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Copper levels in South African nursery soils: Possible effects on the propagation of grapevines
Kobus Conradie *
Key words: Copper, toxicity, nursery soils, grapevine, percentage take.
INTRODUCTION
Fungicides that contain copper are commonly used in the South African nursery industry. Ongoing use of such products, a large portion of which land directly on the soil, may lead to a gradual accumulation of copper levels in nursery soils. High copper levels are known to be toxic to the root development of plants (Reuther & Labanauskas, 1966; Mortvedt et al., 1991; Mozaffari et al., 1996). For citrus, root development may be impeded at levels above 25 mg/kg (Mozaffari et al., 1996), with critical limits for maize and rice at 17 mg/kg and 13 mg/kg respectively (Borkert et al., 1998). Copper toxicities have seldom been reported in production vineyards, but in Spain high copper levels have been found to affect the establishment of new plantings adversely (Giovannini, 1997). Copper toxicity could therefore be especially harmful in nurseries, because poor root development may also have a negative effect on callus formation and percentage take. An investigation into copper levels of South African nursery soils has not been undertaken previously, but values up to 27 mg/kg have already been found in production vineyards (Van Huyssteen et al., 1997). In the "traditional" nursery areas, where grapevines have been propagated on the same soils for 50 years (or longer), high values are to be expected. The precise level at which copper becomes toxic to root development in vines remains unknown.
The first aim of this study was to determine the levels of copper (Cu) that exist in nursery soils, and to investigate the possibility of a link between copper content, callus formation, root development and percentage take. Secondly an attempt was made to identify the exact level at which copper becomes toxic in nursery soils.
MATERIAL AND METHODS
In the first phase of the investigation (1999/2000) 44 nursery soils, randomly distributed over the Western Cape, were sampled to a depth of 300 mm to determine copper content. This data was used to identify 22 representative nursery soils (high, medium and low copper content). Complete chemical analyses were done for these soils. Graftlings were planted according to standard nursery practices, whereafter a "trial site" containing 50 grapevines was identified. The rootstock cultivar was either 99 Richter, or 110 Richter, or 101-14 Mgt. The scion was usually Shiraz or Merlot, but one site each of Colombar, Sauvignon blanc and Pinotage was also included. Leaves were sampled for analysis in February. At the end of the growing season the 50 trial vines at each site were taken out of the soil and analysed for percentage take, shoot and root mass. Leaves and roots were also analysed for mineral composition.
In the 2000/2001 season Chenin blanc (clone SN 1061) was grafted onto 99 Richter (clone RY 13A) according to standard Nietvoorbij procedures and stored in a callus room for 4 weeks. The vines were subsequently planted in 8 L plastic bags in a tunnel. Organic sand with 3% clay was used as a growth medium. The pH (KCL) of the medium (5.6) was comparable to that of the nursery soils used in the preliminary study, while phosphate (30 mg/kg) and potassium (100 mg/kg) levels were sufficient. Copper levels were adjusted to 10, 20, 30, 40, 50 and 100 mg/kg respectively before planting, by making use of copper oxychloride. A concrete mixer was used to ensure proper mixing of soil and copper oxychloride. A control, to which no copper was added, was also included. Each treatment was replicated five times, with 8 vines per replication (therefore 40 vines per treatment). Vines were regularly fed with Polyfeed. The rate of shoot growth was measured every two weeks. Leaves were sampled twice (March and May) for analysis. Vines were uprooted at the end of the season (June). Percentage take was determined, as well as shoot and root mass. Shoots, roots and soils were analysed for mineral composition.
RESULTS AND DISCUSSION
Soil analyses for 44 nursery soils investigated in the first phase: In some cases soils that were sampled had been used to propagate vines for more than 30 years, in other cases soils had been used for less than five years. Producers could not always provide exact information about the number of years that vines had been propagated on specific soils. The copper levels of the soils were usually below 10 mg/kg, while values of 15-20 mg/kg were only observed in two cases (Table 1). In the latter instances vines had been propagated on the soils in question for more than 30 years. Values above 20 mg/kg were not observed in any of the instances. It was obviously not possible to sample all South African nursery soils in this study, but the values in Table 1 should be fairly representative of the existing situation. In essence it boils down to the fact that high copper levels could occur in certain nursery soils, but such high values should not be common.
Table 1. Copper levels in
selected South African nursery soils. Soil
depth (mm) Number of
soils in specific class (n = 44) I (0 - 5
mg/kg) II (5 - 10
mg/kg) III (10 - 15
mg/kg) IV (15 - 20
mg/kg) 0 – 150 150 - 300 22 24 18 14 2 4 2 2
Plant reaction to copper in 22 nursery soils where monitoring was done: Percentage take (not shown) was high throughout, while no correlation between the copper content of soils and percentage take could be observed. On the contrary, for the soil with the highest copper content a very high percentage take (86%) was observed. However, root mass tended to be a reduced when soil Cu increased. Even though the copper content of soils used in this study was mostly below 10 mg/kg (Table 1), it still appeared to have an impact on root development in certain instances. In a multiple regression, done for soil analyses vs root mass, copper in the 150-300 mm soil layer was identified as the factor that could explain most (36%) of the variation in root mass. It must be emphasised, however, that root mass was nowhere reduced to unacceptably low levels, and that the vines could mostly be classified as Class I. Moreover root mass was also lower in soils with high resistance/high calcium content/high magnesium content. Such soils, the clay content of which was generally also relatively high, occurred mainly outside the Wellington vicinity (Swellendam and Bonnievale). Inferior root development may therefore also have been caused by other factors such as soil structure, soil chemistry and/or cultivation practices.
Levels of leaf Cu, as well as root Cu, were relatively low and seldom exceeded 25 mg/kg (Table 2). Despite the low levels, soil Cu correlated significantly with root Cu, as well as leaf Cu. In a multiple regression the variations in copper content of leaves could largely (68%) be explained by variations in copper content of topsoils (0-300 mm). Consequently it looked as though the copper absorbed by the grapevine was not contained in the roots only, but also distributed to the leaves. It is also known, however, that copper oxychloride, applied as leaf sprays, may attach itself very strongly to the leaf surface, without necessarily being "absorbed". High leaf Cu, for soils with high copper contents, could therefore also have been caused by spraying residues. Uptake of copper was moreover higher in soils with higher resistance (i.e. lower salt content) and lower for soils with higher sodium contents. Essentially it meant that Cu uptake will be lower for brackish soils with high sodium contents. It did not look as if soil pH as such had a big effect on the uptake of copper, although the uptake of phosphate, manganese and zinc in soils with a higher pH was impeded. In this regard it is important to note that soil pH varied only from 4.7 to 6.7 in the current investigation. Uptake of copper will definitely be inhibited at higher pH levels (Reuther & Labanauskas, 1966).
On the whole the phosphate content of the nursery soils was high, with values often exceeding 100 mg/kg. In the case of 110 Richter especially, when grafted to Merlot, leaf analyses indicated excessive uptake of phosphate. There was no clear indication that the uptake of other elements, such as copper and zinc, was suppressed by the high levels of phosphate, but in most instances phosphate fertilisation could be downscaled fruitfully.
Table
2. Copper levels in roots and leaves of grapevines in South African
nurseries. Organ Nubmer
of samples in specific class (n = 22) I (5 - 10
mg/kg) II (10 –
15 mg/kg) III (15 - 20
mg/kg) IV (20 - 25
mg/kg) V (>
25 mg/kg) Roots Leaves 5 13 9 4 4 1 3 1 1 3
Tunnel study: Graftlings planted in the tunnel took well (Table 3). The rate of shoot growth, percentage take, shoot mass and root mass did not differ between treatments, but root Cu correlated with soil Cu. However, soil Cu had no effect whatsoever on copper levels in shoots and/or leaves. It appeared, therefore, as though high levels of soil Cu did result in an accumulation of copper in the roots, but in this study copper apparently did not translocate to the above-ground organs. This result therefore stands in marked contrast to that of the field trial, which revealed a link between soil Cu and leaf Cu. Seeing that no copper was sprayed in the tunnel study, the high values of the field trial can probably be ascribed to spraying residues. Fruthermore, the field trial was conducted on a large variety of soil forms, which meant that factors such as salinity, soil acidity and texture could also have had an effect on copper status. High levels of soil Cu may also disturb existing populations of soil microbes, which in turn could impact on root development. Such disturbance of populations will occur over a long period of time, suggesting that it may not have occurred in the case of the tunnel study (where copper levels were only adjusted shortly before the start of the experiment). Increased soil Cu reduced the iron content of leaves and shoots, especially where copper levels exceeded 50 mg/kg. Similar trends have been reported in the past (Reuther & Labanauskas, 1966). Although no significant differences in uptake of the other macro and micro-elements could be observed (not indicated), copper levels in excess of 50 mg/kg should thus be considered as a potential problem.
Table 3. Effect of
different soil Cu levels on percentage take, shoot mass, root mass,
as well as on copper and iron content of nursery grapevines. Copper
content of soil (mg/kg) Percentage
take (%) Shoot mass
(g/vine) Root mass
(g/vine) Copper
content
(mg/kg) Iron
content (mg/kg) Roots Leaves Shoots Leaves Shoots 0.5 11.7 22.5 31.3 39.3 52.3 102.0 94 a (1) 86 a 89 a 81 a 83 a 89 a 94 a 12.2
a 14.2
a 13.5
a 19.7
a 14.4
a 12.9
a 13.4
a 26.5
a 32.7
a 32.9
a 39.4
a 28.8
a 36.6
a 34.4
a 15.6
a 37.6
ab 58.0
ab 84.4
bc 126.4
cd 139.2
d 205.0
e 4.4
a 4.6
a 4.6
a 4.0
a 4.7
a 4.8
a 4.1
a 9.2
a 6.4
a 8.6
a 6.2
a 7.0
a 8.0
a 7.2
a 305
a 305
a 303
a 266
ab 252
ab 256
ab 208
b 60
a 55
a 51
a 49
a 51
a 49
a 29
b
CONCLUSIONS
Copper levels in South African nursery soils rarely exceed 20 mg/kg. At these concentrations percentage take and/or callus formation are not affected in commercial nurseries. It is possible, however, that root development might be impeded at levels as low as 10 mg/kg under certain propagation conditions. An increase in soil pH might limit the negative effects of high copper levels. However it looks as though pH (KCL) will have to be increased to at least 6.5 before a positive effect might be expected, thus resulting in the uptake of other trace elements such as zinc being impeded.
In a tunnel study, where soil copper was adjusted to levels up to 100 mg/kg, high levels resulted in an increase in root copper and a reduction in leaf Fe, but percentage take, root development and leaf copper were not affected. A specific level for copper toxicity, above which plant growth will be negatively affected, was therefore not obtained. The effect of high levels of soil copper on populations of soil microbes deserves further investigation.
REFERENCES
Borkert, C.M., Cox, F.R. & Tucker, M.R., 1998. Zinc and copper toxicity in peanut, soybean, rice, and corn in soil mixtures. Comm. Soil Sci. Plant Anal. 29, 2991-3005.
Giovannini, E., 1997. Toxidez por combre em vinhedos. Pesq. Agrop. Ga£cha 3, 115-117.
Mortvedt, J.J., Cox, F.R., Shuman, L.M. & Welch, R.M., 1991. SSSA Book Series 4. Micronutrients in Agriculture. Soil Science Society of America, Madison. 760 pp.
Mozaffari, M., Alva., A.K. & Chen, E.Q., 1996. Relation of copper extractable from soil and pH to copper content and growth of two citrus rootstocks. Soil Sci. 11, 786-792.
Reuther, W. & Labanauskas, C.K., 1966. Copper. In: H.R. Chapman (ed.). Diagnostic criteria for plants and soils. University of California, Division of Agricultural Sciences, 157-179.
Van Huyssteen, I., Louw, P.J.E. & Conradie, W.J., 1997. The effect of copper sprays on the copper content of soil and grape must - a case study. Proceedings of 21st Congress of South African Society for Enology and Viticulture, 27 - 28 November, Cape Town. 1p.
ACKNOWLEDGEMENT
Partial funding of this investigation by Winetech, technical input by Isabella van Huyssteen and the rest of the Soil Science division, as well as the collaboration of participating producers are much appreciated.
Further enquiries may be addressed to: Dr Kobus Conradie, tel (021) 809-3025, email kobusc@infruit.agric.za.
Summary
Uittreksel
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