10.2: Soil Plant Relations
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Natural Processes
Soil plays a key role in plant growth. Beneficial aspects to plants include providing physical support, heat, water, nutrients, and oxygen. Heat, light, and oxygen are also obtained by the atmosphere, but the roots of many plants also require oxygen. Elemental nutrients, dissolved in soil water solution, are derived from soil minerals and organic material (see Figure below).
Figure \(\PageIndex{1}\): Soil-Plant Nutrient Cycle. Figure illustrates the uptake of nutrients by plants in the forest" soil ecosystem. Source: U.S. Geological Survey.
Plants mainly obtain nutrients from dissolved soil solutions. Though many aspects of soil are beneficial to plants, excessively high levels of trace metals (either naturally occurring or anthropogenically added) or applied herbicides can be toxic to some plants.
The ratio of solids/water/air in soil is also critically important to plants for proper oxygenation levels and water availability. Too much porosity with air space, such as in sandy or gravelly soils, can lead to less available water to plants, especially during dry seasons when the water table is low. Too much water, in poorly drained regions, can lead to anoxic conditions in the soil, which may be toxic to some plants. Hydrophytic vegetation can handle anoxic conditions and is thus suitable to poorly drained soils in wetland areas.
Nutrient Uptake by Plants
Several elements obtained from soil are considered essential for plant growth. Macronutrients, including C, H, O, N, P, K, Ca, Mg, and S, are needed by plants in significant quantities. C, H, and O are mainly obtained from the atmosphere or from rainwater. These three elements are the main components of most organic compounds, such as proteins, lipids, carbohydrates, and nucleic acids. Oxygen generally serves as an electron acceptor and is required by roots of many plants. The other six elements (N, P, K, Ca, Mg, and S) are obtained by plant roots from the soil and are variously used for protein synthesis, chlorophyll synthesis, energy transfer, cell division, enzyme reactions, and osmotic or ionic regulation.
Micronutrients are essential elements that are needed only in small quantities, but can still be limiting to plant growth since these nutrients are not so abundant in nature. Micronutrients include iron (Fe), manganese (Mn), boron (B), molybdenum (Mo), chlorine (Cl), zinc (Zn), and copper (Cu). There are some other elements that tend to aid plant growth but are not absolutely essential.
Micronutrients and macronutrients are desirable in particular concentrations and can be detrimental to plant growth when concentrations in soil solution are either too low (limiting) or too high (toxicity). Elemental nutrients are useful to plants only if they are in an extractable form in soil solutions, such as an exchangeable cation, rather than in a solid mineral grain. As nutrients are used up in the microenvironment surrounding a plant's roots, the replenishment of nutrients in soil solution is dependent on three aspects: (a) the rate of dissolution/alteration of soil minerals into elemental constituents, (b) the release rate of organically bound nutrients, and (c) the rate of diffusion of nutrients through the soil solution to the area of root uptake.
Many nutrients move through the soil and into the root system as a result of concentration gradients, moving by diffusion from high to low concentrations. However, some nutrients are selectively absorbed by the root membranes, such that elemental concentrations of solutions within plants may differ from that in soil solutions. Most nutrients exist as exchangeable cations that are acquired by roots from the soil solution—rather than from mineral or particle surfaces. Inorganic chemical processes and organic processes, such as the action of soil microorganisms, can help to release elemental nutrients from mineral grains into the soil environment.