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

Soil Horizons Caused by vertical gradients Leaching: washing material from upper to lower layers Weathering: greater at surface Biotic effects: greater at surface

Soil Horizons Major horizons: O: organic matter at surface A: surface soil. High organic matter E: zone of eluviation (material washed out of layer), leaching strong

Soil Horizons Major horizons: B: subsurface soil. Zone of illuviation (deposition of material from above). Zone of alteration (chemical changes produce secondary soil minerals/clays)

Soil Horizons Major horizons: B: subsurface soil. Hardpan: layer cemented soil grains Claypan: layer dense clay Both: interfere with water penetration, root growth Humic layer: layer organic matter from E horizon

Soil Horizons Major horizons: C: weathered parent material R: unweathered parent material

Soil Horizons Layers subdivided using numbers Fig. 4.5

Soil Forming Factors Soil = function (climate, organisms, relief, parent material, time) S = f (cl, o, r, p, t) Zonal soils: Typical for climate & organisms “Soil climax:” equilibrium with climate/organisms Intrazonal soils: deviate due to relief, parent material Azonal soils: too young (not enough time to mature)

Climate (cl) Moisture & temperature most important More water & higher temperatures accelerate soil development Higher temperature: More weathering Deeper soil, more clay

Climate (cl) More moisture: Deeper soil More clay More OM Greater CEC (potential fertility)

Organisms (o: plants) Plants influence soil and vice versa How plants influence soil? 1) Roots Depth: record 120 m (394 ft) for fig tree (Echo Caves, South Africa) Amount (biomass/unit volume/yr) Size: woody (shrub/tree) vs. fibrous roots (grasses)

Organisms (plants) How plants influence soil? 2) Base cycling Keeps nutrients “in play”

Organisms (plants) Fertile island effect: under desert shrubs surface soil fertile Ex, creosote bush: soil under shrubs--more nutrients

Organisms (plants) Fertile island effect due to: a) base cycling b) stemflow (rainwater flowing down stem) concentrates nutrients in dust into soil below shrub Result: annual plants bigger & healthier under shrubs

Organisms (plants) How plants influence soil? 3) Litter acidity Ex: compare soils under spruce (conifer) vs hardwood

Relief (r) Topographic position affects soil Primary effects: S = f (cl, o, r, p, t) Topographic position affects soil Primary effects: Drainage: movement water through soil. Causes leaching Runoff: movement water over soil surface. Causes leaching Erosion: removal soil by water

Relief (r) Topographic position affects soil properties In general: Hilltop soils shallow, less profile development Basin soils deeper, more clay/salts, more profile development

Relief (r) Erosion not always harmful Example: pygmy forest, Mendocino County, California

Relief (r) Each terrace 100,000 yr older than previous At 300,000 yr, hardpan (layer of cemented iron) develops 1 foot below surface

Relief (r) No drainage when wet, little water holding when dry Trees stunted

Relief (r) Canyon sides: erosion prevents hardpan formation…. full sized forest

Parent Material (p) Within climate area, parent material major influence on vegetation and soil Ex, serpentine soil High Mg, low Ca Lots Ni, Cr Shallow, stony

Parent Material (p) Extreme cases, form serpentine “barrens”

Parent Material (p) Within climate area, parent material major influence on vegetation and soil Ex, granite outcrop soil High silica, lots sand (coarse texture) Soil dry (water drains fast) Little nutrients (little clay: low CEC) Forest on granite in Australia

Time (t) General trends (as time increases): pH decreases S = f (cl, o, r, p, t) General trends (as time increases): pH decreases organic matter increases clay content increases soil depth increases