the release–fixation of the low-K-status soil. Here, K buffer power, β, andFCincreased with time. A first-order model described the time series of exchangeable-K levels during the K-fixation-capacity experiments. However, a parabolic diffusion model was superior at explaining the variance.FCand β were linearly related (r2 = 0.92;P< 0.01), butFCsystematically overestimated β. This shows thatFCdid not appear as a perfect surrogate of β. [ABSTRACT FROM AUTHOR]

The release of non-exchangeable potassium from 24 calcareous soils of divergent mineralogy, from southern Iran, was examined. Sand, silt and clay particles were fractionated after dispersion with an ultrasonic probe. Samples were extracted with 0.01 M CaCl2 for 30 successive 2-h periods. The clay fraction released the largest amount of K in each soil. Cumulative K released ranged from 175 to 723, 35 to 128, and 71 to 146 mg kg−1 contributing 20–90, 4–39 and 2–54% for clay, silt and sand fractions, respectively. The lower proportion of K released from sand and silt fractions can be explained by the presence of a high content of CaCO3 and quartz in these fractions. The release kinetics for the non-exchangeable K data showed that parabolic diffusion and power function were the best fitting kinetic models. This indicated that slow diffusion of K from the mica interlayer positions is the main rate-controlling process. Cumulative K released and constant b values of parabolic diffusion model correlated significantly with the mica content of the clay fraction. [ABSTRACT FROM AUTHOR]

Batch experiments were employed to examine the influence of oxalic acid on release kinetics of potassium (K) from soils along with adsorption and desorption of soil K+. The soils used were three rice soils from high-, mid-, and low-altitude zones. The results showed that soil K extracted using 0.2 mol L–1 oxalic acid was similar to that of 1 mol L–1 boiling nitric acid (HNO3). The relation between K release (y) and concentrations of oxalic acid (c) could be best described as y = a + b log c, whereas the best-fit kinetic equation of K release was y = a + b √t. The K release for soils was in the order Bonbagh > Ganasthan > Kreeri. An oxalic acid solution with low pH was able to release more K. Oxalic acid decreased soil K+ adsorption and increased desorption, the effect of which tended to be greater at lower pH. [ABSTRACT FROM AUTHOR]

The study was conducted to evaluate different forms of K, and release and fixation characteristics of 12 representative calcareous Vertisols of southern Iran. Mineralogical analysis indicated that smectite was the main clay mineral in the Vertisols, but some Vertisols had chlorite, illite, and palygorskite, as well as smectite in the clay fraction. Soluble, exchangeable, HNO3-extractable, and total K ranged from 0.4 to 11.7 mg kg-1, 127.1 to 435.6 mg kg-1, 839.7 to 1942.4 mg kg-1, and 4971.5 to 11427.3 mg kg-1, respectively. Cumulative K released to 0.01 M CaCl2 during 1496 h ranged from 167.7 to 244.9 mg kg-1 for surface soils and from 109.1 to 198.0 mg kg-1 for subsurface soils. Generally, the studied soils fixed 14-38% of the added K. The K fixation in different soils correlated significantly (r = 0.90, n = 42, p < 0.01) with the smectite content in the clay fraction and cumulative K released correlated significantly with illite content in the clay fractions (r = 0.79, n = 42, p < 0.01). Results indicated that clay mineralogy of Vertisols is a key factor affecting dynamics of K in the soils and thereby sustainable management of K in crop production. Generally, we concluded that calcareous Vertisols in southern Iran have sufficient K for plant uptake, but with intensive crop production and without application of K fertilizers, these soils will be transformed to K depleted soils in the future. Thus, application of K fertilizers in long term management of plant production may be required. [ABSTRACT FROM AUTHOR]

Nutrient cycling is an important part of integrated nutrient management. The effects of crop (potato, sunflower, rape) residues, weed residues, fruit waste, vegetables waste, tree leaves, sheep manure, and poultry manure on K release in five calcareous soils was investigated. The soils and plant residues were incubated for 4 weeks before initiating the kinetics study. In residue amended soils, the percentage of K released over 10 successive extractions with 0.01 M CaCl2 ranged from 28% of potato residues (average of five soils) and tree leaves to 75.8% of sheep manure. The kinetics of K release from amended soils and plant residues was analyzed by fitting the experimental data into four kinetic models (parabolic diffusion, power function, Elovich, and first order). The constant rate (mg kg-1 min-1) of K for amended soils was defined as the release rate (average of five soils) and decreased in the following order: vegetables waste (168.2) > sunflower (139.3) > poultry manure (137.9) > potato (146.9) > rape (122.3) > sheep manure (101) fruit waste (103.3) > tree leaves (78.3) > weeds residues (74.6). The release of K consisted of two phases. Phase 1 indicates the high rate of K release which can be considered to supply K to crop for short period. On the other hand, phase 2 indicates the slow release of K that will supply K to crop for longer period. The present study demonstrated that organic residues improved soil K fertility of calcareous soils by increasing NH4OAc-extractable K and release rate and could be an alternative indigenous source of K. [ABSTRACT FROM AUTHOR]

Abstract: The rate of potassium (K) release from soils can significantly influence their K fertility. Sodium (Na), calcium (Ca), and magnesium (Mg) in poor quality (sodic or saline) irrigation water participate in ion-exchange processes resulting in displacement and release of K from minerals into solution. This study determined the effect of sodium adsorption ratio (SAR) and Ca:Mg ratio of water on K release of some calcareous soils in western Iran. Nine different solutions at a total electrolyte concentration of 100 mmolc l−1 and three levels of SAR (5, 15, 45) each with Ca:Mg ratios of 1:3, 1:1, or 3:1, prepared using solutions of NaCl, CaCl2, and MgCl2 were used to extract K from the soils. Significantly different quantities of K were extracted by the solutions. The maximum (average of five soils) (985 mg kg−1) and the minimum (387 mg kg−1) K were extracted by an SAR 5 solution with a Ca:Mg ratio of 1:3 and an SAR 45 solution with Ca:Mg ratio of 3:1, respectively. The importance of Mg versus Ca can be related to the specific ion effect. The kinetics of K release from soils consisted of two phases, an initial rapid phase followed by a slow phase of K release from soils. The two phases of K release are characteristic of a diffusion-controlled process. Based on the correlation coefficients, power function, parabolic diffusion, and Elovich equations adequately described K+ release, whereas a first order equation did not. The K release rate for the soils was estimated by parabolic equation from the above solutions. The constant b (mg kg−1 min−1/2) in the parabolic equation was defined as the release rate and for the Ca:Mg ratio of 1:3 was 96.5 , 55.9, and 35.1.for the SARs of 5, 15, and 45, respectively. The results imply that K extraction from soils could be increased during use of saline irrigation water containing high Mg concentration. The additional K released may be more readily available to plant roots but could also be leached down below the root zone. The results suggest that long-term use of saline irrigation water with a high Mg content could lead to enough leaching of K from soil under saline and sodic conditions that K fertilization management may need modification. [Copyright &y& Elsevier]

The effect of sesquioxides on the mechanisms of chemical reactions that govern the transformation between exchangeable potassium (Kex) and non- exchangeable K (Knex) was studied on acid tropical soils from Colombia: Caribia with predominantly 2: 1 clay minerals and High Terrace with predominantly 1: 1 clay minerals and sesquioxides. Illite and vermiculite are the main clay minerals in Caribia followed by kaolinite, gibbsite, and plagioclase, and kaolinite is the major clay mineral in High Terrace followed by hydroxyl-Al interlayered vermiculite, quartz, and pyrophyllite. The soils have 1.8 and 0.5% of K2O, respectively. They were used either untreated or prepared by adding AlCl3 and NaOH, which produced aluminum hydroxide. The soils were percolated continuously with 10mM NH4OAc at pH 7.0 and 10mM CaCl2 at pH 5.8 for 120 h at 6 mL h-1 to examine the release of Kex and Knex. In the untreated soils, NH and Ca2+ released the same amounts of Kex from Caribia, whereas NH released about twice as much Kex as Ca2+ from High Terrace. This study proposes that the small ionic size of NH4+ (0.54 nm) enables it to enter more easily into the K sites at the broken edges of the kaolinite where Ca2+ (0.96 nm) cannot have access. As expected for a soil dominated by 2: 1 clay minerals, Ca2+ caused Knex to be released from Caribia with no release by NH4+. No Knex was released by either ion from High Terrace. After treatment with aluminum hydroxide, K release from the exchangeable fraction was reduced in Caribia due to the blocking of the exchange sites but release of Knex was not affected. The treatment increased the amount of Kex released from the High Terrace soil and the release of Knex remained negligible although with Ca2+ the distinction between Kex and Knex was unclear. The increase in Kex was attributed to the initially acidic conditions produced by adding AlCl3 which may have dissolved interlayered aluminum hydroxide from the vermiculite present, thus exposing trapped K as exchangeable K. The subsequent precipitation of aluminum hydroxide when NaOH was added did not interfere with the release of this K, and so was probably formed mostly on the surface of the dominant kaolinite. Measurement of availability of K by standard methods using NH4 salts could result in overestimates in High Terrace and this may be a more general shortcoming of the methods in kaolinitic soils. [ABSTRACT FROM AUTHOR]

An attempt has been made to develop a method to characterize soils with respect to potential potassium (K) supply capacity and release of non-exchangeable K during cropping and its contribution towards K uptake through a simulation model. The model is based on the equation of continuity (a mathematical statement derived on the basis of principle of conservation of mass of solute and water in a soil element) with the assumption that nutrient flux from soil to root proceeds by mass flow and diffusion and influx into root follows Michaelis–Menten kinetics. Potassium fixation or release in soil has been simulated by incorporating a sink and source function, respectively, to the equation of continuity with the hypothesis that K release takes place in soil when labile K level goes below release threshold level (RTL) and fixation takes place when labile K level goes above fixation threshold level (FTL). Different soil and root parameters required for this model have been determined in solution culture and pot culture experiment, and a computer program has been written in visual basic to run the model. The model was run to characterize a soil with respect to its maximum supply of K to wheat crop from different depths and the contribution of non-exchangeable K, therein, and compared with the data of a field experiment. It has also been shown by simulation that maximum response towards applied K occurs at a particular value of available K, which shifts towards higher value as RTL increases. [ABSTRACT FROM AUTHOR]

In a pot experiment with five harvests of ryegrass the potassium (K) uptake from soil was studied in a clay and a silt soil, with (K1) and without (K0) K fertilizer. Ammonium acetate lactate-extractable K (K-AL) was rapidly depleted, and in K0 the K-AL level stabilized at 30 and 100 mg K kg[sup -1] in the silt and clay soil, respectively. Corrected for different ''minimum'' values, the K-AL value predicted the K uptake by ryegrass from AL extractable K very well. In the silt soil the K release from reserve K (total K release from soil minus K release from K-AL) was small, whereas in the clay soil there was a substantial release from reserve K. Part of the reserve K in the clay soil was easily releasable and contributed to luxury consumption of K in the first crop. Acid-soluble K (K-HNO[sub 3] minus K-AL) was a good parameter by which to assess the ability of the soil to supply ryegrass with reserve K. The results were compared with the results of the field experiments from which the soils were collected. The difference in K release between silt and clay soil was larger in the pot than in the field experiments, but without K fertilization the K-AL values levelled off at the same values in field and pot experiments. [ABSTRACT FROM AUTHOR]

The influence of hypoosmotic shock on the solute content of growing Escherichia coli K-12 cells was investigated at 37°C. Within 20 s after the shock the cells had released most of their osmolytes K, glutamate and trehalose. This release was specific and not due to rupture of the cell membrane, since under these conditions i) the cells neither lost protein nor ATP, ii)[C]-labeled sucrose did not enter the cytoplasm from the periplasm, and iii) except for their glutamate and aspartate level, which decreased, the amino acid pool of alanine, lysine and arginine of the cells remained approximately constant. Within a minute after the shock the cells started to reaccumulate parts of their previously released glutamate, aspartate and K, but not trehalose and resumed growth within 10 min after the shock. Experiments with K-transport mutants showed that none of the genetically-identified K transport systems is involved in the K-release process. Reaccumulation of K took place via the uptake systems TrkG and TrkH. The possibility is discussed that the exit of solutes after hypoosmotic shock occurs via several stretch-activated channels, which each allow the release of a specific osmolyte. [ABSTRACT FROM AUTHOR]

The autonomic receptors of the rat sublingual gland were characterized by radioligand binding and by specific functional responses involving the release of K and the generation of cyclic AMP in vitro. Both muscarinic cholinergic and alpha-adrenergic receptors were present in moderately high density in the sublingual gland, as judged by the binding of the specific radioligands H-QNB and H-clonidine ( B in pmol/g tissue=18.4±2.4 and 9.9±1.3, respectively). By contrast, although alpha and beta-adrenergic receptors were detected, they were not present in large numbers. The B (pmol/g tissue) for the binding of H-prazosin and H-DHA were, respectively, 3.2±0.6 and 3.6±0.4. Stimulation of muscarinic cholinergic receptors with carbamylcholine (2×10 M) caused a net release of K from sublingual slices incubated in an enriched, oxygenated Krebs Ringer bicarbonate medium of 35±7% after 10 min of incubation. Norepinephrine, phenylephrine, clonidine and isoproterenol did not induced K release from the slice preparation, but actually reduced the basal or unstimulated release of K. As in the submandibular and parotid glands, the release of K from sublingual slices had two components, a passive efflux and an active uptake which depended on the activation of an ouabain-sensitive Na, K ATPase. The release of K was also dependent on the presence of Ca (2.7 mM) in the incubation medium. Stimulation of beta-adrenergic receptors with isoproterenol (10 M) caused a 9-fold increase in the content of glandular cyclic AMP (from 4.0±0.6 to 36.0±2 pmol/mg/10 min). A similar increase was observed with norepinephrine (10 M) in the presence of phentolamine (10 M). Clonidine at concentrations of 10 and 10 M reduced the cyclic AMP content to below basal levels. The rat sublingual gland has functional cholinergic receptors and can release K in vitro in the presence of cholinergic agents. In contrast to the other major salivary glands, it has a low density of alpha-adrenergic receptors and fails to release K upon stimulation with alpha-agonists. The functional significance of a low density of beta-adrenergic receptors is still unclear, although there is a definite glandular cyclic AMP response upon stimulation with isoproterenol. Abundant alpha-adrenergic receptors in the sublingual gland are apparently negatively coupled to adenylate cyclase. [ABSTRACT FROM AUTHOR]

Amylase and K release were measured in slices obtained from innervated and sympathetically denervated rat parotid glands. Amylase release induced by noradrenaline was found to be increased after chronic denervation. The postjunctional component of supersensitivity to noradrenaline was evidenced by an increase in the maximal response to this agonist. The maximal response of denervated glands was blocked by propranolol 10 M. On the other hand, chronic denervation did not modify the α-adrenoceptor-mediated response, K release, either in the presence or in the absence of ouabain. It is concluded that, after sympathetic denervation, postjunctional supersensitivity develops for the β-adrenoceptor-mediated but not for the α-adrenoceptors-mediated effects of noradrenaline. [ABSTRACT FROM AUTHOR]