Soils: the substrate upon which many plants depend for water and nutrients

1. Soils: the substrate upon which many plants depend for water and nutrients --primary medium to support plants --also acts as shelter for billions of micro and macro organisms --bacteria, insects, worms, rodents: all help mix the soil and keep it healthy --nutrients are cycled from living to nonliving components, soil always in a state of flux

2. Soil Processes • Parent material • Climate • Vegetation • Topography • Age

3. Weathering Parent material • Vegetation Climate

4. Parent Material Granite (igneous rock): mineral content includes Al+, Fe+, Ca+, K+, Mo+ (molybdenum) --provides diverse nutrients to vegetation Serpentine (also igneous): mineral content is Cr (chromium), Mg+, Ni+ in high concentrations --limited nutrients = limited plant growth

5. Weathering --can be physical or chemical --physical includes freeze/thaw, scouring by wind and sand to break down rock

6. Weathering --chemical occurs when water dissolves substances in rock from chemical reactions --can release salts, calcium sulfides --rocks react and erode differently based on mineral content, e.g., limestone vs. granite

7. Water and pH Pure water spontaneously associates and disassociates constantly H2O H+ + OH- --1 per 550 million molecules does this every instant Water is rarely pure. Other substances added to it will break bonds and form new ones with either H+ or OH- Acids: substances that leave excess H+ Bases: substances that leave excess OH- e.g., NH3 (ammonia) is a base: H2O + NH3 NH4 + OH- CO2 dissolves in water: CO2 + H2O H2CO3 (carbonic acid) H2CO3 disassociates in soil to H+ + HCO3- making the soil more acidic, influences weathering rate of parent material

8. pH scale is from 1-14 1-6 is acidic, higher amounts of H+ 7 is neutral, or pure water 8-14 is basic, higher amounts of OH- Most rain is at pH 5, or slightly acidic --will weather limestone quickly, especially if acidity increases --higher acidity will also affect soil processes, bacterial decay will slow, etc.

9. Vegetation --plants absorb and store nutrients until released with decay --storage time can be short or long, depending on species of plant --some vegetational debris can make soils more acidic with decay, e.g., pine needles, and influence weathering rates of parent material Topography --important for drainage, sedimentation, e.g., valley bottoms vs. slopes where soils accumulate --also rainshadow effect Age --time it takes to weather bedrock, release nutrients and develop soils --as soil develops, it forms horizons based on water and nutrient flow

10. surface litter mineral soil zone of leaching (eluviation) Figure 4.1 (EFB) zone of accumulation (illuviation) weathered parent material unweathered parent material

11. Soil Patterns Grasslands: grass species short-lived, rapid decay every year, warm areas but low rainfall Forests: trees are longer-lived species, slower decay, more acidity in soil from pine needles, more leaching of nutrients may occur

12. Soil Texture Based on particle sizes of soil and can influence soil horizon development Three size classes: sand: 0.05 – 2.0 mm diameter silt: 0.002 – 0.05 mm clay: < 0.002 mm The relative proportion of each in a soil affects drainage Chart shows how soils can be classified by texture

13. Figure 4.2 (EFB)

14. Why is clay loam best? Water moves through soil as either: 1. Gravitational: drains out 2. Capillary: held between grains 3. Hygroscopic: strong adhesion to surface of smallest soil particles Field capacity: maximum amount of capillary and hygroscopic water a soil can hold --determined 1 to 3 days after a soaking rain, after gravitational water drains through --most plants can absorb capillary water down to -15 bar water potential (permanent wilting point)

15. Micelles Charged clay particles --formed by crystallization of minerals that weather from bedrock, pH dependent to have charge --negative charges capture and hold cations (+ ions) in soil, don’t leach away e.g., Ca+, Na+, K+, Mg+, Fe+, Si+, NH4+ (ammonium) --plants break bonds and can absorb these nutrients --problem is that each cation varies in strength of bond it forms, with H+ as strongest e.g., H+ Ca+ Mg+ K+ Na+ strong weak --thus, H+ ions can displace all others off micelles and cause more leaching. If acidity increases, this can be a problem

16. Negative ions also can be held by micelles: may attach to a cation held on micelle, which acts as a ‘bridge’ e.g., NO3- (nitrate), PO4- (phosphate),SO4- (sulfate) Figure 4.7 (EFB) For these reasons, then, clay loam is best—it maximizes field capacity and has lots of micelles to hold nutrients

17. Figure 4.8 (EFB)

18. Soils classifications by processes that form them Podzolization: highly acidic soils, e.g., pine forests, resulting in highly leached A and E horizons, dark B --typical of cold, northern coniferous forests, where rainfall exceeds evaporation Laterization: deeply weathered soils where decay is rapid and high Fe content, e.g., tropical forests --rainfall exceeds evaporation, no humic buildup and little leaching to A horizon --high Fe content gives reddish color, hardens like bricks when exposed to sun, drying, when forests are cleared

19. Soils classifications by processes that form them Salinization: salt buildup in top soils from reverse leaching, e.g., dry arid regions and deserts --evaporation exceeds rainfall, causes reverse leaching from ground water --can be salts, but also carbonates (alkali) that form hard crust on surface, limiting plant growth