Importance of Soil Chemical Properties
Rivka Fidel
- Define: pH, EC, solution
- Explain the importance of soil chemical properties and surfaces
- Sort the solution, surfaces, and solids in order of relative chemical reaction rates
Why are soil chemical properties important?
Soil chemical properties have widespread impacts on countless soil processes, and therefore their importance cannot be overstated. The chemicals in soil, from solid minerals to nutrients and other salts in solution, affect not only plant growth and microbial diversity but also soil greenhouse gas emissions and the transport and breakdown rate of nearly any chemical from nutrients to hazardous substances. Therefore, understanding soil chemistry is vital to understanding and managing soil.
Chemicals are found everywhere in the soil – in fact, soil is made of chemicals. As such, soil contains countless chemicals, reacting with each other constantly. The enormous diversity of both chemicals and their reactions in soil can become quite overwhelming but becomes much more understandable when we organize them according to phase: solids, liquids (here, aqueous solutions), and gases. Of particular importance in this course are the solids and liquids, and their interactions at the solid-solution interface.
Soil Solids
You may recall that an average soil is about 50% solid particles, where some of these are mineral and some are organic. In soil science, minerals are inorganic, meaning they do not contain carbon (with the major exception of carbonates). Organic particles on the other hand, do contain carbon – specifically carbon atoms bonded to other carbon atoms, or to hydrogen atoms (thus, carbonates are excluded, as CO32- has only C-O bonds).
Minerals are important not only for providing nutrients as they weather and dissolve, but also for providing structure to the soil and surfaces for other chemicals to adhere or “adsorb” to (more on this later). Some minerals weather faster, whereas others resist weathering, and as such their chemical properties affect the overall rate of soil formation.
Organic particles release nutrients into the solution as they break down during the decomposition process (distinct from weathering, see Soil Life or Soil Organic Matter chapter). Organic matter also has a very high surface area and water holding capacity, helping it to hold onto water and nutrients.
Both mineral and organic particles are found in a wide range of sizes, from >2 mm down to less than 1 micron (that’s 0.001 mm). Different size fractions of these particles behave differently in soil, especially in the way they affect the soil solution.
The Soil Solution
The soil solution is all liquid water within the soil and the solutes dissolved within. Solutes include ions, as well as neutrally charged species like sugars and select acids and bases. Gases such as carbon dioxide (CO2) and oxygen (O2) can also dissolve into the solution and react with other solutes, as well as water. During the dissol