Geology, particle size distribution and clay fraction mineralogy of selected vineyard soils in South Africa and the possible relationship with grapevine performance

[摘要] ENGLISH ABSTRACT:This study is an integral part of a multidisciplinary research project concerning the effects ofsoil and climate on wine quality. The motive, which led to the setting up of this project, wasthat producers could not determine beforehand whether a specific location would yield winesof high or low quality. If a specific cultivar were to be planted at the wrong location, then itwas likely that wine of table quality would result, rather than the export quality wine that wasintended. The long term objectives of this multidisciplinary project were the compilation ofguidelines by means of which different sites may be classified according to their potential forthe production of high quality wines, and the identification of the most important climatic andsoil factors responsible for differences in wine quality and character. In this multidisciplinaryproject, measurements (soil water, leaf water potential, cane mass and yield) were madeunder dry land conditions in Sauvignon blanc vineyards at six different localities: five in theStellenbosch district (Simonsberg, Kuils River, Helshoogte, Papegaaiberg and Devon Valley)and one in Durbanville. Each vineyard was owned by a private commercial producer. Thevines were approximately 10 years old in all cases, and were trained on a hedge system.Measurements were made in plots, each of which contained 20 vines. Two different soiltypes were identified at each locality. Vine growth and wine quality differed markedly onthese contrasting soils, even though they were located in close physical proximity. Themeasurements that were made at high and low production plots at each locality during thisstudy were obtained from points which were not more than 60 m metres apart. An automaticweather station was erected halfway between the two, contrasting, experimental plots.Within the overall scope of the multidisciplinary project, the study which forms the subject ofthis thesis, concentrated on the effects of soil parent material as a soil forming parameter andas a possible predetermining character with regard to vine growth and wine character. Fromliterature it was clear at the outset of this work that the geology of the coastal wine region isvery complex and varies over short distances. The geological history indicates different typesof rock formation and rock forming process (sedimentary, igneous as well as metamorphic),plate tectonic activity, mountain building, erosion and weathering, over a period ofapproximately 1 000 million years. The present landscape includes a coastal plane, hills, anderoding mountains.Statistical analyses indicated that the soils from the different localities could mainly becharacterised in terms of differences in their sand size fractions. Soils from Durbanville aredominated by fine sand and correlates with the underlying phyllitic shales. Soils from KuilsRiver contain significantly more coarse sand when compared with the other sites. Thisappears to be a reflection of the underlying coarse granitic material, and implies that in situweathering played an important role in soil development. .The data did not, however, provethat the Kuils River soils formed solely from underlying rocks. The gravel and stone fractionfor the Kuils River soils were nevertheless correlated with those of the underlying parentmaterial. Soils from Helshoogte and Simonsberg (both of which are underlain by granites),Papegaaiberg and Devon Valley (both underlain by hornfels) were not significantly dominatedby any particular sand fraction. Soils from these localities therefore did not only reflect theunderlying material as a source of soil parent material. This implied mixing of parent materialand/or the incursion of eolian sand at Helshoogte, Simonsberg and Devon Valley. A marineincursion may have affected the soil parent material at Papegaaiberg.In order to obtain more information concerning the origins and possible mixing of parentmaterials prior to and during soil formation, samples from the different soil horizons in eachprofile were subjected to a chemical analysis. Particular emphasis was placed on potassium,which is an extremely important nutrient mineral element, from the viewpoints of vine growthand wine quality. Because the soils used in this study were all located in productionvineyards, the probability that fertiliser residues would have contaminated the soils was high.The lower soil horizons were considered to be least affected by this contamination andtherefore most likely to be indicative of the natural soil chemical composition. However, thelower horizon K content of the soils in this study could not be reliably correlated with anyknown or predicted characteristic that might link the soil parent material with local rock types.At Durbanville, both soils contained small quantities of K in the lower horizons, reflecting theunderlying phyllitic shales, but at Devon Valley and Papegaaiberg, the lower horizonscontained more K than expected. The soils at these localities are situated on hornfels,containing low quantities of K. The large quantities of K in the soils may have indicated thatthese soils are situated close to a granite/Malmesbury contact zone. Soils from Kuils River,Simonsberg and Helshoogte are situated on K-rich porhyritic granites and it was expectedthat these soils would contain relatively large quantities of K in the lower horizons. This,however, was not the case. It was therefore concluded that dilution with K-poor material hadtaken place. Such material could have been derived from higher-lying sandstones, or fromeolian processes during the Cenozoic. Alternatively, the K content of the soil might havebeen depleted by long continued leaching.A semi-quantitative analysis of the minerals in the soil clay fractions was also carried out.The objective was to identify the clay minerals that were present in the different soil horizonsand to relate the minerals to weathering conditions. Evidence linking the minerals in the clayfractions of the soil samples with the mineralogical composition of the soil parent materialswas sought. The clay fraction mineralogy data indicated that all soils in the study area are inan advanced stage of weathering and are dominated by kaolinite, and in certain soils quartz.It was difficult to relate these minerals directly with soil parent material because the primaryminerals originating from the soil parent materials have been extensively broken down. Thesimultaneous presence of quartz and gibbsite in the clay fraction of both soils at Simonsberg,Helshoogte and Durbanville as well as one soil form from bath Kuils River and Simonsberg,indicated non-uniform distribution of clay fraction minerals, indicating that different stages ofweathering were present during soil formation. This could have been a result of mixing ofparent materials, but may also reflect different periods of weathering of the same material.Both soils at Papegaaiberg, both soils at Devon Valley and other soils at Simonsberg andKuils River indicated uniform clay fraction mineralogy distribution, mainly because theabsence of gibbsite is related to the presence of quartz in the clay fraction.The soil characteristics, as determined in this study, were also compared with vine growth,wine quality and wine character, as obtained in the broader multidisciplinary research project.For most soils in this study, an increase in clay fraction kaolinite was associated with areduction in vegetative growth, overall wine quality, and fresh vegetative character. Anincrease in clay fraction quartz was associated with higher overall wine quality. Increasedshoot growth also affected fresh vegetative character positively. Better growth occurred onhigher altitudes and this resulted, for Sauvignon blanc, in higher wine quality. Winesproduced from vines situated on both phyllitic shales and porhyritic granites showed highquality (Durbanville and Helshoogte), but both were related to low clay fraction kaolinitecontent and high altitude. It was not possible to relate parent material directly with vinegrowth, wine quality and/or wine character. The lowest quality wines, however, wereproduced from vines situated on hornfels (Papegaaiberg and Devon Valley), both containinghigh quantities of clay fraction kaolinite and situated on low altitudes. High levels of K in soilscontaining high levels of clay fraction kaolinite may have been partly responsible for low winequality obtained on such soils.

[发布机构] Stellenbosch University