Copper in cultivated soils of 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 2S0 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 Cutol 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) + Cuox) were more plentiful than those of Zn (Zn +Zn ), An acetic acid ammonium acetate Na.EDTA solution v py ox 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.

The average percentages of Cu py and Cu ox were 18% and 12% of Cu tol in mineral soils and 34% and 19% of Cu tol 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 acidammonium acetate -Na.EDTA solution v py ox 7 2 used in routine soil testing extracted 56% and 71% of the sum of Cu py + Cu ox in mineral and organogenic soils, respectively. In soil profiles, Cu EDTA was higher in the plough layer than in the subsoil but a few soils rich in Cu 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 industrial areas of Finland have been analyzed for the fractions of Cu (Räisänen and Hämäläinen 1991) but the fractional distribution of Cu in cultivated soils of the country is unknown.
An ammonium acetateacetic acid -Na 2 EDTA solution (AAAc-EDTA, pH 4.65) is used to extract Cu in soil testing in Finland. Recently, Jokinen et al. (1993) found that this extractant dis-solved 40% of total Cu in organogenic soils. However, it is not known, either in organogenic or mineral soils, to what extent the secondary reserves, the potential source of plant-available Cu, are extracted by this solution. This information would be important in order to be able to translate the soil testing results into a quantitative estimate 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 fractions. The fractions of soil Cu were also compared 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). However, 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 investigation.
To determine total Cu, the soil was digested with a mixture of HNO,, HCI0  (Lakanen and Erviö 1971), which is the method used in soil testing in Finland.

Total copper
In mineral soils, total Cu (Cu toi , mg kg -1 ) increased with increasing clay content (r = o.B7'*'). In a few heavy clay soils, Cu io| 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 the tot number of very low contents of Cu| t was higher among the peat soils. When expressing the results as milligrams per dm 3 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, Cu tot (mg dnr 3 ) decreased with increasing organic C (r = -0.52").

Fractions of soil copper
In the 16 representative soil samples, Cu tot and the sum of the fractions (Cu +Cu +Cu )  were 28.9 mg kg~' and 33.0 mg kg ', respectively. 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 determination of Cu was discontinued and the rest of the res results of Cu were calculated as the difference res Cu -(Cu +Cu ).
lot v py ox 7 The concentration of Cu extracted with pyrophosphate (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 dnr 3 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 Cu py ; only in 13 soils was Cu equal to or higher than Cu . Cu was at ox°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 Cu to| (Table 2), while Cu <>x correlated with Cu , Cu rps , clay and poorly crystalline 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 Cu py and Cu <x correlated most closely (P = 0.001) with Cu  (Niskanen 1989). *, ",'" Significant at P = 0.05, 0.01 and 0.001, respectively. Not significant (P > 0.05). 1 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.
In mineral soils, 29% of Cu occurred in the 7 tot secondary fractions (Cu py , Cu ox ), while these fractions constituted 53% of Cu in the organogenic soils (Table 3). Even though in some organogenic soils more than half of soil Cu was in the form of Cu ,Cu was usually relatively the most py 7 res J J abundant fraction in both soil groups. In mineral soils, the percentage of Cu py correlated weakly (r = o.39***) with organic C content.
Copper extracted with AAAc-EDTA Copper extracted with AAAc-EDTA (Cu EDTA , mg dm -3 ) was highest in clay and mull soils ( Table 1). The lowest result (0.9 mg dnr 3 ) occurred in a Sphagnum peat soil which had been cultivated for five years. Cu EDTA constituted on average 16% of Cu tm 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 + Cu ox , mg dm -3 ), respectively. Cu EDTA correlated most strongly with Cu py and Cu ox , in organogenic soils also with Cu tot (Table 4). According to the regression analysis, According to the (3 coefficients (Table 5), Cu py was the dominant soil factor explaining the variation of Cu EDTA in both soil groups. In organogenic soils, Cu ox and Al ox appeared to be slightly more important variables than in mineral soils, but this conclusion becomes less reliable due to the small number of organogenic soils in the material. Poorly crystalline Fe oxide (Fe ox ) correlated with the secondary Cu fractions (Cu py , Cu ox ) and therefore Fe ox was not a statistically significant variable with Cu and Cu , whether or not py ox' Al >x was in the equation. In organogenic soils there was a negative correlation (r = -0.45**) between 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 decreases 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 Cu lo( (> 100 mg kg" 1 ) of the entire material. Within the fine-textured mineral soil profiles P 1, P 2 and P 3, Cu tot increased with increasing clay content 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 profiles 1,3, 5 and 6, Cu EDTA was highest in the deepest layers, two-to-four times that in the plough layer while in the three remaining profiles, Cu E dta was highest in the plough layer.
In 14 sample pairs consisting of the plough layer (A p ) and the subsoil (B) sample, Cu EDTA was significantly higher (t = 3.375** in the t test for paired measurements) in the plough layer. The means and ranges were as follows: Range Mean 5.0 2.2-8.9 A p 2.7 0.6-6.2 B There were three pairs in which Cu EDTA in the subsoil was equal to or slightly lower (0.3-1.0 mg dnr 3 ) than in the plough layer. Of 15 sample pairs one pair not included in the above means had a heavy clay subsoil richer in Cu EDTA (18.4 mg dm ') than the organogenic plough layer (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 Zn tm (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 ', Zn u| 41 mg kg -'). In two thirds of the soils the reserves of Cu in the secondary fractions (Cu py + Cu ox ) 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 between 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 pointed out that in organogenic soils the correlation coefficients between Cu and Zn as well as bepy py tween Cu EDTA and Zn EDTA 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 Cu tot 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 Cu tot 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 Cu crv" EDTA than those of Jokinen and Tähtinen (1987) who deliberately included soils where plants had shown symptoms of Cu deficiency. The mean Cu E[)TA was also higher than in some other research materials (2.8 mg dm 3 , Tares 1978, Sillanpää 1982). Like in Sippola and Tares (1978), Cu EDTA was higher in clay soils than in the other mineral soil classes. In organogenic soils,   mean Cu EDTA was similar to that reported by Jokinen et al. (1993). The present soils also exhibited nearly the average Cu EDTA reported in routine soil testing in 1986-1988 in over 60000 samples of mineral soils coarser than silt (mean 4.2 mg dm -3 ) and in over 25000 organogenic soils (mean 5.2 mg dnr 3 ). The material of this study represents fairly well the average cultivated soils of Finland, even though there was only one soil classified as 'poor' in Cu (Cu EDTA below 1 mg dnr 3 ) according to the interpretation of Sillanpää (1982).
The fraction of water-soluble and exchangeable Cu is too small to satisfy the needs of the plants (McLaren andCrawford 1973, Liang et al. 1991), and this readily plant-available form is replenished from other secondary fractions, especially from that bound by organic matter (Liang et al. 1991). It was therefore considered appropriate in this study to include water-soluble, exchangeable and specifically adsorbed Cu, together with Cu bound mainly by organic matter, in Cu p> and not to extract them separately as is commonly done in fractionation procedures. Cu py , expressed as percentages of Cu |oi , was in mineral soils at the same level as the sum of water-soluble, exchangeable, specifically adsorbed and organically bound Cu in soils of Saskatchewan, Canada (18.4% of Cu , Liang et al. 1991). Also v 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 andCrawford 1973, Shuman 1985), the relative sizes of the secondary 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 percentage of Cu res and the higher ones of Cu py and Cu ox as compared to the mineral soils can be explained by the smaller quantity of mineral material, the source of Cu . res res Copper extracted with AAAc-EDTA has correlated rather closely with Cu supply to plants in pot experiments (Sillanpää 1982, Erviö andSippola 1993). On the basis of the observation that the content of Cu py explained a great deal of the variation of Cu EDTA 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 according to McLaren and Crawford (1973) Cu ox is of minor importance as a source of plantavailable Cu. The latter assumption is supported also by the results of the present study where Cu ox relatively poorly explained the variation of U EDTA" 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 Cu tot , 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 extracted a higher proportion of the potentially plantavailable Cu (Cu +Cu ) than in mineral soils. v py ox' Thus, low Cu EDTA in organogenic soils implies for certain a scarcity of Cu and a probable requirement of Cu fertilization.
A higher Cu EDTA in the plough layer, as compared 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 1982). On the other hand, investigation of the soil profiles revealed that soils rich in Cu had abundant reserves of Cu EDTA 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 weatherable Cu-containing minerals. A similar vertical distribution of Zn crvTA has earlier been observed EDTA in the same soil profiles (Yli-Halla 1993).
In a recent study, carried out with the same soil samples (Yli-Halla 1993) 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. (1990Liang et al. ( , 1991. The difference between the distribution 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 residual tot fraction. According to Mullins et al. (1982), fertilizer Cu and Zn are accumulated in forms extractable with pyrophosphate and oxalate. The relative abundance of secondary Cu fractions as compared to those of Zn as well as the poor correlation between the fractions of Cu and Zn in organogenic soils can partly be explained by addi-tions in Cu fertilizers, applied commonly in Finland since the 1950'5. Zinc fertilization, as a rarer and a more recent practice, has probably contributed to a smaller increase in soil Zn content. Ample application of fertilizer Cu may also explain why even Cu tot was equal to Zn in organogenic 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 abundance 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 concentration of Cu in cereal crops and 3-6 times that in forage crops. However, the size of the secondary 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.