master the garden: soil basics

composition, texture, structure, and profile

https://www.nationalgeographic.org/encyclopedia/soil-composition/
nature of soil

Soil. A natural body embracing the topsoil, but also the subsoil and other layers above its parent rock – is a complex material that supports all life on earth and is a product of the environment – of the rock waste which is its parent material, of the climate under which it weathers, of the kind of topography upon which it is situated, of the vegetation and other organisms with which it develops, and of the length of time during which it has been developing. 1966 in An Encyclopaedia of New Zealand, edited by A. H. McLintock

It is the outermost layer of the earth’s crust and depending on where you are standing, it may be only a few inches deep or many feet in depth and it has been many years, even thousands, in the formation.

This naturally deposited unconsolidated material which covers the earth’s surface and whose chemical, physical and biological properties support life by maintaining temperature, providing a reservoir for water, a source of oxygen, carbon and mineral nutrients satisfactory for plant growth and support of many forms, is one of the most important elements of an ecosystem.

The factors involved in the formation of natural soils are:

  1. Living Matter – plants, animals, and microorganisms
  2. Climate – cold, heat, snow, rainfall, and wind
  3. Parent Material – the ‘fineness’ of particle size as well as their chemical and mineralogical composition
  4. Relief – slope and landform
  5. Time – in some cases thousands of years
soil composition

Each area has a unique soil recipe and this mix of soil ingredients, that varies from place to place, contains both biotic (all the living and once-living things in soil, such as plants and insects) and abiotic factor (all nonliving things—for example, minerals, water, and air). The abiotic factors of soil composition can influence the biotic factors and is key in determining what plants can grow in an ecosystem. The basic components that make up the composition of soil are minerals, organic matter, water and air – of which typical soils consist approximately of 45% mineral, 5% organic matter, 20-30% water, and 20-30% air.

The dynamic nature of soil renders the graphic above a generalization because soil is complex and can change and fluctuate daily depending on a number of properties such as water supply, cultivation practices, and even the soil type. The solid phase of soil (minerals and organic matter) are generally stable in nature, but, if the organic matter is not managed properly, it can be depleted from the soil. The gas and liquid phase of soil (water and air) are the most dynamic properties with the relative amounts of water and air in the soil constantly changing as the soil wets and dries.

Because soils are formed in a variety of ways and under an unlimited number of conditions, it is hard to give a number for the types of soils we have on earth. However, they can be grouped according to what they are made of.

soil texture

The texture of mineral soils – typical of the average garden or farm – effects drainage, aeration, the amount of water a soil can hold – its erosion potential, and even the amount of nutrients that can be stored.

  • Sand – Sand is the largest of the three soil particle sizes (sand, silt and clay) ranging from 0.05 to 2 millimeters in diameter and you can see the individual particles with the naked eye. Sandy soils are often referred to as ‘light soils’ – due to high portions of sand and very little clay (sand is lighter in weight than clay). They warm quicker in spring and summer and tends to be acidic, low in nutrients; partly due to the low surface area and low charge that does attract and retain water or nutrients, and because it is quick to drain water, which can easily wash nutrients away. Wet sandy soil feels gritty when rubbed between two fingers and does not maintain its shape when formed into a ball.

A gram of coarse sand contains about 1000 particles. But a gram of clay contains about 90 billion particles.

  • Silt – Silt particles are medium textured soils – intermediate in size between sand and clay about 0.002 to 0.05 mm in diameter and the fine texture is easily compacted and prone to washing away with rain. Silt is light in weight and drains well – with a high fertility rating.
  • Clay – Clay particles, smallest of the three soil particle sizes, are less than 0.002 millimeters in diameter. A heavy soil type that benefits from high nutrients, clay soils can remain wet and cold in winter and dry out in summer and feel sticky when wet, and can be formed into a ball. Individual clay particles are not visible to the naked eye and often accumulate in the lower soil layers (the subsoil) as particles travel with soil water or mechanical sorting down through the topsoil. Topsoil is generally higher in sand, silt, organic matter, and microorganisms. Subsoil is often higher in clay and salts. Clay particles are plate-shaped and can align in sheets which can compact and form hard soil layers called pans.
  • Organic Matter – although important in the biological, chemical, and physical aspect of soil, is not generally considered in the textural make-up of mineral soils. Organic matter is that fraction within soils which results from the decomposition of plant and animal matter through the action of fungi and bacteria. It is an extremely important constituent of soils and provides the natural home for millions of bacteriological organisms which are vital to the biological and chemical reactions required for sustaining plant life. Benefits of organic matter in soil include:
    • Aids in moisture retention in the soil
    • Supplies nutrient elements for plant use (particularly nitrogen and sulfur)
    • Increases the nutrient holding capacity of soil
    • Enhances soil aggregation and aeration
    • Improves soil tilth which makes a soil friable
    • Aids in the reduction of soil erosion
  • Loam – A soil with roughly equal influence from sand, silt, and clay particles. These soils are fertile, easy to work with and have good drainage. Depending on the dominant composition soil, they can be either sandy or clay loam – they are considered a perfect balance of the negative to positive characteristics of the composition soils.
  • Peat – Peat soil is high in organic matter and retains a large amount of moisture. This type of soil is rarely found in a garden and often is imported into a garden or farm as a soil amendment.
  • Chalk – Chalk soil can be either light or heavy, but will always be highly alkaline due to the calcium carbonate (lime) within its structure and because these soils are alkaline they will not support the growth of ericaceous plants that require acidic soils to grow.
class names

The texture of a soil, expressed by the use of class names, (i.e., clay, sandy clay, silt loam, loam sand, etc.) is based on the relative proportions of these ingredients in a given soil. The different class names are shown in the textural triangle below.

photo – truegreen lawn care
soil structure

Soil structure refers to the grouping or ‘clumping’ of soil particles (sand, silt, clay, organic matter, and fertilizers) into porous compounds called aggregates or peds. Soil structure also refers to the arrangement of these aggregates separated by pores and cracks. Structure influences the movement of water down through the soil profile. This spatial arrangement of the individual particles – their aggregates and pores – play a vital role in determining plant and vegetation performance and unlike soil texture, soil structure is not permanent. It can be manipulated by cultivation practices.

The ped has a specific shape depending on the composition and the condition in which it is formed (wetting and drying out – freezing and thawing…) basic types of aggregate arrangements are:

Types of soil aggregates (Image from Victorian Resources on line at http://vro.dpi.vic.gov.au/dpi/vro/vrosite.nsf/pages/soilhealth_soil_structure)
soil profile

The soil profile or horizons is made up of distinct horizontal layers; these layers are called horizons. They range from rich, humus and topsoil to the rocky layers of subsoil and bedrock and like a history, each profile tells a story about the life of a soil. Most soils have three major horizons (A, B, C) and some have an organic horizon (O). The horizons are:

  • O  The Organic or humus is mostly organic matter and can be a thin, thick or not even a present top layer in some soils.
  • A  The Topsoil is mostly minerals from parent material with humus and mineral particles incorporated. Dark and rich in color, the A layer provides a beneficial material for seeds to germinate, roots to grow, and other organisms to live.

It takes at least 100 years to form 2.5 cm of topsoil

  • Eluviated layer is leached of clay, minerals, and organic matter, as water moves through the soil (process of eluviation) leaving a concentration of sand and silt particles of quartz or other resistant materials. This layer is light in color; beneath the A Horizon and above the B Horizon and is found in older and forest soils – but can be missing from the soil horizon also.
  • The Subsoil is rich in minerals that leached from the A or E horizons and accumulated here. It contains clay and mineral deposits like iron, aluminum oxides, and calcium carbonate, from layers above it when mineralized water moves down through the soil.
  • C The Parent Material or sometimes referred to as substratum, is the layer beneath the B Horizon and is composed of broken bedrock. Plant roots do not penetrate into this layer and very little organic matter is found.
  • The unweathered Bedrock layer is the foundation for all other layers. It is a mass of rock such as granite, basalt, quartzite, limestone or sandstone made up of igneous, sedimentary, or metamorphic rock that forms the parent material for some soils – if the bedrock is close enough to the surface to weather.
normaals iStockphoto
12 orders of soil taxonomy

There are many different types of soils, and each one has unique characteristics, like color, texture, structure, and mineral content. The depth of the soil also varies. The kind of soil in an area helps determines what type of plants can grow. There are 12 types of soil:

Alfisols – These sols result from weathering process that leaches clay minerals and other constituents out of the surface layer and into the subsoil, where they hold and supply moisture and nutrients to plants. They formed primarily under forest or mixed vegetative cover and are productive for most crops.

Alfisols make up about 10% of the world’s ice-free land surface

photo – USDA

Andisols – Form from the weathering process that generate minerals with little orderly crystalline structure. These minerals can result in unusually high water and nutrient holding capacity. As a group, Andisols tend to be highly productive soils. They include weakly weathered soils with much volcanic glass as well as more strongly weathered soils. They are common in cool areas with moderate to high precipitation, especially those areas associated with volcanic materials.

Andisols make up about 1% of the world’s ice-free land surface

photo – USDA

Aridisols – Soils that are too fry for growth of mesophytic plants. The lack of moisture greatly restricts the intensity of the weathering process and limits most soil development processes to the upper part of the soils Aridisols often accumulate gypsum, salt, calcium carbonate, and other materials that are easily leached from soils in humid environments. Aridisols are common in the deserts of the world.

Aridisols make up about 12% of the world’s ice-free land surface

photo – USDA

Entisols – Soils that show little or no evidence of pedogenic horizon development. Entisols in areas of recently deposited parent materials or in areas where erosion or deposition rates are faster than the rate of soil development; such as dunes, steep slopes, and floodplains. They occur in many environments

Entisols make up 16% of the world’s ice-free land surface

photo – USDA

Histosols – Histosols have a high content of organic matter and no permafrost. Most are saturated year round, but a few are freely drained. Histosols are commonly called bogs, moors, peats, and mucks. Histosols form in decomposed plant remains that accumulate in water, forest litter, or moss faster than they decay. If the soils are drained and exposed to air, microbial decomposition is accelerated and the soils may subside drastically.

Histosols make up about 1% of the world’s ice-free land surface

photo – USDA

Inceptisols – Soils of semiarid to humid envoironments that generally exhibit only moderate degrees of soil weathering and development. Inceptisols have a wide range in characteristics and occur in a wide variety of climates.

Inceptisols make up about 17% of the world’s ice-free land surface

photo – USDA

Mollisols – Soils that have a dark colored surface horizon and have a relatively high in organic matter content. The soils are base rich throughout and therefore quite fertile. Mollisols characteristically form under grass in climates that have moderate to pronounced seasonal moisture deficit. They are extensive soils on the steppes of Europe, asia, North america, and South america.

Mollisols make up about 7% of the world’s ice-free land surface

photo – USDA

Oxisols – Are highly weathered soils of tropical and subtropical regions. They are dominated by low activity minerals, such as quartz, kaolinite, and iron oxides. They tend to have indistinct horizons. Oxisols characteristically occur on land surfaces that have been stable for a long times. They have low natural fertility as well as a low capacity to retain additions of lime and fertilizer.

Oxisols make up about 8% of the world’s ice-free land surface

photo – USDA

Spodosols – Soils that form from weathering processes that strip organic matter combined with aluminum (with or without iron) from the surface layer and deposit them in the subsoil. In undisturbed areas, a gray eluvial horizon that has the color of uncoated quartz overlies a reddish brown or black subsoil. Spodosols commonly occur in areas of coarse-textured deposits under coniferous forests of humid regions. They tend to be acid and infertile.

Spodosols make up about 4% of the world’s ice-free land surface

photo – USDA

Ultisols – Soils in humid areas. they formed from fairly intense weathering and leaching processes that result in a clay-enriched subsoil dominated by minerals, such as quartz, kaolinite, and iron oxide. Ultisols are typically acid soils in which nutrients are concentrated in the upper few inches. They have a moderately low capacity to retain additions of lime and fertilizer.

Ultisols make up about 8% of the world’s ice-free land surface

photo – USDA

Gelisols – Soils that have permafrost near the soil surface and/or have evidence of cryoturbation (frost churning) and/or ice segregation. Gelisols are common in higher latitudes or at high elevations.

Gelisols make up about 9% of the world’s ice-free land surface

photo – USDA

Vertisols – Soils with a high content of expanding clay minerals. They undergo pronounced changes in volume with changes in moisture. They have cracks that open and close periodically, and show evidence of soil movement in the profile. Because they swell when wet, vertisols transmit water very slowly and have undergone little leaching. They tend to be fairly high in natural fertility.

Vertisols make up about 2% of the world’s ice-free land surface

photo – USDA

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glossary

Aerobic – relating to, involving, or requiring free oxygen

Aggregation – the process by which individual particles of sand, silt and clay cluster and bind together to form peds

Anion – A negatively charged ion. Plant nutrient examples include nitrate NO3, phosphate H2PO4, and sulfate SO4 2-

Aspect – Direction of exposure to sunlight

Anaerobic – relating to, involving, or requiring an absence of free oxygen

Biosolids – A byproduct of wastewater treatment sometimes used as a fertilizer

Capillary force – Action by which water molecules bind to the surfaces of soil particles and each other, thus holding water to fine pores against the force of gravity

Cation – a positively charged ion. Plant nutrient examples include Ca, and potassium K

C:N ratio – The ratio of carbon to nitrogen in organic materials. Materials with a high C:N ratio are good bulking agents in compost piles, while those with a low C:N ratio are good energy sources

Cold Composting – A slow composting process that involves simply building a pile and leaving it until it decomposes. The process may take months or longer. Cold composting does not kill weed seeds or pathogens

Compaction – Pressure that squeezes soil into layers that resist root penetration and water movement. Often the result of foot or machine traffic

Compost – The product created by the breakdown of organic waste under conditions manipulated by humans

Cover crop – a crop that is dug into soil to return organic matter ans nitrogen to the soil. Also, called green manure

Cation Exchange Capacity (CEC) – a measure of how many cations can be retained on soil particle surfaces. Negative charges on the surfaces of soil particles bind positively-charged atoms or molecules, but allow these to exchange with other positively charged particles in the surrounding soil water.

Chemotrophic – use chemicals as their energy source instead of light

Cyanobacteria – also called blue-green algae, are microscopic organisms found naturally in all types of water. These single-celled organisms live in fresh, brackish (combined salt and fresh water), and marine water. These organisms use sunlight to make their own food

Decomposition – the breakdown of organic materials by microorganisms

Ericaceous plants – Ericaceous trees and shrubs are those that belong to the plant family Ericaceae. A key characteristic of these shrubs is the fact that they need infertile or acidic soil. The word ericaceous is also sometimes used to describe any plant that likes acid soil, regardless of family.

Fertilizer – a natural or synthetic product dded to the soil to supply plant nutrients

Fertilizer analysis – the amount of nitrogen, phosphorus, as P2O5 and potassium, as K2O in fertilizer expressed as a percent of the total weight. N always is listed first, P second, and K third

Green Manure – same as a cover crop

Hot composting – a fast composting process that produces finished compost in 6 to 8 weeks. High temperatures are maintained by mixing balanced volumes of energy materials and bulking agents, keeping the pile moist, and turning it frequently to keep aerated

Humus – the end product of decomposed animal and vegetable matter

Igneous rock – (from the Latin word for fire) form when hot, molten rock crystallizes and solidifies. Igneous rocks are divided into two groups, intrusive or extrusive, depending upon where the molten rock solidifies

Immobilization – the process by which microorganisms use available nitrogen as they break down materials high in C:N ratio, thus reducing the amount of nitrogen to plants

Infiltration – the movement of water into the soil

Ion – an atom of molecule with either positive or negative charge

Leaching – movement of water and soluble nutrients down through the soil profile

Legumes – A legume is a plant in the family Fabaceae, or the fruit or seed of such a plant. The seed is also called a pulse. Some of the most common legumes are: peanuts, soybeans, alfalfa, clovers, and vetches.

Metamorphic rock – Rock that was once one form of rock but has changed to another under the influence of heat, pressure,  or some other agent without passing through a liquid phase

Micronutrient – a nutrient used by plants in small amounts (iron, zinc, boron, molybdenum, manganese, copper, and chlorine) Also called a trace element

Mycorrhizae – beneficial fungi that infect plant roots and increase their ability to take up nutrients from the soil

Nitrifier – a microbe that converts ammonium to nitrate

Nitrogen fixer – the conversion of atmospheric nitrogen into plant available forms by Rhizobia bacteria

Nitrogen fixation – a process by which gaseous nitrogen N2 is converted to NH3 (ammonia) through biological fixation through high-energy physical processes.

Nitrogenase  – functions in the absence of oxygen. The exclusion of oxygen is accomplished by many means. Some bacteria live beneath layers of oxygen-excluding slime on the roots of certain plants. The most important soil dwelling bacteria, Rhizobium, live in oxygen-free zones in nodules on the roots of legumes and some other woody plants. Aquatic filamentous cyanobacteria utilize oxygen-excluding cells called heterocysts

Organic fertilizer – natural fertilizer material the has undergone little or no processing. Can include plant, animal, and /or mineral materials

Organic matter – any material originating from a living organism

Pathogen – disease causing organism. pathogenic soil organisms include: bacteria, viruses fungi, and nematodes

Ped – a cluster of individual soil particles

Permeability – The rate at which water moves through a soil

pH – a measure of the acidity or alkalinity. Values from 0 – 7 indicate acidity, a value o 7 is neutral, and values from 7- 14 indicate alkalinity. Most soils have a pH between 4.5 and 9

Potash – the form of potassium listed in most fertilizer analyses K2O

Primary nutrient – a nutrient required by plants in a relatively large amount

Processed fertilizer – a fertilizer that is manufactured or is refined from natural ingredients to e more concentrated and more available to plants

Primary productivity – in ecology – the rate at which energy is converted to organic substances by photosynthetic producers (photoautotrophs), which obtain energy and nutrients by harnessing sunlight, and chemosynthetic producers (chemoautotrophs), which obtain chemical energy through oxidation

Quick release fertilizer – a fertilizer that contains nutrients in plant-available forms such as ammonium and nitrate

Secondary nutrients – nutrient needed by plants in a moderate amount including sulfer, calcium, and magnesium

Sedimentary rock –  types of rock that are formed by the accumulation or deposition of mineral or organic particles at the Earth’s surface, followed by cementation

Soil colloids – made up of the smallest particles of humus and clay in the soil. Only particles less than 0.001 mm are officially defined as colloids. The clay particles are known as Inorganic Colloids. Soil colloids are the most active portion of the soil and determine the physical and chemical properties of a soil.

Rhizobium bacteria – a genus of bacteria associated with the formation of root nodules on plants. These bacteria live in symbiosis with legumes. They take in nitrogen from the atmosphere and pass it on to the plant, allowing it to grow in soil low in nitrogen.

Rhizoshere – thin layer of soil immediately surrounding plant roots

Saprophytic heterotrophs – A saprophyte or saprotroph is an organism which gets its energy from dead and decaying organic matter. This may be decaying pieces of plants or animals. This means that saprophytes are heterotrophs. Some fungi are parasites on living organisms, but most are saprophytes.

Soil profile – soil horizons. The arrangement of these horizons in a soil is known as a · soil profile.

Water-holding capacity – the ability of a soil’s micropores to hold water for plant use

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