Ch. 4: Soils, Nutrition etc.

Soil Definition: Natural body with layers (horizons) Mineral + organic matter (OM) Differs from parent material: substance from which soil derived

Weathering Factors Mineral component: generated by weathering rock

Soil Texture A: Sand & silt Major particle sizes (know these) B: clays

Textural triangle Distribution particles by size class: texture Loam: mix sand, silt, clay Texture important: fertility, water availability

Soil Structure Particles form peds (visible when soil dry) Affect water + root penetration How important??

Organic matter (OM) Humus: partly decomposed OM Usually negatively charged: carboxyl groups (-COOH) phenols

Soil Fertility Defn: Ability soil hold & deliver nutrients Determined by texture, organic matter, pH Holding soil….

Soil Fertility Texture: clays important Huge surface area Negatively charged: hold cations useful for plants (Ca++, K+, Mg++, Zn++) Huge surface area Surface of clay in top 10 cm 0.5 ha soil would cover continental U.S.!!

Soil Fertility Humus has negative charge: clay & humus hold cations

Soil Fertility Cation Exchange Capacity: amount negative charge per unit soil Units? centimoles (+) charge/kg dry soil (cmolc/kg) Represents “potential fertility”

Soil Fertility Examples: Great Plains prairie soil: 30 cmolc/kg NE US conifer forest soil: 2 cmolc/kg

Soil Fertility H+ (and Al+++) also attracted to negative charge. Take up space: not useful to plants. Base saturation (BS): % exchange sites occupied by “good” cations (bases: Ca++, Mg++, K+) plus Na+

Soil Fertility BS, pH & CEC determine “actual fertility” 1) High CEC + high BS: more fertile 2) If BS low: pH low (lots H+)

Soil pH Measure H+ ion concentration Most AL soils: 4.5-5.1 (strongly acid) Black Belt soils: 7.9-8.4 (alkaline)

Soil pH pH effects: 1) H+ damages roots (@ extreme pH values) 2) soil microflora Acid favors fungi (incl. mycorrhizal) Alkaline favors bacteria 3) soil structure (sometimes)

Soil pH 4) nutrient availability. Major pH influence! Nutrient deficiency: Acid: N, P, Ca, Mg, K, S Alkaline: Fe, Mn, Zn, Cu, Co, B

Soil pH 4) nutrient availability. Major pH influence! Nutrient toxicity: Acid: Fe, Mn, Zn, Cu, Co Alkaline: Mo

Soil pH Plant sensitivity & nutrient needs matter Black Belt lab (#4): Black Belt soil: no pines Fe deficiency likely