Soils defined

What is soil made of?
 
Soil (in layman's terms, regarded as 'the skin of the planet' and technically known as the 'pedosphere') is defined as the top layer of Earth's crust. The pedosphere is a unique, relatively immobile sphere that is easily impacted by human activities. In contrast to the other spheres of the Earth system, the pedosphere can neither quickly intermix (as the atmosphere does), nor rapidly move laterally along the landscape (as water does), nor clearly be separated into individual units and avoid undesirable environmental changes (as the biota can be and does), nor escape rapid human and biological perturbations (as is characteristic of the lithosphere).
 
 
Soil is often seen as an inert medium, merely a support for human activities. However, soil is more than that: it is a dynamic, living system which comprises a matrix of organic and mineral constituents enclosing a network of voids and pores which contain liquids and gases. Unlike air and water, soil is mostly owned as personal property, which renders soil conservation or protection policies difficult to enforce and requires acceptance by landowners and managers.
 
Several definitions of soils have been put forward as the term itself has a different meaning when interpreted at various administrative dimensions: at national, European Commission (EU-DGEnv) and International policy levels. In the layman's mind, the soil is a very concrete thing, namely, the "dirt" on the surface of the earth. To the soil scientist, or pedologist, the word "soil" conveys a somewhat different meaning, but no universally accepted definition exists.

The term refers to a natural substance located in the so-called 'critical zone' of the Earth's crust and which is composed of weathered rock particles (minerals), organic matter, water and air. This critical zone is a holistic framework for integrated studies of water with soil, rock, air and biotic resources in the near-surface terrestrial environment. A typical sample of mineral soil comprises 45% minerals, 25% water, 25% air and 5% organic matter - however these proportions can vary.

Soil characteristics vary also in depth and across different landscapes.



Soil formation (pedogenesis)

Soil is a precious natural resource and its development is a long temporal process (it is estimated that the formation of a layer of 30 cm of soil takes from 1,000 to 10,000 years). Pedogenesis is the combined effect of physical, chemical, biological, and anthropogenic processes on soil parent material. Soil genesis involves processes that develop layers or horizons in the soil profile. These processes involve additions, losses, transformations and translocations of material that compose the soil. Minerals derived from weathered rocks undergo changes that cause the formation of secondary minerals and other compounds that are variably soluble in water. These constituents are moved (translocated) from one area of the soil to other areas by water and animal activity. The alteration and movement of materials within soil causes the formation of distinctive soil horizons.
 
This resource is lost forever through various natural and/or specific human induced activities which trigger its erosion. The latter process can specifically be accentuated through brief yet heavy storms hitting and displacing large amounts of soil (mainly from the upper horizon section, but in some severe cases, almost if not the entire profile section). It is a cross-cutting theme affecting issues related to food security, climate change desertification and biodiversity.
 
Soils can also be severely degraded or partially compromised through insensitive sealing of field parcels for development purposes (in some cases the condition is reversible subject to type of materials used to reduce land permeability) but also through chemical diffuse or point-source contamination, compaction (mainly through the use of heavy agricultural machinery, salinisation (including also chemical processes of sodification, alkalinsaion and acidification), organic matter depletion, landsliding or soil displacement and biodiversity fertility losses.
 
The following "life"cycle factors shape soil character diversity:

Passage of time: The estimation of relative age or degree of maturity of soils is universally based on horizon differentiation. In practice, it is generally maintained that the larger the number of horizons and the greater their thickness and intensity the more mature is the soil. However, it should be kept in mind that no one has ever witnessed the formation of a mature soil. In other words, our ideas about soil genesis as revealed by profile criteria are inferences. They are theories, not facts. This accounts for the great diversity of opinion as to the degree of maturity of specific soil profiles. It is well known that certain eminent pedologists take objection to the general belief that chernozems are mature soils; others consider brown forest soils and gray-brown-podsolic soils merely as immature podsols. The list of controversial soil types is quite long. Whatever the correct interpretation may be, it is evident that the issues center around the factor time in soil formation.

Parent Material: The lower strata of a soil profile, especially those immediately below the B horizon, are commonly designated as C horizons. In many soils of northern regions, such as podsols, the selection of the C horizon offers no difficulties, because the A and B horizon" are sharply differentiated from the unweathered material. In temperate regions, the selection of the C horizon is often ambiguous, especially if the soils are derived from sedimentary deposits. In practice, the difficulties are circumvented by distinguishing several C horizons that are designated as C1, C2, C3, or D1, E1, etc. In the tropics, where rock decomposition may occur to great depth, the problem of the proper choice of the C horizon is often so complex that doubt is cast on the universal applicability of the simple ABC scheme of soil-profile description.

Topography (relief): Physiographers and geomorphologists have no generally accepted definitions of topography and relief. In the present discussion, the terms are used synonymously and denote the configuration of the land surface. Of the topographic designations commonly employed in pedology, the following are prominent: level or flat, undulating, rolling, hilly, and mountainous. Topography as a soil-forming factor has not received the attention it deserves. It is true, of course, that a considerable amount of information on runoff and erosion in relation to slope is at hand, but it deals primarily with the removal and the destruction of soil and not with soil formation. The topography or relief characterized by the inclination of the surface determines the rate of precipitation runoff and rate of formation and erosion of the surface soil profiles. Steep slopes allow rapid runoff and erosion of the top soil profiles and little mineral deposition in lower profiles.

Climate (including climate change): Quantitative functional analysis is only possible if the soil property and the conditioning factors investigated can be expressed in numerical terms. The factor climate is so complex that no single numerical value can be assigned to a given climate. It becomes necessary to work with individual climatic components, the most important of which are moisture (m) and temperature (T).

Organisms (including vegetation):  It is a universally known truism that microorganisms, plants, and many higher animals affect and influence the properties of soil. Micro-organisms, including fungi and bacteria, effect chemical exchanges between roots and soil and act as a reserve of nutrients. Humans can impact soil formation by removing vegetation cover with erosion as the result. They can also mix the different soil layers, restarting the soil formation process as less weathered material is mixed with the more developed upper layers. Some soils may contain up to one million species of microbes per gram (most of those species being unknown), making soil the most abundant ecosystem on Earth. 

On the other hand, vegetation impacts soils in numerous ways. It can prevent erosion caused by excessive rain and the resulting surface runoff. Plants shade soils, keeping them cooler and slowing evaporation of soil moisture, or conversely, by way of transpiration, plants can cause soils to lose moisture. Plants can form new chemicals that can break down minerals and improve soil structure. The type and amount of vegetation depends on climate, land form topography, soil characteristics, and biological factors. Soil factors such as density, depth, chemistry, pH, temperature and moisture greatly affect the type of plants that can grow in a given location. Dead plants and fallen leaves and stems begin their decomposition on the surface. There, organisms feed on them and mix the organic material with the upper soil layers; these added organic compounds become part of the soil formation process.

For all these reasons, soil, as a relatively immovable and formed in situ natural body, is fated to react, endure, and record environmental changes by being transformed according to the interactions of climatic, biotic, and anthropogenic forcing, as conditioned by geologic and topographic setting, over geological and biological time scales. This makes the monitoring of soil change an excellent (albeit complex) environmental assessment, because every block of soil is a timed “memory” of the past and present biosphere-geosphere dynamics. This memory takes multiple forms, such as micromorphology, profile features, and various soil physical,chemical, and biological properties. Learning to “decode” soil features and their changes into environmental information is as valuable as reading the records of ice cores for atmospheric conditions and interpreting tree rings for eco-climatic dynamics.

Further information on the physical and bio-chemical processes associated with soil formation can be found here.

Baseline soil surveys and sampling assessment generally examine physical properties shaping this resource, namely: profile's texture, structure, density, porosity, consistency, temperature, colour and resistivity.

Classifying Soils

Soil biomes are highly varied both in terms of their structure, spatial arrangement (as profiles), colour, depth and a wide range of other physio-chemical variables. 

Further information may be sought through the following links: 

World Reference Base for Soil Resources

FAO/ UNESCO classification

Unified Soil Classification System

AASHTO Soil Classification System

USDA Soil Taxonomy

Maltese Soils 

There are different types of soils in the Maltese Islands despite the archipelago's minute territorial extent. A detailed soil survey for Malta, the first of its kind, carried out by D.M.Lang in 1956-7 and published in 1960, concluded that Maltese soils are mainly derived from local geology, that is rock structure, geomorphology and stratigraphical setup, are highly calcareous and closely related chemically and with the natural and cultural landscapes prevailing at the time.
 
 
It classified Maltese soils formations as Carbonate, Xerorendzinas, Terra and soil complexes, further subdivided into subtypes (series) named after the localities where the first examples were noted. One of the most important products made possible through this detailed scientific assessment was the mapping of the entire territory.
 
 

Maltese Soil Information System (MALSIS)

The MALSIS database initiative (project reference: LIFE 00/TCY/MT/000036) classified the local soil characteristics according to the World Reference Base for Soil Resources (WRB Classification system) and has identified 19 soil units, from 7 soil reference groups (Arenosols, Calcisols, Cambisols, Leptosols, Luvisols, Regosols and Vertisols). Typical profiles for each reference group, established from non-Maltese soil survey assessments, are shown underneath:

 
Broad description of Taxonomic Units (local reference groups)

Arenosols (AR) - the deep sandy soils developed in residual sands, in situ after weathering of old, usually quartz-rich material or  rock, and soils developed  in  recently deposited sands as occur on beach lands. In the Maltese Islands, this type of soil is present in localised areas, in Ramla Valley in Gozo, and in Armier, Malta. 
 
Calcisols (CL) - these are the dominant soil group in the Maltese Islands. They are recognised by the presence of secondary CaCO3 concentrations as coatings on soil structure faces. The calcic horizons may be present in the lower topsoil and/or the subsoil/substrate horizons. Soil units  (secondary calcium carbonates) identified  within  this  group  include  Endoleptic  Calcisols,  Epileptic  Calcisols,  and Hypocalcic Calcisols.  These  (lime-rich) soils generally developed  in  dry  areas.  Dryness,  and  in  places  also  stoniness  limit  the suitability  of  these  soils  for  agriculture,  however,  if  irrigated,  drained,  and fertilised, Calcisols can be highly productive under a wide variety of crops.
 
Cambisols (CM) – in Malta, the Cambisols are soils with limited development showing only a distinct subsoil with a significantly different (browner or redder) colour to the topsoil but no characteristic calcic or reddish clay argic horizons.
 
Leptosols (LP) - Calcari-Lithic Leptosols occur mainly on the vertical cliff faces where a very thin  weathered  layer  of  soil  overlies  rock  at  less  than  10  cm  depth.  These are the most common form of Leptosols,  found on relatively undisturbed garigue (both on  level and very steep slopes), where rock occurs at 10 to 25 cm depth.  Leptosols are  shallow  soils over  rock or  gravelly material whose development  is  often  limited  by  erosion.  Shallowness  affects  cropping  by influencing the range and type of cultivations which can be carried out but also by  restricting nutrient uptake,  root growth and,  in  the  case of  fruit  trees,  root anchorage.
 
Luvisols  (LV)  -  in Malta,  the  reddish  clay  Luvisols are  the  result of  soil development under different  climatic  conditions  to  those of  the present age. They probably formed during the wetter climates associated with Glacial advances  in Northern Europe  (Pleistocene Stadials). They  are  now  relict  soils  and  all  contain  secondary  CaCO3  concentrations  reflecting  the current  predominant  pedo-climatic  regime  in  the  Islands.  The  shallow  eroded  remnants  of former  Luvisols  in  relatively  undisturbed  garigue  are  classified  as  Chromi-Calci-Epileptic Luvisols. Other soil units identified include the Endolepti-Chromi-Calci  Luvisols  and Chromi-Calcic Luvisols. The latter are the deepest form of Luvisols where relatively thick Pleistocene colluvial material underlies more recent terrace or colluvial material within 1 m depth.  Luvisols are normally fertile soils suitable for a wide range of uses, but certain  types  require artificial  internal drainage and careful  timing of cultivations. In  Malta,  these  soils  include  the  ‘soil  pockets’  formed  on  karst landscape.
 
Regosols (RG) – a group that includes ‘other’ soils, with very limited development in virtually unaltered parent material, showing no dark coloured topsoil and no distinct subsoil horizons. In  Malta,  Spolic  Regosols  have  been  described;  these  soils  are  situated  on made  ground terraces overlying urban waste material. 
 
Vertisols (VR) - the cracking claysoils, are restricted to the Blue Clay outcrop in Malta. These soils are recognised by their very clayey nature, the presence of deep, wide cracks during the dry months and the presence of slightly gleyed and rusty mottles. 
 
The latter two soil groups, the heavy cracking clays (Vertisols) found on  the Blue Clay,  and  the  deep  sandy  soils  developed  in  recently  deposited sand  beaches,  are  mostly  vulnerable  to  soil  degradation  especially  if  not managed  in  a  sustainable  way,  and  deserve  to  be  designated  as  soils  of conservation value.
 
Calcisols, which occupy approximately 27% of total country area, Luvisols and Leptosols  are  the most  common  groups.
 
Further information about MALSIS project, which was carried out and managed by the Agriculture Department within the then Ministry for Resources and the Environment (MRAE), with technical and consultancy advice provided by UK's Cranfield University (including other technical information derived from systematic sampling of local soil characteristics emerging from its database established for this purpose), can be found here.
 
A series of thematic maps, prepared by the then Agriculture Department's National Soils Unit, were generated about various soil attributes investigated during this 2004 nation-wide sampling assessment: