Unit 5: Soil chemistry

Table of Contents

Soil composition

Soil is a complex mixture of four main components. A typical "ideal" loam soil consists of:

Relation between organic carbon and organic matter

Soil Organic Matter (SOM) is a complex mix of carbon-based compounds, including living microbes, dead plant/animal "detritus," and stable humus.

Soil Organic Carbon (SOC) refers only to the carbon atoms within the SOM.

It is difficult to measure SOM directly, but easy to measure SOC. A long-standing convention, known as the Van Bemmelen factor, is used to convert between them.

Formula:

% Soil Organic Matter (SOM) = % Soil Organic Carbon (SOC) × 1.724

This factor is based on the assumption that SOM is, on average, 58% carbon (1 / 0.58 ≈ 1.724). This is a general approximation.

Inorganic and organic components in soil

Inorganic Components

These are the non-living, mineral parts of the soil derived from parent rock. They provide the soil's structure (texture) and are the source of most plant nutrients (Ca, Mg, K, P, etc.).

Organic Components

This is the Soil Organic Matter (SOM). It is the single most important indicator of soil health. It can be divided into:

  1. Living Biomass: Bacteria, fungi, insects, earthworms, plant roots.
  2. Detritus (or "Active" SOM): Freshly dead and non-decomposed organic material (e.g., a new leaf, a dead insect). This is the "food" for the soil.
  3. Humus (or "Stable" SOM): The final product of decomposition. (See next topic).

Soil humus

Humus is the stable, dark-colored, amorphous (shapeless) component of soil organic matter. It is what's left after microbes have decomposed the "easy" parts of detritus. It is recalcitrant, meaning it is very resistant to further decomposition and can last in the soil for hundreds of years.

Importance of Humus:

Cation and anion exchange reactions in soil

This is the most important chemical process in soil for plant nutrition.

Cation Exchange Reactions

Both clay particles and humus particles are naturally negatively charged.
Because opposites attract, these negative sites (called "exchange sites") attract and hold onto positively charged ions (cations) from the soil water.

Important nutrient cations include: K⁺, Ca²⁺, Mg²⁺, NH₄⁺
Problematic acidic cations include: H⁺, Al³⁺

The "Exchange": The plant root releases H⁺ ions into the soil. This H⁺ "kicks" a nutrient (like K⁺) off the exchange site, and the K⁺ is then absorbed by the root.

Reaction: [Soil]⁻-K⁺ + H⁺(from root) ⇌ [Soil]⁻-H⁺ + K⁺(to root)

Cation Exchange Capacity (CEC): This is a measurement of the soil's ability to hold nutrients. It is the total negative charge available in the soil.

Anion Exchange Reactions

Anions (negatively charged ions) are generally repelled by the negatively charged soil particles. This is why anions like nitrate (NO₃⁻) leach very easily from soil, causing water pollution.

Some soils (especially acidic, weathered soils) can have some positive exchange sites that hold onto anions like phosphate (PO₄³⁻) and sulfate (SO₄²⁻). This is called Anion Exchange Capacity (AEC), but it's much less common than CEC.

Nitrogen, phosphorus and potassium in soil

These are the three primary macronutrients (NPK) required by plants in the largest amounts.

Nitrogen (N)

Phosphorus (P)

Potassium (K)

Know the function and the main available form for each NPK nutrient.

Phenolic compounds in soil

Phenolic compounds are a large class of organic chemicals that contain a benzene ring bonded to a hydroxyl (-OH) group. They are very common in plants.