Larry Zibilske, Ph.D. Texas Plant & Soil Lab Edinburg, TX

1. Soil Science Fundamentals Larry Zibilske, Ph.D. Texas Plant & Soil Lab Edinburg, TX

2. Soil A Living, Breathing Dynamo Filled with Complex, Interacting Populations of Organisms

3. Dirt What’s left when nutrients have been depleted; unable to support large populations of organisms

4. Soil Constituents • Mineral Matter • Air • Water • Organic Matter

5. 1. Soil Minerals

6. Mineralogy • Rocks (a noun, not a verb) • Contain elements in crystal form • Practically inert forms of oxygen, silicon, aluminum, iron, calcium, magnesium, copper, zinc…many elements except nitrogen.

7. Mineralogy Soil comes from rocks by weathering Over a long time, rain, ice, atmospheric acids, break rocks into smaller and smaller particles

8. Horizons Soils develop from bottom up O=Organic Layer A=“Plow Layer” B=Subsoil C=Parent Material

9. Soil Forming Modifiers • “Residual” soils are rare • Most have been transported from other places • Water • Wind

10. Soil Particles • Categorized by size • Sand: 2 mm down to 0.05 mm • Silt: 0.05 mm down to 0.002 mm • Clay: < 0.002 mm

11. Categorized by size • --as if they were bank accounts-- • Sand: $1,000,000 down to $25,000 • Silt: $25,000 down to $1000 • Clay: < $1,000

12. Soil Texture • How much of each particle size are in the soil • % Sand, silt, clay • Clay dominates texture issues • Loam not dominated by any one size

13. Clay soils can be tightly packed. What does this suggest?

14. Sandy soils are difficult to pack. What does this suggest?

15. Soil Properties Cation Exchange Capacity “CEC”

16. CEC is the capacity of a soil to hold and exchange cations with soil water

17. Cations are chemical elements that have been ionized and now carry a positive (+) charge. Examples: Ca2+, Mg2+, NH4+

18. Anions are chemical elements that have been ionized and now carry a negative (-) charge. Examples: SO42-, NO3-, PO43-

19. The origins of CEC • Clay minerals are often layered crystals • Weather to very small flat, platy particles with lots of (-) charges on their edges • Imperfections inside the crystals also cause (-) charges. • Organic matter greatly affects CEC…more on this later.

20. Soil Particle edges have (-) charges Negative charges attract positively charged ions (cations)

21. CEC is a “balance”

22. Percent Base Saturation • Shows up on soil test reports • Proportion of bases occupying CEC • Ca, Mg, K, Na (not H or Al) • Index of fertility and management • Calcium should dominate: • Ca 80%, Mg 15%, K 5%, Na 1%

23. Soil differences in CEC Sandy Soils: 0-3 LS to SL Soils: 3-10 Loam Soils: 10-15 Clay Loams: 15-30 Clay Soils: >30

24. Organic Matter: 200! Relative importance of mineral particles and organic matter: CEC=(% OM x 200) + (% Clay x 50)

25. Example: Soil with 2% OM and 10% Clay (0.02 x 200) + (0.1 x 50)= 9

26. Soil pH (Soil Reaction) The balance between acidity and alkalinity in soil Technically, the negative log of the hydrogen ion concentration in the soil.

27. pH scale: 0-14 7.0 is neutral < 7.0 is acidic > 7.0 is alkaline

28. Affects solubility of soil nutrients Direct affects on plants

29. Where does all this acid come from? • Lack of basic elements (Ca, Mg,) in soil minerals from which the soil is generated. • Leaching of basic elements from the soil • Plant and microbe activity produces acidic end products

30. Why do other soils have little acid (alkaline soils)? • Formed from minerals with high amounts of basic elements • Leaching losses are less due to arid climate • Hostile growing conditions for plants and microbes means little biological acid formed

32. Effects of low soil pH (high acidity) Increased levels of several nutrients: Copper, Iron, Manganese, Zinc Leaching of bases from soil Soil structure problems

33. Effects of high soil pH (low acidity) Low solubility of metallic nutrients: Copper, Iron, Zinc, Manganese Marked reduction in P solubility (Ca, Mg) Soil structural problems

34. 2. Soil Air

35. Soil Particles mean Pore Space • Spaces between soil particles allow air in and waste gases out • Pore space size determines soil aeration capacity • Air shares pore space with water • More water, less air

36. Pore size is very important • Sandy soil: Higher proportion of larger pores – easier air movement • Clay soil: Higher proportion smaller pores – slows air movement

37. Why is aeration important? • Oxygen is needed for plant roots • Oxygen is needed for many soil microbes • Affects chemical reactions: • Oxidation-reduction reactions affect many soil nutrients (N, S, Mn, Fe, Zn, etc.)

38. Balance between aerobic and anaerobic conditions • Both conditions exist in soils at the same time. How is that? • Natural process that alternates, often after rain events.

39. Soil Air Contents Nitrogen: 79% Oxygen: 5-15% Carbon Dioxide: 0.3-4% Trace gases: very small amounts

40. Aeration and Soil Biology Respiration vs. Fermentation • Respiration: Oxygen Present; high metabolic efficiency • Fermentation: Oxygen Absent; lower metabolic efficiency

41. Fermentation ≠ Anaerobic Respiration

42. Changes in Oxygen Status • Follow saturating rains • Follow heavy irrigation • Follow tillage (no-till?) • Compaction reduces (pore size declines)

43. 3. Soil Water

44. Solution containing dissolved and suspended materials • Nutrients, some nutrients • Salts • Dissolved organic material

45. Carries nutrients to plants • Carries toxins/endproducts away • from plants • Movement of some microbes

46. Movement • Gravity; percolating water • Capillarity; adheres to surfaces surface, moves under tension

47. Soil Air vs. Soil Water

48. Plant Available Water • Held in soil under tension, but not too much. • Plants must use energy to pull water away from soil

49. Drying soil has thinner water films; plants work harder

50. The thinner the water film on soil particles, the more energy is needed to pull it away. • Plants must generate this energy