Potassium Release Rate Research Articles

Potassium-Release Characteristics and Their Correlation with Bean (Phaseolus vulgaris) Plant Indices in Some Calcareous Soils

The rate of potassium (K) release from soils can significantly influence K availability to plants. There are few studies about K-release kinetics and K availability in calcareous soils of central Iran. The objective of this study was to study the release rates in 15 different soils from Chaharmahal-Va-Bakhtiari Province through successive extractions with 10 mM citric acid over periods of 2–2016 h and the correlation of K-release characteristics with bean plant indices. The results showed that there was wide variation in the cumulative K release among soils. The cumulative K release ranged from 169 to 1199 mg kg−1. The amounts of K released after 2016 h was not significantly correlated with bean plant indices. Plots of cumulative amounts of K released show a discontinuity in slope at 168 h. Thus, two equations were applied to segments of the total period (2 to 168 and 168 to 2016 h). The amount of K released after 168 h ranged from 62 to 879 mg kg−1. The amount of K released after 168 h was significantly correlated with bean plant indices. In the segment 1 parabolic diffusion, first-order and power function equations and in the second segment parabolic diffusion, first-order, zero-order, and power function equations described K-release kinetics. Release rate constants in the segment 1 were significantly correlated with bean plant indices; however, in the segment 2 release rate constants were not significantly correlated with bean plant indices. The results of our research showed that K-release characteristics after an initial fast reaction play a role in supplying available K and dilute citric acid solution can be used for estimating of K-release kinetics in the studied soils.

Kinetics of Potassium Release from Calcareous Soils Under Different Land Use

The rate of potassium (K) release from soils can significantly influence K fertility of soils. The aim of this study was to assess the impact of different land uses on K release from calcareous soils, within agricultural areas of western Iran. The kinetics of K release in 0.01 M CaCl2 was studied in surface samples of 62 calcareous soils planted to pasture, orchard, garlic, leafy vegetables, wheat, and potato. Trend in K release kinetics was similar between land use types. We found that samples collected from pasture gave the lowest K release (245.2 mg kg−1), whereas the highest values were measured on samples from leafy vegetables (449.6 mg kg−1). Different models were used to describe K release. In general, Elovich, parabolic diffusion and power equation were found to be appropriate for modeling K release. The K release rate for the soils was estimated by parabolic equation and it was significantly different between land use types. The release rate was lower for pasture and wheat (averaged 16.1 and 25.1 mg kg−1 min−1/2, respectively) than leafy vegetables (27.4 mg kg−1 min−1/2) and potato (27.0 mg kg−1 min−1/2). When the rate of K released found from parabolic model regressed on K forms and soil properties, NH4OAc-K, CaCl2-K, pH, and EC appeared to be the most important K forms and soil properties influencing K release rates in these calcareous soils. The different K release and NH4OAc-K or CaCl2-K prior to K application both should be taken into account for potassium fertilizer application in different land uses.

Application of Kinetic Models in Describing Soil Potassium Release Characteristics and Their Correlations with Potassium Extracted by Chemical Methods

Potassium (K) release characteristics in soil play a significant role in supplying available K. Information on K-release characteristics in soils of central Iran is limited. The objectives of this study were to determine K release characteristics and correlations of K release rate constants with K extracted by different chemical methods in surface soils of ten calcareous soils of central Iran. The kinetics of K release in the soils was determined by successive extraction with 0.01 mol L−1 CaCl2 in a period of 2–2 017 h at 25±1 °C. Soil K was extracted by distilled water, 0.5 mol L−1 MgNO3, 0.002 mol L−1 SrCl2, 0.1 mol L−1 BaCl2, 0.01 mol L−1 CaCl2, 1 mol L−1 NaCl, 1 mol L−1 boiling HNO3, 1 mol L−1 NH4OAC, Mehlich 1, 0.002 mol L−1 SrCl2 + 0.05 mol L−1 citric acid, and ammonium bicarbonate-diethylenetriamine pentaacetic acid (AB-DTPA). A plot of cumulative amounts of K released showed a discontinuity in slope at 168 h. Thus, two equations were applied to two segments of the total reaction time (2–168 and 168–2 017 h). Cumulative amounts of K released ranged from 55 to 299 mg kg−1 in 2–168 h and from 44 to 119 mg kg−1 in 168–2 017 h. Release kinetics of K in the two time segments conformed fairly well to parabolic diffusion, simplified Elovich, and power function models. There was a wide variation in the K release rate constants. Increasingly higher average concentrations of soil K were extracted by distilled water, Mehlich 1, SrCl2, CaCl2, SrCl2 + citric acid, AB-DTPA, MgNO3, NaCl, NH4OAc, BaCl2, and HNO3. Potassium release rate constants were significantly correlated with K extracted. The results of this study showed that information obtained from mathematical modeling in two reaction time segments can help to estimate the K-supplying power of soils.

Efeito da Moagem de Alta Intensidade e da Irradiação Gama Sobre a Liberação de Potássio da Moscovita

eFFect oF the high energY milling AnD gAmmA irrADiAtion on the releAse oF potAssium From muscovite Brazil has a large and growing dependence on imports of potassium for use in agriculture. The development of potassium alternative sources has strategic importance. The grinding of rocks rich in structural potassium occurring in various regions of the country should be considered as a possibility, because the finer particles has shown accelerate the release of potassium. The objective of this paper was to investigate the effect of high energy milling and gamma irradiation on the rate of release of potassium from muscovite. It was observed that it is possible to obtain a particle size ranging from 10 to 11 µm by grinding with a planetary mill of high energy at 800 revolutions per minute for 6 hours. The ground samples in these conditions showed a rate of potassium release two times higher than a sample pulverized by conventional grinding with a particle size of 92µm. No effect of gamma irradiation until 600 kGy dose

KINETIC RELEASE OF POTASSIUM RESERVE IN SOME CALCAREOUS SOILS IN NORTHERN OF IRAQ

This study was conducted to reveal the conditions and parameters which control dissolution and release of indogentic potassium in some Hammam Al-Alil soils northern Iraq, representing Aridisols order. Three pedons were selected having different soil series namely (462CCE, 453CCW and 451CCW). The study included using successive extraction with 6.25M HCl, under reflux conditions for 100 mint. to determe the amount and rates of release exchangeable and non-exchangeable potassium in some calcareous soil periphery of semictite and illite minerals according to the amount of release of mica-K. by using a kinetic diffusion parabolic equation. The results showed that amount of released potassium was reduced by increasing extraction time, and the variation in chemical, physical and mineralogical properties of the soil samples have a great effect on each amount, rate, and pathway reaction of potassium release between soil series, also the results showed that the pathway reaction of K-release consist of two segments according to the parabolic diffusion equation. The first segment represented exchangeable phase that was more than the second phase (non-exchangeable) in its amount and rate of release . Fertility evaluation of the results according to amount and rate of K-release from mica-K referred that all soils samples has a moderate reserve with low release rate, therefore the K-reserve of soils could be mapped by soil series using this method of classification such mapping would provide data on the extent availability of reserves in both short term (i.e.) the crop over a growing season and long term.

Potassium release kinetics and its correlation with pinto bean (Phaseolus vulgaris) plant indices

The rate of potassium (K) release from soils is a dynamic process, and it is important to the evaluation of soil K availability to plant. The objectives of this study were to determine K release characteristics and the correlation of these parameters with bean plant indices in surface soils of 15 calcareous soils. The kinetics of K release was determined by successive extraction with 0.01 mol/L CaCl<sub>2</sub> in a period of 2–2017 h at 25 ± 1°C. The results showed that the cumulative K‏ release ranged from 111 to 411 mg/kg. A plot of cumulative amount of K released showed a discontinuity in slope at 168 h. Thus, two equations were applied to segments of the total reaction time (2 to 168 and 168 to 2017 h). The amounts of K released during 2–168 and 168–2017 h ranged from 55 to 299 and 44 to 119 mg/kg, respectively. Release kinetics of K conformed fairly well to parabolic diffusion, simplified Elovich and power function models in two segments. Potassium released after 2–168 and 168–2017 h and K release rate constants were significantly correlated (P < 0.05) with bean plant indices, while correlation between K extracted by using 1 mol/L NH<sub>4</sub>OAc (ammonium acetate) and plant indices was not significant. The results of this research showed that information obtained from K release studies in laboratory and kinetics equation parameters can help to estimate the K supplying power of soils.  

Continuous Cropping, Fertilization, and Organic Manure Application Effects on Potassium in an Alfisol under Arid Conditions

Effects of continuous cropping and addition of organic manures (farmyard manure, FYM, and groundnut shells, GNS) along with inorganic fertilizers on nonexchangeable potassium (K) release kinetics in a K-deficient Alfisol were studied in a 20-year manurial experiment under arid conditions. There was a depletion in available K under continuous cropping without K input (control) as compared to other treatments such as 100% nitrogen–phosphorus–potassium (NPK), 50% NPK + 4 metric tons (MT) groundnut shells ha−1, 50% NPK + 4 MT FYM ha−1, and 100% organic (i.e., 5 MT FYM ha−1). Over 20 years of cropping without K input, available K was reduced from 155 kg ha−1 (in 1985) to 82 kg ha−1 (in 2005), showing a negative balance of 73 kg ha−1. Soil in control plots showed available K in the deficient range (<50 mg kg−1), whereas four other fertilizer and manurial treatments were greater than the critical limit. Considerable improvements in nonexchangeable K-release parameters such as step K and cumulative K release were observed in manured plots over control. Parabolic diffusion and first-order kinetic equations explained K release from soils. Potassium-release rates were drastically reduced in control plots, and there were increased release rates with continuous addition of manures. Results suggest that soils with groundnut shells or FYM (4 MT ha−1) along with 50% inorganic fertilizer additions could maintain greater K release rates after 20 years of cropping as compared to cropping without K input.

Effect of sodium and magnesium on kinetics of potassium release in some calcareous soils of western Iran

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 mmol c 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, CaCl 2, and MgCl 2 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.

Kinetics of K release from soils of Brazilian coffee regions: effect of organic acids

Kinetic studies on soil potassium release can contribute to a better understanding of K availability to plants. This study was conducted to evaluate K release rates from the whole soil, clay, silt, and sand fractions of B-horizon samples of a basalt-derived Oxisol and a sienite-derived Ultisol, both representative soils from coffee regions of Minas Gerais State, Brazil. Potassium was extracted from each fraction after eight different shaking time periods (0-665 h) with either 0.001 mol L-1 citrate or oxalate at a 1:10 solid:solution ratio. First-order, Elovich, zero-order, and parabolic diffusion equations were used to parameterize the time dependence of K release. For the Oxisol, the first-order equation fitted best to the experimental data of K release, with similar rates for all fractions and independent of the presence of citrate or oxalate in the extractant solution. For all studied Ultisol fractions, in which K release rates increased when extractions were performed with citrate solution, the Elovich model described K release kinetics most adequately. The highest potassium release rate of the Ultisol silt fraction was probably due to the transference of "non-exchangeable" K to the extractant solution, whereas in the Oxisol exchangeable potassium represented the main K source in all studied fractions.

Effect of hyperosmolality and cations on iodinated contrast medium-induced potassium release from human blood cells

Potassium release from blood cells is a contrast medium-induced phenomenon. The purposes of the study were to (1) assess the effect of hyperosmolality and of adding sodium ions and calcium ions to a solution on potassium release from human blood cells and (2) reevaluate the possibility of hemolysis as a cause of potassium elevation. Fresh human blood was mixed with a test solution to examine the temporal changes in the whole blood potassium levels and to calculate the potassium release rate. Test solutions included 5%, 20%, and 50% glucose; 0.9% and 10% NaCl; and 50% glucose mixed with various amounts of sodium and calcium ions. We also measured serum glutamine oxaloacetic acid transaminase (GOT) and serum lactate dehydrogenase (LDH) to evaluate the possibility of hemolysis. Hyperosmolality using glucose solutions promoted higher potassium release. The average +/- SD potassium release rates were 7.3 +/- 2.4 micromol/min with 5% glucose, 13.5 +/- 2.3 micromol/min with 20% glucose, and 128.4 +/- 44.9 micromol/min with 50% glucose. The solutions including sodium ions showed lower release rates. The addition of sodium and calcium ions into 50% glucose significantly lowered the potassium release rates. No significant elevation of GOT or LDH was observed, and the possibility of hemolysis was eliminated. Hyperosmolar glucose solution promoted potassium release, but the presence of sodium ions in the hypertonic solution inhibited it. In addition, there is no possibility of hemolysis as a cause of potassium release.

Release Kinetics of Nonexchangeable Potassium in Calcareous Soils

Release rates of nonexchangeable potassium (K) were determined for 10 intensively cultivated soils of the Hamadan area in western Iran. Calcium (Ca)‐saturated soil samples were equilibrated with 0.01 M CaCl2 and 0.01 M citric acid for 2–2759 h. Plots of cumulative K+ released by different soils vs. time consisted of two parts, suggesting two different rates of K+ diffusion from soils. Four mathematical models (first‐order, parabolic diffusion, power function, and Elovich equation) were used to describe cumulative K+ release. The power function and first‐order equation described the K+ releases kinetics best as evidenced by the highest correlation coefficients and the lowest values of the standard error of the estimate (SE). The parabolic diffusion equation also described the data satisfactory, indicating diffusion‐controlled exchange.

A cation exchange resin method for measuring long-term potassium release rates from soil

The aim of the present study was to determine long-term K release rates from soil by use of a modified resin method, where vigorous agitation and soil–resin separations were avoided to minimise the dissolution of the soil minerals. Resins saturated with Ca2+ or H+ were tested; Ca2+ because it is the most dominating cation on the soil exchange complex, and H+ because it is an important K+ counter-ion released from plant roots. The tested soil: resin systems included two rates of agitation; no agitation, and gentle agitation where the soil particles visually did not move relative to each other. The resin beds were kept physically separated from the soil particles by use of a specially designed extraction tube. Three soils of the same mineralogical and textural origin, but exposed to different K input levels during more than 80 years, were used. A pot experiment with ryegrass was carried out using the same three soils. Vigorous shaking of the resin–soil systems was not needed to obtain a sufficiently high and rapid K release to the resin; even without agitation long-term K release rates could be determined. The accumulated K release was in all combinations of agitation rate, resin saturation ion and soil K status proportional to the square root of extraction time after 1000 h of extraction, indicating diffusion controlled K release from the minerals. Resin extractable K was closely correlated with ryegrass K uptake, indicating similarity in the extraction process. In contrast, K extracted with ammonium acetate or nitric acid did not provide information about the ability of the soils to release K in the long term. Based on the criteria :(i) substantial K adsorption under slow K release conditions; and (ii) achievement of a stable long-term K release within a limited extraction period, the Ca-saturated resins with gentle agitation and H-saturated resins without agitation are concluded to be best suited for routine laboratory work.

Potassium release rates from ustisols and their application

Second-order equations were used to characterize the potassium release rate for 20 low-hilly ustisols derived from Quaternary red clay in Zhejiang province, China. This was done under the condition of electric field strength of 44.4 and 88.8 V cm−1. The values of the initial K release rate (v0) ranged from 1.17 to 21.23 and from 1.93 to 61.58, with an average of 5.36 and 9.54 mg kg−1 min−1 under the electric field strength of 44.4 and 88.8 V cm−1, respectively. Six indices, including the relative grain yield, relative total dry matter yield and K uptake in NP treatments of 20 corn field experiments and available K, HNO3 soluble K and slowly available K of soils were used to assess the practical applicability of K release rates. The correlation analysis showed that v0 was very significantly correlated (P=0.01) with the above six indices, and their correlation coefficients were 0.6275**, 0.5645**, 0.6624**, 0.7277**, 0.7843** and 0.6299**, respectively, under the electric strength of 44.4 V cm−1. The v0 was related to relative total dry matter yield (P = 0.05, r = 0.5445*) and very significantly correlated to the other five indices (relative grain yield, K uptake in NP treatment, available K, HNO3 soluble K and slowly available K), with the correlation coefficients of 0.6064**, 0.7216**, 0.7523**, 0.8202** and 0.6686**, respectively, under the electric strength of 88.8 V cm−1. From the results, we conclude that v0 can be used to estimate the supplying power of soil K to annual crops such as corn, and to characterize soil K fertility.

Potassium release characteristics of some soils of Uttar Pradesh hills varying in altitude and their relationship with forms of soil K and clay mineralogy

Release rates of non-exchangeable potassium were determined for 15 soils occurring widely in different altitude in intensively cultivated soils of U.P. hills. For this, the method of Oliver and Diarisso [Oliver, R., Diarisso, D., 1985. Cine tique de dissolution du potassi um de divers soils per allaque a chaud a Li acide nitrique normal c.r. ATP Dynamique des cations, Bull. CIRAD Montpellier, France Section, N: 1–13.] was adopted involving successive extraction contact duration with 1 N HNO 3 as an extractant on water bath at 85°C. The soil series were then grouped under three classes with respect to altitude and exchangeable K contents, namely, 1250, 1650 and 2250 m AMSL with exchangeable K group 40±5, 90±15 and 140±15 mg kg −1 soil, respectively. The cumulative K release decreased with increase in altitude and exchangeable K content and the values ranged from 473 to 527, 300 to 336 and 235 to 287 mg kg −1 soil, respectively. All values of the initial linear rapid K release part ( P 1) and subsequent slow K release part ( P 2) indicated that P 1 is generally greater than P 2 and the values of P 1 and P 2 decreased with increase in altitude. Hence, it is advisable to consider the amount of exchangeable K, non-exchangeable K content and clay mineralogy while recommending the potassium fertilizer in different altitude.

Decomposition of and nutrient release from ruminant manure on acid sandy soils in the Sahelian zone of Niger, West Africa

In ago-pastoral systems of the semi-arid West African Sahel, targeted applications of ruminant manure to the cropland is a widespread practice to maintain soil productivity. However, studies exploring the decomposition and mineralisation processes of manure under farmers’ conditions are scarce. The present research in south-west Niger was undertaken to examine the role of micro-organisms and meso-fauna on in situ release rates of nitrogen (N), phosphorus (P) and potassium (K) from cattle and sheep–goat manure collected from village corrals during the rainy season. The results show that (1) macro-organisms played a dominant role in the initial phase of manure decomposition; (2) manure decomposition was faster on crusted than on sandy soils; (3) throughout the study N and P release rates closely followed the dry matter decomposition; (4) during the first 6 weeks after application the K concentration in the manure declined much faster than N or P. At the applied dry matter rate of 18.8 Mg ha −1, the quantities of N, P and K released from the manure during the rainy season were up to 10-fold larger than the annual nutrient uptake of pearl millet ( Pennisetum glaucum L.), the dominant crop in the traditional agro-pastoral systems. The results indicate considerable nutrient losses with the scarce but heavy rainfalls which could be alleviated by smaller rates of manure application. Those, however, would require a more labour intensive system of corralling or manure distribution.

Measuring interlayer potassium release rates from soil materials. I. Critical evaluation on the use of resin and other batch procedures for determining kinetic data

AbstractLaboratory procedures are evaluated for their principal suitability to derive unbiased kinetic data of interlayer K release (days to weeks, Luvisol ‘Ohlendorf’, loess, Ap horizon). The final intention is to relate K release rates to their main governing soil environmental variables rather than to conventional procedural standards. Of actual interest are technical aspects of quantifying the influence of solute K (CK), which is considered to be the most important among those variables. The relevant range is that of a few μM K, similar to the rhizosphere of plants where diffusion‐controlled, non‐destructive K release is thought to be of importance. Tested non‐resin batch techniques, e.g. the time dependent shift of K‐Ca exchange isotherms, are considered to be unsuitable because the derived release rates were markedly biased by shaking the suspensions. Generally, the batch technique just showed that additional proton induced release of nonexchangeable K (Knex) is negligible above pH 4. Ca‐resin procedures extracted considerable amounts of Knex but most of it had to be attributed to the addition of fresh (K‐free) exchange solution rather than to the presence of the resins. Further, an active regulation of CK in the intended range was only hardly possible via the resin/soil ratio but was inexact and only achieved with inacceptable effort. Further modifications, e.g. containing the resins in dialysis tubes, improved the technique but did not eliminate the main problems: Ca‐resin does not act as a real sink for K. H‐resins did act as a sink but, due to other constraints, are also rejected: CK is too low (&lt; 1 μM, no effective regulation possible) and the pH of the suspension steadily declines (&lt; 3 after one week). The one promising procedure among those tested was a continuous (non‐resin) percolation technique (&gt; 900 h) which permitted a flexible regulation of CK by the percolation rate.

Measuring interlayer potassium release rates from soil materials. II. A percolation procedure to study the influence of the variable ‘solute K’ in the &lt; 1…10 μM range

AbstractRelease rates of nonexchangeable K from Ap material of a Luvisol (‘Eckerde’ loess, 15% clay) are determined using a percolation procedure which avoids the common artifacts due to shaking or stirring soil suspensions and thus provides less biased kinetic data. CaCl2 solution (10 mMc. 20 °C. pH 5.8) is percolated through packages of soil aggregates (0.5–1 mm grain size, 0.5 g samples) with 0.02 to 25 ml h−1. Solute K (CK) was varied between &lt; 1 and &gt; 10 μM and is shown to have a dominant influence on the rates of interlayer K release. These increase exponentially below 3.5 μM (no steady state but steady decrease of release rates in each sample). The difference between the related CK between moderate and high release rates is as small as 1 μM K. The average rates, of a 10‐day‐interval, starting after 1.3 times the exchangeable K had been removed, are 40 μmol K kg−1 soil d−1 at 4 μM CK and 240 μmol kg−1 d−1 at 3 μM CK, respectively. It is concluded that larger quantities of interlayer K become plant available in the studied soil if the of soil solutions gets below 3.5 μM (for 10 mMc Ca, 20 °C, pH 5.8), probably because the dominant dioctahedral illites start to join the release process below this critical limit. The higher K concentration range was accounted for by K‐Ca exchange isotherms which, by alteration of shape, indicate that Knex release becomes measurable below 10 to 20 μM K. It is further argued that existing diffusion or reaction kinetics approaches towards K release are incomplete because the influence of solute K is not considered.

RELEASE KINETICS OF NONEXCHANGEABLE POTASSIUM IN SOILS USING SODIUM TETRAPHENYLBORON

Sodium tetraphenylboron (NaBPh 4 ) can extract both exchangeable K + and nonexchangeable K + held in the interlayer of phyllosilicate minerals in soils and is a potential soil test method for plant-available K + . Eleven midwestern soils were used to (i) compare the ability of various kinetic equations to model nonexchangeable K + release by NaBPh 4 , (ii) determine the contribution ofparticle size fractions to nonexchangeable K + release, and (iii) measure changes in mineralogy associated with K + release by NaBPh 4 . Nonexchangeable K + release in most soils was almost complete in 96 h and was well described by Elovich, simplified Elovich, parabolic diffusion, and power function equations (r 2 = 0.962 to 0.997). However, neither form of the Elovich model described the early phase of K + release adequately. A rate index defined as the product of the two rate parameters of the power function equation was related to illite K + (r 2 = 0.995). Fit of the data to the parabolic diffusion model yielded two separate straight lines, suggesting two different rates of K + diffusion from illite. Potassium release rates increased as particle size decreased, indicating that in whole soils, fast early release is primarily from the fine and coarse clay fractions, and slower late release is mainly from the coarse clay and fine silt fractions. X-ray diffraction analysis showed that expansion of the 1.01-nm basal spacing to 1.23 nm was nearly complete in the 96-h NaBPh 4 treatment. These results show that mathematical models can be used to infer the mechanism(s) ofnonexchangeable K + release by NaBPh 4 and estimate the relative K + -supplying power of soils.

Release Rates of Phosphorus, Ammonium, and Potassium in Clinoptilolite‐Phosphate Rock Systems

AbstractControlled release of P, NH4, and K can be accomplished in synthetic soils through dissolution and cation‐exchange reactions by phosphate rock (PR) and the zeolite mineral, clinoptilolite (Cp). The power‐function equation has previously been found to adequately describe transport kinetics of the nutrient release process. Research was conducted to evaluate factors affecting P, NH4, and K release rates in Cp‐PR systems. Data was collected using a continuous‐flow thin‐disk technique. Results were evaluated with the power‐function equation. Materials included a Texas Cp and North Carolina and Tennessee PRs. Variables examined included the Cp/PR ratio, the mole fraction of NH4 and K on the Cp, and the type of PR. The power‐function model indicated that initial release rates and cumulative nutrient release in Cp‐PR mixtures were increased by using a higher Cp‐PR ratio and by using a more reactive PR. Nutrient release rates were also affected by changes in the proportion of NH4 and K occupying Cp exchange sites and reflected ion‐selectivity characteristics of Cp. The results indicate that Cp and PR may be used to obtain a controlled release of NH4, P, and K in a synthetic soil.

Muscovite dissolution kinetics as a function of pH and time at 70°C

The dissolution rate of muscovite at 70°C was determined using a single-pass, flow-through apparatus. Dissolution rate data were obtained at approximately unit pH interval over the pH range 1.4 to 11.8 in 11 separate experiments, each lasting 50 days. The muscovite dissolution rates were defined by the silica, aluminum and/or potassium release rates to solution. Under most pH conditions the dissolution at steady-state was congruent or nearly so. Speciation-solubility calculations made using the geochemical modeling code EQ3NR indicate that the fluids were maintained far from equilibrium with respect to muscovite and undersaturated with respect to possible secondary minerals.Under all pH conditions we initially observe a transiently elevated dissolution rate. The dissolution rate gradually approaches a limiting (steady-state) value, which we interpret as the dissolution rate of the bulk mineral. From pH 1 to approximately pH 5, the limiting rate for muscovite dissolution at 70°C varies negatively with pH, and the rate constant, k, is 10−14.7 mol/cm2 · s. Under acidic conditions the dissolution rate is proportional to−0.37 pH. As pH is increased at 70°C, the rate of dissolution becomes essentially pH independent, and k is 10−16.6 mol/cm2 · s. At about pH 7 and higher the limiting rate for muscovite dissolution varies positively with hydroxyl ion activity, and the dissolution rate is proportional to +0.22 pH. Under alkaline conditions k is 10−18.1 mol/cm 2 · s at 70°C.