Soil Formation and Composition
Soil-Forming Factors
Parent material provides mineral content and affects texture. Climate affects weathering rates through temperature and precipitation. Organisms contribute organic matter and aid decomposition. Topography affects drainage, erosion, and microclimate. Time determines degree of profile development.
Soil Composition
Ideal agricultural soil contains approximately 45% mineral matter, 25% water, 25% air, and 5% organic matter. Mineral particles include sand (0.05-2.0 mm), silt (0.002-0.05 mm), and clay (<0.002 mm).
Soil Texture
Determined by proportions of sand, silt, and clay. The soil textural triangle identifies 12 textural classes. Sandy soils drain quickly but hold few nutrients. Clay soils hold water and nutrients but drain poorly. Loam soils balance drainage and nutrient retention.
Soil Structure
Arrangement of soil particles into aggregates (peds). Types include granular (rounded, ideal for agriculture), blocky, platy, prismatic, and columnar. Good structure improves aeration, drainage, and root penetration. Compaction destroys structure.
Soil Profiles and Horizons
O Horizon
Organic layer of decomposing plant material. Present in forests and undisturbed areas.
A Horizon
Topsoil with organic matter mixed with minerals. Darkest mineral horizon. Most biological activity occurs here.
E Horizon
Zone of eluviation (leaching). Light colored. Materials leach downward.
B Horizon
Subsoil accumulating materials from above. Clay, iron, and aluminum accumulate here.
C Horizon
Weathered parent material. Little biological activity.
R Horizon: Bedrock.
Soil Chemistry
Cation Exchange Capacity (CEC): Measure of soil's ability to hold positively charged nutrients. Clay and organic matter have high CEC. Sandy soils have low CEC. Higher CEC means greater nutrient-holding capacity.
Soil pH: Measures acidity or alkalinity on a scale of 0-14. Most crops prefer pH 6.0-7.0. pH affects nutrient availability. Acid soils (low pH) may have aluminum toxicity. Alkaline soils (high pH) may have iron, manganese, and phosphorus deficiency.
Lime raises pH. Sulfur lowers pH. Buffer pH indicates lime requirement.
Essential Plant Nutrients: Macronutrients needed in large amounts include nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S). Micronutrients needed in small amounts include iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), chlorine (Cl), and nickel (Ni).
Nitrogen promotes vegetative growth and is mobile in plants (deficiency appears in older leaves first). Phosphorus supports root development and energy transfer. Potassium regulates water and disease resistance.
Soil Fertility Management
Soil Testing
Determines nutrient levels and pH. Proper sampling techniques include collecting multiple cores, sampling to appropriate depth (typically 6 inches for row crops), avoiding unusual areas, and mixing samples thoroughly.
Fertilizer Calculations
Fertilizer grade indicates percentage of N-P₂O₅-K₂O. A 10-10-10 fertilizer contains 10% nitrogen, 10% phosphorus (as P₂O₅), and 10% potassium (as K₂O). To calculate amount needed: pounds of nutrient needed ÷ percentage in fertilizer × 100 = pounds of fertilizer needed.
Organic Matter Management
Improves soil structure, water-holding capacity, and CEC. Sources include crop residues, cover crops, manure, and compost. Decomposition releases nutrients over time.
Soil Water
Types of Soil Water
Gravitational water drains freely through soil. Capillary water is held against gravity and available to plants. Hygroscopic water is held too tightly for plant use.
Field Capacity
Water content after gravitational water drains. Permanent Wilting Point: Water content when plants cannot extract more water. Available Water: Difference between field capacity and permanent wilting point.
Drainage
Removes excess water. Poor drainage causes root diseases, nutrient deficiencies, and restricted root growth. Methods include surface drainage, subsurface tile drainage, and raised beds.
Irrigation Methods
Surface irrigation (flood, furrow) is low cost but inefficient. Sprinkler irrigation has moderate efficiency. Drip/trickle irrigation has highest efficiency and delivers water directly to root zone. Irrigation scheduling should be based on crop needs, weather, and soil moisture monitoring.
