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The influence of Calcium and Magnesium soil applications on the performance and Potassium uptake of Vitis vinifera L. cvs. Cabernet Sauvignon and Cabernet franc in the Paardeberg area
Gerhard Engelbrecht1 & Dawid Saayman2
INTRODUCTION
The effect of fertilisation on the performance of grapevines has been researched by various authors (Somers, 1975; Conradie & Saayman, 1989a; Conradie & Saayman, 1989b; Ruhl, Fuda & Treeby, 1992; Jackson & Lombard, 1993; Champagnol,1994; Etourneaud, 1996; Gallego, 1999; Daverede & Garcia, 2000). One of the aspects of grapevine performance is the quality of juice and wine produced by the grapevine. According to Boulton (1980a) one of the most important factors in the determination of the quality of juice and wine is pH. However, the pH of wines is sometimes excessively high, throughout the world and also in the Western Cape, and little is known about the extent to which this may be influenced by fertilisation. This investigation studied the effect of different Ca and Mg soil applications on the performance of Cabernet Sauvignon and Cabernet franc and the pH of juice, specifically with regard to soils of granitic origin, inter alia, since there are indications that granite as mother material may give rise to high K levels in plants (Wooldridge, 1985).
MATERIAL AND METHODS
Experimental procedure
A random block design field trial, with 12 factorial treatment combinations and two repetitions, was conducted in the Paardeberg area on each of two farms, viz. Meerlus and Kersfontein. According to Siegfried (1993) Paardeberg forms part of the Malmesbury Batholith, which consists of six different granites and a quartz porphyry gangue. The farms were selected on the premise that the mother material of the soils would most likely be granite. On Meerlus a five-year-old Vitis vinifera L. var. Cabernet franc vineyard, grafted onto 99 Richter (Vitis Berlandieri var. Las Sorres x Vitis rupestris var. du Lot) was used, and on Kersfontein a five-year-old Vitis vinifera L. var. Cabernet Sauvignon vineyard, grafted onto 101-14 Mgt. (Vitis riparia x Vitis rupestris). Two profile pits on each of Meerlus and Kersfontein were used for the typing of the soils.
Treatments
Although three canopy management and four chemical soil applications took place, this report is concerned only with the latter. Soil applications of 5 t ha-1 CaSO4.2H2O and equivalent quantities of Ca(OH)2 and MgSO4.7H2O, with a control treatment of no applications, were given once during February 1998 (two weeks before the harvest). These treatments were scattered evenly over the surface and worked into the soil with spades to a depth of 10 cm, whereafter 20 mm irrigation was given.
Sampling
One year after the application of the chemical soil treatments, an auger was used to take soil samples for each site at 0-30 cm, 30-60 cm and 60-90 cm depths. These samples were analysed for applicable characteristics using standard analytical methods of the Soil Science Department at Stellenbosch University. Soil descriptions were given of two profile pits made in each vineyard.
Bunches were counted and weighed and representative berry samples taken for skin and juice analyses. Some of the berries were crushed by hand and the juice allowed to remain in contact with the skins and lees at 15oC for 24 hours (skin contact juice), whereafter the juice was separated from the remainder of the berries. The juice was analysed for applicable characteristics using standard methods of the Viticulture and Oenology Department.
Canopy measurements
Canopy density measurements were taken using the point-quadrant method and light measurement within and outside the canopy using a LI-COR 191 SA "Line Quantum Sensor" light meter. Total length per shoot (main shoot length plus lateral shoot length) of eight randomly selected shoots per site was measured after v‚raison.
Soil
The soil of the Meerlus vineyard was identified as yellow brown, gravelly (80%) Oakleaf/Tukulu soils, with respectively 6% and 9% clay in the A and B horizons, underlaid at 1050 mm by heavier texture (60% clay) material. On Kersfontein a yellow brown Oakleaf soil, with respectively 16% and 34% clay in the A and B horizons, was underlaid by lighter texture (25% clay, 25-30% fine gravel) material.
Only the Ca(OH)2 application increased the pH of the 0-30 cm soil layer significantly (Table 1), while at the same time reducing the extractable H thereof (Table 3). Although Ca(OH)2 is the most motile liming matter (Kotze & Joubert, 1978), in this investigation it only increased the pH of the 0-30 cm soil layer significantly. This corresponds with the findings of Kotze & Deist (1975), indicating that the effect of Ca(OH)2 on pH is limited mainly to the depth of application.
Calcium sulphate significantly reduced the pH(H2O) of the 0-30 cm soil layer. This can possibly be attributed to a higher salt concentration in the soil solution that was thus obtained, as indicated by the significantly lower Rs. It is known that an added salt is able to displace nitrogen ions from the soil colloid in order to reduce the soil pH (Mengel & Kirkby, 1987a).
According to Table 2 the significant increase in extractable Ca effected by CaSO4 and Ca(OH)2, was also limited to the 0-30 cm soil layer. On the other hand MgSO4 resulted in a significant increase in extractable Mg even in the 60-90 cm soil layer on Kersfontein. Possible explanations for the differences in depth between the Mg and Ca movements, are the higher solubility of MgSO4 compared to CaSO4 and Ca(OH)2 (Hodgman, 1950) and the weaker adsorption of Mg to the soil colloid, compared to Ca (Mengel & Kirkby, 1987a). Both these factors will play a role in making Mg more motile in the soil than Ca. It is not known, however, why MgSO4 significantly increased the extractable Mg to a depth of 60-90 cm on Kersfontein only, despite a higher clay content in the soil than Meerlus.
Only the MgSO4 soil application, and only on Kersfontein, was able to reduce the extractable K of the 0-30 cm soil layer (Table 2). This reduction in K possibly occurred because the Mg displaced the K on the soil colloid and by so doing increased the concentration of K in the soil solution and the subsequent leaching thereof, resulting in a significant reduction in the extractable K of the 0-30 cm soil layer. A lower soil pH will increase the concentration of K in the soil solution (Thomas & Hipp, 1968) and such an increase could possibly promote the leaching of this cation (Mengel & Kirkby, 1987a). It is possible that the significantly lower soil pH of the 0-30 cm soil layer on Kersfontein, compared to Meerlus (Table 2), contributed to the MgSO4 having significantly reduced the extractable K of the 0-30 cm soil layer on Kersfontein only (Table 2). It seems therefore that the higher solubility of MgSO4 compared to the other fertilisers, caused Mg to be a more effective exchanger of K than Ca.
Both Ca(OH)2 and CaSO4 were ineffective as far as the reduction of the K content of the soil layers is concerned (Table 2), but appear to have reduced, just like the MgSO4, the percentage K saturation in the top soil (Table 3) due to the resulting high percentage Ca and Mg saturation (data not shown). The increase in soil pH, together with the low solubility of Ca(OH)2, limited the effectiveness of this lime to displace K from the soil colloid. This limitation possibly contributed to the fact that Ca(OH)2 did not significantly reduce the extractable K of the soil layers (Table 2).
The K content of the 0-30 cm soil layer on both Kersfontein and Meerlus was still much higher than the usual average of 4% K saturation (Table 3), generally considered to be optimal for viticulture. According to Conradie (1994) a K content of 70-80 mg kg-1 (0.18-0.21 cmolc kg-1) is optimal for viticulture on soils with a history of lime application. According to Table 2 the K content of the 0-30 cm and 30-60 cm soil layers on both Kersfontein and Meerlus exceeds this average.
Magnesium sulphate significantly reduced the extractable Ca and CaSO4 the extractable Mg of the 0-30 cm soil layer (Table 2). The increase in soil pH through Ca(OH)2 (Table 1) appears to have limited the leaching of Mg and so too the extractable Mg (Table 2).
The chemical soil treatments had no significant influence on the P content of the soils.
Plant
Despite obviously longer shoots the canopy on Kersfontein was less dense than on Meerlus, as can be seen from the percentage full sun, the percentage shade bunches and LLC data (Table 4). Although there are indications that Mg-sulphate applications resulted in a lower percentage of full sun in the foliage, this was not reflected by total shoot length, shade bunches or LLC.
Table 5 indicates that the chemical soil treatments had no effect on the measured bunch characteristics. The production per vine of the Kersfontein Cabernet Sauvignon was significantly lower, however, than that of the Meerlus Cabernet franc and may be linked to lower bunch mass, with larger berries.
According to Table 6 it seems that the sugar contents on Kersfontein were generally higher than on Meerlus. Ca-hydroxide and Mg-sulphate soil applications significantly increased the sugar content on Kersfontein. In contrast Ca and Mg soil applications on Meerlus did not have any significant effect on the sugar content of juice compared to the control. The latter result confirms that of Ruhl et al. (1992), Daverede & Garcia (2000) and Gallego (1999), who did not find any effect on the sugar content of juice following MgSO4, CaCl2 and CaCO3 applications.
Table 1 indicates that the Control topsoil pH on Kersfontein (pH(KCl) of 4.7) was significantly lower than on Meerlus (pH(KCl) of 5.5). Trenching of the soil on Kersfontein was not as deep and consequently the root system of these vines was not as deep as on Meerlus. From the above it seems, therefore, that the physical and chemical soil characteristics on Kersfontein were less optimal for viticulture than on Meerlus. One might therefore assume that if the application of Ca or Mg to the soil had a positive effect on the sugar content of juice, it would be more prominent in the case of Kersfontein. However, the specific mechanism involved in the increase of the sugar content of juice by Ca(OH)2 and MgSO4 on Kersfontein (Table 6) is not known.
Only on Meerlus diid the application of Ca and Mg significantly reduce the pH of the juice, while MgSO4 resulted in a significant increase on Kersfontein (Table 6). According to Boulton (1980a) the pHs of juice and wine are determined mainly by the tartaric/malic acid (TA/MA) ratio, titratable acids (TiA), Na and K content thereof. Seeing that Ca and Mg fertilisation on Meerlus did not indicate any significant influence on the Na and K content of juice (data not shown), one may assume that Ca and Mg fertilisation reduced the juice pH by increasing the TiA (this data is not available for Meerlus) and/or TA:MA (this data is not available either) ratio of the juice.
Daverede & Garcia (2000) also obtained a significant reduction in juice pH in vines that were hydroponically grown, following the application of an excess of Ca as CaCl2 and ascribed it to a K/Ca antagonism, which significantly reduced the K content and therefore also the pH of juice.
No obvious K/Ca antagonism occurred following the CaSO4 treatment on Meerlus, as there was no significant reduction in the K content of either the petioles or the juice (data not shown). This could possibly be because the applied Ca concentration in the soil was too low and because Ca was neither properly distributed nor sufficiently deep (Table 2).
The concentration of K in the soil solution may also have an influence, however, on a K/Ca antagonism. According to Maathuis & Sanders (1996) active uptake of K in higher plants has a greater selectivity for K than in the case of passive uptake. One might therefore conclude that the possibility of a K/Ca antagonism would increase with an increase in passive K uptake. Epstein (1973) also proved that a high external Ca concentration was able to limit the passive uptake of K. According to Maathuis & Sanders (1996) passive uptake will be dominant at high external K concentrations in the millimolar area, while active uptake will be dominant at lower concentrations in the micromolar area. As a result of the exhaustion zone of K that forms around the surface of roots, K is absorbed into the micromolar concentration area in most soils. If one therefore assumes that the active uptake of K also played an important role on Meerlus, this may have inhibited the possibiliy of a K/Ca antagonism. The occurrence of a K/Ca antagonism as described by Daverede & Garcia (2000) can also be explained, since the importance of passive K uptake will possibly increase under hydroponic conditions, thereby increasing the possibility of a K/Ca antagonism.
It seems therefore that the CaSO4 soil applications on Meerlus reduced the juice pH by promoting the synthesis and/or retention of certain acids. The reason for this is not known, however, as it contradicts research by Daverede & Garcia (2000) who found that CaCl2 fertilisation significantly reduced the malic acid content of juice, while it had no significant influence on TA and TiA content of juice.
Gallego (1999) experienced a significant increase in TiA and reduction in K content of juice in reaction to CaCO3 fertilisation on four different soil types. Although it was not significant, a reduction in juice pH was also found on all four different soil types involved. The reduction in juice pH is also explained by a K/Ca antagonism.
As in the case of the CaSO4 treatment on Meerlus, it is unlikely that the significant reduction in juice pH on Meerlus by the Ca(OH)2 treatment may be ascribed to a K/Ca antagonism, in that Ca(OH)2 fertilisation did not result in a significant reduction in the K status of the vine and juice K on Meerlus (data not shown). It is also unlikely that the increase in soil pH would have reduced the K concentration of the soil solution to such an extent that it limited the K uptake, as the uptake of K is not dependent on the K concentration in the soil, provided a shortage does not occur (Boulton, 1980b) or passive K uptake does not play an important role (Maathuis & Sanders, 1996). As a result of the high K concentration and percentage saturation of the topsoil, especially on Meerlus (Table 2; 3), and the fact that the increase in soil pH is limited to a 0-30 cm soil layer only (Table 1), it is unlikely that K uptake would be hampered significantly.
As in the case of CaSO4, it seems that Ca(OH)2 reduced the juice pH significantly by promoting the synthesis and/or retention of certain acids on Meerlus.
Soil applications of MgSO4 on Meerlus also reduced the juice pH significantly (Table 6). This corresponds with results obtained by Ruhl et al. (1992), who also found a significant reduction in wine pH of Chardonnay in reaction to MgSO4 fertilisation. According to the authors the reduction was to be expected due to the occurrence of a possible K/Mg antagonism in vines, as described by Loué, Gaynard & Morard (1987), and reported in Ruhl et al. (1992). However, the expected antagonism did not occur in the trial of Ruhl et al. (1992), as there was no significant difference in the K content of wines. Mengel & Kirkby (1987b) also discussed the possibility that an increase in Mg fertilisation might reduce the K content of plants by means of a K/Mg antagonism. In the MgSO4 treatment on Meerlus there probably did not occur any K/Mg antagonism, seeing that there wasn't any significant reduction in the K content of juice (data not shown). According to Conradie & Saayman (1989a) no Mg induced K deficit (K/Mg antagonism) has been identified in any SA vineyard up to now. The opposite trend is, however, a common occurrence in leaf analyses (Conradie, 1994; Garcia et al. 1999).
With regard to MgSO4 soil applications, these also seem to have promoted the synthesis and/or retention of certain acids on Meerlus in order to bring about a significant reduction in juice pH.
At this stage one can only speculate as to why Ca and Mg fertilisation reduced the juice pH on Meerlus only, while increasing it significantly on Kersfontein, since there are too many differences between Meerlus and Kersfontein. Research by Ruhl et al. (1992) indicates that the influence of fertilisation on wine pH may differ from one cultivar to the next, as MgSO4 fertilisation in a trial only reduced the wine pH of one out of three cultivars significantly. Conradie (1983) also proved that liming is able to influence the K content of shoots differently, depending on the rootstock cultivar and the increase in soil pH that is brought about. The relationship between the K content and pH of juice and wine (Boulton, 1980a) allows one to conclude that the possible effect of liming on juice and wine pH will differ, depending on the increase in soil pH and rootstock cultivar.
According to Jackson & Lombard (1993) the pH and sugar content of juice usually increase along parallel lines during ripening. The significantly higher sugar content of juice from the Ca(OH)2 and especially the MgSO4 treatments on Kersfontein (Table 6) suggests earlier ripening and may serve as an explanation for the significant increase in pH and inclination to lower acids in the case of the MgSO4 treatment.
CONCLUSIONS
Calcium hydroxide and especially MgSO4 applications may be used successfully, given the conditions of the field trial, to increase the sugar content of the juice of Cabernet Sauvignon/101-14 Mgt significantly, probably due to earlier ripening, as reflected by higher pHs. Magnesium sulphate also appears to be the most effective soil application to reduce absolute K contents of the soil, but the effectiveness thereof is clearly dependent on the soil and there is no obvious explanation for this phenomenon. Under the conditions of the field trial, calcium and Mg fertilisation could also be used successfully to reduce the juice pH of Cabernet franc/99R significantly. The reason for this reduction is not known either. Further research is required to identify the specific mechanisms involved in these reactions.
The K saturation of the soils in question was so high that bigger concentrations of Ca and/or Mg ions will probably be necessary in the soil solution to induce a K/Ca or K/Mg antagonism successfully in order to achieve lower K levels in the juice and wine. Although magnesium sulphate appeared to have been the most promising soil application, excessively high applications of Mg salts may be harmful to the soil structure. Further research into this matter is required, also regarding the possibility of bunch directed Ca or Mg spraying at different phenological stages.
REFERENCES
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THOMAS, G.W. & HIPP, B.W., 1968. Soil Factors Affecting Potassium Availability. In: V.J. Kilmer, S.E. Younts & N.C. Brady (Eds). The Role of Potassium in Agriculture. Am. Soc. Agron. Madison, Wis.
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