Coca-Colaā (CC) Extraction for the Evaluation of Available
Micro-Nutrients in Soils for Oilseed Rape
Haneklaus, S., Fleckenstein, J. and Schnug, E.
Institute of Plant Nutrition and Soil Science, Federal Agricultural Research Center Braunschweig-Voelkenrode, Bundesallee 50, D-38116 Braunschweig, Germany, e-mail: ewald.schnug@fal.de
Coca-ColaÓ (CC) is not only ubiquitous and perhaps the most popular beverage in the world but also an excellent extractant for the determination of available micro-nutrients in soils with the advantages of a ready to use extraction solution, an environmentally friendly extractant, a safe handling and worldwide availability in comparison with traditional chemicals. It was the aim of the investigations presented in this contribution to determine available micro-nutrient concentrations in soils by different extraction methods and to derive critical values for available Fe, Mn, Zn and Cu contents in soils for oilseed rape.
Keywords: Coca-ColaÓ, soil analysis
Soil analysis is a common way to determine the micro-nutrient supply of oilseed rape and various methods are employed for individual elements in different countries. The use of CC as an extractant requires a high quality standard of the CC product in terms of chemical composition which is achieved by a controlled and standardized production process. The main ingredients of CC are highly purified water, phosphoric acid, sugar and carbon dioxide. Extensive research on samples from all over the world showed that the composition of the major constituents in CC was very consistent (Schnug et al., 1998). Methods of soil analysis are generally validated setting soil analysis data in relation to plant analysis data or crop yield. CC extraction provided closer correlations between soil and Mn in wheat than traditional standard methods (Schnug et al., 1996). In this contribution the results of the determination of plant available Fe, Mn, Zn and Cu contents in the soils extracted with DTPA, AAAc-EDTA and CC and critical values for available Fe, Mn, Zn and Cu contents in the soil are presented for oilseed rape.
Field surveys were conducted on the Isle of Rugen in Mecklenberg-Vorporren in 1993 and 1994 during which a total 420 plant and soil samples were taken. Allocation of and navigation to sampling positions was carried out by a Global Positioning System (GPS). This enabled the allocation of sampling positions chosen in 1993 and thus, in 1994, sampling of neighboring fields where oilseed rape was grown. Both plant and soil material were taken in a representative area of the field of about 100 m2.
Plant analysis: Young, fully differentiated leaves of oilseed rape were taken at the beginning of stem elongation. The plant material was dried in a ventilated oven at 85° C until constant weight was achieved. The plant samples were finely ground to a particle size < 0.12 mm. For the determination of micro-nutrients (iron (Fe), manganese (Mn), zinc (Zn), and copper (Cu)), a dry ashing procedure (470°C) was applied to the plant material (Schnug & Haneklaus, 1996). The final elemental determination was carried out by AAS and ICP-OES.
Soil analysis: Top soil samples (0 - 0.2 m) were air dried, passed through a sieve of 2-mm mesh width and homogenized. Available Fe, Mn, Zn and Cu were extracted in CC (Schnug et al., 1996), 0.5N NH4-Ac + 0.02M EDTA at pH 4.65 (AAAc-EDTA; (Cottenie, 1980)) and DTPA (Lindsay & Norvell, 1978).
The results of plant analysis revealed that Mn and Zn were minimum factors in the mineral nutrition of oilseed rape (Table 1). In 1994, 54% of all samples had Zn contents below the critical nutrient value of 33 µg g-1 Zn which is necessary in order to obtain maximum yield (Schnug and Haneklaus, 1992). Differences in the Fe, Zn and Cu concentrations in the leaf tissue of oilseed rape were significant (t-test) between the two years of investigation.
Table 1. Micro-nutritional status of oilseed rape on the Isle of Rugen (n=210).
|
Element |
CNV1 |
1993 |
1994 |
||||||
|
|
|
mean |
min |
max |
share of samples <CNV (%) |
mean |
min |
max |
share of samples <CNV (%) |
|
Fe |
50 |
97 |
52 |
149 |
- |
88 |
63 |
132 |
- |
|
Mn |
30 |
61 |
26 |
176 |
4 |
63 |
22 |
183 |
3 |
|
Zn |
33 |
43 |
27 |
73 |
11 |
33 |
19 |
50 |
54 |
|
Cu |
4.5 |
10.2 |
5.4 |
15.3 |
- |
9.2 |
5.7 |
13.8 |
- |
|
note: 1CNV = critical nutrient value (µg g-1) for achieving maximum yield (5 t ha-1; Schnug and Haneklaus, 1992)) |
|||||||||
DTPA, AAAc-EDTA and CC are well acknowledged agents for the determination of plant available micro-nutrients in soils (Lindsay and Norvell, 1978; Sillanpää, 1990; Schnug et al., 1996). The descriptive results of these methods applied to a total set of 220 inceptisol samples from 1993 and 1994 on the Isle of Rugen are shown in Table 2.
Table 2. Descriptive statistics for DTPA, AAAc-EDTA and CC extractable micro-nutrients in the soil on the Isle of Rugen (n=210).
|
1993 |
Fe |
Mn |
Zn |
Cu |
||||||||
|
|
DTPA |
AAAc- EDTA |
CC |
DTPA |
AAAc- EDTA |
CC |
DTPA |
AAAc- EDTA |
CC |
DTPA |
AAAc- EDTA |
CC |
|
mean |
71.0 |
236.4 |
5.7 |
15.1 |
92.1 |
26.7 |
1.2 |
2.4 |
0.5 |
0.8 |
1.7 |
0.18 |
|
min |
16.9 |
43.7 |
0.2 |
3.1 |
18.4 |
1.1 |
0.1 |
0.9 |
0.1 |
0.1 |
0.2 |
0.10 |
|
max |
204 |
708.5 |
12.6 |
38.6 |
209.5 |
65.4 |
3.7 |
6.3 |
1.4 |
1.7 |
4.3 |
0.41 |
|
CV (%) |
49.8 |
56.6 |
36.5 |
46.7 |
48.2 |
44.0 |
48.5 |
47.4 |
57.8 |
41.8 |
42.0 |
36.2 |
|
1994 |
Fe |
Mn |
Zn |
Cu |
||||||||
|
|
DTPA |
AAAc- EDTA |
CC |
DTPA |
AAAc- EDTA |
CC |
DTPA |
AAAc- EDTA |
CC |
DTPA |
AAAc- EDTA |
CC |
|
mean |
70.0 |
230.7 |
7.9 |
10.4 |
97.3 |
25.2 |
1.5 |
2.8 |
0.6 |
0.8 |
1.8 |
0.23 |
|
min |
23.6 |
16.5 |
3.2 |
1.5 |
6.6 |
1.2 |
0.5 |
0.7 |
0.1 |
0.2 |
0.5 |
0.11 |
|
max |
166.4 |
791.2 |
16.1 |
24.1 |
244.4 |
70.4 |
6.0 |
9.5 |
1.8 |
1.7 |
4.5 |
0.40 |
|
CV (%) |
41.2 |
67.4 |
28.8 |
48.6 |
53.3 |
52.5 |
61.6 |
61.3 |
68.4 |
41.1 |
40.0 |
29.9 |
The mean Mn and Zn contents extracted by DTPA were significantly (t-test) higher and lower respectively in 1993 than in 1994. The mean Fe and Cu contents extracted by CC were significantly (t-test) higher in 1994 than in 1993. The mean amounts of Mn (10.4 and 15.1 µg g-1 Mn) extracted by DTPA are in agreement with values determined by Schnug et al. (1996) in the same landscape and those of Flueh (1988), Schnug and Pissarek (1981) in Schleswig-Holstein. The mean amounts of Fe, Zn and Cu extracted by DTPA (Table 2) were lower than those determined by Schnug and Pissarek (1981) in Schleswig-Holstein (2.5 µg g-1 Zn; 96.8 µg g-1 Fe; 1.2 µg g-1 Cu).
The relationship between plant available micro-nutrient contents in the soil and plant nutrient status was established for the different extraction methods in order to derive critical nutrient thresholds in soils (Table 3).
Table 3. Correlation coefficients for the relationships between available Fe, Mn, Zn and Cu contents in soil extracted by DTPA, AAC and CC and pH and total Fe, Mn, Zn and Cu concentrations in the younger, fully differentiated leaves of oilseed rape.
|
1993 |
DTPA |
AAAc-EDTA |
CC |
pH |
Fe |
0.009 |
-3.5e-4 |
0.076 |
-0.166 |
|
Mn |
0.131 |
-0.242* |
0.221 |
-0.563*** |
|
Zn |
-0.007 |
-0.131 |
0.337** |
-0.234* |
|
Cu |
0.073 |
0.186 |
0.113 |
-0.072 |
|
1994 |
DTPA |
AAAc-EDTA |
CC |
pH |
Fe |
0.165 |
0.149 |
0.201 |
-0.155 |
|
Mn |
0.537*** |
0.136 |
0.470*** |
-0.358*** |
|
Zn |
0.034 |
0.071 |
-0.080 |
0.083 |
|
Cu |
0.031 |
0.015 |
0.031 |
-0.141 |
|
note: *** = p <0.001; ** 0 p < 0.01; * p < 0.05; all other p > 0.05 |
||||
The results were not consistent in the two years of investigations as DTPA and CC showed a positive correlation to the plant Mn status only in 1994 (Table 3). The correlation coefficients of both relationships were much lower than those determined for wheat on the Isle of Rugen by Schnug et al. (1996). This is probably related to the higher Mn uptake efficiency of oilseed rape in comparison with winter wheat. In 1993 soil pH was strongly related to the Mn content in the plant while this effect was less so strong in 1994 (Table 3). In 1993 only CC was positively correlated with the Zn supply of the oilseed rape crop. AAAc-EDTA extraction was inversely related to the Mn status of the crop in 1993 (Table 3); this pseudo-relationship was most likely due to the 3.5 to 9 times higher extraction force of AAAc than CC and DTPA (Table 2).
Multiple regression analysis revealed that the integration of soil pH next to plant available Mn and Zn contents in the soil, respectively extracted by DTPA and CC improved the relationship to the plant Mn and Zn status (Table 4).
Table 4. Polynomial and multiple regression equations for the relationship between plant available Mn and Zn extracted by CC and DTPA and pH in soil and plant nutrient contents.
|
|
Regression equations |
Correlation coefficient (r) |
|
1993 |
Zn-P1 = 11.040*CC + 38.111 |
0.337** |
|
|
Zn-P = 6.977*CC 3.80*pH + 61.252 |
0.361** |
|
1994 |
Mn-P = 3.144*DTPA + 30.215 |
0.537*** |
|
|
Mn-P = 2.734*DTPA 11.085*pH + 99.721 |
0.562*** |
|
1994 |
Mn-P = 1.067*CC + 36.359 |
0.470*** |
|
|
Mn-P = 0.877*CC 11.681*pH + 109.889 |
0.500*** |
|
note: 1Zn-P = Zn concentration in the plant tissue (µg g-1) |
||
Critical values for plant available Mn and Zn by DTPA, CC and AAAc-EDTA extraction were calculated employing the critical nutrient values of plant analysis (see Table 1) and polynomial regression equations obtained from the actual data set (Table 4). The critical values for plant available Mn and Zn in the investigated inceptisols are 18 µg g-1 Mn and 0.4 µg g-1 Zn for CC and 7.4 µg g-1 Mn for DTPA. While the threshold for Mn extracted by CC is the same as for winter wheat, the corresponding value for Zn is about three times lower (Schnug et al., 1996).
Conclusion
The results of the field surveys conducted on the Isle of Rugen revealed that Mn and Zn were minimum factors in the mineral nutrition of oilseed rape. While only 3 to 4% of all plant samples showed an insufficient Mn supply in the two years of investigations, 1993 and 1994, respectively, 11 and 54% of all samples had Zn concentrations below the critical nutrient value of 33 µg g-1 required in order to achieve maximum yield. The relationship between soil analysis and plant nutrient status was less clearly defined from the data presented here and the use of this procedure alone may result in errors if used for the evaluation of the micro-nutrient supply of oilseed rape. Including soil pH values in addition to the available micro-nutrient contents during multiple regression analysis improved the correlation with the plant nutrient status. Thus a correction of soil analysis data for pH as recommended by Sillanpää (1980) could be applied to improve the expressiveness of the results. An important aspect of substituting traditional extraction solutions in favor of CC could be time and costs. Both are distinctively higher for chemical extractants than for CC because they require the purchase of pro analysi chemicals, the careful preparation of the extraction solution and its regular disposal.
Acknowledgements
The authors cordially thank Dr. R. Walker (SAC, Aberdeen) for improving the language of this paper.
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