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Bioavailability and the Soil Solution

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Plants and soil organisms take up only those nutrients (and other elements) that are available to them in the soil solution.

By G. E. Warrington and E. O. Skogley. 1997
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Bioavailability is much like the weather--everyone talks about it, but no one does anything about it! Let's start by defining bioavailability as "those chemicals in the soil that are present in forms and amounts that plants (or other organisms) can take up during the time they are growing." Ah, you say this is obvious--but what does it really mean in practice? --and how is it measured?

What is a Chemical?

The term chemical refers to any organic or inorganic substance. Various combinations of only a few elements (C, H, O, N, S, P, Cl, F, Br, and I) make up practically all organic substances. Organic chemicals are, therefore, compounds with various arrangements of inorganic elements that give them specific properties.

Examples of organic substances are fuels or their components such as BTEX (benzene, toluene, ethyl benzene, xylene), polyaromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), chlorinated solvents, many pesticides (like DDT or atrazine), and PCP (pentachlorophenol, a wood preservative fungicide). Many different organic and inorganic chemicals make up complex organic substances like sewage sludge and municipal waste compost. Plants and soil organisms cannot take up most organic molecules until they are broken down into tiny units, or to their individual inorganic elements.

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Chemicals Entering the Food Chain

Fig 1 - Organisms are exposed to chemicals in the "real" environment.Plants and soil organisms are exposed to all the chemicals where they are growing. This may be in soil, groundwater, material between the root zone and groundwater (known as the vadose zone), or free water in wells, lakes, and streams (see Fig 1). Just the presence of some (particularly organic) chemicals may adversely affect organisms. Generally, however, it is more important to know if organisms can take up these chemicals, thus introducing them into the food chain where they may end up affecting humans.
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Chemical Status

Organisms can not absorb all forms of a chemical. Most chemicals are absorbed only when present as ions. Chemicals exist mostly in combined states--organic compounds for organic chemicals and mineral substances for inorganic chemicals. Nearly all soil nutrients (except N) are present in minerals that make up soil solids. Soil N is usually in organic compounds (soil organic matter) in the soil. For elements to become bioavailable, they must be converted from combined forms to the ionic state.

For example, N in soil organic matter must be converted to the ions NH

4+ or NO3- to be available for uptake. Nutrients present in soil minerals must also be in ionic forms (e.g. H2PO3- or K+). The same is true whether the element is a required nutrient or if it is present in the medium as a heavy metal such as Lead, Cadmium, Mercury, etc. Mineral and organic solids in the soil are the storehouse for nutrients and other elements.
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Time - Rate of Availability

To illustrate the time factor, consider a plant that requires only a small amount of some nutrient. Early in the growth of this plant, that chemical may be present in the plant at a level too low to be detected by the analytical method used, so bioavailability will not register. When the plant continues to take up the chemical over time, eventually there will be enough of that chemical in the plant to detect, and its level of bioavailability revealed.

Diffusion Distance

Next, the location of chemicals, relative to where organisms are growing, is very important. To be bioavailable, chemicals in the soil must be close to living organisms. This aspect of the system is called positional availability. If nearby, ions can diffuse toward roots or be swept along with the water being taken up, becoming available.
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Soil Solution

Fig 1 - Plant roots receive a continuous supply of cations and anions.This is the liquid water in a moist soil. The soil solution regulates bioavailability by 1) letting ions form and 2) providing a channel for ion movement to the plant root and be absorbed. To see how this works, consider a plant root absorbing ions from the soil solution that contacts the root (see Fig. 2).

When a plant takes up nutrient ions from the solution surrounding its roots, that solution becomes deficient in those nutrients unless one of two processes occurs; a) more solution moves from other portions of soil toward the plant root, or b) more ions move through the soil solution toward the root. Process a) is called "mass flow" and is caused by plant uptake of water, and b) is "ion diffusion," caused by the concentration gradient created when roots take up the ions directly surrounding them. Active plant roots continually grow into new soil volumes to minimize distances for water or ions to move and thus help keep plants supplied with nutrients.

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Measuring Bioavailability

The best measure of bioavailability is to analyze an organism after it has grown in a medium and determine the amount of each target chemical ended up in the organism. Next best are UNIBEST, Inc. resin capsules 1/ that are designed to mimic the action of plant roots. Capsule values reflect active, dynamic components of each medium and its bioavailability system as it functions over some period of time.

Methods that extract soil chemicals with other chemicals do not reflect how elements in a fresh soil respond to continuous removal by biological activity over time. Traditional analytical methods provide only a "snapshot" of chemical conditions when the soil was sampled.

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For a detailed description of the way a resin capsule works, see  "How a PST-1 Resin Capsule Works."
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The following technical papers have more details about this topic.

Skogley, E.O., A. Dobermann, G.E. Warrington, M.F. Pampolino and M.A. Adviento. 1996. Laboratory and field methodologies for use of resin capsules. Sciences of Soils Vol.1, 1996 - http://www.hintze-online.com/sos/1996/Toolbox/Tool1/abstract.html

Skogley, E.O. and A. Dobermann. 1996. Synthetic Ion-exchange Resins: Soil and Environmental Studies. J. Environ. Qual. 25:13-24.

Dobermann, A., H. Langner, H. Mutscher, J.E. Yang, E.O. Skogley, M.A. Adviento and M.F. Pampolino. 1994. Nutrient Adsorption Kinetics of Ion Exchange Resin Capsules: A Study With Soils of International Origin. Commun. Soil Sci. Plant Anal. 25:1329-1353.

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Gordon Warrington
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