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For remediating polluted soils, phytoextraction of metals received considerable attention in recent years, although slow removal of metals remained a major constraint in this approach. We, therefore, studied the effect of selected organic and inorganic amendments on the solubility of zinc (Zn), cadmium (Cd), and lead (Pb) in polluted soil and enhancing the efficacy of phytoextraction of these metals by Indian mustard (Brassica juncea cv. Pusa Vijay). For this purpose, a greenhouse experiment was conducted using a metal-polluted soil to evaluate the effect of amendments, viz. green manure (T2), EDTA (T3), sulfur (S)+S oxidizing bacteria (Thiobacillus spp.) (T4), metal-solubilizing bacteria (Pseudomonas spp.) (T5), and green manure + metal-solubilizing bacteria (T6), on solubility and bioavailability of Zn, Cd, and Pb. Distribution of metals in different soil fractions revealed that Cd content in water soluble + exchangeable fraction increased to the extent of 34.1, 523, 133, 123, and 75.8% in T2, T3, T4, T5, and T6 treatments, respectively, over control (T1). Cadmium concentrations in soil solution as extracted by Rhizon sampler were recorded as 3.78, 88.1, 11.2, 6.29, and 4.27 μg L−1in T2, T3, T4, T5, and T6, respectively, whereas soil solution concentration of Cd in T1 was 0.99 μg L−1. Activities of Cd (pCd2+) in Baker soil extract were 12.2, 10.9, 6.72, 7.74, 7.67, and 7.05 for T1, T2, T3, T4, T5, and T6, respectively. Cadmium contents in shoot were recorded as 2.74, 3.12, 4.03, 4.55, 4.68, and 4.63 mg kg−1 in T1, T2, T3, T4, T5, and T6 treatments, respectively. Similar trend in Zn and Pb content with different magnitude was also observed across the different amendments. Cadmium uptake by shoot of mustard was enhanced to the extent of 125, 62.5, 175, 175, and 212% grown on T2-, T3-, T4-, T5-, and T6-treated soil, respectively, over T1. By and large, free ion activity of metals as measured by Baker soil test proved to be the most effective index for predicting Zn, Cd, and Pb content in shoot of mustard, followed by EDTA and DTPA. Among the metal fractions, only water soluble + exchangeable metal contributed positively towards plant uptake, which explained the variation in shoot Zn, Cd, and Pb content to the extent of 74, 81, and 87%, respectively, along with other soil metal fractions. Risk to human health for intake of metals through the consumption of leafy vegetable (mustard) grown on polluted soil in terms of hazard quotient (HQ) ranged from 0.64 to 1.10 for Cd and 0.11 to 0.34 for Pb, thus rendering mustard unfit for the human consumption. Novelty of the study mainly consisted of the use of natural means and microorganisms for enhancing solubility of metals in soil with the ultimate aim of hastening the phytoremediation.

A case study was undertaken to assess the risk of sewage-irrigated soils in relation to the transfer of trace elements to rice and wheat grain. For this purpose, peri-urban agricultural lands under the Keshopur Effluent Irrigation Scheme (KEIS) of Delhi were selected. These agricultural lands have been receiving irrigation through sewage effluents since 1979. Sewage effluent, groundwater, soil, and plant (rice and wheat grain) samples were collected with GPS coordinates from this peri-urban area. Under wheat crop, sewage irrigation for four decades resulted into a significant buildup of zinc (141 %), copper (219 %), iron (514 %), nickel (75.0 %), and lead (28.1 %) in sewage-irrigated soils over adjacent tube well water-irrigated ones. Under rice crop, there was also a significant buildup of phosphorus (339 %), sulfur (130 %), zinc (287 %), copper (352 %), iron (457 %), nickel (258 %), lead (136 %), and cadmium (147 %) in sewage-irrigated soils as compared to that of tube well water-irrigated soils. The values of hazard quotient (HQ) for intake of trace toxic elements by humans through consumption of rice and wheat grain grown on these sewage-irrigated soils were well within the safe permissible limit. The variation in Zn, Ni, and Cd content in wheat grain could be explained by solubility-free ion activity model (FIAM) to the extent of 50.1, 56.8, and 37.2 %, respectively. Corresponding values for rice grain were 49.9, 41.2, and 42.7 %, respectively. As high as 36.4 % variation in As content in rice grain could be explained by solubility-FIAM model. Toxic limit of extractable Cd and As in soil for rice in relation to soil properties and human health hazard associated with consumption of rice grain by humans was established. A similar exercise was also done in respect of Cd for wheat. The conceptual framework of fixing the toxic limit of extractable metals and metalloid in soils with respect to soil properties and human health hazard under the modeling framework was established.

In an incubation experiment, effect of ten levels of sludge (0, 1.12, 2.24, 4.48, 8.96, 17.9, 35.8, 71.6, 142, 285 g kg−1), two levels of moisture (field capacity and 2.5 cm standing water) and two levels of temperature (20 and 35 °C) on the release of copper (Cu), iron (Fe), manganese (Mn), nickel (Ni) and lead (Pb) was evaluated in acid and alkaline soils. Application of sludge was more effective in enhancing ethylene diamine tetraacetic acid (EDTA) extractable metals in acid soil than alkaline soil. On an average, maximum increase in the release of EDTA extractable Cu, Fe, Mn, Ni and Pb were 2.5, 2.6, 3.8, 4.7, 1.7 fold in sludge treated soil over control. There was a decrease in EDTA extractable Cu, Ni, and Pb by 64.4, 23.0, and 14.0 %, respectively under submergence as compared to that under field capacity. On the contrary, EDTA extractable Fe and Mn increased by 14.1 and 13.7 %, respectively under submergence. On an average, amount of EDTA extractable Cu, Fe, Mn, Ni, and Pb increased by 52.5, 80.8, 40.9, 27.3, and 55.1 %, respectively at 35 °C than that at 20 °C.

Widespread nutrient deficiencies have emerged as the major soil-related constraints, with sulfur (S) being one among them, for sustaining rice–wheat productivity in many parts of the north-western Indo-Gangetic Plain (IGP) and the Western Himalayan region (WHR). Therefore, soils from different agricultural development blocks (ADBs) of Meerut and Jyotiba Phule Nagar (J.P. Nagar) Districts in the Upper Gangetic Plain (UGP) zone, Sonipat, and Panipat Districts in Trans-Gangetic Plain (TGP) zone, and New Tehri District in Garhwal zone of WHR were analyzed for their available S-status. Farmers’ fertilizer management practices revealed that fertilizer use was highly unbalanced, and use of S fertilizers was generally negligible. Deficiencies of S were noticed in 19–47 % of the soil samples. On-farm experiments at these sites showed that rice (Oryza sativa L.) yields improved by 0.84–1.90 t ha−1 with the additions of 30–45 kg S ha−1 on S-deficient soils of IGPs, whereas the crop response varied from 1.16 to 1.39 t ha−1 on WHR soils. Also, the residual effect of 30–45 kg S ha−1 was noticed in succeeding wheat (Trititicum aestivum L.). Averaged over S rates, 37–49 % of the applied S was recovered in the system at different locations. Skipping S application decreased the available S content of the soils (0.6–2.4 mg kg−1) compared with initial content. Substantial yield gain and economic returns due to the use of S suggested for inclusion of S in the fertilizer schedules for these soils.

We attempted to develop a protocol for fixing the maximum permissible limit of sludge in agricultural lands based on transfer of metals from sludge-amended soils to human food chain. For this purpose, spinach was grown as a test crop on acid and alkaline soils with graded doses of sludge (0, 1.12, 2.24, 4.48, 8.96, 17.9, 35.8, 71.6, 142 and 285 g kg(-1) of soil) in a pot experiment. Biomass yield of spinach was increased due to sludge application in both acid and alkaline soils. Among the chemical extractants, EDTA extracted the highest amount of metals from sludge-amended soil followed by diethylenetriaminepentaacetic acid (DTPA) and CaCl2. Elevated levels of Zn, Cu, Fe, Mn, Ni, Cd and Pb in spinach were observed due to sludge application over control. Application of sludge was more effective in increasing metal content in spinach grown on acid soil than alkaline soil. Solubility-free ion activity model as a function of pH, organic carbon and extractable metal was far more effective in predicting metal uptake by spinach grown on sludge-amended soils as compared to that of chemical extractants. Risk in terms of hazard quotient (HQ) for intake of metals through consumption of spinach by humans grown on sludge-treated soils was computed for different metals separately. In a 90-day pot experiment, safe rates of sludge application were worked out as 4.48 and 71.6 g kg(-1) for acid and alkaline soils, respectively.

Risk assessment of metal-contaminated soil depends on how precisely one can predict the solubility of metals in soils. Responses of plants and soil organisms to metal toxicity are explained by the variation in free metal ion activity in soil pore water. This study was undertaken to predict the free ion activity of Zn, Cu, Ni, Cd, and Pb in metal-contaminated soil as a function of pH, soil organic carbon, and extractable metal content. For this purpose, 21 surface soil samples (0-15 cm) were collected from agricultural lands of various locations receiving sewage sludge and industrial effluents for a long period. One soil sample was also collected from agricultural land which has been under intensive cropping and receiving irrigation through tube well water. Soil samples were varied widely in respect of physicochemical properties including metal content. Total Zn, Cu, Ni, Cd, and Pb in experimental soils were 2,015 ± 3,373, 236 ± 286, 103 ± 192, 29.8 ± 6.04, and 141 ± 270 mg kg(-1), respectively. Free metal ion activity, viz., pZn(2+), pCu(2+), pNi(2+), pCd(2+), and pPb(2+), as estimated by the Baker soil test was 9.37 ± 1.89, 13.1 ± 1.96, 12.8 ± 1.89, 11.9 ± 2.00, and 11.6 ± 1.52, respectively. Free metal ion activity was predicted by pH-dependent Freundlich equation (solubility model) as a function of pH, organic carbon, and extractable metal. Results indicate that solubility model as a function of pH, Walkley-Black carbon (WBC), and ethylenediaminetetraacetic acid (EDTA)-extractable metals could explain the variation in pZn(2+), pCu(2+), pNi(2+), pCd(2+), and pPb(2+) to the extent of 59, 56, 46, 52, and 51%, respectively. Predictability of the solubility model based on pH, KMnO4-oxidizable carbon, and diethylenetriaminepentaacetic acid-extractable or CaCl2-extractable metal was inferior compared to that based on EDTA-extractable metals and WBC.

High fertilizer use efficiency in rice (Oryza sativa L.)-based cropping systems in Indo-Gangetic Plains (IGP) is not achieved because farmers’ fertilizer practices (FFP) and blanket state recommendations (BSR) do not take into account the site-specific variability of nutrient availability in the soil. We evaluated the site-specific nutrient management (SSNM) option against existing FFP, blanket state recommendation (BSR), improved BSR (IBSR i.e., 25 % higher than BSR), and state soil testing laboratory recommendation (STLR) in six pre-dominant rice-based cropping systems namely wheat–rice (W–R), mustard–rice (M–R), potato–rice (P–R), garlic–rice (G–R), chickpea–rice (C–R) and berseem fodder–rice (B–R) in terms of yield gain, economics, nutrient harvest index, soil fertility, and apparent nutrient balances. System rice equivalent yield (SREY) improved by 15.7, 9.5, 13.9 and 30 %, in SSNM over BSR, IBSR, STLR and FFP, respectively. SSNM involved additional cost of ₹ 2940–5291 ha−1 over BSR and ₹ 5492–10120 ha−1 over FFP under different cropping systems but contributed higher added net return of ₹ 23236–75056 ha−1 and ₹ 39577–143899 ha−1 over BSR and FFP, respectively. The output: input ratio and nutrient harvest index for N, P, K were also highest in SSNM. At the end of the experiment, soil available N, Olsen-P and available K content were either maintained or improved over their initial values in SSNM treatments, whereas soil available K in FFP declined by −1.65 % and increased over initial content in BSR and STLR in 0–15 cm soil profile depth. After three crop cycles, apparent N and P balances were positive in all the cropping systems and fertilizer treatments; only exception was a negative N balance in C–R and B–R systems in different fertilizer treatments. The apparent K balances were negative in all the cropping systems irrespective of nutrient management options. But, the magnitude of negative balance was lower in plots received SSNM treatment as compared to other nutrient management strategies, indicating a potential for improving yields, nutrient use efficiency and farm profit without deteriorating soil fertility in different rice based systems in IGP.

A field experiment was conducted during 1986–87 on a sandy loam acid Alfisol of Barapani, Meghalaya, India to study the effect of liming on boron nutrition of pea and corn grown in a sequence. Lime and boron were applied to pea and their residual effect was studied on corn. Application of 1.5 kg B ha−1 significantly increased the pod and stover yield of pea. A sharp yield depression occurred at higher B rates. Liming accentuated B deficiency in the absence of B treatment and it cured toxicity arising from excess B supply. A combination of B at 1.5 kg ha−1 and lime at 3.0 t ha−1 was optimum for pea, but this level of B application was insufficient to meet B requirements of succeeding corn. With higher rates of B fertilization, the residual effect of B on corn yield was significant. Concentration of B in shoot, pod, grain and stover of crops increased with the rate of B application. With increasing liming rate, B concentration decreased. Addition of B increased the hot-water soluble B in soil. The availability of native and added B decreased sharply with increasing liming rate.