Week 7 (Soil Chemistry) Study Guide

What is the soil solution?

The water between soil particles where ions move and many chemical reactions occur.

What is the approximate composition of idealized soil?

50% solid material (about 45% minerals and 5% organic matter) and 50% pore space containing air and water.

Why is clay more chemically active than sand?

Clay has more surface area providing more sites for chemical reactions and nutrient adsorption.

What three major chemical properties of soil?

pH, redox status, and electrical conductivity

What does soil pH measure?

How acidic or basic the soil is, based on hydrogen ion activity. Lower pH is more acidic while higher pH is more basic.

What do redox conditions describe?

How readily substances gain or lose electrons through oxidation-reduction reactions.

What is electrical conductivity (EC) in soil?

How well soil solution conducts electricity. It generally increases as the concentration of dissolved ions increases.

What is adsorption?

The process in which ions or other substances stick to the surface of soil particles.

What are soil colloids?

Very small soil particles, commonly including clay and organic matter, with high surface area relative to volume and high chemical reactivity.

What are phyllosilicate clays made of?

Layers of silica tetrahedra and aluminum or magnesium containing octahedra.

What is the difference between 1:1 and 2:1 clay structures?

A 1:1 clay has one tetrahedral layer and one octahedral layer. A 2:1 clay has two tetrahedral layers surrounding one octahedral layer.

Why can 2:1 clays hold more water and nutrients than many 1:1 clays?

Their layered structure can create interlayer spaces that hold water and nutrients.

What is isomorphic substitution?

When one ion in a clay mineral structure is replaced by another of similar size, potentially creating a permanent net negative charge if the replacement has a lower positive charge.

What does CEC refer to?

The capacity of soil to hold and exchange positively charged ions (cations) on negatively charged sites.

What soil properties generally increase CEC?

More clay, especially 2:1 clay, and more organic matter generally increase CEC.

What is soil buffering capacity, and how does it relate to CEC?

Buffering capacity is the soil's ability to resist changes in pH or chemical conditions. Higher CEC generally means greater buffering capacity.

What is base saturation?

The percentage of cation exchange sites occupied by base cations, such as calcium, magnesium, potassium, and sodium.

How do you calculate base saturation?

Base saturation (%) = (base cations / total cation exchange capacity) × 100

How does soil pH affect plant nutrient availability?

It changes the availability of nutrients. Many major nutrients are most available around pH 6–7, though the ideal range depends on the crop and nutrient.

How do lime and sulfur affect soil pH?

Lime raises soil pH, making soil less acidic. Sulfur lowers pH, making soil more acidic.