Diethylenetriaminepentaacetic Acid–Extractable Micronutrients Status in Soil under a Rice–Wheat System and Their Relationship with Soil Properties in Different Agroclimatic Zones of Indo‐Gangetic Plains of India

Available micronutrient status of zinc, copper, manganese, and iron (Zn, Cu, Mn, and Fe) in surface soil samples under a rice–wheat system collected from farmers' fields in 40 districts representing different agroclimatic zones of the Indo‐Gangetic Plain (IGP) were determined. The selection of farmers, villages, blocks, and districts within an agroclimatic zone was made on the basis of a multistage statistical approach. In Trans‐Gangetic Plains, the diethylenetriaminepentaacetic acid (DTPA)–extractable Zn ranged from 0.11 to 5.08, Cu ranged from 0.22 to 4.72, Mn ranged from 2.9 to 101.2, and Fe ranged from 1.05 to 97.9 mg kg^?1. In the Upper Gangetic Plains, the DTPA‐extractable Zn ranged from 0.04 to 2.53, Cu ranged from 0.06 to 4.32, Mn ranged from 11.1 to 421.0, and Fe ranged from 3.48 to 90.2 mg kg^?1. In the Middle Gangetic Plains, the DTPA‐extractable Zn ranged from 0.17 to 8.60, Cu ranged from 0.09 to 7.80, Mn ranged from 3.0 to 155.1, and Fe ranged from 9.22 to 256.7 mg kg^?1. In the Lower Gangetic Plains, the DTPA‐extractable Zn ranged from 0.04 to 3.46, Cu ranged from 0.21 to 4.38, Mn ranged from 9.54 to 252.2, and Fe ranged from 3.60 to 182.5 mg kg^?1. The intensively cultivated Trans‐Gangetic transect representing the midplain and Siwalik had more available micronutrients than the arid plain. Midplain and arid plain showed 17 to 20% of soil samples were low to medium in Zn and 5 and 8% were low in Fe. In the Upper Gangetic Plains, only 25% samples were deficient in Zn, especially in central and southwest plains. In the Middle Gangetic Plains, 20 to 30% of samples were deficient in Zn, and very few samples were deficient in other micronutrients. In the Lower Gangetic Plains, a majority of the samples were medium to high in micronutrients except in Barind and Rarh Plains where 30% of samples were deficient in Zn. In the Lower Gangetic Plains, the available micronutrients were plentiful. Available micronutrients increased with increase in organic C content and decreased with increase in sand content, pH, and calcium carbonate. These soils are alluvial in nature, and there was no definite pattern of micronutrient distribution with depth in the profile. However, there was more accumulation in the Ap horizon than in the B horizon.     

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.     

Effect of Potassium Nutrition of Different Varieties of Rice on the Redox Status in Microzone Rhizosphere Soils

Being divided into three groups-strong,moderate and weak-according to the different kinetic parameters (Fmax,km,Cmin) of potassium uptake by crops,21 cultivars of rice have been studied to find out the relationships between their potassium nutrition and the oxidation-reduction status in the rhizosphere soils. Results show that,with no application of K fertilizer,there were higher contents of active reducing substances and ferrous iron in rhizosphere soils planted with cultivars,such as Zhongguo 91,week in absorbing potassium than in soils cropped with cultivars,Shanyou 64,stronger in absorbing potassium.As a result of K application,however,these toxic substances were decreased appreciably in the soil,particularly in the root zone where weakly K-absorbing sultivars were growing,and the parameter of soil redox (pH pE) was increased,the most striking example of this being found in the rhizosphere soil where the more strongly K-absorbing sultivars were growing.On and close to the root surface in soils where rice plants were supplied with potassium fertilizer,rather more iron oxide had been accumulated compared with rice receiving no potash,and even greater amounts of red iron oxide precipitated on the rice root in neutral paddy soils.As shown by the concentration distribution of active reducing substances and ferrous iron in a microzone of the profile,the redox range of rice roots supplied with potassium may extend as far as several centimeters from the root surface.It can thus be seen that potassium nutrition exerts its effect first on the morphological properties of rice roots and their exudation of oxygen,then on the content of soluble oxygen and the count and species of oxygen-consuming microbes in the rhizosphere soil,and finally on the redox status of the soil.     

Nitrogen‐Supplying Capacity of Soils for Rice and Wheat and Nitrogen Availability Indices in Soils of Northwest India

Abstract

Quantitative assessment of soil nitrogen (N) that will become available is important for determining fertilizer needs of crops. Nitrogen‐supplying capacity of soil to rice and wheat was quantified by establishing zero‐N plots at on‐farm locations to which all nutrients except N were adequately supplied. Nitrogen uptake in zero‐N plots ranged from 41.4 to 110.3 kg N ha^?1 for rice and 33.7 to 123.4 kg N ha^?1 for wheat. Availability of soil N was also studied using oxidative, hydrolytic, and autoclaving indices, salt‐extraction indices, light‐absorption indices, and aerobic and anaerobic incubation indices. These were correlated with yield and N uptake by rice and wheat in zero‐N plots. Nitrogen extracted by alkaline KMnO₄ and phosphate borate buffer and nitrogen mineralized under aerobic incubation were satisfactory indices of soil N supply. For rice, 2 M KCl and alkaline KMnO₄ were the best N‐availability indices. Thus, alkaline KMnO₄ should prove a quick and reliable indicator of indigenous soil N supply in soils under a rice–wheat cropping system.     

Crop Residue Retention and Nutrient Management Practices on Stratification of Phosphorus and Soil Organic Carbon in the Soybean–Wheat System in Vertisols of Central India

The long-term crop residue retention coupled with external nutrient inputs are crucial for maintaining soil phosphorus (P) and soil organic carbon (SOC) in Vertisols of Central India. A study was conducted to evaluate the long-term effect of three wheat residue management practices (residue burning, incorporation, and surface retention) in combination with three supplementary nutrient inputs (SNI) [control, fertilizer, and farmyard manure (FYM)] on stratification of P and SOC in the soybean–wheat system in Vertisol. The wheat residue either incorporated or retained on the soil surface increased the availability of P and SOC content as compared to the common practices of residue burning. Residue retention or incorporation increased stratification of P and soil organic carbon over the residue burning. Irrespective of the nutrient treatments, greater stratification ratio of SOC and P were registered under wheat residue incorporation or retention compared to residue burning. It is evident from the study that wheat residue incorporation or retention plus addition of FYM could be an effective strategy for increasing the soil fertility in a soybean–wheat system of Vertisols of Central India.     

Fixation and Recovery of Added Phosphorus and Potassium in Different Soil Types of Pulse‐Growing Regions of India

Abstract

Fixation and recovery of added phosphorus (P) and potassium (K) were studied in different soil types of pulse‐growing regions. Amounts of P and K fixed increased in all the soils irrespective of type and texture. With the increase in levels of added P and K, maximum P fixation was observed at lower levels of added P (50 mg kg^?1). Alfisols showed maximum P‐fixation capacity (92.7%), followed by Vertisols (86.5%) and Inceptisols (76.6%) at 50 mg kg^?1 added P. However, K fixation increased with increasing levels of added K up to 200 mg kg^?1, and thereafter fixation either decreased or was maintained at similar levels. Vertisols showed higher K fixation than Inceptisols and Alfisols. Fertilizer P requirement per unit increase in available P in soil was highest in Bangalore (3.23) and lowest in Delhi (2.38). Fertilizer K requirement per unit increase in available K in soil was highest in Raipur and Gulbarga (1.75) and lowest in Ranchi (1.28).     

Distribution of Forms of Zinc and Their Association with Soil Properties and Uptake in Different Soil Orders in Semi‐arid Soils of Punjab,India

Abstract

Profiles of semi‐arid–zone soils in Punjab, northwest India, were investigated for different forms of zinc (Zn), including total, diethylenetriamine penta‐acetic acid (DTPA)-extractable, soil solution plus exchangeable (Zn), Zn adsorbed onto inorganic sites, Zn bound by organic sites, and Zn adsorbed onto oxide surfaces. Irrespective of the different fractions of Zn present, its content was higher in fine‐textured Alfisols and Inceptisols than in coarse‐textured Entisols. In general, the higher content of Zn was observed in the surface horizon and then decreased in the subsurface horizons. However, none of the forms of Zn exhibited any consistent pattern of distribution. Organic matter and size fractions (clay and silt) had a strong influence on the distribution of different forms of Zn. Based upon the linear coefficient of correlation, the soil solution plus exchangeable Zn, adsorbed onto inorganic sites, and DTPA‐Zn increased with increase in organic carbon but decreased with increase in pH and calcium carbonate content. Total Zn increased with increase in clay and silt content. Among the different forms, Zn bound by organic sites, water soluble plus exchangeable Zn and Zn adsorb onto oxide (amorphous surfaces) were all correlated with DTPA extractable Zn. The uptake of Zn was more in recent floodplain Entisols than very fine textured Alfisols and Inceptisols. Among the different forms soil solution +exchangeable and DTPA‐extractable Zn was positively correlated with total uptake of Zn.     

Potential of Carbon Dioxide Biosequestration of Saline–Sodic Soils during Amelioration under Rice–Wheat Land Use

Potential for carbon dioxide (CO₂) biosequestration was determined during the reclamation of highly saline–sodic soils (Aridisols) after rice (2003) and wheat (2003–2004) crops at two sites in District Faisalabad, Pakistan. Two treatments were assessed: T₁, tube-well brackish water only; and T₂, soil-applied gypsum at 25% soil gypsum requirement?+?tube-well brackish water. The irrigation water used at both sites had different levels of salinity (EC 3.9–4.5 dS m^?1), sodicity (SAR 21.7–28.8), and residual sodium carbonate (14.9 mmol_c L^?1). Composite soil samples were collected from soil depths of 0–15 and 15–30 cm at presowing and postharvest stages and analyzed for pH, EC_e, and sodium adsorption ratio (SAR). After rice harvest, there was no significant effect of gypsum application on EC_e, pH, and SAR at both sites, except pH at 0–15 cm depth decreased significantly with gypsum at site 1. After wheat harvest, EC_e, pH, and SAR decreased significantly with gypsum at site 1, whereas the effect of gypsum on these parameters was not significant at site 2. Compared to initial soil, EC_e and SAR in soil decreased considerably after rice or wheat cultivation, particularly at site 1, whereas pH increased slightly due to cultivation of these crops. For rice, the total CO₂ sequestration was significantly increased with gypsum application at both sites and ranged from 1499 to 2801 kg ha^?1. The total sequestration of CO₂ was also significantly increased with gypsum application in wheat at both sites and ranged from 2230 to 3646 kg ha^?1. The amounts of CO₂ sequestered by crops due to gypsum application were related to seed and straw yield responses of rice and wheat to gypsum, which were greater at site 1 than site 2. Also, the yield response to applied gypsum was greater for rice than wheat at site 1, whereas the opposite was true at site 2. Overall, the combined application of gypsum with brackish water reduced soil EC_e and SAR compared to brackish water alone, particularly at site 1. Our findings also suggest that the reclamation strategies should be site specific, depending on soil type and quality of brackish water used for irrigation of crops. In conclusion, the use of gypsum is recommended on brackish water–irrigated salt-prone soils to improve their quality, and for enhancing C biosequestration and crop production for efficient resource management.     

Comparison of Soil Physical Properties between a Permanent Fallow and a Long-Term Rice–Wheat Cropping with Inorganic and Organic Inputs in the Humid Subtropics of Eastern India

A comparison was made between a long-term rice–wheat cultivation with fertilizer nitrogen–phosphorus–potassium (NPK) or added organics [farmyard manure (FYM), paddy straw (PS), green manure (GM)] and a permanent fallow on bulk density (BD), saturated hydraulic conductivity (K_sat), available water capacity (AWC), maximum water-holding capacity (MWHC), aggregation, and soil organic carbon (SOC) dynamics on an Inceptisol of humid subtropics of eastern India. Continuous cropping caused a net decrease in SOC content. Undisturbed fallow was comparable to soils with FYM, PS, and GM amendments in structural and hydrophysical properties. Maximum WHC and AWC values were in the order of FYM followed by PS, GM, fallow, NPK, and control. The relative efficacy of the organics for physical buildup was FYM > PS > GM, which increased structural indices. This study represents further steps toward understanding the ecological importance of fallow management and integrated use of balanced fertilizer and organics.     

Efficiency of Phosphogypsum and Mined Gypsum in Reclamation and Productivity of Rice–Wheat Cropping System in Sodic Soil

In this study, efficacies of mined gypsum and phosphogypsum (PG), when applied at equivalent doses, were compared for sodic soil reclamation and productivity of rice–wheat system. Application of PG, followed by karnal grass as first crop, resulted in the greatest reduction of soil pH and exchangeable sodium percentage (ESP) followed by PG applied at 10 Mg ha^?1 alone. Application of PG at 10 Mg ha^?1 resulted in greater yields of both rice and wheat than other treatments. Ditheylenetriaminepentaacetic acid (DTPA)–extractable micronutrients of PG-treated soil were greater than in mined gypsum–treated soil. A greater portion of applied P entered the calcium (Ca)–phosphorus (P) fraction in PG-treated soil, which also resulted in more soluble P than the mined gypsum–treated soil. Phosphogypsum effected greater increase in aggregation, soil organic carbon, microbial biomass carbon, and aggregate associated carbon and decrease in zeta potential, leading to increased hydraulic conductivity and moisture retention capacity in soil over mined gypsum–treated soil.     

Using Carbon Management Index to Assess the Impact of Compost Application on Changes in Soil Carbon after Ten Years of Rice–Wheat Cropping

Soil organic carbon (C) is a major determinant for the sustainability of agricultural systems. The changes in C pools (active or total) reflect the changes in an agricultural system. The C management index (CMI) can be used to monitor the soil over time, and it also tells whether a new system or practice is declining or rehabilitating the soil. Carbon management index was calculated for a long‐term experiment after 10 cycles of rice–wheat cropping to assess the influence of rice straw compost application either alone or in combination with inorganic fertilizers on soil C buildup. Total and labile C was greater in rice straw compost–amended soil as compared to unamended control or soils receiving inorganic fertilizers only. Application of rice straw compost increased the mean CMI (47.1) as compared to control (21.0). Labile C was positively related to mean weight diameter, and seemingly it plays an important role in the maintenance of physical fertility of soils and thus sustainability of the cropping system.     

Effect of Sewage Sludge and Rice Straw Compost on Yield,Micronutrient Availability and Soil Quality under Rice–Wheat System

ABSTRACT

There is a need to improve the way in which crop residues and industrial organic wastes are managed and also to study their potential use in agriculture for improving soil fertility and biological activity. This study evaluated the effects of integrated use of organic (sewage sludge (SS) and rice straw compost (RSC)) and inorganic fertilizers on crop yield, soil enzymes activity, macro- and micro-nutrients availability under rice–wheat cropping system after three consecutive years of cropping in a subtropical semi-arid area. Different combinations of inorganic nitrogen and two doses of organic sources (SS and RSC) were applied to the soil. The results revealed that substitution with 50% N through RSC significantly increased the yield and biochemical properties as compared to inorganic fertilizers (NPK) alone. Micronutrients availability was found increased in treatment having substitution of 50% N through SS @10 t ha^?1. All the enzymatic activities viz. dehydrogenase, fluorescein diacetate (FDA), phosphatase, phytase, and urease) were found to be maximum by substitution of 50% N through RSC. Also, a significant positive correlation was found between soil enzymes (dehydrogenase and FDA) and organic carbon as well as crop yield. Thus, the study demonstrated that substitution of 50% inorganic nitrogen through organic sources will be a better alternative for improving soil quality and productivity.     

Methods to Estimate the Protection of Soil Organic Carbon within Macroaggregates, 1: Does Soil Water Status Affect the Estimated Amount of Soil Organic Carbon Protected inside Macroaggregates?

The additional mineralized soil organic carbon (SOC) after soil crushing is considered to be the amount of SOC protected within aggregates (>200 μm). This study investigated the effect of soil moisture in crushed and uncrushed soil samples on the calculated amounts of protected SOC in five tropical soils (Arenosol, two Ferralsols, Nitisol, and Vertisol). No differences in soil moisture optimum were observed between crushed and uncrushed soil samples, except in clayey soils with high SOC contents and high SOC mineralization rates (Nitisol and Vertisol). Crushing the soil increased soil respiration by 0.9 to 2.4 times. Soil moisture seemed to be a confounding factor in estimation of the SOC-protected amount only in soil with a high amount of protected SOC or with a low macroaggregate stability (Ferralsol and Vertisol). In these soils, the amount of protected SOC could be influenced by the method used to estimate it.     

Carbon Stock in Sandy and Loamy Soils of Coniferous–Broadleaved Forests at Different Succession Stages

Eurasian Soil Science - A comparative assessment of the carbon stock in loamy soils (Albic Retisols) of coniferous–broadleaved forests on the Moskvoretsko-Oksky Plain and sandy soils (Albic...     

Effects of Rice Husk Ash and Bagasse Ash on Phosphorus Adsorption and Desorption in an Alkaline Soil under Wheat–Rice System

Rice husk ash (RHA) and bagasse ash (BA) are available in large quantities in South Asian countries growing rice and sugarcane. Land application of RHA and BA is likely to influence chemistry of soil phosphorus (P) and thereby P adsorption and desorption. Laboratory studies were carried out to investigate the short-term and long-term effects of RHA and BA application on P adsorption and desorption in an alkaline soil under a wheat–rice system. Addition of RHA or BA (10 Mg ha^?1) resulted in a significant decrease in P adsorption compared to the control. The decrease in P adsorption was lower when RHA and BA were applied to either rice or wheat as compared with when applied to both the crops. The BA was more effective in reducing P adsorption than RHA because of its greater P concentration. Fresh addition of RHA and BA at 1% (dry-weight basis) showed a small effect on P adsorption as compared to their long-term application. The Frendulich isotherm equation gave better fit with the experimental data than the Langmuir equation and is reliable to describe the P quantity/intensity relationships of this soil as affected by the additions of RHA and BA. The P-adsorption capacities (revealed from the Langmuir isotherms) of the unamended control, RHA, and BA (applied to both wheat and rice) were 256, 313, and 385 mg kg^?1, respectively; the corresponding bonding energies for the three treatments are 0.0085, 0.0041, and 0.0026 L kg^?1, respectively. Desorption of P was minimum in the control plots and maximum with BA followed by RHA, especially when applied to both the crops.     

Root Growth and Soil Water Dynamics in Relation to Inorganic and Organic Fertilization in Maize–Wheat

Agricultural productivity is increasingly becoming dependent upon soil fertility, which is generally thought to be supplemented through the application of nutrients mainly through inorganic fertilizers. The present study aims to characterize the soil physical environment in relation to long-term application of farmyard manure (FYM) and inorganic fertilizers in a maize–wheat cropping system. The treatments in both the maize and wheat systems included a control (without any fertilizer or FYM), FYM (farmyard manure at 20 t ha^?1), N₁₀₀ (nitrogen at 100 kg ha^?1), N₁₀₀P₅₀ (nitrogen and phosphorus at 100 and 50 kg ha^?1), and N₁₀₀P₅₀K₅₀ (nitrogen, phosphorus, and potash at 100, 50, and 50 kg ha^?1). The treatments were replicated four times in a randomized complete block design in sandy loam soil. The root mass density in surface layers of both the crops was lower in FYM and higher in inorganic fertilizer plots. The root length density was found to be highest in FYM-treated plots and lowest in control plots. The periodic soil matric suction during wheat following maize remained highest in FYM plots followed by that in N₁₀₀ plots in all the layers. The soil water storage of wheat at harvest (rice–wheat) was highest (21.1 cm) in control and lowest (17.8 cm) in FYM-treated plots. The soil water status, root growth, and crop performance improved with balanced fertilization.     

Changes in Nitrogen,Phosphorus, and Potassium in a Long‐Term Continuous Maize–Wheat Cropping System in India

In a long‐term maize–wheat rotation at the Punjab Agricultural University, Ludhiana, India (subtropical climate), the effects of nitrogen (N), phosphorus (P), and potassium (K) addition on soil fertility and forms of inorganic P and K in the plow layer of an alkaline sandy loam soil were measured after 11 and 22 years of cropping. The treatments comprised four rates of N (0, 60, 120, and 180 kg N ha^?1) as urea, three rates of P (0, 17.5, and 35 kg P ha^?1) as single superphosphate, and two rates of K (0 and 33 kg K ha^?1) as muriate of potash. The treatments selected for the present study were N₀P₀K₀, N₁₂₀P₀K₀, N₁₂₀P_17.5K₀, N₁₂₀P₃₅K₀, N₁₂₀P_17.5K₃₃, and N₁₂₀P₃₅K₃₃. A significant year × treatment interaction in decreasing available N [alkaline potassium permanganate (KMnO₄)–oxidizable N) status of soils was found in all the treatments. Available P (Olsen P) in the control plot decreased over time whereas in plots with added P, available P increased significantly after years 11 and 22, with the greatest increase in the N₁₂₀P_17.5K_o treatment. Compared to the initial values, continuous P fertilization resulted in greater total P and chloride P concentrations after 11 and 22 years. Although sodium hydroxide (NaOH) P and sulfuric acid (H₂SO₄) P increased in P‐treated plots from the start of the trial to year 11, they decreased from year 11 to year 22. Among these inorganic P forms, chloride P was significantly positively correlated with P uptake (r = 0.811*). When only N and P were applied, available K [ammonium acetate (NH₄OAc)–extractable K] significantly decreased over time. In plots without K addition, water‐soluble and exchangeable K decreased from their initial status. Compared to year 11, water‐soluble K increased, whereas exchangeable K decreased after year 22 in plots receiving no K fertilizer. Compared with NPK treatments, a significant decrease of total K in NP treatment plots suggests the release and uptake of nonexchangeable K. Water‐soluble K and exchangeable K were not correlated with K uptake. These results suggest that long‐term application of P fertilizers resulted in the accumulation of P in the soil, which could have resulted in saturation of P binding sites. Of the soil inorganic P fractions, only chloride P appears to be a good indicator of plant‐available P. The gradual loss in native soil K and release of nonexchangeable K indicates the need for adding K fertilizer to maintain soil fertility.     

Potassium Fixation and Release Characteristics in Rhizosphere and Nonrhizosphere Soils for a Rapeseed–Rice Cropping Sequence

Potassium (K) fixation and release in soil are important factors in the long-term sustainability of a cropping system. Changes in K concentration and characteristics of K fixation and release in rhizosphere and nonrhizosphere soils in the rapeseed (Brassica napus L.)–rice (Oryza sativa L.) rotation were investigated using a rhizobox system. The concentrations of different forms of K in both rhizosphere and nonrhizosphere soils decreased with plants compared to without plants, regardless of K fertilizer application. Potassium uptake by crops mainly came from the rhizosphere soil. In the treatment without K fertilizer (–K), the main form of K supplied by the soil to the crops was 1.0 mol L^?1 nitric acid (HNO₃) nonextractable K, followed by nonexchangeable K, and then exchangeable K. In the treatment with K fertilizer (+K), the main K forms supplied by the soil to the crops were exchangeable K and nonexchangeable K. The amount and rate of K fixation after one cycle of the rapeseed–rice rotation was greater in rhizosphere soil than in nonrhizosphere soil. The amount and rate of K fixation of soil in the +K treatment were significantly less than in the –K treatment. The cumulative amounts of K released with 1.0 mol L^?1 ammonium acetate (NH₄OAc) and 1.0 mol L^?1 HNO₃ extraction increased with the increasing numbers of extractions, but the K-releasing power of soil by successive extraction decreased gradually and finally became almost constant. The release of K was less in rhizosphere soil than in nonrhizosphere soil. The release of K in the +K treatment was similar to that in the –K treatment in rhizosphere soil, but the K release in nonrhizosphere soil was greater with the +K than the –K treatment. Overall, the information obtained in this study will be helpful in formulating more precise K fertilizer recommendations for certain soils.     

Prediction of Yield and Nitrogen Uptake of Wheat from Net Nitrogen Balance after Rice in Rice–Wheat Cropping System

Net nitrogen (N) balance after rice (Oryza sativa L.) and its relationship with yield and N uptake of succeeding wheat crop was studied in a greenhouse. Three urea-enriched green manures, namely dhaincha (Sesbania aculeata L.), cowpea (Vigna unguiculata L.), and guar (Cyamopsis tetragonoloba L.) were compared with split application of urea in a rice–wheat cropping sequence. After rice, a negative N balance was measured in all treatments; however, the N balance values were greater with urea than with green manures. The N balance was positively correlated with the N content but negatively correlated with lignin content and carbon (C)–N ratio of the green manures. Lignin content was a better index than C/N ratio for predicting the net N balance, which described 82.3% of the total variations. Efficiency of residual N utilization by wheat could be determined by estimating the N balance after rice. Net N balance after rice can be used as a yardstick for the prediction of yield and N uptake by wheat crop.     

Potassium Release in an Aeric Haplaquept as Influenced by Long-Term Rice–Rice Cropping with Different Rates of Fertilizers and Manuring

Different fractions of potassium (K) and the kinetics of K release as influenced by 21 cycles of rice–rice cropping with different rates of fertilizers and manuring were investigated on an Aeric Haplaquept (kaolinitic Inceptisol) soil profile from Bhubaneswar, India. The neutral 1 N ammonium acetate–extractable K in the surface soil layer (0–15 cm) increased from its initial value of 11.2 mg K kg^?1 to 14.8, 14.2, and 17.5 mg K kg^?1 soil in different treatments. However, the nonexchangeable K content in the surface soil layer dropped considerably to a level of 4.8–20.0 mg K kg^?1 soil. Cumulative nonexchangeable K release after 121 h of extraction with 0.01 M calcium chloride (CaCl₂) was <14 mg K kg^?1. The first-order kinetic model best described the nonexchangeable K release. The decrease in pH and increase in iron (Fe) content indicated the possibilities of K supply to plants through the dissolution of soil minerals.