Potassium fixation of some calcareous soils after short term extraction with different solutions

This investigation was done to determine the release of potassium (K) from five calcareous soils of southern Iran using 0.025 M CaCl₂, HCl and citric acid during six successive extractions and to study the K fixation capacity of the soils after K release experiment. Mineralogical study indicated that Vertisols and Mollisols were dominated with smectites; while other soils had illite, chlorite, palygorskite and smectite. Results indicated that citric acid extracted more K than CaCl₂ and HCl (137 vs. 111 and 113 mg kg^?1, respectively). The analysis of calcium (Ca), magnesium (Mg) and K concentrations in the solutions suggests that the exchange of K with soluble Ca and Mg (originated from dissolution of carbonates by acidic solutions) is the main mechanism of K release, but citrate is able to dissolve K-bearing minerals and release K in slightly calcareous soils. Soils with more illite released more K. Potassium fixation capacity of soils increased after extractions of soils with different extractants from 324 to 471 mg kg^?1, with no significant difference. It is suggested to apply more K fertilizers in K-depleted calcareous soils and use of different solutions for extracting K from soil minerals may be a temporary and short term solution.     

棉花钾素营养与土壤钾素供应水平

大田试验和田间调查分别在杭州浙江农业大学实验农场黄松土(Semihy dromorphic soil)和金华盆地红壤上进行.品种分别为钱江9号和协作2号,这两处土壤有效钾含量均较低,棉花(Gosoypium hirsutum L.)生长不良,普遍早衰.根据土壤中交换性钾的含量与棉花生长情况,一般可分为四类:土壤交换性钾>90ppm,棉花生长正常,70-90ppm,潜在缺钾;<70ppm,明显缺钾;<50ppm,严重缺钾.棉花叶片含钾量与土壤交换性钾含量呈正相关,r=0.928,显著性超过1%水准.大田试验表明,施用钾肥能提高棉花叶片含钾量,增加叶面积和叶绿素含量以及CO₂的同化率;钾还能促进叶片中细胞色素氧化酶和硝酸还原酶的活性,提高气孔导度,降低水分的蒸腾速率.缺钾叶片结构松弛,角质层发育不良;正常叶片结构紧密,角质层发育良好.叶片含钾量无论在苗期、初花期、盛花期、花铃期,均与皮棉产量呈正相关,达到显著水准.     

The mineralogy and potassium supplying power of some loessial and related soils of New Zealand

Sixteen Pallic (Aqualfs, Ustalfs, Aquepts, Ochrepts) and related soils collected from the North and South Islands of New Zealand were studied to determine if variations in nonexchangeable K supply could be explained by mineralogical composition. A strong relationship was found which indicates that mica is weathering to vermiculite. Proportions of vermiculite and kandite are generally higher in the North Island than in the South Island. Dominance of either micas or vermiculite or kandite is determined by the stage of weathering and/or the nature of parent materials. Soils that supplied more nonexchangeable K (K_nex) to ryegrass plants contained more mica in the clay fraction than soils that supplied less K_nex. The results confirmed that use of a soil test that includes a measure of K_nex (i.e. acid-extractable K) may be a vital part of identifying variations in plant available K status of the soils. It is concluded that the K supplying power of the Pallic and related soils is related directly to the amounts of mica present in clay fraction and that good K supplying soils will be transformed to K depleted soils as a result of increased weathering and leaching (pedogenic factors) and K exploitation in intensive farming systems.     

Beziehung zwischen dem durch K-Düngung erzielbaren Mehrertrag und dem K-Gehalt der Böden nach Feldversuchen in der Bundesrepublik Deutschland

Relationship between the maximum yield increase from K fertilization and the K content of the soils after field trials in the Federal Republic of Germany Several hundred long term potassium field experiments with crop rotation and with plant residues like straw and sugar beet leaves removed from the fields, on different soils of the FRG were evaluated. The K content in the top soil was determined by the DL and CAL methods (K(lact)), given in mg K₂O/100g soil. No relationship could be found between K(lact) and the maximum possible yield increase for wheat, barley, sugar beets, and potatoes, if K(1act) was >5 mg/100g. It seems unnecessary, therefore, raise the K content of the soils. Occasional significant positive yield response should be less emphazised than average yields. Economical average yield increases on K fertilization were found with sugar beets and potatoes, rarely with barley but never with wheat. In all longterm trials the K amount of fertilizer required for maximum possible yield increase was above the amount of K removed by the crop. 7 to 27year field trials on clayer soils show that the K content of the soil may drop to 5-10mg K20(CAL) without causing increased yield response to K fertilization. The yield responses to K fertilizer are dependent to a high degree on the season (weather conditions), as was shown by high yield increases in exceptionally dry seasons. The large content of plant available Kin the top soils was due to high K fertilization during the past decades and the illitic nature of the soils. From the results of the field trials it may be concluded that an amount of K fertilizer which replaces K removed by the crop is sufficient for all soils with 5-10mg K₂O; for > lOmg K₂O(lact) K fertilization can be less than actual K removal. Because of large variations, the K(lact) figures (chapter 5),are of very limited value for predicting fertilizer requirements. The average K fertilizer consumption used (1981/82) of the FRG was higher than the K removed from soils by 79 kg K₂O/ha (in 1982). Official recommendations of K fertilizer in the FRG as well as the amounts of K actually applied are both in disagreement with the results of fertilizer experiments in the field.     

Forms and profile distribution of potassium in typical soils of the lower Benue Valley of Nigeria

Abstract

Six profiles, derived from Precambrian Basement Complex rocks (mainly gneiss), Cretaceous sediments (mainly shale and sandstone), and Quaternary alluvium, and which are typical of the major agricultural soils in the Lower Benue Valley (Nigeria) were studied with the objective to determine their overall potassium (K) reserves and any relationship between these and other soil properties including their parent materials. Total K in the soils varies from 0.13–27.1 g kg^‐1 with average 6.64 g kg‐¹. This correlates positively with the clay, and negatively with the sand contents of the soils and is also influenced by their parent materials. The order of abundance according to parent material is: alluvium‐ > Basement Complex (gneiss)‐ ≈ shale‐ > sandstone‐derived soils. The concentrations of readily available K (RAK) in the soils are quite low, accounting for only between 0.30 and 7.8% of the total K in the soils and less than 4.0% of their exchange capacities. Based on critical limits established for many Nigerian soils, the soils derived from sandstone are clearly deficient in RAK, while soils developed from gneiss, shale and alluvium parent materials have moderate to sufficient levels for a wide range of crops. Non‐exchangeable or moderately available K (MAK) in the soils is also relatively low (0.020–8.59 mmol_c kg^‐1); while the sandstone‐derived soils have the least MAK, the alluvial soils have the most levels. However, the potassium supplying power (KSP) of the soils may be considered to be generally high. Although this bears no particular relationship to soil parent materials, the sandstone‐derived soils have the lowest KSP. The bulk of the total K reserves in the soils (55–88%) exists as difficultly available or structural K (DAK). The alluvial soils first, then the gneiss‐ and shale‐derived soils next have the highest contents of DAK, while the highly weathered sandstone soils have the lowest. Simple correlation analysis shows that, irrespective of parent material and K form, clay content and CEC are the most important soil properties influencing the overall K supplying status of these soils. It is concluded that in major agricultural soils of the Lower Benue Valley of Nigeria K exists mostly in the lattice structures of K‐bearing minerals, with accumulations in the subsurface horizons. Its plant‐available or supplying status is low on sandstone‐derived soils and moderate to sufficient on soils derived from Basement Complex rocks, shales and alluvium.     

紫色水稻土钾有效性和钾释放的研究

紫色水稻土全钾含量属中等水平,速效钾属中等偏下水平,土壤对钾素的供应总体不足。作物吸收的钾,矿物钾占76.95％,速效钾和缓效钾仅占10.81％和12.24％。土壤矿物钾释放随时间的延长逐渐下降,随土壤颗粒粒径的下降显著增大,在80分钟时,释放量仍高达08～8.8mgkg-1.min-1,80分钟内的累积释放量为土壤缓效钾的1.5～2.0倍;土壤速效钾的形成随时间的延长逐渐趋向稳定。不同土壤矿物钾的释放顺序和土壤速效钾形成顺序的不一致性,是由它们的机理和土壤性状综合造成的,但是,Elovich方程都能较好地拟合土壤矿物钾的释放过程和土壤速效钾的形态过程,方程参数b值能反映过程进行的速度。     

The forms and availability to plants of soil potassium as related to mineralogy for upland Oxisols and Ultisols from Thailand

Potassium (K) deficiency is widespread in crops on highly weathered upland soils under a tropical monsoonal climate. Critical assessment of the forms of K in soils and of the ability of soils to release K for plant uptake is important for the proper management of K in crop production. The relationships between different pools of K were investigated as a function of silt and clay mineralogy for 14 upland Oxisols and 26 upland Ultisols soils from Thailand. Most soils contained no K-minerals in the silt fraction. XRD showed that kaolinite is the dominant clay mineral with variously minor or moderate amounts of illite, hydroxy-Al interlayered vermiculite and smectite present in some soils. For some soils, both conventional and synchrotron XRD were unable to detect illite. Analytical TEM including EFTEM of individual clay crystals showed that clay in the apparently illite-free samples contained very small amounts of illite. Many kaolinite particles appear to contain K which may be present in illite interleaved with kaolinite crystals. A glasshouse K-depletion experiment was conducted to assess the K supply capacity and changes in chemical forms of K and K-minerals using exhaustive K depletion by Guinea grass (Panicum maximum). Potassium deficiency symptoms and mortality of plants occurred on light textured soils, whereas plants survived for six harvests for Oxisols with clay texture, relatively high CEC and higher NH₄OAc-K (exchangeable K plus water-soluble K). There is a strong linear relationship of unit slope between NH₄OAc-K and cumulative K uptake by plants indicating that NH₄OAc-K is a major form of K available to plants. Thus K-bearing minerals contributed little K to plants over the time scale of the experiment and XRD patterns of whole soil samples, silt and clay from soils after cropping mostly showed no change from those for the initial soil. An exception was for a single surface soil clay where a minor amount of smectite was formed from illite by K release to plants.     

Sorption of carbofuran and diuron pesticides in 43 tropical soils of Sri Lanka

To better understand the environmental fate of pesticides in Sri Lankan soils, we studied the sorption behavior of two commonly used pesticides (carbofuran and diuron) in 43 surface soils representing a range of soil physicochemical properties from dry and wet zones of Sri Lanka. For carbofuran, the K(d) (L/kg) values varied from 0.11 to 4.1 (mean, 0.83; median, 0.62) and K(oc) ranged from 7.3 to 120.6 (mean, 41.65; median, 36.1), whereas for diuron K(d) values varied from 0.5 to 75 (mean, 9.6; median, 5.15) and K(oc) ranged from 55.3 to 962 (mean, 407; median, 328). A comparison of sorption data on these tropical soils with published studies (mostly European and north American soils) showed that the ranges of sorption coefficients from Sri Lankan soils were within the wide range of K(oc) values reported in the literature. However, these values for both pesticides in soils from dry zones of Sri Lanka were consistently higher (up to two times) than those from the wet zone. The wide range of K(oc) values in Sri Lankan soils may be due to the possible difference in the nature of soil organic carbon, which needs to be further investigated.     

Effect of NaCl-induced saline sodicity on the interpretation of soil potassium dynamics

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.     

Assessment of Soil Potassium Sufficiency as Related to Quantity‐Intensity in Montmorillonitic Soils

Abstract

No studies have been conducted to evaluate the potassium (K) quantity‐intensity (Q/I) relationships that exist in eastern South Dakota soils and how that may affect K fertility interpretations. The objectives of this study were to i) evaluate the K status of smectite‐dominant soils through quantity‐intensity relationships and (ii) relate the findings to current research on soil K release and plant availability. Soil and plant tissue samples were collected from eight different corn production fields across east‐central South Dakota. Samples were collected from areas where corn plants did or did not exhibit K deficiency symptoms. Quantity‐intensity plots were developed and used to derive the typical Q/I parameters. Little difference existed in Q/I parameters and the form of Q/I plots among field sites. The AR_e ^K and ΔK⁰ values ranged from 0.0013 to 0.0113, and ?0.47 to 0.18 cmol_c kg^?1, respectively, and most sites were considered K insufficient. The predominant phyllosilicate present in the clay‐sized fraction was montmorillonite with an estimated 17% tetrahedral charge. These soils would not be expected to contribute much plant‐available, nonexchangeable K and would be in need of frequent K fertilization. Presumably, these and similar soils, upon K exhaustion, rely heavily on K released from K‐bearing silt‐sized particles and may be highly dependent on surface‐controlled dissolution processes for labile K replenishment. Additional research needs to be conducted concerning the release kinetics of K from K‐bearing minerals of these soils.     

Potassium Dynamics in US Coastal Plain Soils

ABSTRACT

Potassium (K) deficiency in crops in southern US Coastal Plain soils has been documented since the l880s. Long-term soil fertility studies such as Alabama’s “Cullars Rotation” experiment (circa 1911) have been conducted with K since 1911. Other Alabama long-term experiments on several Coastal Plain and related Hapludults, Paleudults, and Kandiudults also contain K variable treatments which have been monitored since 1929. Soil test data from these long-term experiments have allowed us to answer some practical questions regarding K dynamics in Coastal Plain soils. Potassium movement through the soil profile is dependent on the soil’s cation exchange capacity (CEC) but relative accumulation is greater in the plow layer regardless of soil CEC. While subsoil K testing may be useful for identifying situations where subsoil K has been depleted, this extra effort and expense is not necessary for most cropping situations. A crop will remove most of its K from the plow layer if it is present in sufficient quantity based on soil test. Crop depletion of plow-layer K to the point where yield may be reduced is gradual and may take 10–15 years or more depending upon soil CEC and initial soil K concentration. Depletion is most rapid in low CEC soils as would be expected. However, soil test K can vary considerably during the course of a crop season with the lowest soil test K concentrations occurring immediately after harvest.     

Dynamics of Potassium in Representative Soil Series of Southwestern Nigeria

Experiments were carried out to evaluate the dynamics of potassium (K) in six representative soil series of southwestern Nigeria to provide guidelines on soil K management. Quantity–intensity (Q/I) approach was used and the Q/I isotherms obtained revealed that all the soils released K before reaching equilibrium, having a negative intercept. The results showed that labile K from the Q/I evaluation was greater than exchangeable K from ammonium–acetate extraction in all the soils, showing that evaluation of soil K by exchangeable K alone may not give an in-depth understanding of the K dynamics in soil. Hence, it should be used in addition to Q/I parameters for a reliable evaluation. The potential buffering capacity obtained ranged from 0.4983 to 1.4272 cmol kg^?1/(mol L^?1)^1/2, indicating that the soils have a low capacity to maintain K concentration for a long period and hence would require frequent K fertilization.     

Effect of nitrogen,phosphorus and potassium on yields and nutrient levels in carrots grown on sphagnum peat and mineral soils.

Abstract

Carrots, Daucus carota L., were grown on both sphagnum peat and mineral soils from 1969 to 1972 inclusive. Fertilizer treatments consisted of three rates of N, of P and of K applied in all possible combinations.

In practically every instance rates of N, P and K applied to sphagnum peat were reflected in the levels of these nutrients found in carrot leaves. This was not the case with mineral soils. On sphagnum peat there were eight opportunities, and on mineral soils seven, for each of the three applied nutrients to influence yields. On peat N increased yields in three instances, P in one and K in five. On mineral soils N decreased yields in two instances, P decreased them in one and increased them in one while K had no effect.

The results suggest that on sphagnum peat carrots may require N, P and K up to 250, 50 and 150 kg/ha respectively whereas on mineral soils maximum rates would be 25, 25 and 50 kg/ha.     

Organic matter effects on phase partition of 1,3-dichloropropene in soil

The fumigant 1,3-dichloropropene (1,3-D), in combination with chloropicrin, is considered a major replacement to methyl bromide (MeBr). This study was conducted to better understand phase partitioning of 1,3-D and the role of organic matter in its adsorption to soil. Partition of 1,3-D between air and water (K(H)), and between soil and water (K(f)), was determined by quantifying the concentration in both phases upon equilibrium. At 20 degrees C, the K(H) values of (Z)- and (E)-1,3-D were 0.052 and 0.033, respectively. In four California and Florida soils, the K(f) values of 1,3-D isomers ranged from 0.39 to 8.55, and the K(oc) values ranged from 18 to 60. The relatively high K(H) and low K(f) imply that 1,3-D is highly mobile in most soils after subsurface application. Adsorption of 1,3-D in native soils and soils amended with manure compost increased with increasing soil organic matter content. This suggests that organic wastes may be applied to soil to increase 1,3-D adsorption, thus reducing its potential for offsite movement.     

Relationships between symptoms expressed by Norway spruce and foliar and soil elemental status

Surveys conducted from 1987 to 1990 of Norway spruce [Picea abies(L.) Karst.] within 12 plantations across 4 northeastern states revealed symptoms of crown discoloration and defoliation on a site-specific basis. Foliar N. K. and Ca concentrations of most of the sampled trees were above deficiency ranges, while foliar Mg concentrations of most of the symptomatic trees were below the deficiency range within the plantations. Soil pH, exchangeable Mg, K, Ca, and their corresponding percent saturations in soils were lower, while soil Al concentrations were higher for most of the symptomatic trees in comparison to the healthy trees. Foliar concentrations of Mg, Ca, K, P, Al, Mn, Pb, and Zn were positively correlated with concentrations of corresponding soil elements. Knowledge of nutrient deficiency ranges may help diagnose foliar symptoms, but their exclusive use may overly simplify relationships between foliar symptoms and foliar elements. Principal component regression analysis of the data provided assessment of interactions and balances among foliar elements, and among soil elements and their possible influences on crown symptoms. Crown symptoms were not only associated with concentrations of individual elements of foliage and soils, but also associated with interactions and balances between these elements. The influences of individual soil elements on discoloration and defoliation may depend upon other elements in soils. Soil Al may induce crown discoloration and defoliation by interfering with Mg, Ca, and K uptake in acidic soils.     

Genesis and transformation of minerals in the formation of red (Alfisols) and black (Inceptisols and Vertisols) soils on Deccan basalt in the Western Ghats, India

Six noncalcareous pedons from the basaltic terrain of the Western Ghats in Maharashtra, India, were identified for the present study. Of these, two red-soil pedons (Typic Haplustalfs) and one black-soil pedon (Vertic Ustropept) are from the Bhimashankar plateau at an elevation of 1000 m above mean sea level, experiencing a humid (>5000 mm rainfall) tropical climate. The other three pedons of black soils (Typic Chromusterts and Typic Ustropept) are from the semi-arid zone (500–1000 mm rainfall), at an elevation of 825–893 m. Clay mineralogical investigation indicated that interstratified smectite-kaolin (Sm/K) is dominant in red soils whereas smectite is dominant in black soils. The Sm/K is formed by the transformation of montmorillonite, the first weathering product of Deccan basalts in a humid tropical climate. We suggest that the interstratification of kaolin with chloritized smectite may also be an important ephemeral stage during the transformation of smectite to kaolinite. The presence of zeolites provided sufficient bases to prevent the complete transformation of Sm/K to kaolinite. The presence of smectites and zeolites made the formation of black soils possible in microdepressions even in a tropical humid climate. The genesis of Sm/K and smectite in red and black soils, respectively, suggests that these soils formed through a progressive landscape reduction process. The presence of both Sm/K and smectite in black soil clays of semi-arid climate suggests that the smectite of these soils was formed in an earlier humid climate.     

Comparison of the ammonium acetate,Mehlich 3, and sodium tetraphenylboron as extractants to evaluate crop available potassium

ABSTRACT

The 1 M ammonium acetate (NH₄OAc) (AA) is the most widely used method for soil-test potassium (K), but other methods have been also suggested to estimate crop available K. The accuracy of these extractants may be influenced by soil texture and clay mineralogy. This study evaluated the relationships among AA, Mehlich-3 (M3), and sodium tetraphenylboron (TPhB) methods using soils differing in texture and clay minerals from the agricultural area of Uruguay. The M3 and AA extractable K concentrations were highly correlated (R² > 0.97) across soils, although AA extracted slightly higher amount of K than M3. The TPhB method extracted more K than AA and M3, indicating that extracted K from different pools. The slopes of the relationships between TPhB and AA or M3 varied among soils being higher in fine-textured and illitic soils than in coarse soils. These results would be useful for evaluating the feasibility of incorporating M3 into a test program using the existing calibrations of the AA method. In addition, TPhB could be considered a complementary tool to improve the interpretations of the extractants to estimate soil-test K along with other characteristics such as the texture and clay mineralogy.     

不同土壤钾素释放动力学及其供钾特征的研究

应用自行研制的连续流动交换仪研究了取自不同类型的19个土壤样品的钾素释放动力学及其供钾特征。不同土壤钾的释放差异很大,钾的释放持续时间变幅为100～600分钟;最大释放速率变幅为0.46～20.55毫克千克-1分-1;平均释放速率变幅为0.009～1.25毫克千克-1分-1;总释放量变幅为40～430毫克千克-1。供试土壤钾的释放过程可以用一级反应方程拟合,表明钾的释放速率主要受交换位上钾离子饱和度的影响。应用一级方程计算出钾的释放速率常数变幅为5.4810-3～65.6210-3分-1。多数土壤的钾释放过程符合单一的一级反应;而以云母类为主,且含有部分绿泥石的土壤的钾释放过程由两个速率不同的一级反应构成,表明不同吸附位上的钾离子可能有不同的释放机制。用供试土壤连续种植玉米幼苗57茬进行土壤钾素耗竭试验,结果表明应用连续流动交换仪研究得出的土壤钾素释放的动力学参数可以较可靠地评价土壤的供钾特征。释钾速率快,持续时间长,总释放量大的土壤(如青海栗钙土),在耗竭试验中显出很强的供钾能力;而释钾速率慢,持续时间短,总释放量小的土壤(如广东砖红壤)在盆栽试验中供植物吸收的钾量很低。有的土壤(如四川紫色土)钾的总释放量不低,但释放速度快,持续时间短,在盆栽试验     

我国北方一些土壤对外源钾的固定

本文研究了我国北方农区主要土壤的固钾能力。结果表明,在施K量0-4000mg/kg范围内,土壤固钾量与施钾量的关系符合方程y=ax3/2/(x3/2+b)。不同土壤固钾量相差很大,其固钾量与土壤矿物学性质和理化性质(粘粒含量、CEC、非交换性钾和速效钾含量)有关,与原始土壤非交换性钾含量呈显著负相关(不同施钾水平下土壤固钾量与非交换性钾含量之间的相关系数均在-0.6769* *以上)。供试土壤固钾能力表现出一定的地带性分布规律。施K1200mg/kg以下时土壤固钾能力的一般趋势是取自西北的土壤取自东北的土壤取自华北的土壤;而施K1200mg/kg以上时土壤固钾能力,从西向东呈逐渐增加的趋势。取自西北地区的土壤,固钾量(施K量为400～4000mg/kg,下同)平均为133～348mg/kg,最大同钾量平均是372mg/kg;取自华北地区的土壤,其固钾量平均为235～1001mg/kg,最大固钾量平均是1121mg/kg;取自东北地区的土壤,其固钾量平均为199～1254mg/kg,最大固钾量平均是1519mg/kg。     

Mineralselektive K-Freisetzung aus Böden mit Oktodecylammonium-Ionen (nc = 18)

Mineral-selective K release from soils by octodecylammonium- ions (n_c = 18) n_c18-releasable potassium was determined on soils of five different areas. n_c18-releasable K is the potassium which is displaced by octodecylammonium ions (ODA). Because the main sources of this potassium are trioctahedral micas, it is called briefly biotite-K. The investigated soils are from different areas in Bavaria (loess and clay stone derived soils, alluvial soils); partly the locations are K fertilizer trials. Soils of known age are locations from the Niederrhein terraces (Holocene soils). In the 0.2–2 μm (coarse clay) and 2–6 μm (fine silt) fractions the biotite-K content lies between 12 and 40% of total K. At given fractionation the highest amounts of K were released from the coarse clay of the soils. In the loess soils rich in silt the biotite-K pool in the medium and coarse silt fractions was also considerable. Clay soils showing poor K supplying power (K fertilizer trials) in the field are characterized by very low biotite-K quantities in all silt fractions. In the Holocene soils the age of which varies between 4 000 and 8 000 years the ODA releaseable values are greater than in the loess soils. In all fractions of the young Holocene soils the released K is greater than in the same fractions of the old Holocene soils. Calculated to a soil depth of 60 cm the K release in kg/ha reaches from 25 000 (young soil form) to 17 000 (old soil form).