What colour is red earth?
Red earth gets its distinctive color from the presence of iron oxides, also known as iron rust. Specifically, the red hues come from hematite, an iron oxide with the chemical formula Fe2O3. Hematite is a common mineral found in rocks and soils across the world, and it weathers to produce the familiar rusty red coloring.
What causes the red color in soils?
The red color in soils is primarily caused by the presence of iron oxides, particularly hematite (Fe2O3). As rocks weather and break down over time, minerals like hematite are chemically altered and oxidized, causing them to turn red.
Some key factors that contribute to red soil formation include:
– Parent rock material – Iron-rich basalts and other mafic igneous rocks weather into red soils. The iron oxide minerals present in these rocks alter into hematite during weathering.
– Warm, moist climate – Warm temperatures and abundant moisture promote chemical weathering processes that oxidize iron into hematite. Prolonged seasons of wetness and dryness also mobilize and concentrate iron oxides.
– Good drainage – Red soils tend to form in well-drained areas where hematite is not washed away by rainfall or groundwater flow. Poorly drained soils tend to lack oxygen, preventing iron oxidation.
– Organic matter – Soil microorganisms and plant roots release compounds that facilitate iron oxidation. Soils with moderate organic content tend to be redder.
– Time – It takes thousands to millions of years of weathering to transform rocks and minerals into red soils. Older landscapes tend to have more thoroughly oxidized and red soils.
What minerals make soil red?
The main minerals that contribute to red soil color are iron oxide minerals like hematite, maghemite, and goethite. Some key red soil minerals include:
– Hematite (Fe2O3) – The most abundant iron oxide mineral in red soils. Its distinctive blood-red color comes from its hexagonal crystal structure.
– Maghemite (Fe2O3) – A reddish-brown iron oxide that forms as a weathering product in soils. Often found with hematite.
– Goethite (FeO(OH)) – An iron oxyhydroxide mineral that produces yellowish, reddish-brown soils. Goethite forms under conditions where hematite is unstable.
– Lepidocrocite (FeO(OH)) – An orange-red iron hydroxide mineral that arises from weathering and soil formation processes.
– Ferrihydrite – A weakly crystalline hydrated iron oxide that is structurally similar to goethite and hematite. Imparts a red-brown color.
– Ilmenite (FeTiO3) – An iron-titanium oxide with a black streak but contributes some iron oxides upon weathering.
These and other iron oxide minerals accumulate in soils through the breakdown of iron-bearing minerals in rocks. Prolonged oxidation transforms them into highly stable red hematite over time.
What is the chemical composition of red soil?
The chemical composition of red soils is dominated by iron and aluminum oxides. A typical chemical analysis of red soil gives:
– Iron oxides – 5 to 15% iron oxides by weight, primarily as hematite Fe2O3. Provides the characteristic red hues.
– Aluminum oxides – 5 to 25% aluminum oxides by weight, chiefly as gibbsite Al(OH)3. Contributes to texture and structure.
– Silica – Up to 80% silica (SiO2) in the form of quartz sand and silt particles. Gives a gritty/sandy texture.
– Water – 5 to 15% bound water by weight, both adsorbed and in minerals like goethite.
– Organic matter – 1 to 5% organic matter by weight, consisting of plant residues, roots, and soil organisms. Provides nutrients upon decomposition.
– Other oxides – Trace amounts of manganese and titanium oxides.
– Clay minerals – Variable clay content depending on parent material. Clays assist in aggregation.
– Soluble salts – Small amounts of soluble cations (Ca, Mg, Na, K) and anions (sulfate, chloride, nitrate).
The iron gives the red color, while silica sand and quartz provide the grainy texture. Aluminum and clay assist in forming granular structure. Organic matter supplies nutrients.
What are the physical properties of red soil?
Red soils have distinctive physical properties arising from their mineralogy and chemistry:
– Color – Varying hues of red, ranging from reddish-brown, dark red, to vibrant red. Reflects hematite content.
– Texture – Mainly sand to sandy loam texture. High silica content gives a course, gritty feel.
– Structure – Granular or blocky structure in the subsoil. Topsoil tends to be more massive and porous.
– Consistence – Loose and friable when dry. Sticky and plastic when wet.
– Porosity – Moderate to high porosity, with pores between aggregates. Improves drainage and aeration.
– Density – Low to medium bulk density, typically 1.3 to 1.6 g/cm3.
– Permeability – Moderate to rapid permeability and drainage in topsoil and subsoil. Low risk of waterlogging.
– Water retention – Low overall water holding capacity but can store water in pores. Rapidly loses moisture.
– Aggregation – Strong aggregation in subsoil, weaker in topsoil. Clay assists binding of soil particles.
– pH – Slightly acidic to neutral pH, from 5.5 to 7.0. Iron oxides buffer against further acidification.
These properties allow red soils to retain nutrients and moisture while still draining well. The granular structure facilitates root growth and plant anchorage.
What are some global examples of red soils?
Here are some major examples of red soils found across the world:
| Red Soil Type | Location |
| Terra Rossa | Around Mediterranean basin, e.g. southern Europe, North Africa, and Levant |
| Red Mediterranean soils | Spain, southern France, Italy, Greece, Turkey, Syria, Morocco, Tunisia |
| Red Ferralsols | SubSaharan Africa, southeast Brazil, Indonesia, Australia |
| Red Lixisols | Subtropical regions like southern USA, Mexico, southeast Asia |
| Red Nitisols | India, southeast Asia, central Africa, South America |
| Red Ferrosols | Australia, parts of southern Africa |
| Red Latosols | Brazil, tropical Africa, Southeast Asia |
These soils span tropical to subtropical climates with distinct wet-dry seasons promoting iron oxide formation. Parent materials are often iron-rich sedimentary or volcanic rocks. Red soils are highly weathered and nutrient-poor, but structurally stable for agriculture.
How do red soils develop and form?
Red soils develop through a long process of chemical weathering and transformation of iron-bearing parent rocks:
– Initial weathering – Rocks like basalt and shale begin breaking down, releasing iron-rich minerals. With drainage, iron oxidizes into hematite, coloring the parent material.
– Pedogenesis – Over time, distinct layers form in the weathering profile. Hematite accumulates in the subsoil, while leaching concentrates silica and quartz as sand in the topsoil.
– Mineral transformation – Unstable iron minerals transform to more crystalline forms like hematite and goethite. Processes like hydrolysis, hydration, and diffusion facilitate these mineral changes.
– Structural development – Clay minerals and iron oxides gradually bind weathered particles into aggregated peds and granules. This creates distinctive soil structure.
– Organic matter accumulation – Vegetation establishes and plant roots + soil organisms produce organic matter. This supports more iron oxidation through chemical byproducts.
– Oxidation and dehydration – With alternating wet-dry seasons, iron becomes highly oxidized while clays dehydrate into metal oxide lattices. This further reddens and hardens the subsoil.
– Translocation – Soluble salts and silica move upwards while sesquioxides of iron, aluminum and clay are retained, increasing profile differentiation over time.
After thousands to millions of years these processes produce deep, intensely weathered and oxidized red soils.
How do you identify red soils?
Red soils can be identified using some simple field tests:
– Observe color – Look for a vivid red, reddish-brown or yellowish-red soil matrix. Hematite imparts the signature reddish hue.
– Check texture – Rub the soil to check for a gritty, sandy feel from high silica content. Yet the soil still forms clods if compacted.
– Test drainage – Dig a hole and see how fast water drains through the profile. Red soils tend to have good drainage.
– Evaluate structure – Look for granular or blocky peds that break into hard prisms or blocks. This indicates good aggregation.
– Test pH – Use a soil testing kit to determine pH. Red soils tend to be moderately acidic with a pH between 5.5 and 7.
– Shake soil in water – Dispersed clay should give water a red tinge from suspended iron oxides. Settled particles are predominantly sand and silt.
– Look for parent material – Check surroundings for any iron-rich source rocks like basalt, shale, schist or granite.
– Examine the profile – Dig a pit to see if subsurface horizons are redder and more clayey. Increased iron oxide content gives deeper hues.
With training, the distinctive color, texture, and structure makes most red soils easy to identify in the field.
What vegetation grows well in red soils?
Certain types of vegetation thrive in red soils, while others are restricted by the low native fertility:
– Grasses – Most grasses including cereals, lawn grasses, and wild prairie grasses grow well. Graminoids are adapted to lower nutrient soils.
– Legumes – Legumes like beans, peas, peanut and clover grow reasonably well with proper inoculation. Nitrogen fixation helps overcome nutrient limitations.
– Fruit trees – Citrus fruits, peach, plum and tropical fruits can grow well with amendments and fertilizers. The lighter texture provides good drainage.
– Timber trees – Pine, fir, acacia, eucalyptus, teak and exotic conifers can grow in low fertility red soils. Some support extensive forestry.
– Drought-tolerant plants – Succulents like agave and cacti, as well as native scrub and chaparral species, are adapted to the good drainage.
– Early succession species – Grasses, shrubs and fast-growing trees suited for poor soils will establish first until native fertility builds up.
However, red soils do not support more nutrient demanding crops like vegetables, rice, and sugarcane without substantial fertilization. The sandy texture also requires more frequent irrigation for good plant growth.
How are red soils used and managed?
Red soils are used in a variety of ways:
– Cropland – Cereal grains, legumes, fruits, coffee, cashews and other crops can be grown with nutrient and water amendments. Minimum tillage helps retain organic matter. Cover crops and crop rotation also improve fertility.
– Grazing land – The native grasses and shrubs provide grazing for livestock like cattle, sheep and goats. Legumes offer protein-rich forage. Controlled stocking rates prevent overgrazing and erosion.
– Forestry – Timber plantations work well in red soils. Silviculture treatments enhance tree growth while minimizing erosion. Controlled harvesting helps maintain long-term productivity.
– Restoration – Addition of compost, manure and organic residues can gradually improve red soils for horticulture and landscaping uses like parks, gardens and recreational areas.
– Construction – The granular structure provides stable foundations for buildings and roads. Red soils compact well with minimal shrink-swell. Drainage is improved with gravel layers and perforated piping.
– Wastewater filtration – Good permeability makes some red soils suitable for soil-based wastewater treatment systems like drainfields. But high iron may lead to clogging over time.
With proper management red soils can be quite productive agriculturally. Their appearance and drainage also lend well to structural applications.
What are some challenges and limitations of red soils?
Some limitations and management challenges with red soils include:
– Low native fertility – Inherently low levels of essential plant nutrients like nitrogen, phosphorus and potassium. Requires significant fertilizer inputs.
– Moisture stress – Fast drainage reduces water holding capacity. May require more frequent irrigation for good crop yields, especially on coarser-textured red soils.
– Compaction risk – Sandy soils are prone to compaction under heavy machinery traffic and overgrazing. Degrades soil structure.
– Tillage difficulties – High iron oxide content causes soils to be harder to work. Plowing requires significant power and can lead to cloddiness.
– Erosion potential – Limited cohesion on sloping land increases susceptibility to water and wind erosion if groundcover is not maintained.
– Aluminum toxicity – High levels of exchangeable aluminum in strongly weathered red soils can limit crop root growth. Liming helps mitigate this.
– Iron toxicity – Under flooded conditions, reduced iron can reach toxic levels for rice and other waterlogged crops. Proper drainage control is needed.
– Workability issues – Stickiness when wet and rock-hard consistency when dry hamper tillage, planting and harvesting operations. Proper timing is essential.
With care most limitations can be addressed through integrated soil and nutrient management tailored to the local environment.
Conclusion
Red soils derive their distinctive color from iron oxides like hematite and maghemite. These minerals accumulate through weathering of iron-rich rocks in warm, moist climates with seasonal wetting and drying. Well-drained red soils occur on all inhabited continents and support crops with proper management. Key challenges include low fertility, moisture stress, and tillage difficulties. However with care, red soils can be quite productive for growing grasses, legumes, fruits and tree crops. The same iron oxides that color the soils also provide unique physical and chemical properties favorable for plants and structures in the right environment.