D. Damodar Reddy

Effects of repeated manure and fertilizer phosphorus additions on soil phosphorus dynamics under a soybean-wheat rotation

Abstract Soil P availability and efficiency of applied P may be improved through an understanding of soil P dynamics in relation to management practices in a cropping system. Our objectives in this study were to evaluate changes in plant-available (Olsen) P and in different inorganic P (Pi) and organic P (P0) fractions in soil as related to repeated additions of manure and fertilizer P under a soybean-wheat rotation. A field experiment on a Typic Haplustert was conducted from 1992 to 1995 wherein the annual treatments included four rates of fertilizer P (0, 11, 22 and 44 kg ha–1 applied to both soybean and wheat) in the absence and presence of 16 t ha–1 of manure (applied to soybean only). With regular application of fertilizer P to each crop the level of Olsen P increased significantly and linearly through the years in both manured and unmanured plots. The mean P balance required to raise Olsen P by 1 mg kg–1 was 17.9 kg ha–1 of fertilizer P in unmanured plots and 5.6 kg ha–1 of manure plus fertilizer P in manured plots. The relative sizes of labile [NaHCO3-extractable Pi (NaHCO3-Pi) and NaHCO3-extractable P0 (NaHCO3-P0)], moderately labile [NaOH-extractable Pi (NaOH-Pi) and NaOH-extractable P0 (NaOH-P0)] and stable [HCl-extractable P (HCl-P) and H2SO4/H2O2-extractable P (resisual-P)] P pools were in a 1 : 2.9 : 7.6 ratio. Application of fertilizer P and manure significantly increased NaHCO3-Pi and -P0 and NaOH-Pi, and -P0 fractions and also total P. However, HCl-P and residual-P were not affected. The changes in NaHCO3-Pi, NaOH-Pi and NaOH-P0 fractions were significantly correlated with the apparent P balance and were thought to represent biologically dynamic soil P and act as major sources and sinks of plant-available P.

Yield sustainability and phosphorus utilization in soybean–wheat system on Vertisols in response to integrated use of manure and fertilizer phosphorus

Abstract Low native soil phosphorus (P) availability coupled with poor utilization efficiency of added P is a major constraint limiting the productivity of soybean–wheat system on Vertisols in Indian semi-arid tropics. The use of fertilizer P is limited by its high cost, while organic inputs generally cannot provide sufficient P for optimum crop growth due to their low P concentration. We, therefore, evaluated in a 5-year field experiment (1992–1997) the effects of integrated use of manure and fertilizer P on crop yield sustainability, P utilization and soil P fertility under soybean–wheat system on a Typic Haplustert. The treatments consisted four rates each of manure (applied only to soybean) and fertilizer P (applied to both soybean and wheat) arranged in a split-plot design with four replications. Both soybean and wheat crops responded significantly to the application of manure and fertilizer P. For the same level of P input, the yield increases were greater with manure P than with fertilizer P. Further, integrated use of fertilizer P and manure was better than their sole application in increasing and sustaining the productivity of soybean–wheat system. The P uptake by the crops increased with increasing rates of manure and fertilizer P and was relatively larger in soybean than in wheat. The per cent phosphorus recovery by the crops from fertilizer P decreased with increasing fertilizer P rate, while it was improved in the presence of manure. Though the extent of fertilizer P recovery was more or less similar in both the crops, soybean was more efficient than wheat in extracting the soil and manure P. The available P status of the soil showed a significant build-up during successive cropping seasons due to fertilizer P and the increase was strikingly greater when fertilizer P was applied in combination with manure. The integrated use of manure and fertilizer P is a promising strategy to improve soil P fertility status and to obtain higher and sustained productivity of the soybean-wheat cropping system on Vertisols of the semi-arid tropics.

Crop residue addition effects on myriad forms and sorption of phosphorus in a Vertisol.

Crop residues are a vital organic resource and their extensive use in soil management for sustainable agriculture is widely advocated. The effects of soybean residue (SR) and wheat residue (WR) applied alone or in combination with fertilizer P (FP) on dynamics of labile P, distribution of P fractions and P sorption in a Vertisol (Typic Haplustert) were assessed in a 16 week long incubation study. The amount of P added through crop residues, FP or their combination was kept constant at 10 mg P kg(-1) soil. Addition of SR or WR resulted in net increase of labile inorganic (Pi) and organic (Po) P, and microbial P throughout the incubation period, except that the WR decreased labile Pi during the first two weeks due to Pi immobilization. Integration of FP with SR had no added benefit compared to SR alone, while use of FP + WR proved better in ensuring short-term P availability by offsetting initial P immobilization associated with WR alone. Sequential fractionation of soil P at the end of 16 weeks showed that addition of SR and WR alone or in combination with FP favoured a build-up in labile Pi and Po (NaHCO3-Pi and Po), and moderately labile Po (NaOH-Po) fractions at the expense of recalcitrant P (HCl-P). The P sorption capacity of soil and P required to maintain optimum solution P concentration of 0.2 mg P 1(-1) also decreased with addition of these crop residues. The implication of the results of this study is that soybean and wheat residues can potentially improve soil P fertility by increasing labile Pi and Po, and moderately labile Po fractions, decreasing P sorption and concomitantly causing dissolution of recalcitrant P in soil.

Potassium balance and release kinetics of non-exchangeable K in a Typic Haplustert as influenced by cattle manure application under a soybean-wheat system

Potassium balance and non-exchangeable potassium release in 0.01 M CaCl2 medium in a Typic Haplustert were studied in a 7-year-old soybean–wheat rotational experiment involving 4 levels of cattle manure along with the recommended dose of K (66 kgsha.year). Continuous cropping without returning crop residues to the soil led to a negative balance of 66–107 kg Ksha.year. Manuring at the rate of 4 and 8 tsha did not significantly change the negative balance, but application of 16 tsha manure reduced the negative K balance. Continuous cropping without cattle manure reduced cumulative K release from 236 mgskg (initial) to 195 mgskg. Application of manure at 4, 8, and 16 tsha kept K release at 229, 245, and 246 mgskg soil, respectively. A parabolic diffusion equation was the best fit to describe K release. Cropping under the present level of K input resulted in a decline in non-exchangeable K whatever the addition of manure. Manuring at 4 and 8 tsha has accelerated the mining of native K, which increased the unsustainability of the system. K balance, non-exchangeable K release kinetics, soybean–wheat rotation, cattle manure, Typic Haplustert.

Time-dependent zinc desorption in soils.

Abstract Time dependent zinc (Zn) desorption in eight benchmark soils of India was studied in relation to various pH values and ionic strengths. Soil samples were equilibrated in solutions containing 10 μg Zn g‐1 soil at pH 5.5,6.5, and 7.5 for 48 h at 25±2°C, and adsorbed Zn extracted with calcium chloride (CaCl2) for various periods of time. Desorption of Zn decreased with increasing pH, and the desorption rate decreased abruptly at pH 7.5. In contrast, an increase in the equilibration period and ionic strength of the background electrolyte increased Zn desorption. Four rival kinetic models were fitted and evaluated for their suitability for describing the Zn desorption process. Reaction rate constant (ß) calculated from the Elovich model for the different soils ranged from 9.99 to 25 (mg Zn kg‐1)‐1. The different kinetic models tested indicated that Zn desorption in soils was a diffusion controlled process. The desorption was rapid in the first 4 h, followed by slower phase in the rest of the time at all the pH values indicating a biphasic desorption, characteristic of a diffusion controlled process. The ß value for the Elovich equation showed a strong association with soil clay content and cation exchange capacity (CEC). Further, the best prediction of Zn desorption reaction rate constant could be made using multiple‐regression equation with soil clay content and CEC as variables.

Phosphorus Solubilization from Low-Grade Rock Phosphates in the Presence of Decomposing Soybean Leaf Litter

Abstract The natural phenomenon of defoliation of mature soybean leaves onto the soil surface provides a large quantity of easily decomposable organic matter in the form of leaf litter. The potential of decomposing soybean leaf litter (SLL) to solubilize phosphorus (P) from two low‐grade rock phosphates, Jhabua rock phosphate (JRP) and Hirapur rock phosphate (HRP), alone or amended with pyrite, was assessed in an incubation study. Decomposing SLL solubilized P both from JRP and HRP and concurrently increased water‐soluble as well as organic P contents. Amending rock phosphates with pyrite (in a 1∶2 P to S ratio) promoted P solubilization. Approximately 71 to 92% of the total solubilized P was converted to organic P. The rate of P solubilization increased with SLL decomposition time, reaching its peak at 60 days with rock phosphates alone and at 90 days with pyrite‐blended rock phosphates. The maximum P solubilization (as a percentage of total P added) with different rock phosphates and their mixtures with pyrite followed this order: HRP (11.4%)<HRP+pyrite (16.5%)<JRP (20.2%)<JRP+pyrite (26.5%). These findings clearly suggest that the decomposing SLL has the potential to solubilize P from the otherwise insoluble low‐grade rock phosphates and can offer a natural opportunity for direct use of rock phosphates in the cropping systems that have soybean as a component crop.

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

ABSTRACT 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.

Long-Term Wheat Residue Management and Supplementary Nutrient Input Effects on Phosphorus Fractions and Adsorption Behavior in a Vertisol

The management of crop residues coupled with external nutrient inputs is important for improving and conserving soil fertility and productivity. We assessed the long-term effects of three wheat residue management options (RMO) (residue burning, incorporation, and surface retention) in combination with three supplementary nutrient inputs (SNI) [control, fertilizer, and farmyard manure (FYM)] on phosphorus (P) fractions and adsorption behavior of a Vertisol under soybean–wheat system. Wheat residue incorporation and retention improved the labile inorganic P [sodium bicarbonate (NaHCO3-Pi)] by 3.2 and 5.0 mg kg−1 and the labile organic P (NaHCO3-Po) by 2.4 and 4.2 mg kg−1, respectively, as compared to residue burning. The soils under residue incorporation and retention had 38 and 26% more moderately labile organic P [sodium hydroxide (NaOH-Po)], respectively, than the soil under residue burning. The SNI either as fertilizer or FYM further enhanced NaHCO3-Pi, NaHCO3-Po, and NaOH-Po. In contrast, less labile P fractions [hydrochloric acid (HCl)-P and residual-P] remained unaffected by RMO and SNI treatments. Residue retention or incorporation decreased P adsorption over the residue burning for all the three nutrient inputs. The P-adsorption data fitted well to the Langmuir equation (R2 ranged from 0.970 to 0.994). The P-adsorption maximum (b), bonding energy constant (k), differential P-buffering capacity (DPBC), and standard P requirement (SPR) were lower with residue incorporation or surface retention than with residue burning. The SPR followed the order residue burning > incorporation > retention for RMOs and control > fertilizer > FYM for SNI treatments. The NaHCO3-Pi, NaHCO3-P0, and NaOH-Po had negative correlation with P-adsorption parameters and showed positive correlation with soybean P uptake. Wheat residue incorporation or retention plus FYM could be an effective strategy for enhancing the P fertility of Vertisols under a soybean–wheat system.

Simple and Inexpensive Water Extraction Method for Assaying Potassium Concentration in Tobacco Plant Tissue

Potassium (K) in plant tissue is not bound to organic compounds and occurs in soluble forms, thus indicating the ease of its extractability. The conventional methods of plant-sample preparation for K determination are often tedious, time-consuming, and/or require chemicals, making the analysis expensive. In this investigation, we propose a water extraction method for assaying K concentration in tobacco leaf tissue and evaluate it for analytical accuracy and precision in comparison to the established methods, namely, triacid digestion, 1 N ammonium acetate (NH4OAc) extraction, and 0.5 N hydrochloric acid (HCl) extraction. The proposed method entails extracting K from 0.5 g finely ground plant tissue (<0.5-mm sieve) with distilled water at a 1:100 ratio (sample weight to water volume, w/v) by shaking for 20 mins and filtering before K measurement by flame photometry. Results with 25 tobacco leaf samples having a wide range in K concentrations showed very close agreement between the values of K determined by the proposed water extraction method and the established methods. The mean K concentration obtained with water extraction method was within 3 to 6% of those measured by established methods. The correlations between the K values obtained by the established methods and the water extraction method were highly significant (P = 0.01), and the relationships are best described by linear regression equations with high values of R2 (>0.99). The standard errors (SEs) and coefficient of variation (CV) for K measurements by different methods followed the order water extraction < HCl extraction < triacid digestion < NH4OAc extraction. The results suggest that the water extraction method is comparable in accuracy and superior in precision to the established methods for K determination. Being simple, rapid, and inexpensive, the water extraction method could be used as an alternative to the most commonly employed standard, triacid digestion, for routine analysis of K in tobacco plant tissue.

Organic Manure Based Phosphorus Supply Strategies: Effects on Crop Yield and Soil Test Maintenance P Requirement under Soyabean-Wheat System

ABSTRACT Assessing soil test maintenance P requirement of a given production system in relation to P supply strategies is of critical importance for judicious and efficient use of inorganic and organic p inputs. Field experiments with soyabean-wheat annual rotation on a Vertisol were conducted for three years to (i) assess the effects of p supplied through different strategies involving fertilizer and cattle manure on crop yield, P uptake and soil test p status, and (ii) determine soil test maintenance P requirement and its relation to P removal by crops. The treatments for the field experiment consisted of a control (no P check) and nine combinations of three annual P rates (P1= 26, P2 = 39 and P3 = 52 kg P ha−1, with the amount of P applied to soyabean and wheat being in the ratio of 1.6:1.0) and three P supply strategies (PSS-I = fertilizer-P for both soyabean and wheat, PSS-II = cattle manure-P for soyabean and fertilizer-P for wheat, and PSS-III = cattle manure-P + fertilizer-P on 1:1 P basis for soyabean and fertilizer-P for wheat). The soyabean and wheat crops responded significantly to P application under all the P supply strategies. The crop yield increases due to P input were, however, strikingly larger under PSS-II and PSS-III than under PSS-I. Agronomic efficiency of P (yield increment per unit of P applied) also indicated the superiority of the PSS-III and PSS-II over the PSS-I. The P uptake by crops in soyabean-wheat rotation increased significantly and linearly with increasing P rate under all the P supply strategies. During three cycles of annual soyabean-wheat rotation, the soil test P in the plot receiving no P (P0 control) declined from the initial 5.84 to 2.40 mg kg−1, while it showed a marked build-up with P application particularly at higher rates. The annual soil test maintenance P requirement (STMPR) was lower by 27–33% under PSS-II (26.3 kg P ha−1) and PSS-III (24.1 kg P ha−1) than under PSS-I (36.1 kg P ha−1). Despite these differences in STMPR, the crop yields achievable at STMPR were interestingly more or less similar across all the P supply strategies. In terms of P removal by crops, the STMPR equalled or slightly exceeded the P removal by crops under the PSS-III and PSS-II, while it was 1.4 times the removal under PSS-I. Therefore, the use of manure-P as a component of P supply strategies can help reduce the amount of P required to maintain soil P fertility and hence increase P utilization efficiency.