Soil plays a key role in plant growth. Beneficial aspects of plants include providing physical support, water, heat, nutrients, and oxygen (Figure 1). Mineral nutrients from the soil can dissolve in water and then become available to plants. Although many aspects of soil benefit plants, excessively high levels of trace metals (naturally occurring or anthropogenically added) or applied herbicides can be toxic to some plants.
The soil’s solids/water/air ratio 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.
Nutrient Uptake by Plants
Several elements obtained from the 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 rainwater. These three elements are the main components of most organic compounds, such as proteins, lipids, carbohydrates, and nucleic acids. The other six elements (N, P, K, Ca, Mg, and S) are obtained from plant roots from the soil. They are used for protein synthesis, chlorophyll synthesis, energy transfer, cell division, enzyme reactions, and homeostasis (the process regulating the conditions within an organism).
Micronutrients are essential elements needed only in small quantities but can still limit 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). Some other elements tend to aid plant growth but are not absolutely essential.
Micronutrients and macronutrients are desirable in particular concentrations and can harm plant growth when concentrations in the soil solution are either too low (limiting) or too high (toxicity). Mineral nutrients are useful to plants only if they are in an extractable form in soil solutions, such as a dissolved ion, rather than a solid mineral. Due to concentration gradients, many nutrients move through the soil and into the root system, moving from high to low concentrations by diffusion. However, some nutrients are selectively absorbed by the root membranes, making concentrations higher inside the plant than in the soil.