The Central Valley spans the O'Higgins (VI) and Maule (VII) administrative regions as well as the Metropolitana Region. Within it are four subregions: the Maipo Valley, the Rapel Valley, the Curicó Valley and the Maule Valley. This is Chile's most productive and internationally known wine region, due in large part to its proximity to the country's capital, Santiago. It is located directly across the Andes from Argentina's most well known wine region, Mendoza Province The soil of Maipo Valley is noted for high salinity stemming from irrigation from the Maipo river and low potassium level which has some impact on the grapevines. Vineyards in the Maule also suffer from low potassium as well as deficient nitrogen levels. Advances in viticultural techniques have helped vineyards in these regions compensate for some of these effects.