SOIL EXAM 3

Soil Chemical Properties

Properties that influence nutrient availability, pollutant behavior, microbial activity, and soil physical conditions.

Colloids

Mineral or organic soil particles < 0.001 mm in diameter; highly reactive and where most chemical reactions occur.

Clay Fraction

Soil particles ≤ 0.002 mm; the most chemically active portion of soil.

Iron & Aluminum Oxides

amorphous, common in highly weathered soils.

Non-crystalline Silicate Clays

amorphous, common in volcanic soils

Crystalline Silicate Clays

structured clays

Tetrahedral Unit

Silicon-centered structure (Si⁴⁺ surrounded by oxygen).

Octahedral Unit

Aluminum-centered structure (Al³⁺ with OH⁻ groups).

Layer Silicates

Clay minerals made of tetrahedral and octahedral sheets.

1:1 Clays

One tetrahedral + one octahedral sheet (e.g., kaolinite).

2:1 Clays

Two tetrahedral sheets sandwiching one octahedral sheet (e.g., smectite, vermiculite, illite).

2:1:1 Clays

2:1 structure plus an additional layer (e.g., chlorite).

Permanent Charge (Fixed Charge)

Charge caused by isomorphous substitution; not affected by pH.

Isomorphous Substitution

Replacement of one ion with another of lower charge (e.g., Al³⁺ replacing Si⁴⁺), creating net negative charge.

Variable Charge (pH-dependent charge)

Charge that changes with soil pH; common in organic matter, oxides, and 1:1 clays.

Surface Charge

Electrical charge on soil particles that allows nutrient retention

pH

Measure of acidity:

pH = −log[H⁺]

Acidic Soil

pH < 7

Neutral Soil

pH = 7

Basic (Alkaline) Soil

pH > 7

Active Acidity

H⁺ in soil solution; measured by pH meter.

Exchangeable Acidity

H⁺ and Al³⁺ attached to soil colloids.

Residual Acidity

Acidity bound in soil minerals and organic matter; largest pool.

Total Acidity

Sum of active + exchangeable + residual acidity.

Soil Fertility

Ability of soil to supply essential nutrients for plant growth.

 

Soil Productivity

Capacity of soil to produce crops under management conditions.

Essential Element

An element required for plant life cycle, non-replaceable, and involved in metabolism.

Macronutrients

Needed in large amounts (>0.1%)

(C, H, O, N, P, K, Ca, Mg, S)

Micronutrients

Needed in small amounts (<0.1%)

(Zn, Cu, Fe, Mn, B, Cl, Mo, Ni)

Mobile Nutrients

Move within plant (e.g., N, P, K, Mg); deficiency appears in older leaves.

Immobile Nutrients

Do not move easily (e.g., Ca, Fe); deficiency appears in new growth.

Root Interception

Roots physically contact nutrients.

Mass Flow

Nutrients move with water into roots.

Diffusion

Movement from high to low concentration (important for P and K).

Biological Test

Growing plants to evaluate soil fertility.

Nutrient Deficiency Symptoms

Visual signs like chlorosis or necrosis.

Plant Tissue Analysis

Testing plant material for nutrient content.

Soil Analysis

Lab testing of soil samples.

Chlorosis

Yellowing due to lack of chlorophyll.

Necrosis

Death of plant tissue.

Stunting

Reduced growth.

Abnormal Coloration

Red/purple coloration due to stress.

Fertilizer

Material added to supply nutrients.

Law of Minimum

Plant growth is limited by the nutrient in the shortest supply.

4 Rs of Fertilization

  Source, Right rate, Right time, Right placement

Ammonification

Conversion of organic N → NH₄⁺

Nitrification

NH₄⁺ → NO₃⁻

Denitrification

NO₃⁻ → N₂ gas

Immobilization

Microbes take up inorganic N