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It is a work based: definition, structure, classification, uses, problems and current status in Merida - Venezuela soil.

luis trejo

on 14 November 2011

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Transcript of THE SOIL

THE SOIL What is the soil? Soil estructure clasification system Soil chemical
properties The soil uses Problem and current
state of soil in Merida Soil, dirt, sediment, what’s the difference? Depending upon whom you ask, you might get a radically different answer. Some sources state that the only difference between them has to do with their location: soil is the unconsolidated material on the ground, dirt is that same matter on your hands or clothes, and sediment is the same material on the bottom of a river or lake. Soil is a thin layer of material on the Earth's surface in which plants have their roots. It is made up of many things, such as weathered rock and decayed plant and animal matter. Soil is formed over a long period of time. Definition of Soil Structure: The arrangement and organization of primary and secondary particles in a soil mass is known as soil structure. Soil structure controls the amount of water and air present in soil. Plant roots and germinating seeds require sufficient air and oxygen for respiration. Bacterial activities also depend upon the supply of water and air in the soil. Soil is used in agriculture, where it serves as the primary nutrient base for plants; however, as demonstrated by hydroponics, it is not essential to plant growth if the soil-contained nutrients could be dissolved in a solution. The types of soil used in agriculture (among other things, such as the purported level of moisture in the soil) vary with respect to the species of plants that are cultivated. Chemical Properties of Soil

Salinity (EC)
Cation exchange capacity (CEC)
Organic matter
C:N ratio (Carbon to Nitrogen)

Soil pH

A measure of the acidity or alkalinity of a soil.
Neutral = 7.0
Acidic < 7.0
Alkaline > 7.0
Logarithmic scale which means that a 1-unit drop in pH is a 10-fold increase in acidity.

Soil pH and plant growth
Affects availability of plant nutrients (in general, optimal pH is between 5.5-7.5)
Low pH soils (<6.0) results in an increase in Al. Aluminum is toxic to plants
Affects availability of toxic metals (in general, more available in acidic soils)
Affects the activity of soil microorganisms, thus affecting nutrient cycling and disease risk In recent years there has been a strong expansion of agriculture in the north of Merida State, Venezuela with a substantial increase in chemicals but a significant reduction in rural education and agricultural research. The study area is experiencing a time of crisis, both ecologically and economically. The increased use of chemicals and their expansion is threatening the fragile Andean ecology and changing economic conditions are limiting the margins
profit. We see agriculture as an engine while a threat to rural development in the area. The intensive use of pesticides, intensive cultivation of the slopes, and low water prices and foreign labor, are the perfect recipe for serious degradation of water quality, soil fertility and human health . However, there are improved technologies for agricultural production are in better harmony with the environment, and to improve business competitiveness. While our goal is to identify these more favorable, our challenge will be to understand the transaction costs between increased production and environmental damage. Soil structure describes the arrangement of the solid parts of the soil and of the pore space located between them (Marshall & Holmes, 1979). The structure depends on what the soil developed from. The practices that influence soil structure will decline under most forms of cultivation—the associated mechanical mixing of the soil compacts and shears aggregates and fills pore spaces; it also exposes organic matter to a greater rate of decay and oxidation (Young & Young, 2001). The life sustaining ability of soil is best understood by appreciating the complex cycles of decay and erosion. Its natural formation occurs in a series of layers starting at the surface but gradating down to the deepest bedrock. The surface layer is where active decomposition begins. Exposure to atmospheric elements, surface warmth and moisture helps to break organic matter into loose mulch like material. At the microscopic level, this layer is teeming with a diversity of bacterial, fungal and algal life forms. In combination with larger organisms like beetles and worms they provide the additional recycling activity to enable minerals and nutrients to be retrieved from the decaying organic matter and returned to the soil. Another family of soil based micro-organisms are involved in relationships that enable plants to absorb nitrogen from their roots. The benefits of improving soil structure for the growth of plants, particularly in an agricultural setting include: reduced erosion due to greater soil aggregate strength and decreased overland flow; improved root penetration and access to soil moisture and nutrients; improved emergence of seedlings due to reduced crusting of the surface and; greater water infiltration, retention and availability due to improved porosity.
Sustain plant and animal productivity and diversity;
Maintain or enhance water and air quality;
Support human health and habitation
The underlying principle in the use of the term “soil health” is that soil is not just a growing medium, rather it is a living, dynamic and ever-so-subtly changing environment. We can use the human health analogy and categorise a healthy soil as one:
A state of composite well-being in terms of biological, chemical and physical properties;
Not diseased (ie not degraded, nor degrading), nor causing negative off-site impacts;
Providing a full range of functions (especially nutrient, carbon and water cycling) and in such a way that it maintains this capacity into the future. Soil analysis A technical analysis of structure can isolate the important layers of soil, their relationship to each other, aeration and drainage characteristics along with the mineral components characteristic to a particular location. It can also indicate the comparative rates and efficiency for recycling organic material. Information about structure will assist the serious gardener to predict how soils behave under varying seasonal conditions.

Soil type is a classification based on the major particle constituent along with the average pH reading. The most typical examples of soil type are sand, clay, and silt based. In some respects this information has limited value because soils tend to vary significantly across regions even when described to be of similar type. This is where an understanding of structure will provide a clearer picture. with this we can see if the soil is fertile or not, by its soil type, so having porous or non-capacity (Cation Exchange Capacity) Land Clasification "A" Horizons may be darker in color than deeper layers and contain more organic material, or they may be lighter but contain less clay or sesquioxides. The A is a surface horizon, and as such is also known as the zone in which most biological activity occurs. Soil organisms such as earthworms, potworms (enchytraeids), arthropods, nematodes, fungi, and many species of bacteria and archaea are concentrated here, often in close association with plant roots. Thus the A horizon may be referred to as the biomantle. However, since biological activity extends far deeper into the soil, it cannot be used as a chief distinguishing feature of an A horizon. The "O" stands for organic. It is a surface layer, dominated by the presence of large amounts of organic material in varying stages of decomposition. The O horizon should be considered distinct from the layer of leaf litter covering many heavily vegetated areas, which contains no weathered mineral particles and is not part of the soil itself. O horizons may be divided into O1 and O2 categories, whereby O1 horizons contain decomposed matter whose origin can be spotted on sight (for instance, fragments of rotting leaves), and O2 horizons containing only well-decomposed organic matter, the origin or which is not readily visible. The B horizon is commonly referred to as "subsoil", and consists of mineral layers which may contain concentrations of clay or minerals such as iron or aluminium oxides or organic material which got there by leaching. Accordingly, this layer is also known as the "illuviated" horizon or the "zone of accumulation". In addition it is defined by having a distinctly different structure or consistency to the A horizon above and the horizons below. They may also have stronger colors (is higher chroma) than the A horizon. The C horizon is simply named so because it comes after A and B within the soil profile. This layer is little affected by soil forming processes (weathering), and the lack of pedological development is one of the defining attributes. The C Horizon may contain lumps or more likely large shelves of unweathered rock, rather than being comprised solely of small fragments as in the solum. "Ghost" rock structure may be present within these horizons. The C horizon also contains parent material.
R horizons basically denote the layer of partially weathered bedrock at the base of the soil profile. Unlike the above layers, R horizons largely comprise continuous masses (as opposed to boulders) of hard rock that cannot be excavated by hand. Soils formed in situ will exhibit strong similarities to this bedrock layer. Soil of Venezuela SOILS AND TOPOGRAPHY
Six different relief formations are recognized (CATIE/FAO, 2000):
a. the continental platform, located at < 1,000 metres, and that covers 17 percent of the territory;
b. the coastal or Caribbean mountain chain, with altitudes ranging between 200 and 2,765 metres, and covering 3.2 percent of the land;
c. the valleys and hills of the Falcón, Lara and Yaracuy States, which represent 2.6 percent of the territory;
d. the Andean chain, with altitudes from 200-5,007 metres, 5.8 percent of the land;
e. the plains or Llanos, at 40-200 metres, and 25.5 percent of the territory, and
f. the Guyana Shield, located from 100-3,840 metres and covering 45.4 percent of the national territory.
There is considerable variation in Venezuelan soils, partly linked to the geology of each region. Agricultural use of soils is constrained by a number of limitations: 4 percent of the territory is arid, 18 percent has drainage limitations, 32 percent are soils of low fertility, and 44 percent is on steep slopes, thus leaving only 2 percent without limitations (Casanova et al., 1992).
The geologically oldest formation is that of the acid Guyana shield to the south of the Orinoco River, frequently identified as the Pantepui Region, it extends into north-western Guyana and northernmost Brazil. The geology consists of a mainly granitic Precambian base (the Guyana Shield), overlain by younger sedimentary sandstones and quartzites of variable thickness. This gave rise to very infertile, leached soils that include: (a) soils of the flat-topped table mountains ("tepuys") and the Gran Savanna, characteristically very sandy, with extremely low organic matter content; (b) mountain clay-sand soils, derived from granite and gneiss and (c) soils along the Orinoco River, influenced by alluvial sediments
Along the more recent Andean region (the Andes, the Interior Chain and the Coastal Chain), soils are newer than those of the Guyana shield but have been altered by erosion, particularly in the piedmont, where human intervention has been drastic through deforestation.
In the oldest plains or Llanos (Eastern and Central Plains, and the Plains of the Meta River) oxisols predominate, frequently with very superficial horizons and an underlying ferrous layer. The more recent plains (Western Llanos, and South of Lake Maracaibo), some of the best soils are found. These are deep relatively fertile soils, though may have drainage limitations during the peak of the wet season.
The delta of the Orinoco River includes soils limited by salinity and by the presence of high sulphate concentrations.
Utilization of soils along much of the coast is severely limited by low rainfall. Soils are mostly superficial litosols, or poorly developed entisols, very low in organic matter and P.
A large proportion of soils in Venezuela are acid (Table 5) and therefore have low cation exchange capacity, are low in P and frequently in several bases. Region percent soils with pH< 5.5 percent soils pH5.5 to 8.5
Western Venezuela 60-70 30-40
Western Llanos 15-30 70-85
Central Llanos 53-75 25-47
Andean region 53-69 31-47
Region of Zulia 32 66
Central region 19-46 54-77
Entisol - recently formed soils that lack well-developed horizons. Commonly found on unconsolidated sediments like sand, some have an A horizon on top of bedrock.

Vertisol - inverted soils. They tend to swell when wet and shrink upon drying, often forming deep cracks that surface layers can fall into.


Aridisol - dry soils forming under desert conditions. They include nearly 20% of soils on Earth. Soil formation is slow, and accumulated organic matter is scarce. They may have subsurface zones (calcic horizons) where calcium carbonates have accumulated from percolating water.

Many aridiso soils have well-developed Bt horizons showing clay movement from past periods of greater moisture.

Mollisol - soft soils with very thick A horizons.
Spodosol - soils produced by podsolization. They are typical soils of coniferous and deciduous forests in cooler climates.


Ultisol - soils that are heavily leached.

Oxisol - soil with heavy oxide content.

Histosol - organic soils.

Andisols - volcanic soils, which tend to be high in glass content.

Gelisols - permafrost soils.

Soil intrazonal

Intrazonal soils have more or less well-defined soil profile characteristics that reflect the dominant influence of some resident factor of relief or parent material over the classic zonal effects of climate and vegetation. There are 3 major sub-types, 2 of which have 2 further sub-types each.
Calcimorphic or calcareous soils develop from a limestone. It has two sub-types:
Rendzina soils are thin soils with limited available water capacity.
Terra Rossa soilss are deep red soils associated with higher rainfall than Rendzina.
Hydromorphic soils form in wetland conditions. There are two sub-types:
Peat soils form under circumstances that prevent the breakdown of vegetation completely.
Halomorphic soils form due to soil salination.
Soil of arid and semiarid
Distribution: Arid soils are one of the most prevalent soil orders of the world.
Climate: Arid soils are most characterized by their water deficiencies. Most arid soils contain sufficient amounts of water to support plant growth for no more than 90 consecutive days.
Mineralogy: Arid soils typically contain high levels of calcium carbonates, gypsum, as well as sodium.
During soil formation, soluble salts at translocated to a subsurface horizon. However, since there is insufficient rainfall to leach salts from the soil profile, soluble salts accumulate.
Aridisols often have a surface horizon that is pale in color and low in organic matter.
Fertility: Due to limited moisture and accumulated soluble salts, these soils are generally not suited for major crop production. However, if properly managed and irrigated, these soil may become productive. There are notable soils in Maui that are classified as arid soils, but are notable for their unique fertility.
Semiarid agriculture in this context is defined as agricultural production in semiarid areas constrained by low rainfall, poor or low nutrient soils, high temperatures, high solar radiation, and low precipitation.

High transpiration and evaporation rates in vegetated areas are also dominant hydrological features of a semiarid agricultural environment. Therefore, natural fragility of dry land ecosystems renders them extremely vulnerable to inappropriate land use and exploitation. Farmers often over-cultivate the few available areas of fertile land in an attempt to increase production. This implies that these dry land communities are likely to become vulnerable to climatic variations.
Soil fertility

Fertile soil has the following properties:
It is rich in nutrients necessary for basic plant nutrition, including nitrogen, phosphorus and potassium.
Good soil structure, creating well drained soil, but some soils are wetter (as for producing rice) or drier (as for producing plants susceptible to fungi or rot) such as agave.
A range of microorganisms that support plant growth.
It often contains large amounts of topsoil.
Fertilite organic soil management is guided by the philosophy of " feed the soil to feed the plant". This basic precept is implemented throught a series of practices designed to increase soil organic matter, biological activity, and nutrient availability

Shortly fertility

Ultisols (Latin ultimus: Last): Basic Soil Materials With low saturation weatherable red that give the United Nations. They have washed clay materials. Its fertility is low and the poor child in humus. They are found in humid, tropical and temperate. The natural vegetation Services You Can forest, savannah or swamp and marsh flora. His word capacity, Can You accrue Agricultural fertilizer and good management.


Entisols (Latin ent: youth): weakly developed soils on material hauling in mountainous or hilly areas. Its limitations are the poor development of the profile, low fertility and, sometimes, the high salt content. They are found in any climate and vegetation is consistent with the same, although the most characteristic is the alluvial river valleys.
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