The Basics of Soil Chemistry

Core Concepts

In this article, we will be diving into the building blocks of soil chemistry, along with the key chemical processes and reactions that influence plant growth and the overall health of the surrounding ecosystem.

Introduction

Soil has a big responsibility on Earth. It is a complex mixture of minerals, organic matter, water, and air. Each of its components plays an important role in how it functions. The way these various components are arranged gives soil its structure, which affects how it holds water, stores nutrients, and supports life.

One of soil’s most important functions is its ability to influence water movement through the environment. It drives water retention and controls groundwater recharge, an important process for replenishing our drinking water supply. Soil structure and composition allow water to be released slowly into surface water and groundwater supplies, which mitigates severe weather events such as droughts and floods. By slowing the flow of water, soil enables water to remain during periods of dryness. More specifically, its ability to hold on to and gradually release the water prevents rapid surface runoff and flash flooding during heavy rains.

Another important function of soil is its ability to act like a natural filter through its sponge-like properties. As rain falls, the soil absorbs the rain and filters out contaminants. These contaminants become bound to soil components such as organic matter. The contaminants can also be broken down by microbes or transformed through chemical reactions, which helps prevent them from reaching our nearby water supplies. Soil also plays a big role in mitigating climate change through its ability to store carbon, as organic matter in the soil holds carbon in stable forms, helping to keep it out of the atmosphere. Aside from this, soil serves as the foundation for agriculture, and supports a habitat for many organisms both above and below ground.

These soil functions are primarily driven through chemistry. Chemical composition and processes affect the pH, the nutrients available for plant uptake, and the mobility of compounds. Understanding the structure, composition, and chemistry of soil is key to understanding how this vital resource sustains life and protects the environment.

Soil Composition

So, what is soil made of? Well, it contains four main parts: air, water, minerals, and organic matter. A typical soil is roughly 50% solid particles and 50% void spaces, which can be filled by air or water. We refer to this as soil porosity. Porosity plays an important role in how water, nutrients, and contaminants move through the soil profile. The soil’s chemical composition is very important because the many nutrients within it are vital for plant growth. We classify these as macronutrients and micronutrients.

Primary macronutrients include nitrogen, phosphorus, and potassium, as opposed to secondary macronutrients such as calcium, magnesium, and sulfur. Micronutrients include iron, manganese, zinc, nickel, copper, boron, and molybdenum. The difference between primary macronutrients, secondary macronutrients, and micronutrients is the amount needed by plants. Plants need primary macronutrients in the largest quantities because these nutrients are most crucial for their growth. This is followed by secondary macronutrients, then micronutrients, which they require in the smallest quantities. Each of these nutrients serve different functions. For example, nitrogen, a key macronutrient, is essential for amino acid and protein synthesis as well as chlorophyll production. Meanwhile, the micronutrient iron is important for electron transport in photosynthesis.

How Plants Access Nutrients

In order for a plant to be able to use any of these nutrients, they must be present in a soluble, ionic form. Plants absorb macronutrients such as nitrogen, phosphorus, and potassium as ions. For instance, nitrogen is taken up as nitrate (NO₃⁻) or ammonium (NH₄⁺), phosphorus is taken up as phosphate ions (H₂PO₄⁻ and HPO₄²⁻), and potassium is taken up as a cation K⁺. Plants also take up micronutrients as ions, like iron (Fe²⁺) and copper (Cu²⁺). Fertilizers can contribute to nutrient concentrations in soil, but the underlying geology of the bedrock also heavily influences them. For example, limestone-rich soils have higher levels of calcium carbonate, which raises the overall pH. (In the next section, we’ll talk more about why the pH level matters in the first place.)

The amount of organic matter in the soil also provides a lot of nutrients, a function that’s vital for plant growth. Organic matter refers to things like decomposing plant and animal materials. These materials come in a variety of forms. They include biomolecules such as carbohydrates that are rich in starch and cellulose, proteins that are rich in nitrogen, and more complex compounds like lignin. Decomposition releases smaller molecules that have various functional groups, like carboxyl groups and amine groups.

Some of the nutrients provided by organic matter are present in ionic salt forms, such as ammonium (NH₄⁺), that contribute to the overall salinity of the soil. The salinity levels must be suitable for the plant that’s growing in the soil. Too much of anything isn’t good, so if these are present in exceedingly high levels, this can negatively impact soil and plant health.

How pH Shapes Soil

We mentioned pH briefly in the previous section, but how does it affect nutrient availability and retention? At various pH values, the charges of the soil particles change.

Optimal plant growth occurs at a neutral pH of about 6.0 – 7.5. Most soil particles (such as clay and organic matter) are negatively charged. This negative charge allows these particles to hold onto positively-charged cations. This is important because certain nutrients, like potassium, calcium, and magnesium, exist as cations. However, in a more acidic soil, there is an excess of positively charged hydrogen and aluminum ions. These ions compete with the nutrient cations for a negatively charged site on the particle surface. This means that those nutrient cations get displaced, and they are more likely to leach through the soil pores with water movement.

A high degree of acidity (usually below a pH of 5.5) also slows microbial activity like nitrification. This poses a problem because once microbial activity slows, there is less nitrogen in the plant-available form, nitrate. Other nutrients, such as phosphorus, can react with iron and aluminum, forming an insoluble form that is not available for plant uptake.

Highly alkaline soils (usually above a pH of 8.5) come with their own challenges. At high pH, nutrients like iron and manganese become less soluble and precipitate out of the soil, becoming unavailable for plant uptake. Therefore, an optimal pH level helps the soil retain the nutrients that it needs.

Cation Exchange Capacity

Cation exchange capacity (CEC) is another key soil chemistry property that influences nutrient retention. CEC is the soil’s ability to hold onto positively charged cations, like potassium, magnesium, and calcium. As we described above, these cations attach themselves to negatively charged sites on clay and organic matter. A high CEC indicates that the soil is able to store and provide more nutrients to plants.

CEC value can vary across different soil types. For example, sandier ones often have a relatively low CEC. In other words, they cannot hold onto as many nutrients and they may need more frequent fertilization. By contrast, clay soils often have a higher CEC and can retain a lot of nutrients.

Redox Reactions in Soil

Oxygen also significantly affects nutrient availability and changes soil chemistry through redox reactions. In a well-aerated soil, microbes typically use oxygen as their primary electron acceptor during aerobic respiration. This is the most efficient process, and it enables nutrient cycling such as nitrification to occur. However, in soils with low oxygen levels (anaerobic conditions), microbes must use other elements as their electron acceptor. Microbes’ “willingness” to use other electron acceptors is based on the soil’s relative oxidation-reduction potential (Eh). As Eh goes lower and even negative, microbes use other elements in a step-wise manner. This sequence of steps is referred to as the redox ladder.

After all of the oxygen (O₂) is used up, microbes then switch to nitrogen (NO₃^–). We call these denitrifying microbes. Next in the sequence are microbes known as manganese reducers (MnO₂), then the iron reducers (Fe(OH)₃). Following iron and manganese are the sulfate reducers (SO₄^2-). Lastly, in the most reducing conditions are the carbon reducers that use carbon dioxide and produce methane (CO₂). We call these microbes methanogens.

A common example of these processes occurs in wetland soils, which often remain saturated for long periods of time, leading to anaerobic conditions. This gives carbon reducers and methanogens an opportunity to become active. As a result, wetlands can be a natural source of methane emissions.

Sorption and Mobility

At this point, we know about all of these nutrients and soil particles that have varying charges. We’ve also learned about how these chemical properties influence the retention of a compound in the soil. But, how do we measure this? Scientists use the soil adsorption coefficient, also referred to as K_d, to measure a compound’s tendency to attach to soil particles or stay in the soil water solution. A high K_d value means a compound is more likely to stick to the particles. Contrarily, a low K_d value is the exact opposite: the compound is more likely to remain in the soil water solution and move throughout the soil profile.

K_d values significantly depend on the organic matter content. Why? The answer lies in the organic carbon content. K_d values are often proportional to the organic carbon content of the soil. Organic matter, as you may recall, is a large source of soil organic carbon (SOC). As a result, soils with higher organic matter usually have higher K_d values.

SOC can form bonds with a lot of compounds, especially ones that are more hydrophobic, like PFAS for example. These largely hydrophobic compounds, including contaminants, can form strong bonds with soil organic matter. In simpler terms, organic matter provides more binding sites for compounds to bind with. The extent of bonding will determine whether the compound remains in the soil water solution (high K_d value) or passes through it (low K_d value).

Conclusion

Soil is extremely important and supports life of all kinds. It is key in providing food for our population and mitigating climate change. Its mixture of particles like water, air, clays, organic matter, and others play a key role in regulating water flow, as well as influencing nutrient availability and retention. Oxygen also has a significant role in soil nutrient availability and retention through redox reactions. Additionally, organic matter and charged particles change how a compound moves through the soil profile. Understanding soil chemistry is key to understanding all of the functions it provides us.