4.3 Soil and sustainability  (Page 5/8)

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.

Soil health and agricultural impacts: soil as a sustainable resource

Soil health and sustainability

Overall soil health can generally be defined as the capacity of the soil to function in a way that infiltrates water and cycles nutrients to support plant growth. Long term health of native soil is in many cases improved by disturbing the soil less, growing a greater diversity of crops, maintaining living roots in the soil, and keeping the soil covered with residue. Stable soil aggregates are important for soil health as they promote proper infiltration and thus limit the amount of water runoff —this has the added benefit of reducing soil erosion and downstream flooding and sedimentation.

Management of soil on farms may include use of tillage, fertilizer, pesticides, and other tools that may improve soil health if used correctly; however, significant damage to soil may result otherwise. Tillage with a plow or disk is can be physically disruptive to soil fauna and microbes. The complex relations between soil and plant life, which have evolved into a sustainable relationship in the natural world, can be disturbed chemically by misuse or overuse of fertilizers or pesticides. Thus, to maintain soil health, one needs to understand the chemical, biological, and physical processes that operate in the natural soil profile. To the extent possible, we must work with the complexity of processes that function in a healthy soil and limit our disturbances to only those that are clear, practical necessity. Biodiversity is another important aspect to consider, because increasing the biodiversity of plants that are grown in soil can limit disease and pest problems and allow for a better functioning food web. More diversity in plants above ground leads to more diversity in the subsurface food web. Consequently, increasing the diversity of appropriate crop rotation in agricultural lands can ultimately lead to better soil health and limit problems in the long run.