Higher on the slopes, the soils are composed of a mixture of fine elements (quartzite, slate,…) and lower in the valley, the soils get decomposed elements from the slopes. In Bierzo most vineyards grow on brownish humid soils, slightly acid, and therefore with no carbonates (typical of humid climate). The acidity level of the soil ranges from 4 to 8.5, being higher than 6 down in the valley. Most soils are poor in lime oxide, usually less than 3’000 kg/ha. (apart from the lower parts of the valley). Moreover, the maximum content of organic matter and nitrogen is usually between 2% and 4%. The average content is greater down in the valley than on the slopes, due to livestock being more common in the lower areas. Humidity and nitrification rates are reasonable, as the carbon to nitrogen ratio is around 11.9 in the valley and 11 in the meadows. However, phosphorus content is scarce (less than 100 gr), which is emphasised on the slopes; but as far as potassium (K20) is concerned, the rates are reasonable. The average is around 700 kg./ha. in the valley and 685 kg./ha. on the slopes. The key factors of quality are either on terraces with gentle tilt (where vineyards grow close to the river) or on steep slopes. In Bierzo, vineyards grow at an altitude ranging between 450 and 1’000 metres.
Most soils could be classified into 3 different categories: alfisols, inceptisols and entisols.
Inceptisols: Soils of relatively new origin characterized by having only the weakest appearance of horizons or layers produced by soil-forming factors. They are usually arable with appropriate control of erosion or drainage.
Alfisols: Usually characterized by an argillic, a kandic or a natric horizon with a base saturation of 35% or greater. Arable soils with water content adequate for at least three consecutive months of the growing season.
Entisols: Soils characterized by the absence (or almost absence) of horizons (or layers of the soil-forming processes). Shallow and highly resistant to extreme conditions.