Potassium Fixation and Release Characteristics of Some Benchmark Soil Series under Rice–Wheat Cropping System in the Indo‐Gangetic Plains of Northwestern India

Abstract

Potassium (K) fixation and release in soil are important issues in long‐term sustainability of a cropping system. Fixation and release behavior of potassium were studied in the surface and subsurface horizons in five benchmark soil series, viz. Dhar, Gurdaspur, Naura, Ladowal, and Nabha, under rice–wheat cropping system in the Indo‐Gangetic plains of India. Potassium fixation was noted by adding six rates of K varying from 0 to 500 mg kg^?1 soil in plastic beakers while K release characteristics were studied by repeated extractions with 1 M HNO₃ and 1 M NH₄OAc extractants. The initial status of K was satisfactory to adequate. Potassium fixation of added K increased with the rate of added K irrespective of soil mineralogy and soil depth. Soils rich in K (Ladowal and Nabha) fixed lower amounts (18–42%) of added K as compared to Gurdaspur, Dhar, and Naura (44.6–86.4%) soils low in K. The unit fertilizer requirement for unit increase in available K was more in low‐K soils. The study highlights the need for more studies on K fixation in relation to the associated minerals in a particular soil. Potassium‐release parameters such as total extractable K, total step K, and CR‐K varied widely in different soil series, indicating wide variation in the K‐supplying capacity of these soils. K released with 1 M NH₄OAc extractant was 20–33% of that obtained with 1 M HNO₃. Total extractable K using 1 M HNO₃ varied from 213 to 528 mg kg^?1 and NH₄OAc‐extractable K ranged from 71 to 312 mg kg^?1 soil in surface and subsurface layers of different soil series. The Ladowal and Nabha series showed higher rates of K release than Gurdaspur, Dhar, and Naura series, indicating their greater K‐supplying capacity.     

Assessment of Suitable Extractants for Predicting Plant-available Potassium in Indian Coastal Soils

The suitability of seven chemical extractants was evaluated on 24 Indian coastal soils for prediction of plant-available potassium (K) to rice (Oryza sativa L. var. NC 492) grown in modified Neubauer technique. Average amounts of soil K extracted were in descending order: 0.5 M NaHCO₃ > neutral 1 N NH₄OAc > 0.02 M CaCl₂ > Bray and Kurtz No.1 > 1 N HNO₃ > 0.1 N HNO₃ > distilled water. The highest simple correlation with plant K uptake was obtained with 0.1 N HNO₃-K (r = 0.848) and lowest with CaCl₂-K (r = 0.805). Predictive models were developed using plant K uptake as the dependent variable and extractable soil K, sand, silt, soil pH, and electrical conductivity as the independent variables. Based on the final R² and ease of measurement, distilled water, 1 N NH₄OAc, and 0.1 N HNO₃ models were the best predictors of plant-available K in coastal soils when used along with sand or soil pH.     

Extraction of potassium from some kaolinitic soils of the tropics

Abstract

Three methods for soil potassium extraction (M NH₄OAc pH 7, 0.01 M AgTU and 30 % hot H₂SO₄) were compared for a variety of kaolinitic soils of the tropics. The AgTU‐extractable K was much higher than the M NH₄OAc‐extractable K when vermiculite clay was present in the soil. The correlation between both was given by an R value of 0.937. The amounts of K extracted by 0.01 M AgTU and by hot H₂SO₄ were approximately the same. The R value for these two methods was 0.843.

It is suggested that the AgTU extractant could be used for determination of plant‐available K in soil and for testing for the presence or absence of vermiculite clay in soils.     

Soil Test to Predict the Copper Availability in Pasture Soils

Abstract

The proportion of copper (Cu) that can be extracted by soil test extractants varied with the soil matrix. The plant‐available forms of Cu and the efficiency of various soil test extractants [(0.01 M Ca(NO₃)₂, 0.1 M NaNO₃, 0.01 M CaCl₂, 1.0 M NH₄NO₃, 0.1 M HCl, 0.02 M SrCl₂, Mehlich‐1 (M1), Mehlich‐3 (M3), and TEA‐DTPA.)] to predict the availability of Cu for two contrasting pasture soils were treated with two sources of Cu fertilizers (CuSO₄ and CuO). The efficiency of various chemical reagents in extracting the Cu from the soil followed this order: TEA‐DTPA>Mehlich‐3>Mehlich‐1>0.02 M SrCl₂>0.1 M HCl>1.0 M NH₄NO₃>0.01 M CaCl₂>0.1 M NaNO₃>0.01 M Ca(NO₃)₂. The ratios of exchangeable: organic: oxide bound: residual forms of Cu in M1, M3, and TEA‐DTPA for the Manawatu soil are 1:20:25:4, 1:14:8:2, and 1:56:35:8, respectively, and for the Ngamoka soil are 1:14:6:4, 1:9:5:2, and 1:55:26:17, respectively. The ratios of different forms of Cu suggest that the Cu is residing mainly in the organic form, and it decreases in the order: organic>oxide>residual>exchangeable. There was a highly significant relationship between the concentrations of Cu extracted by the three soil test extractants. The determination of the coefficients obtained from the regression relationship between the amounts of Cu extracted by M1, M3, and TEA‐DTPA reagents suggests that the behavior of extractants was similar. But M3 demonstrated a greater increase of Cu from the exchangeable form and organic complexes due to the dual activity of EDTA and acids for the different fractions and is best suited for predicting the available Cu in pasture soils.     

Plant Availability Index by Potassium Exchange Isotherms in Selected Rice Soils of Lesser Himalayas

Potassium (K) exchange isotherms (quantity–intensity technique, Q/I) and K values derived from the Q/I relationship provide information about soil K availability. This investigation was conducted to study Q/I parameters of K, available K extracted by 1 N ammonium acetate (NH₄AOc) (exchangeable K plus solution K), K saturation percentage (K index, %), and the properties of 10 different agricultural soils. In addition, the relationship of mustard plant yield response to the K requirement test based on K exchange isotherms was investigated. The Q/I parameters included readily exchangeable K (ΔK₀), specific K sites (K_X), linear potential buffering capacity (PBC^K), and energy of exchange of K (E_K). The results of x-ray diffraction analysis of the oriented clay fractions indicated that some mixed clay minerals, illite clay minerals, along with chlorite/hydroxy interlayered vermiculite and kaolinite were present in the soils. The soil solution K activity ratio at equilibrium (AR₀) ranged from 8.0 × 10^?4 to 3.1 × 10^?3 (mol L^?1)^0.5. The readily exchangeable K (ΔK₀) was between 0.105 to 0.325 cmol_ckg^?1 soil, which represented an average of 88% of the exchangeable K (K_ex). The soils showed high capacities to maintain the potential of K against depletion, as they represented high linear potential buffering capacities (PBC^K) [13.8 to 50.1 cmol_c kg^?1/(mol L^?1)^0.5. The E_K values for the soils ranged from ?3420 to ?4220 calories M^?1. The percentage of K saturation (K index) ranged from 0.7% to 2.2%. Analysis of variance of the dry matter (DM), K concentrations, and K uptake of mustard plants indicated that there were no significant differences among the adjusted levels of K as determined by the exchange-isotherm curve.     

Mineralogical budgeting of potassium in soil: A basis for understanding standard measures of reserve potassium

The study was conducted to investigate the relationship between some standard measures of soil reserve potassium (K) and soil mineralogy. Eight different agricultural soils from the N temperate and S boreal regions were studied and analyzed both by standard methods (exchangeable K, 2 M HCl‐ and aqua regia–extractable K) and by quantitative mineralogical methods based on X‐ray powder diffraction analysis of spray‐dried bulk soils. Linear regression and multivariate methods were used to assess the relationships between standard measures of soil reserve K and a number of soil chemical, physical, and mineralogical properties. A mineralogical budgeting approach, to estimate total K and its speciation between different mineral phases, is shown to be accurate after validation against total K analyzed geochemically. This approach enabled us to determine that both HCl‐ and aqua regia–extractable K were highly correlated with K in dioctahedral phyllosilicates and extracted 1%–17% and 5%–45% of total K, respectively. Neither extraction showed any obvious relationship to K in feldspar, which is frequently a larger reservoir of K in the soils examined.     

Topsoil and subsoil potassium as affected by long‐term potassium fertilization of corn‐soybean rotations

Abstract

Long‐term potassium (K) fertilization practices are likely to affect the K content of soils. This study assessed the effect of long‐term K fertilization strategies for corn (Zea mays L.) and soybean [Glycine max (L.) Merr.] rotations on extractable K in the soil profile of a major Iowa soil type at two locations. The soil type was a Webster fine‐loamy, mixed, mesic, Typic Haplaquoll at both sites. Soil samples were collected from the 0–15, 15–30, 30–60, and 60–90 cm depths after 17 years (Site 1) or 19 years (Site 2) of K fertilization with combinations of two initial rates and four annual rates. The initial rates were 0 and 1,344 or 1,120 kg K ha^‐1 at Site 1 and 2, respectively, and the annual rates ranged from 0 to 100 kg K ha^‐1. Samples were analyzed for ammonium acetate‐extractable K (STK) and nitric acid (HNO₃)‐extractable nonexchangeable K (HNO₃‐K). Concentrations of STK and HNO₃‐K in the top 0–15 cm soil layer at the two sites were higher for the high initial K rates and were linearly related with the annual K rate. Results for the subsoil layers varied between sites and extractants. At Site 1, annual rates of 30 kg K ha^‐1 or higher resulted in a relative accumulation of HNO₃‐K in the 15–30 cm layer. At Site 2, these rates resulted in relative accumulations of STK in the 30–60 cm layer and of HNO₃‐K in the 60–90 cm layer, but with relative depletions of STK in the 15–30 and 60–90 cm layers. Thus, use of one extractant may not always be sufficient to evaluate cropping and fertilization effects on subsoil K. Long‐term K fertilization of corn and soybean rotations affected extractable K of both the topsoil and subsoil. The effects on subsoil K, however, were smaller compared with effects on the topsoil and varied markedly between sites, subsoil layers, and extractants.     

动力学模型在土壤钾素释放特征描述中的运用及其与化学方法提取的钾素之间的相关性研究

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 ^oC. 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--2017 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.     

我国主要土壤供钾能力的综合评价

本文应用生物耗竭法并结合化学的对我国１１种主要土壤的供钾能力做了综合评价。     

Influence of soil ingestion by earthworms on the availability of potassium in soil: An incubation experiment

Summary An incubation experiment was conducted to study the changes that occur in the K status of soil due to earthworm activity. Samples of Tokomaru silt loam soil were inoculated with the common pasture earthworm species Aporrectodea caliginosa and incubated for 21 days. Aliquots of moist soil were analyzed for exchangeable K by leaching with neutral molar ammonium acetate at 1:50 soil solution ratio. Extraction with boiling 1 M nitric acid at 1:100 soil solution ratio for 20 min was used to determine available non-exchangeable K. The results indicated that the exchangeable K content increased significantly due to earthworm activity but nitric acid-extractable K did not change significantly. It is inferred that earthworms increase the availability of K by shifting the equilibrium among the forms of K from relatively un-available forms to more available forms in the soil chosen for the study.     

Release Kinetics of Nonexchangeable Potassium by Different Extractants from Soils of Varying Mineralogy and Depth

Abstract

Nonexchangeable potassium (K) release kinetics of six major benchmark soil series of India as affected by mineralogy of clay and silt fractions, soil depth and extraction media was investigated. The cumulative release of nonexchangeable K was greater in smectitic soils (353 mg K kg^?1 at 0‐ to 15‐cm depth and 296 mg K kg^?1 at 15‐ to 30‐cm depth, averaged for 2 soils and 3 extractants) than in illitic (151 mg K kg^?1 at 0‐ to 15‐cm depth and 112 mg K kg^?1 at 15‐ to 30‐cm depth) and kaolinitic (194 mg K kg^?1 at 0‐ to 15‐cm depth and 167 mg K kg^?1 at 15‐ to 30‐cm depth) soils. Surface soils exhibited larger cumulative K release in smectitic and illitic soils, whereas subsurface soils had larger K release in kaolinitic soils. Among the extractants, 0.01 M citric acid extracted a larger amount of nonexchangeable K followed by 0.01 M CaCl₂ and 0.01 M HCl. The efficiency of citric acid extractant was greater in illitic soils than in smectitic and kaolinitic soils. Release kinetics of nonexchangeable K conformed fairly well to parabolic and first‐order kinetic models. The curve pattern of parabolic diffusion model suggested diffusion controlled kinetics in all the soils, with a characteristic initial fast rate up to 7 h followed by a slower rate. Greater nonexchangeable K release rates in smectitic soils, calculated from the first‐order equation (b=91.13×10^?4 h^?1), suggested that the layer edge and wedge zones and swelling nature of clay facilitated the easier exchange. In contrast to smectitic soils, higher release rate constants obtained from parabolic diffusion equation (b=39.23×10^?3 h^?1) in illitic soils revealed that the low amount of exchangeable K on clay surface and larger amount of interlayer K allowed greater diffusion gradients, thus justifying the better fit of first‐order kinetic equation in smectitic soils and parabolic diffusion equation in illitic soils.     

Effect of potassium fertilization on soil potassium pools and rice response in an intensive cropping system in China

In order to assess the changes in soil K pools as affected by K‐fertilizer application and the impact of the changes on K balance, grain yield, and K uptake, an experiment was conducted in Central Zhejiang Province, E China, in a continuous double‐cropping rice system. Two sites were selected: (1) the Agricultural Research Institute of Jinhua (ARI) where soil is calcareous and (2) the Shimen Research Farm (SM) where soil is acidic. Eight consecutive crops were grown (1997–2000) in ARI and five consecutive crops (1998–2000) at SM. Treatments included unfertilized control (CK) and three different fertilizer treatments (NP, NK, and NPK). Potassium extracted by ion‐exchange resin decreased from 26 mg kg^–1 to 5–10 mg kg^–1 after eight consecutive seasons of growth at the ARI site. Addition of 100 kg K ha^–1 for each rice crop was not enough to maintain initial K availability, especially in the calcareous soil at ARI site. In treatments with K, a small increase in readily available K was observed only in SM soil. The K extracted by HNO₃ also decreased significantly in the treatments without K addition and was increased slightly in the treatments with K application. In the NP treatment, the decrease in HNO₃‐K was several times greater than resin‐K, indicating that nonexchangeable K may be the major source of K supply to rice. Soil K depletion was greater for hybrid rice than for inbred rice, and this difference in K demand should be taken into account in developing fertilizer recommendations for irrigated rice.     

Mehlich 3 or modified olsen for soil testing in Malawi

Abstract

Soil nutrient extraction methods, which are currently being used in Malawi, are time consuming and require too many resources. The use of a universal soil extractant would greatly reduce resource requirements. The objectives of the study were to (i) compare the universal soil extractants, Mehlich 3 (M3) and Modified Olsen (MO) with ammonium acetate (AA), Bray P1 (BPl), and diethylene triamine penta acetic acid (DTPA) in the amount of nutrients extracted, (ii) determine the relationship among the extractants for the nutrients they extract, and (iii) determine the critical soil‐test levels of phosphorus (P), potassium (K), and zinc (Zn) for a maize crop. Missing nutrient trials involving P, K, and Zn were conducted on thirty sites across Malawi using maize (Zea mays L.). Phosphorus application rates ranged from 40 to 207 kg P₂O₅ ha^‐1. Potassium and Zn were applied at 75 kg K₂O and 10 kg Zn ha^‐1, respectively. Procedures of Cate and Nelson were used to identify soil nutrient critical levels. Results showed that the correlations between M3 and BP1, and MO and BPl were highly significant (r=0.93, 0.94, respectively). Mehlich 3 extractable K and AA extractable K (r=0.90), MO and AA extractable K (r=0.94) were highly significant (P<0.01) and the correlations between M3 and AA and MO and AA extractable calcium (Ca) (r=0.92, 0.90, and 0.94, respectively) were also highly significant (P<0.01). The correlations between M3, MO, and AA extractable magnesium (Mg) (r=0.99) were highly significant (P<0.01). Zinc, copper (Cu), and manganese (Mn) extracted with M3 and DTPA were significantly correlated (r=0.89, 0.87, and 0.95, respectively). Correlations between MO and DTPA extractable Zn, Cu, and Mn were also highly correlated (r=0.89,0.85, and 0.95, respectively). Maize grain yields ranged from 730 to 9,400 kg ha^‐1. Mehlich 3‐P and MO‐P critical levels were 31.5 and 28.0 μg g^‐1, respectively. Mehlich 3 and MO gave a similar critical level of 0.2 cmol kg^‐1 for K while Zn critical levels were 2.5, and 0.8 μg g^‐1 for M3 and MO, respectively. Mehlich 3 and MO were equally effective in separating responsive to none responsive soils for maize in Malawi.     

Evaluation of Universal Extractants for Determination of Some Macronutrients from Soil

Evaluation of nutrient status in soil is important for nutritional, environmental, and economical aspects. The objective of this work was to find out the most suitable universal extractant for determination of available phosphorus (P) and nitrate (NO₃-) and exchangeable potassium (K), calcium (Ca), and magnesium (Mg) from soils using 0.01 M calcium chloride (CaCl₂), 0.01 M barium chloride (BaCl₂), 0.1 M BaCl₂, 0.02 M strontium chloride (SrCl₂), Mehlich 3, and ammonium bicarbonate diethylene triamine penta acetic acid (AB-DTPA) extractants. Composite surface soil samples (0–20 cm) were collected from the Eastern Harage Zone (Babile and Haramaya Districts), Wolaita Zone (Damot Sore, Boloso Bombe, Damot Pulasa, and Humbo Districts), and Dire Dawa Administrative Council by purposive sampling. The experiment was carried out in a completely randomized design (CRD) with three replications. Results indicated that the greatest correlations were found between Mehlich 3 and Olsen method and also between 0.02 M SrCl₂ and Olsen method for available P. The amount of NO₃^– extracted by 0.02 M SrCl₂ was significantly correlated to the amount determined by 0.5 M potassium sulfate (K₂SO₄). The amounts of exchangeable K, Ca, and Mg determined by ammonium acetate (NH₄OAc) method were significantly correlated to the amount determined by universal extractants tested. In general, both 0.02 M SrCl₂ and Mehlich 3 can serve as universal extractants for the macronutrients considered in this study with the former being more economical when NO₃^– is included.     

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.     

Soil micronutrient contents and relation to other soil properties in Ethiopia

Abstract

Alfisols, Vertisols, Inceptisols, Aridisols, Mollisols, and Entisols were sampled (0–30 cm) from 32 locations across Ethiopia. The soils were analyzed for copper (Cu), zinc (Zn), manganese (Mn), and iron (Fe) contents using 0.005 M diethylene triamine pentaacetic acid (DTPA), 0.05 M hydrochloric acid (HC1), and 0.02 M ethylene diamine tetraacetic acid (EDTA) extractants. EDTA extracted more of each micronutrient than DTPA, which extracted greater amounts than HC1. The quantities of EDTA and DTPA‐extractable micronutrients were significantly correlated, and were in the order: Mn>Fe>Cu>Zn. The order of HCl‐extractable micronutrients was Mn>Fe>Zn>Cu. Micronutrient contents of Mollisols, Vertisols, and Alfisols were usually greater than those of the other soils, and Entisols usually had the lowest micronutrient contents. The contents were mostly positively correlated with clay and Fe₂O₃ contents, but negatively correlated with soil pH and A1₂O₃contents. While comparison of DTPA‐ and EDTA‐extractable micronutrients with critical levels showed that most soils had adequate amounts of the micronutrients for crops, the amounts extracted by HC1 were below critical levels in most soils. Since the critical levels that were used in the comparisons were not established in Ethiopia, calibration of the soil contents of these micronutrients with crops grown in Ethiopia is required to identify the most suitable extractant(s).     

An evaluation of chemical methods for extracting copper from rice soils

Abstract

Rice (Oryza sativaL. CV. Lemont) was grown on 19 soils, and eight extractants were evaluated for determining the availability of Cu to rice plants. Correlation analyses were employed as criteria for evaluating methods that would provide the best index of Cu availability. The order of removal of Cu from soils was: 0.5NHC1 + 0.05NA1C1₃> 0.5NHNO₃> 0.5 N HC1 > EDTA + NH₄OAc > 0.1NHC1 > EDTA + (NH₄)₂CO₃? DTPA‐TEA, pH 7.3 >>> 1 N NH₄0Ac, pH 4.8.

Uptake of Cu by rice plants was significantly correlated with soil Cu. Among the eight extractants evaluated, Cu extracted with DTPA‐TEA, pH 7.3 was better related to the concentration (r = 0.563 ) and uptake (r = 0.673 ) of Cu by rice plants grown on the soils with different chemical and physical properties.

A significant negative correlation was found between the concentration of Cu in rice plants and the organic matter content of the soils. Each one percent increase in the organic matter of the soils resulted in a corresponding decrease of approximately one mg/kg in the concentration of Cu in the rice‐plant tissue. Multiple regressions of extractable Cu by eight methods with soil organic matter content accounted for from 53.4 to 70.0% of the variations in the prediction of the concentration of Cu in the rice plants. Combinations of other soil chemical properties measured with extractable Cu did not significantly improve the predictability     

Effect of long‐term fertilization and manuring on potassium release properties in a Typic Ustochrept

A long‐term fertilizer experiment, over 27 years, studied the effect of mineral fertilizers and organic manures on potassium (K) balances and K release properties in maize‐wheat‐cowpea (fodder) cropping system on a Typic Ustochrept. The treatments consisted of control, 100% nitrogen (100% N), 100% nitrogen and phosphorus (100% NP), 50% nitrogen, phosphorus, and potassium (50% NPK), 100% nitrogen, phosphorus, and potassium (100% NPK), 150% nitrogen, phosphorus, and potassium (150% NPK), and 100% NPK+farmyard manure (100% NPK+FYM). Nutrients N, P, and K in 100% NPK treatment were applied at N: 120 kg ha^—1, P: 26 kg ha^—1, and K: 33 kg ha^—1 each to maize and wheat crops and N: 20 kg ha^—1, P: 17 kg ha^—1, and K: 17 kg ha^—1 to cowpea (fodder). In all the fertilizer and manure treatments removal of K in the crop exceeded K additions and the total soil K balance was negative. The neutral 1 N ammonium acetate‐extractable K in the surface soil (0—15 cm) ranged from 0.19 to 0.39 cmol kg^—1 in various treatments after 27 crop cycles. The highest and lowest values were obtained in 100% NPK+FYM and 100% NP treatments, respectively. Non‐exchangeable K was also depleted more in the treatments without K fertilization (control, 100% N, and 100% NP). Parabolic diffusion equation could describe the reaction rates in CaCl₂ solutions. Release rate constants (b) of non‐exchangeable K for different depth of soil profile showed the variations among the treatments indicating that long‐term cropping with different rates of fertilizers and manures influenced the rate of K release from non‐exchangeable fraction of soil. The b values were lowest in 100% NP and highest in 100% NPK+FYM treatment in the surface soil. In the sub‐surface soil layers (15—30 and 30—45 cm) also the higher release rates were obtained in the treatments supplied with K than without K fertilization indicating that the sub‐soils were also stressed for K in these treatments.     

Combination of Different Extractants to Assess Binding Forms of Some Elements in Soil Profiles

Abstract

Three different chemical extractants were evaluated as to their extraction efficiency for copper (Cu), zinc (Zn), lead (Pb), aluminium (Al), iron (Fe), chromium (Cr), manganese (Mn), potassium (K), magnesium (Mg), and calcium (Ca) on forest soil profiles from the Romanian Carpathians. The extractants were hot 14 M nitric acid (HNO₃), 0.05 M hydrochloric acid (HCl), and 0.1 M sodium pyrophosphate. By comparing amounts extracted by 0.05 M HCl and 0.1 M sodium pyrophosphate relative to that dissolved by hot 14 M HNO₃, some conclusions were drawn concerning the chemical forms of the metals in the extractable pool. The amount released by 0.05 M HCl was generally less than 10% of the HNO₃‐extractable fraction but showed considerable variation among the elements studied. The relative amount extracted by pyrophosphate increased with organic‐matter content of the soils for Cu, Zn, Pb, Al, Fe, and Cr; stayed more or less constant for Mn, K, and Mg; and decreased for Ca. These findings are discussed with respect to the different binding forms of the metals in the soil and the processes affecting their mobility. From the present results, the metals were ranked as follows with respect to their ability to form organic complexes in natural soils: Cu>Cr, Pb>Ca>Al>Fe, Zn, Mn, K>Mg. However, the use of cold dilute HCl as a fractionation step may be questionable in cases of soils with a high content of substances possessing large neutralization capacity for protons.