What does leaching mean?

Types Of Leaching: Info On Leaching Garden Plants And Soil

What is leaching? This is a commonly asked question. Let’s learn more about types of leaching in plants and soil.

What is Leaching?

There are two types of leaching in the garden:

Leaching of soil

The soil in your garden is like a sponge. When rain falls, the soil near the top absorbs as much as possible, keeping the moisture available to the plants growing there. Once the soil is filled with all the water it can hold, the water begins to leak downward through the layers of rock and subsoil beneath your garden. When the water sinks down, it takes soluble chemicals with it, such as nitrogen and other fertilizer components, as well as any pesticides you may have used. This is the first of the types of leaching.

What soil type is most prone to leaching? The more porous the soil, the easier it is for chemicals to pass through. Pure sand is probably the best leaching type, but isn’t very hospitable to garden plants. In general, the more sand your garden soil has, the more likely it is that you will have excess leaching. On the other hand, soil with more of a clay component presents less of a leaching problem.

Leaching in plants is more an environmental concern than that of poor drainage. Once your pesticides have leached from the plants themselves down through your soil into the water table, they begin to affect the environment. This is one reason why many gardeners prefer organic methods of pest control.

Leaching of potted plants

Leaching in plants can happen in potting containers. Once the chemicals have drained down through the soil, they can leave a crust of soluble salts on the surface, which makes it hard for the soil to absorb water. Removing this crust with water is the other type of leaching.

Leaching garden plants grown in containers is the process of washing the salts from the surface of the soil. Pour large amounts of water through the soil until it runs freely from the bottom of the planter. Leave the container alone for about an hour, then do it again. Repeat the process until you don’t see any more white covering on the surface of the soil.

Sentence Examples

  • The result is a great destruction of the humus of the soil, and great leaching and washing, especially in the light loams of the hill country of the United States.
  • The ores, having been broken and ground, generally in tube mills, until they pass a 150 to 200-mesh sieve, are transferred to the leaching vats, which are constructed of wood, iron or masonry; steel vats, coated inside and out with pitch, of circular section and holding up to woo tons, have come into use.
  • The leaching is generally carried out with a strong, medium, and with a weak liquor, in the order given; sometimes there is a preliminary leaching with a weak liquor.
  • There can be no question that a deep soil is better for the cottonplant; but the expense of obtaining it, the risk of injuring the soil through leaching, and the danger of bringing poor soil to the surface, have led many planters to oppose this plan.
  • If the burningup of humus and the leaching of the soil could be prevented, there is no reason whyia cotton soil should not produce good crops continuously for an indefinite time.

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Leaching usually refers to the movement of dissolved substances with water percolating through soil. Sometimes, leaching may also refer to the movement of soluble chemicals out of biological tissues, as when rainfall causes potassium and other ions to be lost by foliage.

Leaching occurs naturally in all soils, as long as the rate of water input through precipitation is greater than water losses by evapotranspiration. In such cases, water must leave the site by downward movement, ultimately being deposited to deep groundwater, or emerging through springs to flow into surface waters such as streams, rivers, and lakes. As the subterranean water moves in response to gradients of gravitational potential, it carries dissolved substances of many kinds.

Leaching is a highly influential soil-forming process. In places where the climate is relatively cool and wet, and the vegetation is dominated by conifers and heaths, the soil-forming process known as podsolization is important. In large part, podsolization occurs through the dissolving of iron, aluminum, calcium, organic matter, and other chemicals from surface soils, and the downward leaching of these substances to lower soil depths, where they are deposited. Some solubilized materials may also be altogether lost from the soil, ending up in deep groundwater or in surface water. A different soil-forming process known as laterization occurs under the warm and humid climatic conditions of many tropical rainforests, where aluminum and iron remain in place in the surface soil, while silicate is dissolved and leached downward.

The ability of water to solubilize particular substances is influenced to a substantial degree by the chemical nature of the solution. For example, highly acidic solutions have a relatively great ability to dissolve many compounds, especially those of metals. Aluminum (Al), for instance, is an abundant metallic constituent of soils, typically present in concentrations of 7-10%, but occurring as aluminum compounds that are highly insoluble, so they cannot leach with percolating water. However, under highly acidic conditions some of the aluminum is solubilized as positively charged ions (or cations), particularly as Al3+ and AlOH2+. These soluble ions of aluminum are highly toxic to terrestrial plants and animals, and if they are leached to surface waters in large quantities they can also cause biological damage there. Aluminum ions are also solubilized from soils by highly alkaline solutions, in which they occur mostly as the anion Al(OH)–. A large salt concentration in soil, characterized by an abundance of dissolved ions, causes some ions to become more soluble through an osmotic extraction, also pre-disposing them more readily to leaching.

Soils can become acidified by various human activities, including emissions of air pollutants that cause acidic precipitation, certain types of agricultural fertilization, harvesting of biomass, and the mining of coal and sulfide minerals. Acidification by all of these activities causes toxicity of soil and surface waters through the solubilization of aluminum and other metals, while also degrading the fertility and acid-neutralization capacity of soil by causing the leaching of basic cations, especially calcium, magnesium, and potassium.

Another environmental problem associated with leaching concerns terrestrial ecosystems that are losing large quantities of dissolved nitrogen, as highly soluble nitrate. Soils have little capability to bind nitrate, so this anion leaches easily whenever it is present in soil water in a large concentration. This condition often occurs when disturbance, fertilization, or atmospheric depositions of nitrate and/or ammonium result in an availability of nitrate that is greater than the biological demand by plants and microorganisms, so this chemical can leach at relatively high rates. Terrestrial ecosystems of this character are said to be “nitrogen-saturated.” Some negative environmental effects are potentially associated with severe nitrogen saturation, including an increased acidification and toxicity of soil and water through leaching of aluminum and basic cations (these positively charged ions move in companion with the negatively charged nitrate), nutrient loading to aquatic systems, potentially contributing to increased productivity there, and possibly pre-disposing trees to suffer decline and die back. If the nitrogen saturation is not excessive, however, the growth of trees and other vegetation may be improved by the relatively fertile conditions.

Nitrogen in the Environment: Leaching

Scott C. Killpack and Daryl Buchholz
Department of Agronomy

When soil becomes excessively wet through rainfall, the soil will reach a point where it cannot hold any more water. This happens because the air spaces between soil particles become filled with water. As these air spaces fill, gravity will cause water to move down through the soil profile. (It can also pound or run off the surface of the soil.) As water moves down through the soil, nitrogen can be carried with it. This is called leaching (Figure 1).

Figure 1
Heavy rainfall can cause nitrates (NO3-) to move down through the soil, a process called leaching

Forms of nitrogen that leach

Nitrate (NO3-) is primarily the form of nitrogen that is leached. Nitrates may originate from manures, the decay of plants and other organic materials or from fertilizers. Nitrate is very mobile and is easily moved by water. Other forms of nitrogen, such as ammonium (NH4+), generally do not leach.Unlike nitrate, the ammonium form does not leach because it is attached to the soil and resists movement with water. The forces that cause ammonium to attach to the soil are somewhat like the forces that cause metal objects to become attached to a magnet.

What affects nitrate leaching?

An important factor that can affect the degree of leaching is how much water a soil can hold. For example, by their nature sandy soils cannot hold as much water as clay soils. This means that leaching of nitrates will take place much more easily in a sandy soil compared to a clay soil. Often, leaching is not a significant factor in heavy clay soils.

Other factors that can affect nitrate leaching include amount of rainfall, amount of water use by plants and how much nitrate is present in the soil system.

Impact on water quality

Nitrate leaching can have a direct impact on water quality. Nitrate is very mobile and easily leaches with water. Heavy rains can cause nitrates to leach downward in the soil below the root zone. Whether nitrates continue to leach downward, and into groundwater, depends on underlying soil and/or bedrock conditions, as well as depth to groundwater. If depth to groundwater is shallow and the underlying soil is sandy, the potential for nitrates to enter groundwater is relatively high. However, if depth to groundwater is deep and the underlying soil is heavy clay, nitrates will not likely enter groundwater. In some cases where dense hardpans are present, nitrate leaching will not progress beyond the depth of the hardpan.

Once nitrates get into the groundwater, the greatest concerns are for infants less than one year old and for young or pregnant animals. High levels of nitrates can be toxic to newborns, causing anoxia, or internal suffocation. Seek alternative water sources if nitrate levels exceed the health standard of 10 ppm nitrate-N. Do not boil water to eliminate nitrates. It increases nitrate levels rather than decreasing them. The most common symptom of nitrate poisoning in babies is a bluish color to the skin, particularly around the baby’s eyes and mouth. These symptoms of nitrate toxicity are commonly referred to as the “blue-baby” syndrome.

The potential of nitrates from animal manure and nitrogen fertilizers getting into groundwater can be reduced through good management practices. Applying manure and nitrogen fertilizers when crops are actively growing, and using nitrates for growth and development will reduce the amount of nitrate in the soil system and thus the amount that could potentially be leached. However, little can be done to minimize the leaching of nitrates into groundwater that result from the ongoing decay of organic matter in the soil. In this situation, nitrates are simply a natural biological result of an ongoing cycle of nitrogen transformation processes in the soil.

This material is based upon work supported by the United States Department of Agriculture, Extension Service, under special project number 89-EWQI-1-9203.

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