Report No DWI0741
EFFECTS OF METALS IN SLUDGE ON CROPS (EI 9324 SLD)
Final Report to the Department of Environment
DWI0741
Dec 1986
SUMMARY
(i) BACKGROUND
Utilisation on agricultural land is the major disposal outlet for sewage sludge in the UK, accounting for about 50% of the sludge produced annually. Metals in sludge applied to agricultural land remain in the cultivated layer of top soil. It is important that repeated applications of sludge do not build up metal concentrations in the soil to levels which could adversely affect crops or the animals, including man, which eat them. Field experiments were established at three sites on soils broadly representative of UK agriculture. Controlled applications of sludge were made and soil and crop concentrations of metals were monitored thereafter over a five year period. The results have defined the effect of sludge applications of known metal content on total and extractable concentrations of metals in soil. The results have defined also the extent of transfer of metals from sludge-treated soil into the leaves and edible parts of six crops of major importance to UK agriculture.
(ii) OBJECTIVES
Assist in the formulation of guidelines for the safe disposal of sewage sludge to agricultural land:
(iii) RESUME
Crops to be grown and harvested on an annual rotation over five successive seasons, with the following variables:
Yield from each plot to be measured, and analysis of the following to be made where appropriate, for nutrients, cation exchange capacity, organic matter, total metals (Ni, Cu, Zn, Cd, Pb) and available metals (Ni, Cu, Zn, Cd, Pb - by the methods of the Agricultural Development and Advisory Service of the Ministry of Agriculture, Fisheries and Food methods):
(iv) EFFECTS OF SLUDGE ON SOIL CONCENTRATIONS OF METALS
Mi = 10 x Ml/(r x s) mg/kg
where M1 is the metal loading rate (kg/ha)
r the soil density (g/cm3), air-dried
s the depth of cultivation (cm)
Where the weight of sludge applied exceeds ten percent of the weight of soil in the cultivated layer, then the effect of sludge on soil density should be taken into account.
Amounts of other metals, Ni, Cu, Zn and Pb applied in liquid sludge were also accounted for in the cultivated profile with the exception of Cu and Zn in the calcareous loam where some mobility may have occurred beyond 15 cm depth or there may have been some lateral movement of sludge-treated soil off the plots following cultivation.
unsludged soil |
||||
Ni |
Cu |
Zn |
ZE |
|
non-calcareous |
13 |
39 |
6 |
13 |
calcareous |
4 |
20 |
19 |
12 |
Liquid sludges |
Bed-dried sludge |
|||||||
Ni |
Cu |
Zn |
ZE |
Ni |
Cu |
Zn |
ZE |
|
non-calcareous |
68 |
74 |
87 |
78 |
62 |
57 |
87 |
68 |
calcareous |
22 |
37 |
46 |
33 |
12 |
27 |
26 |
21 |
Crop concentrations of the most plant-available metals (Cd, Zn and Ni) generally followed the pattern of percentage EDTA-tractability being highest on non-calcareous soils and liquid sludge treatments.
(v) EFFECTS OF SLUDGE ON CROP YIELDS
(vi) EFFECTS OF SLUDGE ON CROP COMPOSITION
Crop |
Soil* |
Estimated increase in crop tissue concentration per 1 mg/kg increase in soil content. |
Wheat |
sl |
0.097 |
grain |
c |
0.105 |
cl |
0.046 |
|
Potato |
sl |
0.042 |
tuber |
c |
NS |
cl |
0.028 |
|
Lettuce |
sl |
0.75 |
c |
1.2 |
|
cl |
0.34 |
|
Red beet |
sl |
0.22 |
root |
c |
0.24 |
cl |
0.07 |
|
Cabbage |
sl |
0.034 |
c |
0.051 |
|
cl |
0.044 |
|
Ryegrass |
sl |
0.084 |
c |
0.079 |
|
cl |
0.040 |
* sl = sandy loam, c = clay, cl = calcareous loam NS = relationship not statistically significant
(vii) EFFECTS OF TIME
(viii) EFFECTS OF SOIL CONDITIONS AND SLUDGE TYPE
(IX) RELATION TO CURRENT RECOMMENDATIONS FOR SLUDGE UTILISATION
Crop |
Non-calcareous soils |
Calcareous soils |
Wheat grain |
0.25 |
0.12 |
Potato tuber |
0.11 |
0.07 |
Lettuce |
2.4* |
0.85 |
Red beet root |
0.58 |
0.18 |
Cabbage |
0.11* |
0.11 |
Ryegrass |
0.20 |
0.10 |
* see (viii) 3.
Copies of this report may be available as an Acrobat pdf download under the 'Find Completed Research' heading on the DWI website.