Sludge-treated Soil Research Articles

Effect of soil pH on availability to crops of metals in sewage sludge-treated soils. I. Nickel, copper and zinc uptake and toxicity to ryegrass

The effect of soil pH value on concentrations of Ni, Cu and Zn in ryegrass grown on two sludge-treated soils was examined under field conditions and the maximum permissible soil limit values for these elements were determined which prevent phytotoxicity in crops where sewage sludge is applied to agricultural soils with pH <6.0. Concentrations of all the elements in ryegrass decreased as simple linear functions of increasing soil pH and this response was consistent across the range of pH values measured (pH 4.2–7.0). The response of individual elements tended to differ though, with Cu being less sensitive to changing pH conditions compared with Zn and Ni which responded in a similar mammer. The yield of ryegrass also increased with increasing soil pH value probably due to the effects on Zn uptake as the crop content of Zn exceeded known upper critical tissue concentrations for this element at both sites. The proportional change in metal content of ryegrass at pH 5.0, 5.5 and 7.0 from tissue concentrations at pH 6.0 was calculated to determine the permissible soil concentration values on the basis of current maximum limits set by the Sludge Regulations in the UK for sludge-treated agricultural land at pH 6.0–7.0. The estimated permissible concentrations of Ni and Zn in soil corresponded with the regulatory values at the low pH ranges, but were substantially above the current soil limits at pH 7.0 indicating larger quantities of these elements could be safely applied under alkaline soil conditions. The estimated soil limits for Cu implied that the current Regulations were highly precautionary for this element.

Effect of soil pH on availability to crops of metals in sewage sludge-treated soils. II. Cadmium uptake by crops and implications for human dietary intake

Appropriate pH-related permissible soil-limit concentrations for cadmium in sewage sludge-treated agricultural soils were estimated from the proportional changes in concentrations of cadmium in potatoes, oats and ryegrass grown on two sludge-amended soils and at different pH values. Implications for potential human dietary intake of cadmium were also assessed. Yields of crops increased with increasing soil pH, probably in response to decreasing uptake of zinc as soil pH value was raised. In general, cadmium concentrations in peeled potato tubers, potato peelings, oat straw and ryegrass decreased as simple linear functions of increasing soil pH over the range of pH values measured (pH 3·9·7·6). Cadmium concentrations in potato peel were particularly sensitive to changing pH conditions, whereas cadmium levels in oat grain were independent of soil pH. On the basis that a highly precautionary approach is adopted in setting soil standards for heavy metals, appropriate permissible concentrations of cadmium in sludge-treated agricultural soil which protect the human food chain were determined as 2·0 and 2·5 mg Cd Kg −1 for banded pH ranges of 5·0–5·5 and 5·5–6·0, respectively.

Screening of isolates and strains ofRhizobium leguminosarumbiovarTrifoliifor heavy metal resistance using buffered media

The computer program GEOCHEM-PC was used to calculate the metal ion activities of Cu, Zn, Cd, and Ni in defined media amended with various metal ion buffers at pH 6.6 or 6.0 so that the lowest-observed-effect concentrations (LOECs) of these metals to isolates and strains of Rhizobium leguminosarum biovar trifohi could be determined. Strains from the U.S. Department of Agriculture (USDA) Beltsville (MD) Rhizobium culture collection were generally more tolerant of the metals than any of the isolates from the sludge-treated soil (S-isolates) and those from farmyard manure-treated soil (F-isolates), although the S- were more metal resistant than the F-isolates. All isolates and strains, however, tolerated much larger concentrations in the buffered systems than those found in the solutions of soils from which they originated. Copper toxicity, using iminodiace-tate (IDA), occurred for the F- and S-isolates and USDA strains at concentrations of 16, 47, and 430 μg ml−1, respectively, corresponding to predicted ion activities of 0.002, 0.006, and 0.06 μg ml−1, respectively. The Zn LOECs for the F- and S-isolates, without a buffer, occurred at concentrations of 47 and 207 μg ml−1, respectively, corresponding to predicted ion activities of 37 and 157 μg ml−1, respectively. No toxicity occurred when nitrilotriacetate was used for Cd up to concentrations of 356 μg ml−1 (Cd ion activity of 2.6 μg ml−1). Nickel concentrations up to 0.6 μg ml−1 had no effect with ethylene-bis(oxyethylenenitrilo)tetraacetate (EGTA; Ni ion activity of 0.006 μg ml−1) and up to 186 μg ml−1 with IDA (Ni ion activity of 11 μg ml−1).

Relationships amongst organic matter content, heavy metal concentrations, earthworm activity, and soil microfabric on a sewage sludge disposal site

From 1973 to 1980, sewage sludge precipitated by three chemical treatment processes (aluminum sulphate, ferric chloride and calcium hydroxide) was applied to replicated plots on a silt loam soil at four different rates. The sludge was contaminated by heavy metals associated with industrial activity in the metropolitan areas from where it was collected. The grass on the plots has been mown regularly from 1973 onwards. Soil and earthworm populations were subjected to analysis in 1989 and 1990. Soil pits were dug and sampled to a depth of 50 cm in four plots receiving the highest rates of the three different types of sewage treatment processes (and a control plot treated through the same period with ammonium nitrate) to determine heavy metal contents and soil organic matter distributions with depth. The earthworm fauna of the plots was almost entirely represented by populations of Lumbricus terrestris L. that were analyzed for abundance and biomass following collection using the formalin expulsion method. Earthworm biomass was higher in sludge treatments than in control plots. Cadmium concentrations in earthworm tissues were correlated with soil Cd concentrations Only worms from Al-sludge treatments had elevated tissue concentrations of Cd. Cadmium distributions in sludgetreated soils were correlated with organic matter distributions in the soil profile (for individual profiles). Intact blocks of soil were removed from soil pits for chemical analysis of the soil microfabric by PIXE (proton induced X-ray emission) and electron micro-probe (EMP). Concentrations of several trace metals on the inner walls of the earthworm channels correlated with those added to the soil via the sludge. Water flow through earthworm burrows lined with fecal material, which had higher metal concentrations than the adjacent soil matrix, could move soluble forms of the elements, thus accelerating leaching of metals into the aquifer from surface-applied sludge.

Rhizobium meliloti is less sensitive to heavy-metal contamination in soil than R. leguminosarum bv. trifolii or R. loti

A gradient of soils with increasing metal contamination was constructed by mixing uncontaminated farmyard manure-treated (FYM) and contaminated, sewage sludge-treated (S) soils from the Woburn Experimental Farm, and separate samples of these soils were inoculated with strains of Rhizobium leguminosarum bv. trifolii, R. meliloti and R. loti. The survival of rhizobia in these soils was monitored using most-probable number (MPN) plant infection tests. After 51 days incubation numbers of R. meliloti were similar in all of the soils irrespective of the heavy metal content, whilst the numbers of R. leguminosarum bv. trifolii and R. loti were reduced in soils containing more than 2 3 sludge-treated soil. The numbers of R. leguminosarum bv. trifolii were monitored in the same gradient of soils over a 171 day period and differences in survival between the soils were only apparent from 27 days in soil mixtures containing more than 5 6 sludge-treated soil. After 171 days the number of cells surviving had decreased even in the soil mixture containing 1 6 sludge-treated soil when compared with the uncontaminated FYM-treated soil. The populations of indigenous R. leguminosarum bv. trifolii were also monitored in the soil of the FYM-treated and S-treated plots in the field. Numbers of rhizobia remained at around 4 × 10 4 cells g soil −1 in the FYM-treated soil over 3 yr after clover was removed, but gradually declined in the S-treated soil to 1 × 10 3 cells g soil −1.

Effective Rhizobium leguminosarum biovar Trifolii present in five soils contaminated with heavy metals from long-term applications of sewage sludge or metal mine spoil

The presence of effective strains of Rhizobium leguminosarum biovar trifolii was determined in a range of soils contaminated with heavy metals resulting from long-term applications of sewage sludge and also metal mine spoil, using an in vitro plant inoculation technique with Trifolium repens. Effective Rhizobium were always present in soil from sites where the host plant was established, irrespective of the amount of metal contamination. With no indigenous clover present, effective Rhizobium were also isolated from soils with concentrations of metals which exceeded the maximum permissible concentrations for sludge-treated soil. This may indicate Rhizobium strains had developed tolerance to elevated metal conditions without losing the ability to fix N 2 with white clover. However, elevated concentrations of Cd in soil apparently determined the absence of Rhizobium at one arable site as only half the soil samples taken nodulated white clover whereas increased concentrations of Zn and Cu had no effect in a soil low in Cd. Critical amounts of metals which resulted in a complete absence of Rhizobium in soil and nodulation could not be determined across the range of metal concentrations measured at each site. Therefore, detrimental effects of metals on Rhizobium were apparently site specific and there was no evidence that increased metal concentrations in soil selected for tolerant strains which were ineffective in N 2-fixation with white clover.

Survival of the indigenous population of rhizobium leguminosarum biovar trifolii in soil spiked with Cd, Zn, Cu and Ni salts

Solutions of Zn, Cd, Cu or Ni salts were added to a relatively uncontaminated soil, previously treated with farmyard manure (FYM), at six different concentrations for each metal. A further experiment compared the survival of the indigenous population of Rhizobium leguminosarum bv. trifolii in the same soil treated previously with FYM or metal-contaminated sewage sludge. No decline in rhizobial numbers occurred in either experiment after 2 months' exposure. After 18 months, the number of rhizobia in the control soils had declined by 90%. No rhizobia survived at Zn and Cd concentrations ≥385 and ≥7.1 μg g −1 soil, respectively. At these Zn and Cd concentrations, which are ca 1.3 and 2.4 times the U.K. limits, respectively, there was no N 2-fixation. At the greatest Cu concentration of 225 μg g −1 soil, which is 1.7 times the U.K. limit, there was a 99% decrease in rhizobial numbers compared to the control soil, but sufficient survived to give effective nodules on clover plants. Nickel had very little effect on the rhizobial population, even at the greatest concentration of 54 μg g −1 soil, ca 75% of the U.K. limit. The order of decreasing toxicity to rhizobia in the soil, and hence the indirect effect on N 2-fixation, was Cd > Zn > Cu. The soil treated with metal-contaminated sewage sludge contained 230 rhizobial cells g −1' soil after 18 months exposure. These could nodulate white clover, but were ineffective in N 2-fixation. Zinc and Cu concentrations in this soil were close to the U.K. limits, whereas Ni and Cd concentrations were half and 4 times the U.K. limits, respectively. Clover plants grown in soil spiked with Zn or Cd to give concentrations of ≥385 and ≥7.1 μg g −1 soil, respectively, or in the sludge-treated soil, after 18 months exposure were chlorotic and stunted, and contained less N than the control plants grown in FYM-treated soil. These effects were not caused by phytotoxicity, because they were overcome by the addition of inorganic nitrogen fertilizer. Rather, they resulted from a direct toxic effect of these metals on the indigenous effective rhizobial population. It took 18 months exposure of the moist soil at 20°C, in the absence of clover host plants, before the toxic effect became apparent.

Behavior of zinc and copper in soil with long term application of sewage sludges

Abstract Two kinds of sewage sludge composts were applied separately to a crop field consisting of volcanic ash soil (Andosoil) twice a year at 10 t/ha for 12 years since 1978 and the changes in the concentrations of total and soluble Zn and Cu in the sewage sludge-treated soil were studied. The total amount of Zn in the surface soil of the sewage sludge-treated plots increased year by year. The total Zn concentration in the subsoil of these plots was higher than that of the non-treated plot after 12 years of application of the sewage sludge. Cu accumulated somewhat in the surface soil, but not in the subsoil during this period. These facts suggest that Zn moved down to about 20 cm in the Andosoil within 12 years. Sewage sludge application increased the ratio of soluble Zn to total Zn in both the surface soil and subsoil during a 9 year period.

Chemical characterization of humic substances extracted from organic-waste-amended soils

Humic substances were extracted from a soil treated, in a 4-year experiment, at different rates with a sludge from anaerobic treatment of combined civil and industrial wastes, and with agricultural manure. Elemental and chemical analyses, molecular weight (MW) distribution and infrared (IR) spectroscopy were performed on the purified humic acids (HA). Organic wastes significantly increased the HA content of the treated soils and improved CEC. The C/N, C/H and C/O ratios of HA extracted from the original wastes showed a higher degree of humification for sludge than for manure. This difference was also noticed for the C/N ratio of soil humic extracts, indicating a faster humification process for the sludges in soil. The content of oxygen-containing functional groups was lower than the ‘model’ HA reported in the literature, and even more so for HA from sludges, reflecting their anaerobic formation. MW distribution and E 4/E 6 ratios showed that the sludge material had a higher molecular complexity than manure, supporting the high degree of humification attributed to the former. HA extracted from sludge-treated soils revealed a molecular dimension increasing with the application doses of waste material. Infrared spectra showed that HA formed in soils after waste additions reflected the chemical composition of the original organic material, which was rich in aliphatic groups and peptides for sludge and in carbohydrates for manure. On the basis of this study, it is concluded that not only are organic waste additions able to build up the HA content in soils but the HA formed assume the chemical characteristics and the degree of humification of the original material.

The bioavailability of cadmium to lettuce and cabbage in soils previously treated with sewage sludges

The application of sewage sludges to soils may lead to increased soil-Cd levels. The bioavailability of Cd is determined by the interaction of a number of soil physico-chemical and plant variables, of which pH is the most important. Duplicate samples of sludge-treated soils were transferred to tubs in the field, one of each pair being limed to pH 7±0.5. Lettuce and cabbage were grown to maturity and analysed for Cd. Liming always reduced Cd uptake by the plants. Three soil extractants, 1 M NH4NO3, 0.05 M EDTA-(Na)2 and 0.05 M CaCl2 were used as indices of Cd bioavailability. CaCl2 proved to be the most effective for both lettuce and cabbage. Multiple linear regression equations were derived to describe the uptake and accumulation of Cd by both crops. The relative influence of soil variables differed between the two species. Unlike those of a number of pot experiments conducted in glasshouses, the data from this experiment are comparable with those of crop samples taken from the field.

Effect of solid-phase speciation on metal mobility and phytoavailability in sludge-amended soil

Sequential extraction was utilized for partitioning Cd, Cr, Ni, and Zn, in soil and sludge samples into five operationally-defined fractions: exchangeable, bound to carbonates, bound to Fe-Mn oxides, bound to organic matter and residual. The highest amounts of Cd, Ni, and Zn, expressed as per cent of the total, were found in the Fe-Mn oxide fraction of the sewage sludge. Chromium was significantly associated with the organic fraction of the sludge. The residue was the most abundant fraction for all metals studied in the untreated soil, and for Cd and Ni in the sludge-treated soil. The concentration of exchangeable Cd and Cr was relatively low in the untreated soil and did not change much after sludge application, whereas the concentrations of exchangeable Zn increased about 50 times and the concentrations of exchangeable Ni doubled in the sludge-treated soil. The lysimetric experiment revealed an increase in Zn and Ni uptake by ryegrass and in the percentage of metals leached from the soil profile after massive sludge application. In contrast only negligible changes were observed for Cd and Cr. The assumption that mobility and biological availability are related to metal speciation was confirmed by the agreement between the distribution pattern of Cd, Cr, Ni and Zn in the soils, the uptake of the metals by plants and their capacity for leaching out from the soils.

Behaviour of LAS in Sewage and Sludge Treatment and in Sludge-Treated Soil

Behaviour of LAS in Sewage and Sludge Treatment and in Sludge-Treated Soil

Heavy metal contents of vegetable crops treated with refuse compost and sewage sludge

Refuse compost and sewage sludge were mixed with a loamy sand at various rates in pots and sown withBrassica chinensis, Daucus carota andLycopersicon esculentum in a glasshouse. A commercial fertilizer was also applied to the same soil for comparison. Dry matter production of the three crops and contents of Cd, Cu, Mn, Pb and Zn in the harvested tissues were determined at the end of the experiment. In general, crop yield in refuse compost treatment was improved over that in sandy soil alone, but was less than that in the sludge and fertilizer treatments. Despite the relatively high heavy metal contents of refuse compost, crops grown on compost-treated soils accumulated lower levels of metal than those grown on sludge-treated soils. This is probably due to the high pH and organic matter content of the composted refuse. Higher levels of heavy metals were found in the roots than in the aerial parts ofB. chinensis andL. esculentum, but the reverse was found inD. carota. In the edible tissue of the three crops,L. esculentum accumulated metals to a lesser extent than the other two.

Soil and crop response to nitrogen fertilization of soil containing decomposed sewage sludge

Five years after sewage sludge application, the residual effect of the decomposed sludge on fertilizer N uptake by-maize ( Zea mays L.) was studied. Maize grown from 1975 to 1978 without N fertilization had reduced the N mineralization potential of sludge-treated soil to that of the original soil. In 1979, N uptake from fertilized and non-fertilized sludge plots, and from N fertilized control plots, was compared using ( 15NH 4) 2SO 4. Uptake of soil N was not affected by fertilization. Uptake of fertilizer N was lower in sludge-treated plots (27%) than in control plots (41%), while the proportion of fertilizer N immobilized was higher in the sludge plots (32%) than in the control plots (18%). Stronger soil biological response paralleled a lower N uptake efficiency by maize grown on sludge plots. Fertilizer NH 4-N nitrified more rapidly in the sludge soil (31 g NH 4-N (kg soil) −1 day −1 than in the control plot soil (13 g NH 4-N (kg soil) −1 day −1. Dehydrogenase activity was twice as high as in fertilized sludge soil (140 mg Formazan kg −1) as in control soil (60 mg Formazan kg −1). Maize grain yields were linearly related to N uptake, whereas fodder yields were curvilinearly related, across all treatments. This response indicated that N fertilization induced no toxic side effects in sludge-grown maize.

SEWAGE SLUDGE AS A PHOSPHORUS AMENDMENT FOR SESQUIOXIC SOILS

The objective of this study was to evaluate the effectiveness of sludge P in comparison with inorganic P, as a fertilizer for P-deficient sesquioxic soils. Municipal sewage sludge and monocalcium phosphate (MCP) were applied to two soils—Griffin clay (Typic Haplorthox) and Clovelly sandy clay loam (Tropeptic Haplorthox)—at rates equivalent to 200, 500, and 1,000 kg ha−1 P. Italian ryegrass (Lolium multiflorum) was grown in the soils using a split pot technique, and tops were harvested at 14, 35, 63, 97, 125, 153, 181, and 209 d after commencement of root:soil contact. Yield and concentration of P, N, Ca, Mg, K, and Na in the plant tissue were determined. Both soils exhibited large responses to applied P, with P uptake from sludge treatments being significantly greater (P < 0.05) than MCP treatments in the Griffin soil, and significantly lower than MCP treatments in the Clovelly soil. Rates of P uptake declined rapidly with time in MCP-treated soils, and in sludge-treated soils rates of P uptake increased or declined only slowly. The relative efficiency of sludge P compared with MCP increased from 44 to 90% and 64 to over 100% with time in the Clovelly and Griffin soils, respectively.

The effects of pH and soil type on concentrations of zinc, copper and nickel extracted by calcium chloride from sewage sludge-treated soils

The effects of pH on concentrations of zinc, copper and nickel extracted by calcium chloride from a clay loam and two sandy loam soils that had been treated with sewage sludge were studied. Concentrations of all the metals increased rapidly as pH decreased below a threshold value ranging from 6·2 to 7·0 for Zn, 6·2 to 7·2 for Ni and 4·7 to 5·7 for Cu. Both the total concentrations and the threshold pH values were influenced by differences in soil texture and (between the two light-textured soils) by differences in soil cation exchange capacity. The amount of zinc and copper extracted from the mixtures was considerably less than that extracted from the same quantity of sludge alone.

Cadmium in sludges used as fertilizer.

In intensively populated countries efficient sewage treatment is essential to protect river quality. An inevitable by-product is sewage sludge which has to be disposed of safely and economically. Utilisation of sludge as a fertilizer of agricultural land is the most economic disposal route for inland sewage-treatment works and also benefits farmers by providing a cheap manure. Much of the cadmium in wastewater is concentrated into sludge which consequently contains higher concentration of cadmium than soil does. It is impracticable to reduce cadmium concentrations in sludge below certain levels. When sludge is used on farmland rates of application must be controlled so that cadmium concentrations in soil never reach levels that could significantly contaminate food crops. Cadmium is a principal factor limiting the use of sludge on land. Nevertheless, it is a local problem since agricultural land in general receives more cadmium from aerial deposition and phosphatic fertilizers. The significance of accumulations of cadmium in soil depends mainly on its availability for crop uptake. Investigations are described which have attempted to identify and to determine the availability of forms of cadmium in soil. There is considerable research interest in cadmium in soil solution which is likely to be directly available for crop uptake. Another area of interest is the apparent disappearance of cadmium from sludge-treated soil. Soil analysis often cannot fully account for the cadmium added in sludge. Apart from the effect of soil conditions, especially pH value, crop uptake varies according to the particular crop examined. Highest concentrations of cadmium occur in tobacco, lettuce, spinach and other leafy vegetables. Using crop uptake data from field trials it is possible to relate potential human dietary intake of cadmium, on which hazard depends, to soil concentrations of cadmium, which can be controlled by regulating applications of sludge. This provides an objective basis for limits for cadmium concentrations in soils receiving sludge. Transfer of cadmium via farm animals to meat and dairy products for human consumption is thought to be minimal, even allowing for some direct ingestion of sludge-treated soil by the animals. Evidence from these and other investigations suggests that a loading rate limit of 5 kg Cd/ha (equivalent to a soil concentration of about 3.5 mg Cd/kg) affords adequate protection to the foodchain where sludge is used on agricultural land. More research work is needed to provide a basis for predicting the long-term availability of cadmium introduced to the soil in sludge.

METAL MOVEMENT IN SLUDGE-TREATED SOILS AFTER SIX YEARS OF SLUDGE ADDITION

Movement of the metals Ni, Co, Fe, Mn, Cr, and Hg in Dublin loam soil was studied after 6 yr of sludge addition. Sludges from two sources were added annually at rates ranging from 0 to 225 metric tons per hectare (t/ha) in increments of 45 t. Increasing rates of sludge addition resulted in almost no change in the concentration of metals in the surface soil, with the exception of Cr and Hg, where the metal concentrations in the added sludge were markedly greater than those found in the untreated soil. In these cases metal increased linearly with rate of sludge addition. Surface concentrations of HNO3-extracted Mn were less than those obtained at lower depths. The low values appeared to be due to the failure of HNO3 to extract all the Mn from sludge-treated soils. Only Fe of all elements increased significantly in the profile with time. No element moved more than 5 cm below the zone of sludge incorporation in either the highly acid (pH 4.5) Oakland or the near-neutral (pH 7.3) Pacheco sludge-treated soils. Relative metal availability followed the same order for both sludges, i.e., Mn > Co = Ni ≫ Fe > Cr. Percentage of available metal in Oakland sludge-treated plots was nearly double that in Pacheco sludge-treated plots. Percentage availability of Fe and Cr was extremely low.

Toxicologic studies associated with the agricultural use of municipal sewage sludge and health effects among sewage treatment plant workers

The proposed use of municipal sewage sludges as soil amendments on agricultural land involves potential health risks. Sludges may contain toxic heavy metals, synthetic organics, and pathogens. Studies of animals inhabiting sludge-amended soils, or foraging on crops grown on such soils, have shown an increased deposition of cadmium and polychlorinated biphenyls in animal tissues, hepatic microsomal enzyme induction, altered blood profiles, appearance of mutagens in blood and excreta, and lowered feed intake, feed efficiency, and milk production. Epidemiologic evidence of possible adverse health effects among sewage plant workers exposed to aerosolized pathogens or synthetic organic vapors during wastewater treatment or land spreading of sludge is still inconclusive. Other possible deleterious environmental effects include pollution of groundwater and toxicity to birds consuming worms or insects inhabiting sludgetreated soils.

ChemInform Abstract: CADMIUM‐A COMPLEX ENVIRONMENTAL PROBLEM. PART II. CADMIUM IN SLUDGES USED AS FERTILIZER

In intensively populated countries efficient sewage treatment is essential to protect river quality. An inevitable by-product is sewage sludge which has to be disposed of safely and economically. Utilisation of sludge as a fertilizer of agricultural land is the most economic disposal route for inland sewage-treatment works and also benefits farmers by providing a cheap manure. Much of the cadmium in wastewater is concentrated into sludge which consequently contains higher concentrations of cadmium than soil does. It is impracticable to reduce cadmium concentrations in sludge below certain levels. When sludge is used on farmland rates of application must be controlled so that cadmium concentrations in soil never reach levels that could significantly contaminate food crops. Cadmium is a principal factor limiting the use of sludge on land. Nevertheless, it is a local problem since agricultural land in general receives more cadmium from aerial deposition and phosphatic fertilizers. The significance of accumulations of cadmium in soil depends mainly on its availability for crop uptake. Investigations are described which have attempted to identify and to determine the availability of forms of cadmium in soil. There is considerable research interest in cadmium in soil solution which is likely to be directly available for crop uptake. Another area of interest is the apparent disappearance of cadmium from sludge-treated soil. Soil analysis often cannot fully account for the cadmium added in sludge. Apart from the effect of soil conditions, especially pH value, crop uptake varies according to the particular crop examined. Highest concentrations of cadmium occur in tobacco, lettuce, spinach and other leafy vegetables. Using crop uptake data from field trials it is possible to relate potential human dietary intake of cadmium, on which hazard depends, to soil concentrations of cadmium, which can be controlled by regulating applications of sludge. This provides an objective basis for limits for cadmium concentrations in soils receiving sludge. Transfer of cadmium via farm animals to meat and dairy products for human consumption is thought to be minimal, even allowing for some direct ingestion of sludge-treated soil by the animals. Evidence from these and other investigations suggests that a loading rate limit of 5 kg Cd/ha (equivalent to a soil concentration of about 3.5 mg Cd/kg) affords adequate protection to the foodchain where sludge is used on agricultural land. More research work is needed to provide a basis for predicting the long-term availability of cadmium introduced to the soil in sludge.