Hardpan
Encyclopedia
In soil science
, agriculture
and gardening
, hardpan or ouklip is a general term for a dense layer of soil
, usually found below the uppermost topsoil
layer. There are different types of hardpan, all sharing the general characteristic of being a distinct soil layer that is largely impervious to water
. Some hardpans are formed by deposits in the soil that fuse and bind the soil particles. These deposits can range from dissolved silica to matrices formed from iron oxide
s and calcium carbonate
. Others are man-made, such as hardpan formed by compaction from repeated plowing, particularly with moldboard plows, or by heavy traffic or pollution.
. Acid
soils are most often affected due to the propensity of certain mineral salts, most notably iron and calcium, to form hard complexes with soil particles under acid conditions.
Another major determinant is the soil particle size. Clay
particles are some of the smallest particles commonly found in soils. Due to their structure the spaces between individual clay particles is quite small and already restricts the passage of water, negatively affecting infiltration and hence drainage. Soils with a high clay content are also easily compacted and affected by man-made discharges. Clay particles have a strong negative electrostatic charge and will readily bond to positively charged ions dissolved in the soil-water matrix. Common salts such as sodium
ions contained in wastewater can fulfil this role and lead to a localized hardpan in some soil types. This is a common cause of septic system failure due to the prevention of proper drainage in field.
of water and restricting the growth of plant
root
s. In these situations, the hardpan can be broken up by either mechanical means such as digging or plowing, or through the use of soil amendments. The broadfork
is a manual tool
specifically designed for this task; a digging fork or a spade
might also be used. The chisel plow does a similar job with the help of a tractor
.
The use of soil amendments can also be employed to alter the soil structure
and promote the dissolution of the hard pan. It has been observed that increasing the amount of soil organic matter
through the working-in of manure
, compost
or peat
can both improve local drainage and promote the proliferation of earth worms that can, over time, break relatively thin hardpan layers.
More difficult hardpans may be further improved through the action of both adjusting the soil pH
with lime
if the soil is acidic, and with the addition of gypsum
. This combination can help loosen clay particles bound into a hardpan by the actions of hard salts such as iron
, calcium carbonate
and sodium
, by promoting their mobility through a higher pH while proving a suitable source of exchanging minerals (the gypsum). This works because gypsum salts, although not "soft", are still water permeable and have a larger, more open structure, the results of which do not promote as hard a matrix as was replaced. However, unlike when employing mechanical means, breaking a hardpan through the use of amendments may require action over the course of years, and even then one is by no means assured success. The results are primarily determined by how extensive and / or intractable the hardpan is.
Soil science
Soil science is the study of soil as a natural resource on the surface of the earth including soil formation, classification and mapping; physical, chemical, biological, and fertility properties of soils; and these properties in relation to the use and management of soils.Sometimes terms which...
, agriculture
Agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...
and gardening
Gardening
Gardening is the practice of growing and cultivating plants. Ornamental plants are normally grown for their flowers, foliage, or overall appearance; useful plants are grown for consumption , for their dyes, or for medicinal or cosmetic use...
, hardpan or ouklip is a general term for a dense layer of soil
Soil
Soil is a natural body consisting of layers of mineral constituents of variable thicknesses, which differ from the parent materials in their morphological, physical, chemical, and mineralogical characteristics...
, usually found below the uppermost topsoil
Topsoil
Topsoil is the upper, outermost layer of soil, usually the top to . It has the highest concentration of organic matter and microorganisms and is where most of the Earth's biological soil activity occurs.-Importance:...
layer. There are different types of hardpan, all sharing the general characteristic of being a distinct soil layer that is largely impervious to water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
. Some hardpans are formed by deposits in the soil that fuse and bind the soil particles. These deposits can range from dissolved silica to matrices formed from iron oxide
Iron oxide
Iron oxides are chemical compounds composed of iron and oxygen. All together, there are sixteen known iron oxides and oxyhydroxides.Iron oxides and oxide-hydroxides are widespread in nature, play an important role in many geological and biological processes, and are widely utilized by humans, e.g.,...
s and calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
. Others are man-made, such as hardpan formed by compaction from repeated plowing, particularly with moldboard plows, or by heavy traffic or pollution.
Formation
Soil structure strongly affects its tendency to form a hard pan. One such common soil condition related to hardpan is soil pHPH
In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...
. Acid
Acid
An acid is a substance which reacts with a base. Commonly, acids can be identified as tasting sour, reacting with metals such as calcium, and bases like sodium carbonate. Aqueous acids have a pH of less than 7, where an acid of lower pH is typically stronger, and turn blue litmus paper red...
soils are most often affected due to the propensity of certain mineral salts, most notably iron and calcium, to form hard complexes with soil particles under acid conditions.
Another major determinant is the soil particle size. Clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...
particles are some of the smallest particles commonly found in soils. Due to their structure the spaces between individual clay particles is quite small and already restricts the passage of water, negatively affecting infiltration and hence drainage. Soils with a high clay content are also easily compacted and affected by man-made discharges. Clay particles have a strong negative electrostatic charge and will readily bond to positively charged ions dissolved in the soil-water matrix. Common salts such as sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
ions contained in wastewater can fulfil this role and lead to a localized hardpan in some soil types. This is a common cause of septic system failure due to the prevention of proper drainage in field.
Problems and workarounds
Hardpan can be a problem in farming and gardening by impeding drainageDrainage
Drainage is the natural or artificial removal of surface and sub-surface water from an area. Many agricultural soils need drainage to improve production or to manage water supplies.-Early history:...
of water and restricting the growth of plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...
root
Root
In vascular plants, the root is the organ of a plant that typically lies below the surface of the soil. This is not always the case, however, since a root can also be aerial or aerating . Furthermore, a stem normally occurring below ground is not exceptional either...
s. In these situations, the hardpan can be broken up by either mechanical means such as digging or plowing, or through the use of soil amendments. The broadfork
Broadfork
In farming and gardening, the broadfork, or U-fork is a tool used to manually break up densely packed soil, like hardpan, to improve aeration and drainage....
is a manual tool
Tool
A tool is a device that can be used to produce an item or achieve a task, but that is not consumed in the process. Informally the word is also used to describe a procedure or process with a specific purpose. Tools that are used in particular fields or activities may have different designations such...
specifically designed for this task; a digging fork or a spade
Spade
A spade is a tool designed primarily for the purpose of digging or removing earth. Early spades were made of riven wood. After the art of metalworking was discovered, spades were made with sharper tips of metal. Before the advent of metal spades manual labor was less efficient at moving earth,...
might also be used. The chisel plow does a similar job with the help of a tractor
Tractor
A tractor is a vehicle specifically designed to deliver a high tractive effort at slow speeds, for the purposes of hauling a trailer or machinery used in agriculture or construction...
.
The use of soil amendments can also be employed to alter the soil structure
Soil structure
Soil structure is determined by how individual soil granules clump or bind together and aggregate, and therefore, the arrangement of soil pores between them...
and promote the dissolution of the hard pan. It has been observed that increasing the amount of soil organic matter
Soil organic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...
through the working-in of manure
Manure
Manure is organic matter used as organic fertilizer in agriculture. Manures contribute to the fertility of the soil by adding organic matter and nutrients, such as nitrogen, that are trapped by bacteria in the soil...
, compost
Compost
Compost is organic matter that has been decomposed and recycled as a fertilizer and soil amendment. Compost is a key ingredient in organic farming. At its most essential, the process of composting requires simply piling up waste outdoors and waiting for the materials to break down from anywhere...
or peat
Peat
Peat is an accumulation of partially decayed vegetation matter or histosol. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world...
can both improve local drainage and promote the proliferation of earth worms that can, over time, break relatively thin hardpan layers.
More difficult hardpans may be further improved through the action of both adjusting the soil pH
Soil pH
The soil pH is a measure of the acidity or basicity in soils. pH is defined as the negative logarithm of the activity of hydrogen ions in solution. It ranges from 0 to 14, with 7 being neutral. A pH below 7 is acidic and above 7 is basic. Soil pH is considered a master variable in soils as it...
with lime
Lime (mineral)
Lime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...
if the soil is acidic, and with the addition of gypsum
Gypsum
Gypsum is a very soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4·2H2O. It is found in alabaster, a decorative stone used in Ancient Egypt. It is the second softest mineral on the Mohs Hardness Scale...
. This combination can help loosen clay particles bound into a hardpan by the actions of hard salts such as iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
and sodium
Sodium
Sodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
, by promoting their mobility through a higher pH while proving a suitable source of exchanging minerals (the gypsum). This works because gypsum salts, although not "soft", are still water permeable and have a larger, more open structure, the results of which do not promote as hard a matrix as was replaced. However, unlike when employing mechanical means, breaking a hardpan through the use of amendments may require action over the course of years, and even then one is by no means assured success. The results are primarily determined by how extensive and / or intractable the hardpan is.
See also
- Caliche (mineral)Caliche (Mineral)Caliche is a sedimentary rock, a hardened deposit of calcium carbonate. This calcium carbonate cements together other materials, including gravel, sand, clay, and silt. It is found in aridisol and mollisol soil orders...
- ClaypanClaypanIn geology, a claypan is a dense, compact, slowly permeable layer in the subsoil having a much higher clay content than the overlying material, from which it is separated by a sharply defined boundary. Claypans are usually hard when dry, and plastic and sticky when wet. They limit or slow the...
- Stuart (song) (discusses hardpan formation as a side effect of aerospace facility construction)