Effect of two contrasting soils on grape and wine sensory characteristics in Shiraz

Aims: Berry composition and wine sensory characteristics reflect the origin of grape production and seasonal climatic conditions. The aim of this study was to compare berry and wine sensory characteristics from two contrasting soil types where the vineyard climate, geography, topography, vine and management factors were not different.

Methods and Results: Two adjoining blocks of Shiraz with similar vine age (+/-1 year), identical clone (1654), row orientation (NW, SE) and cordon height were selected for this study. All irrigation, spray and midrow management treatments were identical. Both sites have soils that are texture contrast or duplex brown chromosols. The main distinguishing feature between the two sites being the presence of 10% to 50% ironstone gravel, mainly in the bleached topsoil “E” (or A2) horizon for the “Ironstone” block which is in contrast to the “Sand over clay” block. 

Berry sensory attributes were evaluated using the accepted method of berry sensory assessment (BSA). The method allows for the identification and quantification of berry sensory attributes against standard sensory references by a trained panel. The evaluation of wine sensory attributes was performed using a quantitative descriptive analysis (QDA). Both methods were performed to assess sensory differences in grapes and wine from the two soil types. Berries from the “Ironstone” soil had more intense green/grassy flavour, a higher perception of acidity and greater astringency. This was in contrast to berry samples from the sand over clay soil, which were described as having more intense dried fruit/jammy flavour, a higher perceived sweetness and an elevated toasted flavour. Wines made from fruit from the “Ironstone” soil were found to have more intense red fruit characters, tannin quality and astringency in contrast to the dark fruit, higher colour intensity and confectionary characteristics of the wines made from fruit from “Sand over clay” soils.  Fifty-six soil mineral elements were analysed from each soil horizon, leaf blades, must and wine samples. Results obtained from inductively couple plasma atomic emission spectroscopy (ICP-OES) analysis identified elements some of which were unique to each soil type and some which were in higher concentrations. The differences in the two soils elemental status was translated to leaves, berries and wine from those soils. 

Conclusions: 

Differences were observed in berry and wine sensory characteristics when comparing the fruit harvested from two contrasting soils in close proximity. Soils displayed very similar physical characteristics. Both soils were observed to be texture contrast or duplex brown chromosols. They shared common features of sandy or loamy topsoils (“A” horizons) over brown light clay (LC) to light medium (LMC) “B” horizons with or without highly weathered sandstone in the subsoil or “C” horizon. There was no soil carbonate present at any site and topsoil pH was neutral (pH 6.5-7.5) and decreased slightly to 6.0 in the “B” and “C” horizons.  Root zones, both predicted and observed were not significantly different.

Slight differences were observed between the soils with measures of readily available water (RAW), topsoil depth and a unique layer of gravel in the ironstone soil all of which have been associated in previous research with water movement and plant water availability in soils. Analysis of the chemical composition and concentration of soils, vines, grapes, musts and wines demonstrated distinct differences in the chemical characteristics between the two soil sites. This study was able to investigate soils with different soil chemistries and sensory characteristics for berries and wine in isolation from other known influences including viticultural, environmental, many other soil, and winemaking factors. 

The application of elements to vines in a controlled environment in future work may provide a link between soil chemistry and grape and wine sensory attributes. 

Significance and Impact of the Study: Soil elemental composition is a contentious aspect of terroir especially in relation to the relative importance afforded to climate and soil physical characteristics in previous research. This trial was able to isolate soil for analysis to observe unique elemental compositions in varying concentrations in relation to differences in berry and wine sensory outcomes. The mechanisms by which soil elements might influence sensory outcomes of wines is not widely understood and future research could lead to soils and wines being paired for desired sensory outcomes.