Introduction

Soil and irrigation water of irrigated areas are becoming increasingly saline, particularly in semi-arid areas. In addition to the negative effects of high salt content, there is ample evidence that Na and Cl toxicity can decrease vineyard growth and yield (Moolman et al., 1999; De Clerque et al., 2001). Therefore, Na and Cl uptake and accumulation in grapevines should be as low as possible. Leaching of excessive or toxic ions from the root zone is not always a practical solution, particularly in soils with poor internal drainage, or where the leachate can pollute natural resources. The objectives of the project were to determine (i) how salinity affects grapevines and (ii) how ion balances in soil, e.g. the sodium adsorption ratio (SAR), affect ion uptake and accumulation in grapevines.

Experimental background

The study was carried out in 13 full-bearing vineyards during the 2000/2001 season near Robertson in the Breede River Valley. Chardonnay and Ruby Cabernet, both grafted onto 110 R and 101-14 Mgt rootstocks were used for the study. Each vineyard had a poor performing patch that was presumably caused by high salinity or Na (Fig. 1). Soil samples were collected in the poor patches, as well as in the better parts. The pH (KCl), electrical conductivity (ECe), water soluble K, Na, Ca, Mg, Cl and SO4 in the 0 – 30 and 30 – 60 cm layers were determined by the soil laboratory at ARC Infruitec-Nietvoorbij. The adsorption ratios for sodium (SAR) and potassium (PAR) were calculated from the basic cations. Leaf and juice N, P, K, Na, Ca, Mg, Cl and SO4 contents were determined at harvest by a commercial laboratory (Bemlab, Strand). Yield, number of bunches per grapevine and bunch mass were determined. Juice pH, TSS and TTA were determined by the winery at ARC Infruitec-Nietvoorbij. Cane mass per grapevine was measured at pruning in August 2001.

Soil_figure1
FIGURE 1. One of the poorly growing patches where soil chemical status and grapevine response were measured in the Robertson area. Insert shows leaf damage caused by high soil salinity.
FIGURE 2. The relationship between juice pH and total titratable acidity (TTA) for four scion/rootstock combinations. Dashed lines indicate the boundary lines.

The boundary line approach was used to determine trends in vineyard reaction to salinity-associated soil properties. Boundary lines for a specific crop can be obtained by sampling the variation that occurs naturally under a range of conditions within a specific region (Walworth et al., 1986). To obtain a boundary line, a scatter plot of plant response against a plant growth factor is created. In most cases, it would be possible to draw a line, or lines, which confines the data. Therefore, the boundary line describes the sole effect of a plant growth factor where no other limiting factors occur. More complete details of the experiment procedures were reported earlier (Myburgh & Howell, 2014).

Results and discussion

The following is only a summary of the boundary line trends. The actual boundary line plots were published earlier (Myburgh & Howell, 2014).

Leaf macro element content

Under the prevailing conditions, there was no relationship between leaf N and P and any of the measured soil variables (data not shown). Leaf K increased linearly with soil pH(KCl) to a maximum at pH(KCl) 7.5. This was followed by a linear decline (Table 1). Similarly, leaf Ca reached a maximum at pH(KCl) 7. Since leaf Ca decreased linearly as soil Na:Ca increased from 0.1 to 15, it suggested that high soil Na supressed Ca uptake and/or translocation to the leaves.

Vegetative growth

Cane mass increased linearly as the soil pH(KCl) increased from 3.5 to 8.2 (Table 2). Under the prevailing conditions, vegetative growth of Chardonnay/110 R tended to be more sensitive for soil pH than the other scion/rootstock combinations. Cane mass did not show a distinct ECe threshold value, but declined at a rate of 14% per dS/m (Table 2). Maximum cane mass occurred when leaf Ca was 2.4%. Cane mass increased linearly as leaf K increased. However, cane mass of Chardonnay/110 R remained constant at approximately 0.6 kg per grapevine when leaf K levels exceeded 1.5%. Cane mass decreased non-linearly as leaf Na increased from 0.01 to 0.2%. Cane mass also decreased as leaf Cl increased from 0.1 to 0.85%, but this trend was linear compared to the response to leaf Na. The foregoing trends clearly indicated the toxic effects of high Na and Cl on vegetative growth.

Yield

Large variation, from 2 to 25 ton/ha, occurred in the yield. Yield tended to remain constant up to a ECe threshold value of 1.8 dS/m (Table 3). Above this threshold value, yield declined linearly at a rate of 13% per dS/m. Sodium had a negative effect on yield above an SAR threshold value of 3 (Table 3). Above the threshold value, yield decreased linearly as SAR increased to 13.

Juice TTA, pH and K

The Chardonnay juice generally had a higher TTA and lower pH than the Ruby Cabernet juice (Fig. 2). Under the prevailing conditions, juice pH increased as the TTA decreased. Near the upper boundary layer, juice K was higher than near the lower boundary line (Fig. 2). At a given TTA, a juice K increase from around 900 to 1 600 mg/ℓ increased juice pH approximately by 0.4 units. The soil (K + Ca):Na should preferably be lower than two to reduce the risk of excessively high Na in juice and wine (Table 4). However, juice Na decreased as cane mass increased (Table 4), which indicated that vegetative growth might be a strong sink for Na.

Conclusions and recommendations

The boundary line approach can be useful to determine effects of plant growth factors on grapevine response. The selection of plots covered most of the possible variation in soil and vineyard conditions. Under the prevailing conditions, the four scion/rootstock combinations responded almost similarly to the salinity-associated variables. Results confirmed that the ideal soil pH(KCl) for vineyards is around 6.0. The salinity threshold for vineyards in the Breede River Valley is between 0.7 and 1.5 dS/m. The SAR must be below 3, and the soluble Na content in the soil should not exceed 5 mg/kg, to reduce the Na toxicity risk. If gypsum is used to reduce soil Na, it must be applied judiciously to avoid soil SO4 accumulation. The latter increases the risk of K and Mg deficiencies. Under the prevailing conditions, Cl and B toxicity probably contributed to reduced vegetative growth. Therefore, soil Cl and B must be kept as low as possible, but care should be taken that B is not reduced to deficient levels.

Serious saline or sodic soil conditions were the exception rather than the rule for vineyards in the Robertson area. Therefore, future research must be carried out in areas where the topography is such that over-irrigation of vineyards on higher land may cause salinisation of lower lying vineyards and/or natural water resources. Under such conditions, salinity could manifest on a more extensive scale. This could also increase the negative effects of salinity on the ion composition in grapevines, as well as on growth and yield.

Summary

High levels of soil salinity or sodicity may have negative effects on grapevine growth, yield and quality. Soil chemical status and grapevine responses were measured in poor patches in 13 vineyards near Robertson in the Breede River Valley. Chardonnay and Ruby Cabernet, both grafted onto 110 R and 101-14 Mgt were used in the study. The boundary line concept was used to determine vineyard responses to salinity-associated soil properties. Under the prevailing conditions, the four scion-rootstock combinations responded almost similarly to the salinity-associated soil properties. Results confirmed that the ideal pH (KCl) for vineyard is 6. The salinity threshold for vineyards in the Breede River Valley must be between 0.7 and 1.5 dS/m (decisiemens per metre) to avoid growth and yield losses. The SAR should be below 3, and the water soluble Na content in the soil should not exceed 5 mg/kg to reduce the Na toxicity risk. If gypsum is used to reduce soil Na, it must be applied judiciously to avoid soil SO4 accumulation. The latter increases the risk of K and Mg deficiencies. Under the prevailing conditions, Cl and B toxicity apparently contributed to reduced vegetative growth. Therefore, soil Cl and B should be kept low. However, care should be taken that B is not reduced to levels that will cause deficiencies in vineyards.

Acknowledgements

Winetech for partial funding, the ARC for infrastructure and other resources, as well as the Soil and Water Science staff at ARC Infruitec-Nietvoorbij for technical support.

References

De Clerque, W.J., Fey, M.V., Moolman, J.H., Wessels, W.P.J., Eigenhuis, B. & Hoffman, J.E., 2001. Experimental irrigation of vineyards with saline water. WRC Report No. 522, 695/1/01, Water Research Commission, Pretoria.

Moolman, J.H., De Clerque, W.P., Wessels, W.P.J., Meiri, A. & Moolman, C.G., 1999. The use of saline water for irrigation of grapevines and the development of crop salt tolerance indices. WRC Report No. 303/1/99, Water Research Commission, Pretoria.

Myburgh, P.A. & Howell, C.L., 2014. Use of boundary lines to determine effects of some salinity-associated soil variables on grapevines in the Breede River Valley. South African Journal of Enology and Viticulture 35, 234 – 241.

The Non-Affiliated Soil Analysis Work Committee, 1990.

Walworth, J.L., Letzsch, W.S. & Sumner, M.E., 1986. Use of boundary lines in establishing diagnostic norms. Soil Science Society of America Journal 50, 123 – 128.

This article originates from research funded by Winetech and the final report of project WW 04-13, “Determining the effect of ion ratios in saline soils on ion accumulation in the grapevine”, can be downloaded from http://www.sawislibrary.co.za/dbtextimages/finalreport12.pdf.

– For more information, contact Philip Myburgh at myburghp@arc.agric.za.

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