Copper in cultivated soils of Finland Markku Yli-Halla Yli-Halla,M. 1994. Copper in cultivated soils of Finland. Agricultural Science in Finland 3: 487-495. (Department of Applied Chemistry and Microbiology, P.O. Box 27, FIN-00014 University of Helsinki, Finland.) Soil samples from the plough layers of 105 fields in different parts of Finland were analyzed for Cu fractions. Vertical distribution of Cu was also studied in a smaller material. Total Cu (Cu tot , HN03 -HCI0 4-HF-H 2 S0 4 digestion) in the surface soil ranged 6.9-97.4 mg kg ' (mean 37.1 mg kg ') and was highest in clay soils (mean 59.0 mg kg ') and lowest in fine sand and moraine soils (mean 18.3 mg kg -1 ). Copper in the water-soluble, exchangeable and mainly organically bound fraction was extracted with 0.1 M K 4P 207 (Cu py ), and Cu bound by poorly crystalline Fe, A 1 and Mn oxides (Cu ) was dissolved subsequently with 0.05 M oxalate (pH 2.9). The average percentages of Cupy and Cuox were 18% and 12% of Cu tol in mineral soils and 34% and 19% of Cutol in organogenic soils, respectively. Residual Cu (Cu res ) incorporated in mineral lattices was calculated to constitute 70% and 47% of Cu|ol in mineral and organogenic soils, respectively. In two thirds of soils the potentially plant-available reserves of Cu (Cu p) + Cu ox ) were more plentiful than those of Zn (Zn +Zn ), An acetic acid - ammonium acetate - Na.EDTA solutionv py ox7 2 used in routine soil testing extracted 56% and 71% of the sum of Cupy + Cuox in mineral and organogenic soils, respectively. In soil profiles, CuEDTA was higher in the plough layer than in the subsoil but a few soils rich in Cu wl had abundant reserves of CuEDTA below the rooting depth of annual field crops. Key words: total analysis, sequential extractions, pyrophosphate extraction, oxalate extraction, ammonium acetate-acetic acid-EDTA extraction, vertical distribution of Cu, zinc Introduction Soil Cu is commonly divided into fractions with different extractants applied sequentially (McLaren and Crawford 1973, Shuman 1979, 1985, Liang et al. 1991). It is assumed that each solution dissolves a specific fraction retained by a given mechanism or soil constituent; Cu in soil solution, exchangeable, specifically adsorbed, complexed by organic matter or by Fe, Al and Mn oxides and residual Cu incorporated mainly in the lattices of primary minerals (Viets 1962). The residual fraction is considered unavailable to plants, while the other ones, collectively called secondary fractions, are, at least to some extent, sources of plant-available Cu (Gallardo-Lara and Torres-Martin 1990, Liang et al. 1991). A few sediment samples mainly from polluted in- dustrial areas of Finland have been analyzed for the fractions of Cu (Räisänen and Hämäläinen 1991) but the fractional distribution of Cu in cul- tivated soils of the country is unknown. An ammonium acetate - acetic acid - Na2EDTA solution (AAAc-EDTA, pH 4.65) is used to ex- tract Cu in soil testing in Finland. Recently, Joki- nen et al. (1993) found that this extractant dis- 487 Agricultural Science in Finland 3 (1994) https://www.c-info.fi/en/info/?token=Aro3b84kbtnO4EvI.nBu2gtl814sWCdLFzA-mLg.u9WSnCG3kRhJVejfuZ1yFyI5WwI3YxnaXXxVbylHowW6OborbhCeQBq6at8cV001YsmIZ89_yeJlx_Ngg2fOuCTj2aIC4qwzIz0WBb3fSCe3Sd7O5YxnC0oApjQ-capEbrhyM1t5kGPicaobD1efVZZwStY2GbniPQKx3gPpkd7tvhrpDSSqIW38rMlGOebIUUw-rjavXhOGJYQOiN5mPx_0PPsArrPPNU9cwy9Z7NtQKjShozlWOzkxH6dCJZ1jxmdR84TNMA solved 40% of total Cu in organogenic soils. How- ever, it is not known, either in organogenic or mineral soils, to what extent the secondary re- serves, the potential source of plant-available Cu, are extracted by this solution. This information would be important in order to be able to trans- late the soil testing results into a quantitative es- timate of plant-available Cu. The purpose of the present study is to examine the distribution of soil Cu into different fractions using a simplified procedure of McLaren and Crawford (1973). The extraction power of AAAc - EDTA was studied and the results obtained by this method were related to the secondary frac- tions. The fractions of soil Cu were also com- pared to those of Zn obtained in a previous study (Yli-Halla 1993). Material and methods The distribution of Cu into various fractions was studied in 105 soil samples collected from the plough layers (A p horizons) of cultivated fields in Finland. The material consisted of 25 clay soils, 20 silt and very fine sand soils, 26 fine sand and moraine soils, 14 mull soils and 20 peat soils. The vertical distribution of Cu was studied on seven soil profiles of cultivated fields as well as on 15 pairs of samples from the plough layer (A p horizon) and from the respective subsoil (30- 35 cm). All the samples have been described in detail in an earlier study (Yli-Halla 1993). How- ever, a moraine (soil 53) and a fine sand soil (soil 71) of the surface soil material of the previous study were not included in the present investiga- tion. To determine total Cu, the soil was digested with a mixture of HNO,, HCI0 4 , HF and H,S04 (Yli-Halla 1993). Water-soluble and ex- changeable Cu as well as Cu bound mostly by organic matter were extracted as one fraction with 0.1 M K 4P,0 7 (pH 10), and Cu bound by poorly crystalline Fe, A 1 and Mn oxides was extracted by a 0.05 M oxalate solution (0.026 M ammoni- um oxalate, 0.024 M oxalic acid, pH 2.9) se- quentially after the pyrophosphate extraction (Yli- Halla 1993). In 16 representative soil samples, the residue remaining after the sequential pyro- phosphate and oxalate extraction was further di- gested with a mixture of HNO,, HCI04, HF and H,SO, to determine the residual Cu (Cu ) but in24 v res' most soil samples Cu res was calculated as total Cu minus the sum of Cu extracted with pyro- phosphate and oxalate, i.e. Cu io -(Cu py + Cu ox). All the digestions and extractions were carried out in duplicate. The Cu concentration of the ex- tracts was determined by atomic absorption spec- trophotometry. In order to allow a relevant com- parison of Cu ,Cu ,Cu and Cu between ther tot’ py 7 ox res mineral and organogenic soils, the results, origi- nally expressed as milligrams per kilogram of soil, were in some instances transformed into mil- ligrams per dm3 of soil by multiplying them with the bulk density. Copper was also extracted with a solution containing 0.5 M 0.5 M CH,COOH and 0.02 M Na,-EDTA at pH 4.65 (Lakanen and Erviö 1971), which is the method used in soil testing in Finland. Results Total copper In mineral soils, total Cu (Cu toi, mg kg -1 ) in- creased with increasing clay content (r = o.B7'*'). In a few heavy clay soils, Cuio| approached 100 mg kg~', while in some fine sand soils it was very low (< 10 mg kg - ') (Table 1). Mull and peat soils had a similar concentration of Cu but thetot number of very low contents of Cu| t was higher among the peat soils. When expressing the re- sults as milligrams per dm3 of soil, the averages were 26.8 and 14.7 mg dm-3 in mull and peat soils, respectively, being of the same level as the fine sand and moraine soils. In organogenic soils, Cutot (mg dnr3 ) decreased with increasing organ- ic C (r = -0.52”). Fractions of soil copper In the 16 representative soil samples, Cutot and the sum of the fractions (Cu +Cu +Cu )v nv ox res'py ox res- 3488 Agricultural Science In Finland 3 (1994) Table I. Total Cu (Cu lo| ) and Cu in fractions extracted with pyrophosphate (Cu py ) and oxalate (Cu ox ) and in the residual fraction (Cu tes ), and Cu extracted with AAAc-EDTA (CuEDTA ) as well as the bulk density of the plough layer soil samples. 1 Soil class and Bulk density 2 Cu Cu Cu Cu Cuc„_.J tot py ox res EDTAnumber of samples kg dnr’ mg kg -1 mg dm'5 Clay mean 0.96b 59.0" 9.2" 6.2" 43.8" 7.7" n= 25 range 0.76-1.14 31.6-97.4 2.9-26.9 2.4-20.8 12.4-77.8 2.5-26.0 Silt, very mean 1.00b 30.7 b 4.3b 3.1 b 23.4b 4.2 b fine sand, n= 20 range 0.77-1.16 20.6-45.9 0.4-11.9 1.3-5.0 7.7-36.8 1.5-10.6 Fine sand, mean 1.14" 16.3 C 3.3b 2.1b 10.8C 3.7b moraine, n= 26 range 0.89-1.43 8.4-31.0 1.1-8.4 0.7-5.4 3.5-21.5 1.3-9.1 Mull mean 0.65' 41.4 b 15.3" 7.8" 18.3bc 8.1" n = 14 range 0.53-0.77 21.0-80.5 3.9-26.2 2.8-14.9 4.9-40.2 3.0-14.1 Peat mean 0.37“ 38.5b 12.9" 7.7" 5.9"b n = 20 range 0.25-0.55 6.9-73.6 0.8-34.3 0.6-17.4 4.6-38.6 0.9-14.9 ' The means in each column have been tested separately. Means marked with the same superscript within a column do not differ at P = 0.05. 2 Determined by Yli-Halla (1993). were 28.9 mg kg~' and 33.0 mg kg ', respective- ly. The difference between these figures in the individual soils ranged from -0.7 to 13.0 mg kg 1 (median 4.0 mg kg'). Owing to the satisfactory recovery of Cu in the fractions, the determina- tion of Cu was discontinued and the rest of theres results of Cu were calculated as the difference res Cu -(Cu +Cu ). lot v py ox 7 The concentration of Cu extracted with pyro- phosphate (Cu >y , Table 1) was highest in mull and peat soils, but when expressing the results as mg dm 3 of soil, the mean of 9.8 mg dnr3 places the mull soils at the same level as clay soils. The mean of 5.1 mg dm-3 in peat soils equals that in silt and very fine sand soils. In most soils the concentration of Cu extracted with oxalate (Cu ,v OX 7 Table 1) was smaller than Cupy; only in 13 soils was Cu equal to or higher than Cu . Cu was atox ° py ox the same level in peat, mull and clay soils and substantially lower in coarse mineral soils. Both in mineral and organogenic soils, Cu py and Cu ox correlated highly with each other. In mineral soils, Cu py correlated highly significantly (P = 0.001) also with clay and Cuto| (Table 2), while Cu <>x correlated with Cu , Cu rps, clay and poorly crys- talline Fe oxide (Fe ox ). However, the partial cor- relation between Cu and Fe , after the eliminä- OX ox 7 tion of the effect of clay, was not significant (P = 0.05). In organogenic soils, both Cupy and Cu) and soil properties in mineral and organogenic soils. The calcula- tions have been carried out with the logarithmic (log |0) transformations of the concentrations of Cu (mg dm 3 ) and Fe and Al (g dnr 3 ). Mineral soils Organogenic soilsSoil characteristic Cu Cupy ; Cu Cupy * Clay 0.52"' 0.68‘" Organic C 0.32" 0.09"' -0.52" -0.45" FeJ 0.35" 0.44'" 0.50" 0,48" Al“' 0.24n! 0.23ns 0.47" 0.22" 5 Cu 0.38” 0.62'" 0.61'" 0.67'" CulM 0.57"' 0,80‘" 0.95'" 0.91'" Cu 0.87'" - 0,90'" Extracted with 0.05 M oxalate (0.029 M ammonium oxalate, 0.021 M oxalic acid) at pH 3.3 (Niskanen 1989). *, ",'" Significant at P = 0.05, 0.01 and 0.001, respectively. Not significant (P > 0.05). 489 Agricultural Science in Finland 3 (1994) Table 3, Copper extracted with pyrophosphate (Cu py ) and oxalate (Cu ox) as well as residual Cu (Cu res ) as percent- ages of total Cu. 1 CuSoil class Cu Cu resPV % of total Cu Clay mean 16.5b 10.5b 73.0 a n= 25 range 6.3-51.3 5.3-22.1 33.3-88.1 Silt, very mean 14.8b 10.6b 75.4a fine sand range 1.1-32.0 3.7-24.0 44.0-95.2 n = 20 Fine sand, mean 20.3b 13.l b 66.6a moraine range 9.0-37.6 9.7-25.4 37.1-81.6 n = 26 Mull mean 37.0a 19.0a 44.0b n= 14 range 18.6-62.5 12.6-24.8 14.1-63.5 Peat mean 31.1* 19.6“ 49.3 b n= 20 range 11.6-50.1 8.7-35.8 16.0-68.7 1 The means in each column have been tested separately. Means marked with the same superscript within a col- umn do not differ at P =0.05. In mineral soils, 29% of Cu occurred in the7 tot secondary fractions (Cupy , Cuox ), while these frac- tions constituted 53% of Cu in the organogenic soils (Table 3). Even though in some organogen- ic soils more than half of soil Cu was in the form of Cu ,Cu was usually relatively the mostpy7 res J J abundant fraction in both soil groups. In mineral soils, the percentage of Cupy correlated weakly (r = o.39***) with organic C content. Copper extracted with AAAc-EDTA Copper extracted with AAAc-EDTA (CuEDTA , mg dm-3) was highest in clay and mull soils (Table 1). The lowest result (0.9 mg dnr3) oc- curred in a Sphagnum peat soil which had been cultivated for five years. CuEDTA constituted on average 16% of Cutm in mineral soils and 42% in organogenic soils. In mineral and organogenic soils, AAAc-EDTA extracted 56% and 71% of the secondary Cu (Cu py + Cuox, mg dm-3 ), respec- tively. CuEDTA correlated most strongly with Cu py and Cuox, in organogenic soils also with Cutot (Table 4). According to the regression analysis, Table 4. Correlation coefficients (r) between AAAc-EDTA- extractable Cu and other indices of soil Cu. The correla- tion coefficients have been calculated using the logarithms (log 10) of the results (mg dnr3 of soil). r Mineral soils Organogenic soils 0.78’" 0.87"’ 0.76’" 0.45" 0.88"" o.Bl*** 0.56"’ 0.35" Cupy Cu ox Cu ” toi Cu Significant at P = 0.01 and 0.001, respectively. CtiEDTA (mg dnr3 ) increased with increasing Cu py (mg dnr3 ) and Cu ox (mg dm-3) and with decreas- ing poorly crystalline A 1 oxide (Al ox , g dm-3). The equations, calculated with the logarithms (log |0 ) of the results, were as follows: Mineral soils: log Cuc_T. = 0.66 log Cu + 0.23 log Cu° EDTA ° py ° ox -0.24 log Al +0.14° OX R 2 = 0.82"* Organogenic soils: •°g CUEDTA =0 43 >°g CU py +0 34 >°g CUo, -0.31 log AI +0.28° OX R 2 = 0.86*" According to the (3 coefficients (Table 5), Cu py was the dominant soil factor explaining the vari- ation of CuEDTA in both soil groups. In organo- genic soils, Cu ox and Al ox appeared to be slightly Table 5. t-Values of the regression coefficients and beta coefficients (ff) of the independent variables explaining the variation of log CuEDTA in mineral and organogenic soils. Independent Mineral soils Organogenic soils variable I I [ P t P_ log€ur> 6.88"' 0.73 2.88" 0.61 log Cu” 2,05' 0.22 2.18* 0.42 log Al ' -3.83"' -0.21 -4.51"’ -0.43 ", "‘ Significant at P = 0.05, 0.01 and 0.001, respec- tively. 490 Agricultural Science in Finland 3 (1994) more important variables than in mineral soils, but this conclusion becomes less reliable due to the small number of organogenic soils in the ma- terial. Poorly crystalline Fe oxide (Fe ox ) correlat- ed with the secondary Cu fractions (Cu py , Cuox ) and therefore Fe ox was not a statistically signifi- cant variable with Cu and Cu , whether or notpy ox’ Al >x was in the equation. In organogenic soils there was a negative correlation (r = -0.45**) be- tween log Al and organic C. The appearance of A 1 in the above regression equation thus means that the extractability of Cu with AAAc-EDTA increases with increasing organic C and decreas- es with increasing mineral material. Vertical distribution of soil Cu Except for profile 7(P 7), Cu toI was highest in all the profiles at the bottom (Table 6). The heavy clay layers in P 3 and the Carex peat sample taken from the bottom of P 5 had the highest Culo( (> 100 mg kg” 1) of the entire material. With- in the fine-textured mineral soil profiles P 1, P 2 and P3, Cutot increased with increasing clay con- tent towards the deeper layers. In P 7 generally poor in Cu , the highest Cu tin the plough layer may originate from Cu fertilization. In the pro- files 1,3, 5 and 6, CuEDTA was highest in the deepest layers, two-to-four times that in the plough layer while in the three remaining profiles, CuE dta was highest in the plough layer. In 14 sample pairs consisting of the plough layer (Ap ) and the subsoil (B) sample, CuEDTA was significantly higher (t = 3.375** in the t test for paired measurements) in the plough layer. The means and ranges were as follows: RangeMean 5.0 2.2-8.9A p 2.7 0.6-6.2B There were three pairs in which Cu EDTA in the subsoil was equal to or slightly lower (0.3- 1.0 mg dnr3 ) than in the plough layer. Of 15 sample pairs one pair not included in the above means had a heavy clay subsoil richer in CuEDTA (18.4 mg dm ’) than the organogenic plough lay- er (8.4 mg dm 3 ). Comparison of soil Cu and Zn In mineral soils, the reserves of Cu|ot (36 mg kg ') were substantially smaller than those of Zntm (94 mg kg -1 , for detailed results see Yli-Halla 1993), but in organogenic soils the two elements occurred in the same quantities (Cu to| 40 mg kg ', Znu| 41 mg kg- '). In two thirds of the soils the reserves of Cu in the secondary fractions (Cu py + Cuox) were larger than those of Zn. There were only 3 clay soils and 5 silt soils but as many as 16 coarse mineral soils and 8 peat soils where the secondary reserves of Zn exceeded those of Cu. Accordingly, Cu EDTA was lower than Zn EDTA only in 21 soils. The correlation coefficients be- tween the various indices of soil Cu with those of Zn were poorer in the organogenic soils than in the mineral soils (Table 7). It should be point- ed out that in organogenic soils the correlation coefficients between Cu and Zn as well as be-py py tween CuEDTA and ZnEDTA were not statistically significant. Discussion In total Cu (Cu tot ), the present soils corresponded to other soil materials from Finland (Baghdady and Sippola 1983, Koljonen and Malisa 1991, Jokinen et al. 1993). They contained more Cutot than the silty and sandy soils of England (mean 20.3 mg kg” 1, range 5.2-63.5 mg kg”', McLaren and Crawford 1973) and clay and silt soils of Saskatchewan, Canada (mean 20.9 mg kg ', range 6.5-39.0 mg kg” 1, Liang et al. 1991). Values of Cutot as high as those commonly found for the heavy clay soils in the present study are seldom reported in unpolluted cultivated soils elsewhere. The mineral soils studied were richer in Cucrv„EDTA than those of Jokinen and Tähtinen (1987) who deliberately included soils where plants had shown symptoms of Cu deficiency. The mean CuE[)TA was also higher than in some other research ma- terials (2.8 mg dm 3 , Sippola and Tares 1978, Sillanpää 1982). Like in Sippola and Tares (1978), CuEDTA was higher in clay soils than in the other mineral soil classes. In organogenic soils, 491 Agricultural Science in Finland 3 (1994) Table 6. Total Cu (Cu,ol ) and Cu extracted with AAAc-EDTA (Cu EDTA ) in samples taken from various depths in seven soil profiles. 1 Profile I: Tarvasjoki loam (0-38 cm)/clay loam (38-120 cm) Depth ClayDepth Clay Cu tol cm % mg kg" 1 0-30 25 25. l f 32-38 29 27.0' 38—46 34 33.4“ 50-60 41 40.5' 65-80 51 51.2” 85-100 56 47.5b 105-120 55 48.0b HSD 1.63 Profile 3: Vihti silty clay (0-60 cm)/ heavy clay (60-120 cm) Depth Clay Cu, cm % mg kg" 1 0-30 50 46.9' 30-40 53 55.5“ 40-60 54 61.0' 60-80 72 67.7b 80-100 88 108.9” 100-120 76 109.2” HSD 5.13 Profile 5: Sotkamo, Carex peat (20-120 cm). mineral soil mixed in the plough layer Depth Org. C Cu, cm % mg kg" 1 0-20 9 32.5' 20-30 52 23.8* 30-40 49 27.6f 40-60 52 35.4' 60-80 47 46,8“ 80-100 40 55.7' 100-120 31 65.5b 120-130 32 107.3” HSD 2.94 Profile 7; Muhos ' U ! n: \ mg dm-1 2.5' 2.4' 2.7“' 3.2“ 3.8' 4.8 b 9.8” 0.61 CU EDTA mg dm"' 5.7' 5.7' 4.1“ 6.4' 9.3 b 10.5“ 0.79 CU EDTA mg dm" 3 10.5b 2.4* 2.7* 3.6 f 5.2' 7.4“ 9.8' 20.5“ 0.61 Profile 2: Vihti, silt Depth Clay Cu lot CuEDTA cm % mg kg" 1 mg dm-5 0-27 17 29.5“ 2.9” 30-40 11 34.0‘“ 1.2“ 40-50 12 37.5“ 1.3“ 50-70 17 48.5 b 1.6“ 70-90 6 26.5“ 0.9' 90-100 16 48.5b 1.6“ 110-120 27 62.6“ 2.0b HSD 7.67 0.18 Profile 4: Sotkamo, fine sand Depth FS 2 Cu|ol Cu EDTA cm % mg kg 1 mg dm 3 0-30 55 13.l b 3.5* 30-40 63 4.7' 1.0“ 40-45 52 5.8“' l,4b 45-60 67 6.7“ 0.9“ 60-80 75 8.0“ 0.7“ 80-110 82 12.6b 1.3"“ 110-120 64 15.4“ 1.0““ HSD 1.43 0.31 Profile 6: Jokioinen, Carex peat (0—40 cm)/ mud (40-50 cm)/ heavy clay (50-80 cm) Depth Org. C Cu, o, CuEDTA cm % mg kg" 1 mg dm"3 0-25 31 51.2' 6.4“ 30-40 30 83.2' 11.5b 40-45 17 89. l b 15.0“b 50-70 1 79.9“ 17.0“ 70-80 1 95.3“ 17.5“ HSD 1.98 4.05 Carex peat (0-110cm)/fine sand (110-125 cm) Depth Org. C Cu|o, cm % mg kg" 1 CU EDTA mg dm 3 0-30 45 41.9“ 23.3” 0.9“ 0.8“ 1.3' 1.8b 30-50 55 2.7' 50-70 52 1.3' 70-90 56 4.9' 90-110 54 11.4b 110-125 0.2 2.8' 0.7“ 0.29HSD 3.61 Each profile was tested separately for Cu, , and CuEDTA . Means marked with the same superscript within a column do not differ at P = 0.05. 2 Fine sand, 0.06-0.2 mm 492 Agricultural Science in Finland 3 (1994) Table 7. Correlation coefficients (r) between various indi- ces of soil Cu and Zn, calculated using the logarithms of the results (mg dnr3 of soil). Mineral Organogenic soils soils Correlation soilscoefficients between 0.48"’ 0.65’" 0.89’” 0.87"’ 0.49’" 0.20" sCu and Zn _ p> , p> Cu and Zn OX ox 0.42" 0.34‘ 0.51" Cu and Znres res Cu,. and Zn., lot lot 0.07 nsCUEDTA anJ ZllEDTA *, ", ■” Significant at P = 0.05, 0.01 and 0.001, respec- tively. n ! Not significant (P > 0.05). mean CuEDTA was similar to that reported by Jo- kinen et al. (1993). The present soils also exhib- ited nearly the average Cu EDTA reported in routine soil testing in 1986-1988 (Viljavuuspalvelu - Soil Testing Service, Ltd., unpublished data) in over 60000 samples of mineral soils coarser than silt (mean 4.2 mg dm-3 ) and in over 25000 organo- genic soils (mean 5.2 mg dnr3 ). The material of this study represents fairly well the average cul- tivated soils of Finland, even though there was only one soil classified as ‘poor’ in Cu (CuEDTA below 1 mg dnr3) according to the interpretation of Sillanpää (1982). The fraction of water-soluble and exchange- able Cu is too small to satisfy the needs of the plants (McLaren and Crawford 1973, Liang et al. 1991), and this readily plant-available form is replenished from other secondary fractions, es- pecially from that bound by organic matter (Liang et al. 1991). It was therefore considered appro- priate in this study to include water-soluble, ex- changeable and specifically adsorbed Cu, togeth- er with Cu bound mainly by organic matter, in Cu p> and not to extract them separately as is com- monly done in fractionation procedures. Cupy , ex- pressed as percentages of Cu|oi , was in mineral soils at the same level as the sum of water-solu- ble, exchangeable, specifically adsorbed and or- ganically bound Cu in soils of Saskatchewan, Ca- nada (18.4% of Cu , Liang et al. 1991). Alsov tot’ ' Cu and Cu in the soils of Canada (11% and ox res v 71% of Cu , respectively) were equal to those in texturally similar soils of the present study. In other studies (McLaren and Crawford 1973, Shuman 1985), the relative sizes of the second- ary fractions have been higher and those of Cu rcs slightly lower (Cu res 53% and 65%, respectively) than the relative sizes in the mineral soils of this investigation. In organogenic soils, the lower per- centage of Cures and the higher ones of Cupy and Cu ox as compared to the mineral soils can be ex- plained by the smaller quantity of mineral mate- rial, the source of Cu .resres Copper extracted with AAAc-EDTA has cor- related rather closely with Cu supply to plants in pot experiments (Sillanpää 1982,Erviö and Sip- pola 1993). On the basis of the observation that the content of Cu py explained a great deal of the variation of CuEDTA especially in mineral soils, it can be concluded that AAAc-EDTA dissolves Cu from the same reserves as does pyrophosphate. Also Cu bound by poorly crystalline oxides (Cu ox ) can be plant-available (Gallardo-Lara and Torres-Martin 1990, Liang et al. 1991) but ac- cording to McLaren and Crawford (1973) Cu ox is of minor importance as a source of plant- available Cu. The latter assumption is supported also by the results of the present study where Cu ox relatively poorly explained the variation of UEDTA" In Finland, Cu deficiency in crop production has been reported especially in peat soils (Tainio 1963, Tähtinen 1971). Even though quite a few peat soils may be low in Cutot , the present results demonstrate that by far all of them are not poor in Cu EDTA . Therefore soil testing is necessary to recognize the soils where Cu fertilizers should be applied. In organogenic soils, AAAc-EDTA ex- tracted a higher proportion of the potentially plant- available Cu (Cu +Cu ) than in mineral soils.v py ox' Thus, low CuEDTA in organogenic soils implies for certain a scarcity of Cu and a probable re- quirement of Cu fertilization. A higher CuEDTA in the plough layer, as com- pared to the B horizon, can partly be attributed to fertilizers, manures, atmospheric deposition and uplift of Cu by plant roots from below the plough layer. The higher content of organic matter may also enhance the solubility of Cu (Sillanpää 493 Agricultural Science in Finland 3 (1994) 1982). On the other hand, investigation of the soil profiles revealed that soils rich in Cu had abundant reserves of CuEDTA also in the layers below the rooting depth of annual field crops. There, Cu released from primary minerals is not within the reach of plant roots and has obviously remained where the mineral was weathered. In the soils poor in Cu |oi this phenomenon was not observed, probably owing to the lack of weather- able Cu-containing minerals. A similar vertical distribution of Zn crvTA has earlier been observedEDTA in the same soil profiles (Yli-Halla 1993). In a recent study, carried out with the same soil samples (Yli-Halla 1993), 90% of Zn |oi in mineral soils occurred as Zn , while in the presentres’ r investigation only 71% of Cu occurred as Cu rt . Accordingly, the percentages of the secondary fractions of Cu were higher than those of Zn. Similar conclusions can be drawn also from the results of Shuman (1979, 1985) and Liang et al. (1990, 1991). The difference between the distri- bution of Cu and Zn was even wider in the mull soils of the present study where 47% of Cu iot and as much as 80% of Zn occurred in the residualtot fraction. According to Mullins et al. (1982), fer- tilizer Cu and Zn are accumulated in forms ex- tractable with pyrophosphate and oxalate. The rel- ative abundance of secondary Cu fractions as com- pared to those of Zn as well as the poor correla- tion between the fractions of Cu and Zn in orga- nogenic soils can partly be explained by addi- tions in Cu fertilizers, applied commonly in Fin- land since the 1950’5. Zinc fertilization, as a rarer and a more recent practice, has probably contributed to a smaller increase in soil Zn con- tent. Ample application of fertilizer Cu may also explain why even Cu tot was equal to Zn in orga- nogenic soils, while in mineral soils Cu iot was much lower than Zn iot . However, the secondary Cu fractions were more abundant than those of Zn also in mineral soils, and it is very unlikely that clay soils in particular have received either Cu or Zn in chemical fertilizers. The relative abun- dance of Cu in the secondary fractions therefore suggests that Cu minerals have weathered at a higher rate than those containing Zn. The sufficiency of plant nutrients in soil can, to some extent, be assessed by comparing the need of a plant for the plant-available reserves. In the study of Yläranta and Sillanpää (1984), the Zn concentration was 5-11 times the concen- tration of Cu in cereal crops and 3-6 times that in forage crops. However, the size of the second- ary Cu fractions in two thirds of the soil samples of the present study was higher than that of Zn. Therefore, the reserves of plant-available Cu in average soils may be more abundant as related to plant uptake than those of Zn. Acknowledgements. The author wishes to thank Kemira Oy Espoo Research Centre for carrying out the chemical analyses of this investigation. References Baghdady, N. H. & Sippola, J. 1983. Total heavy metal recovery by aqua regia in soils of different origin. Annales Agriculturae Fenniae 22: 175-185. Gallardo-Lara, F. & Torres-Martin, M. 1990. 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Jr. 1962. Chemistry and availability of mi- cronutrients in soils. Journal of Agricultural and Food Chemistry 10: 174-178. Yli-Halla, M. 1993. Plant-availability of soil and ferti- lizer zinc in cultivated soils of Finland. Agricultural Science in Finland 3: 197-270. Yläranta, T. & Sillanpää, M. 1984. Micronutrient con- tents of different plant species grown side by side. Annales Agriculturae Fenniae 23: 158-170. Manuscript received April 1994 SELOSTUS Kupari Suomen viljelysmaissa Markku Yli-Halla Helsingin yliopisto Viljelysmaiden muokkauskerroksen kuparivaroja tutkittiin määrittämällä kuparin (Cu) kokonaismäärä sekä eri tavoin maahan sitoutuneita kuparin fraktioita. Kuparin kokonais- määrä (6,9 - 97,4 mg kg ') oli suurin savimaissa (keskiar- vo 59,0 mg kg 1) ja pienin karkeassa hiedassa ja moreeni- maissa (18,3 mg kg 1). Kivennäismaissa kuparin koko- naismäärä oli vuorosuhteessa savespitoisuuden kanssa. Vesiliukoisen ja vaihtuvan kuparin sekä orgaanisen ainek- sen ja rauta- alumiini- ja mangaanioksidien sitoman kupa- rin summa oli kivennäismaissa 30 % ja eloperäisissä maissa 53 % kokonaismäärästä. Näitä osuuksia voidaan pitää po- tentiaalisesti kasveille käyttökelpoisina. Kummassakin maalajiryhmässä mineraalirakenteisiin sitoutuneen,kasveil- le käyttökelvottoman kuparin osuus oli suuri (kivennäis- maissa 70 %, eloperäisissä maissa 47 %). Viljavuusana- lyysissä kupari uutetaan happamalla ammoniumasetaatti- etikkahappo-EDTA-liuoksella, pH 4,65 (Cu EDTA ). Tämä liuos uutti vaihtuvasta, orgaanisen aineksen ja oksidien sitomasta kuparista kivennäismailla 56 % ja eloperäisillä mailla 71 %. Muokkauskerroksen CuEDTA -varat olivat lähes poikkeuk- setta suuremmat kuin jankon, mutta varsinkin savimailla juuristovyöhykkeen alapuolisissa maakerroksissa oli run- saasti CuEQTA . Perinteisesti turvemaiden on sanottu tarvit- sevan kuparilannoitusta. Vaikka tässäkin aineistossa mo- nessa turvemaassa kuparin kokonaismäärä oli pieni, oli useimmissa turvemaissa melko runsaasti Cuc„T> . TästäEDTA syystä kuparilannoitus ei saa millään maalajilla olla auto- maattinen viljelytoimi vaan sen on perustuttava maa-ana- lyysiin. 495 Agricultural Science in Finland 3 (1994)