Soil Composition, Horizons, and Types
Soil Factors
The bedrock is the mineral substrate from which the soil is formed, affecting its color, texture, structure, permeability, and acidity.
The climate is the most influential factor in pedogenesis. Rainfall supplies water for biological activity and chemical leaching or dissolution, dragging items from the topsoil. Soil can be zonal (closely linked to climate) or azonal (related to factors other than climate).
The topography also influences soil composition: plain areas accumulate coarse soils, while steep slopes are conducive to erosion and landslides.
Living beings act in different ways. Plants maintain soil fertility through their roots, taking bases from lower strata and returning them to the soil surface. Microflora (bacteria, fungi, etc.) fix nitrogen. Animal action is primarily mechanical.
Humans also impact the soil, potentially destroying or altering it, but also improving it with fertilizers and proper management.
Time is another factor, as soil formation is a slow process requiring centuries.
Soil Horizons and Profiles
Soils are made of layers or horizons, each with distinct physical, chemical, and biological characteristics. The entire set of horizons constitutes the soil profile.
At depth are the D and C horizons. The D horizon consists of consolidated bedrock, and the C horizon of altered or broken rock.
On the surface is the A horizon, with an A0 layer of litter, a dark A1 layer of decaying organic matter, and a lighter A3 layer, a zone of leaching where substances are carried to lower layers by precipitation.
Between them lies the B horizon, a zone of accumulation with a more intense color, containing leachates from upper layers and particles from the C horizon.
Soil Types
Soil classifications are based on origin, evolution (genetic and European classifications), or characteristics (analytical classifications, such as American).
Zonal Soils
a) Soils of Oceanic Climate
On silicate rocky soil, acidity increases.
The moist brown earth is dominant. With favorable topography, it is suitable for farmland if acidity is counteracted with lime. On increased slopes, it can be used for pasture.
Rankers are in higher areas or steep slopes, subject to heavy erosion, and are thin. They are only suitable for pasture and forest, not for crops.
On rocky limestone soil, acidity is lower.
The limestone brown earth is dominant, giving excellent yields in crops like beans, corn, or permanent grassland.
The terra fusca appears on hard limestone and in mountainous areas.
b) Soils of Mediterranean Climate
In silicate rocks, the most characteristic is the southern brown earth, poor in nutrients, acidic, lacking humus, and easily erodible.
In limestone-dominant areas, the fundamental process is the formation of an argillic horizon, a slow process caused by calcium carbonate dissolution.
The Mediterranean red soil is rich in nutrients, excellent for all crops.
The terra rossa on hard limestone has a clay horizon resting directly on bedrock, causing frequent rock outcrops that hamper mechanization.
In clays and loams, vertisols or black soil arise, characterized by abundant clays that swell when wet and shrink when dry, forming cracks filled with surface materials.
In arid Mediterranean areas, soil depends largely on bedrock geology.
The most characteristic is the gray ground serosem subdesert, found in the Ebro Valley, southeast peninsula, and Canary Islands. It is light gray, almost always dry, rich in limestone, and poor in humus. Sparse vegetation leaves large tracts of land bare.
Azonal and Intrazonal Soils
Azonal soils lack well-defined characteristics due to insufficient time or steep slopes hindering profile development.
Intrazonal soils have specific features within a homogeneous area due to local factors unrelated to climate. In Spain, common types include: brown limestone and limestone with rendzinas (woody crops, cereals, pulses, and irrigated gardens); flood soils on riverbanks (garden crops); endorreic flooded areas (poor in nutrients, some strawberry cultivation); sandy soils (arid and unproductive due to infiltration); salt marshes (evaporation causes salt precipitation); and volcanic soils.