Siena province or Provincia di Siena is is the most prolific Tuscan province in terms of wine production. It has five DOCGs (see below).
Provincial capital (‘capoluogo’): Siena.
Communes: Abbadia San Salvatore. | Asciano. | Buonconvento. | Casole d’Elsa. | Castellina in Chianti. | Castiglione d’Orcia. | Cetona. | Chianciano Terme. | Chiusdino. | Chiusi. | Colle Val di Val d’Elsa. | Gaiole in Chianti. | Montalcino. | Montepulciano. | Monteriggioni. | Monteroni d’Arbia. | Monticiano. | Murlo. | Piancastagnaio. | Pienza. | Poggibonsi. | Radda in Chianti. | Radicofani. Radicondoli. | Rapolano Terme. | San Casciano dei Bagni. | San Gimignano. | San Quirico d’Orcia. | Sarteano. | Siena. | Sinalunga. | Sovicille. | Torrita di Siena. | Trequanda.
Vineyard area: 44,790 ha of vineyards and mixed crops, 14,410 ha of specialised vineyard cultivation (Project Corine Land Cover, 1995) cited by Costantini et al.
Climate | Costantini et al (1985) report ‘Long term mean annual precipitation in the province of Siena ranges from 630 to 1275 mm. Rainfall distribution in the territory is greatly influenced by relief. Mount Amiata in the south west and the “Dorsale medio Toscana” along the western side of the province, as well as the so called “Chianti hills” eastward, collect a great deal of rain, while the two large valleys located in between the two ranges, that is the Elsa river and Orcia river valleys, show the minimal values. Long term mean annual air temperature ranges from 10.2°C, on Mount Amiata, to 14.8 in the lower part of the Elsa and Orcia river basins. However, the south facing slope of Montalcino presents the peculiarity of having a warm climate. Mean annual soil temperature at 0.5 m has a rather different pattern than air temperature, because it indeed reflects, besides climatic conditions, the dominance of coarse textured soils in the eastern part of the province and fine ones in the western part. If we put elevation, rainfall and climatic layers in the GIS, on surfaces covered by vineyards, we note that the hilly belt of 200-400 m is preferred by most farmers (fig. 5). This belt receives an average of 700-800 mm, it has a long term mean annual air temperature of about 13.5 °C, and soil temperature around 14.8 °C (fig. 6).
All parameters vary greatly, more so, however, within VCA than between VCA, except for Bianco Val di Chiana, which covers a very limited area within the province of Siena. The least constant variable is the elevation, while soil temperature fluctuates less, because of the mitigation effect operated by soil on temperature excursions.
If the main feature showed by orographic and climatic data is the variability of the territory and vineyards, some peculiarities can also be highlighted. Nobile di Montepulciano, in particular, emerges because of the lower extremes in rainfall, whereas Montalcino does so because of soil temperatures. At 15.1°C, the soil temperature of the Brunello di Montalcino vineyards is also one of the most elevated.
On the other hand, vineyards of the VCA Orcia, as well as those of non DOC areas, show the widest range of extreme values of rainfall and air temperature. Soil temperature of Brunello di Montalcino vineyards is also one of the more elevated, 15.1°C. It is interesting to note that Orcia has the lowest average rainfall, 710 mm, and non DOC areas the highest, 833 mm. Orcia vineyards have also the lowest mean annual air temperature, 13.1°C, but a rather high mean soil temperature, 15 °C. Val d’Arbia vineyards have the highest mean annual air temperature, 14.1 °C, with a rather limited variability, from 13.3 to 14.4°C.
Geology: Costantini et al (1985) report the geology of the province of Siena is rather variegated and complex, as in many other prime viticultural areas of Italy (Cita et. al., 2004). Marine clays and sands are the most diffused lithological types, followed by different kinds of limestone, marls and turbidites, and sandstone (fig.7).
The vineyards of Montepulciano and Chianti Colli Senesi DOCG are rather uniform in lithology, being dominated by Pliocene marine sand (fig. 8). Vineyards of Bianco Vergine della Val di Chiana are all constituted by alluvial sediments, while in the Arbia VCA we have a predominance of marine clay and alluvial sediments. The geology of Montalcino is more differentiated, but we can infer that the most typical formation is conglomerate, while Chianti Classico DOCG is the most heterogeneous VCA, where however, marly limestone, as well as marls and turbidites are the main litho types, all of which belong to the so called “flysh formation”.
The Orcia VCA has a geology which is partly similar to that of Chianti Classico, due to the dominance of marls and turbidites, and partly to that of Chianti Colli Senesi, because of the abundance of marine sand.
Vineyards in non-DOC areas, finally, are mostly planted on marine clay or alluvial sediments.
See Soil series and phases on marine pliocene deposits in San Gimignano (Siena). Preliminary investigation for the definition of areas suitable for Vernaccia D.O.C.G.]. [Italian]  by Costantini E.A.C. Lulli L. Catone I. Pinzauti S. Istituto Sperimentale per lo Studio e la Difesa del Suolo, Florence (Italy) [Corporate Author] Office National Interprofessionnel des Cereales, Paris (France). Ministere de l’ Agriculture, Paris (France). Service Regional de l’ Amenagement des Eaux Nord-Pas-de-Calais, Lille (France). Institut Superieur d’ Agriculture de Lille (France). [Corporate Author]