Soil physical and hydrological properties

The amount of water, air and nutrients available for plant growth is influenced by the soil physical and hydrological properties and by management practices.

The physical-hydrological soil parameters are required as input data in different models commonly used to make predictions, estimates and assessments of the phenomena that affect the movement of water in the soil/groundwater/surface stream system. They also serve as a yardstick for environmental assessments in relation to the filtering function performed by soils for contaminants to groundwater.

The main soil physical and hydrological properties are:

• Texture
• Structure
• Saturated hydraulic conductivity (Ksat)
• Consistence
• Bulk density
• Available water capacity (AWC)

Texture

The particles that compose the soil can be divided by size categories. There is a great variability in the particle size: coarser particles (>2mm in diameter) form the skeleton, whereas the finer ones form the fine earth.

The fine earth fraction is further divided in sand (0.05 mm - 2 mm in diameter), silt (0.002 mm - 0.05 mm in diameter) and clay (<0.002 mm in diameter).  

Texture indicates the relative content of fine earth particles. Emilia-Romagna region follows the classification system according to Soil Survey Division Staff, 1993.

Different combinations of sand, silt and clay are grouped in textural classes.

Texture affects the chemical and physical behaviour of soils. Texture is important for engineering and agronomic interpretations, influences the hydrological qualities of soil and is used for soil classification. Main soil properties affected by texture are drainage, water holding capacity, soil aeration, susceptibility to erosion and cation exchange capacity.

Structure

Structure is the arrangement of the grain-size particles into aggregates which have easily distinguishable shapes. The aggregates can have laminar, granular, angular or prismatic form or soil may be structureless (single sand graines or shapeless resistant masses).

Structure is formed by the continuous alternation of freezing/thawing, in the transition from the wet state to the dry one, by microorganisms activity and root pressure and through cementing by clay, organic matter, iron and aluminum compounds.

Structure increases infiltration of water, thus reducing runoff and erosion, increases the amount of plant available water; improves seedling emergence, root growth and rooting depth and increases permeability because of large continuous pore presence.

Saturated hydraulic conductivity (Ksat)

Saturated hydraulic conductivity (Ksat) refers to the ability of a soil to transmit water when a soil is fully saturated. It’s expressed as cm of water/hour and depends on the properties of both porosity (pore geometry) that the fluid (stickiness and density).

If Ksat is high the water moves fast, if low water moves slowly. Ksat is a parameter used to describe hydrological behaviour of soils such as permeability, infiltration, water balance and hydrological soil groups. Hydrological soil groups gather together soils with similar physical and runoff characteristics.

The factors that affect Ksat are:

• Texture: pores size decreases with smaller particles size: as a result fine textured soils are less permeable;

• Organic matter increases aggregates stability;

• Structure is related to pore continuity. A well-structured soil is more permeable than a poor-structured one;

• Root and worm channels;

• Pores size and continuity: the more pores are large and continuous the more water moves quickly.

Consistence

Consistence describes the resistance of a soil, at different moisture content, to mechanical stress or handling. It expresses cohesive and adhesive forces holding soil particles together: plasticity and stickiness. Consistence is an indicator of the tendency of a soil to develop compacted zones, crusts, cemented layers that create issues of water stagnation, seedling emergence and root growth.

The main factors that affect consistence are water content, texture, type and amount of clay and soil density.

Bulk density

Density is the soil mass per unit volume and is expressed as g/cm³. Soil bulk or dry density is the ratio of the mass of the solid phase of the soil to its total volume (solid and pore volumes together), whereas soil particle density considers only the volume of the solid phase of the soil. Bulk density is always lower that particle density and can change considerably depending on texture, water content and structure.

This parameter affects engineering applications and plant growth. It is used to compute shrinkage/swelling potential, available water capacity, total porosity and also saturated hydraulic conductivity. It is used to convert measurements from a weight basis to a volume basis (e.g. transformation of organic carbon content from percentage to Mg/ha). It is an indicator of how well plant roots are able to extend into the soil.

Available water capacity (AWC)

Available water capacity is the volume of water that should be available to plants if the soil were at field capacity. It is expressed in cm of water for each soil horizon or layer and it’s the difference between field capacity and permanent wilting point water contents. AWC is an important factor in the choice and management of irrigation systems, drainage systems, to predict water resources for a better choice of agricultural crops and also to predict yields.

It depends on organic matter content, soil texture, bulk density and soil structure.

Available Maps

Geological, Seismic and Soil Survey has produced three maps  regarding the following parameters:

• Texture: percentage content of sand, silt, clay, skeleton and USDA textural classes (0-30 cm layer);

• Saturated hydraulic conductivity (Ksat):  1:50,000 scale map of soils in the plain  grouped into 6 Ksat classes according to the Soil Survey Manual 1993;

• Hydrological Soil Groups according to National Engineering Handbook USDA-NRCS, 2009.

Bulk Density parameter is available in other documents such as:

•  Bulk density map in the 0-30cm layer:  raster file in the SOC stock package (download  from Minerva site);

• Soil Inventory: in the genetic soil horizons section among others parameters there is the bulk density data (only for the the soils of the plain);

• Benchmark local sites: bulk density and Ksat values, derived mostly by pedotransfer functions, are always present.

Web maps

• Soil Catalog of Emilia-Romagna
  The website provides access to 1:50k soil map (plain and hills) and point data (extension service soil analyses and shallow water table measurement sites). A selection of derived thematic maps is available as well. It’s possible to draw plots, to identify a soil type using a special tool and to calculate the fertilization plan of your plots.

Contact us

Geological, Soil and Seismic Risk Area
e-mail - phone +39 051 5274792

• Tarocco Paola, Phone: +39 051-5274515