This document discusses factors that influence plant growth through healthy soil management. It begins by explaining that organic agriculture focuses on building healthy soil as the foundation for sustainable agriculture. The document then outlines characteristics of ideal soil, such as being fertile, well-drained, and high in organic matter. It describes the components of soil, including minerals, organic matter, water, and air, and explains how soil texture, organic matter, and cation exchange capacity impact soil health and nutrient availability for plants. The summary emphasizes that managing soil organic matter and texture are essential for supporting plant growth.

1. What Makes Plants Thrive?
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4. “What Makes Plants Thrive?”
 Why start the discussion with soil?

5. Organic Agriculture (USDA National
Organic Program)
 Recognition that healthy soils are the
foundation of sound, sustainable agriculture
system
 primary focus of organic farming is to use
practices that build healthy soils

6. National Organic Program
Section 205.203(a)
 Select and implement tillage and cultivation
practices that maintain or improve the
physical, chemical, and biological condition of
soil and minimize soil erosion

7. Soils: More Than “Just Dirt”
 Aim of organic and sustainable farming: to
improve the soil
 Encourage soil organisms
 Supply nutrients in sufficient quantity without
oversupplying
 Maintain tilth and minimize compaction
 Build organic matter

8. Outline for today
 What is soil?
 Soil textures
 Soil organic matter
 Soil fertility management in relation to above
factors

9. What is a soil?
 Characteristics of an ideal soil
 Components of a soil
 Appearance of a soil
 Soil texture/ soil type

10. Characteristics of an ideal soil
 What are characteristics of ideal soils?
 Fertile
 Deep
 Well drained/ aerated
 High in organic matter
 Friable
 soil is easily worked

11. Components of Soil
Mineral
Organic
Water
Air

12. Soil Air
 Oxygen supports soil life
 Roots
 Microbes
 Air can be displaced by water

13. Management Question:
 The management of what important factor to
related to plant growth must be considered in
relationship to soil air?

14. Irrigation!
 Saturated soils – over-irrigation, poor drainage
 Water fills in all the air spaces – no room left for air
 Affects root growth/health, soil biological activity,
available nitrogen

15. Mineral and organic components
 A substance with a complex structure
consisting of:
 Broken down rock particles (mineral component)
 Organic matter
 Ideal soil is about 5-6%
 Function of organic matter
 Source of food for soil microorganisms
 Source of nutrients for plants
 Holds minerals against loss from soil due to leaching
 Holds water where plants can access it

16. Soil Profile
 Topsoil
 It is the darkest layer of the
soil because it contains
organic matter and humus.
 Contains majority of feeding
roots of plants
 Subsoil
 Low in nutrients
 Contains few
microorganisms
 Structure of subsoil is
important for drainage

17. Example of a
soil profile
• Note the dark
organic horizon at
the surface and
changes in color
and structure down
the profile

18. Soil texture
 Determined by mineral component
 Coarseness or fineness of soil

19. Soil texture
 Clay
 Soil particles are less than
0.002 mm in size
 Does not drain easily and is
difficult to work
 Root growth is poor due to
small spaces between soil
particles

20. Soil texture
 Silt
 Particle size is between
0.002 to 0.05 mm
 Feels silky to the touch

21. Soil texture
 Sand
 Sand particles range in
size from 0.2 mm for
the very finest sand to 2
mm for the coarsest
 Feels gritty if rubbed
between your fingers
 Warms up and dries
early in spring
 Low in nutrients

22. Soil Types
 Most soils are a mixture of different soil textures
 Often a soil type will be dominated by a particular texture
 Can group soil types by how well they drain and major
texture class they contain
 Heavy soils
 Contain a high proportion of clay
 Light soils
 Contain a high proportion of sand
 Important to know the soil type because it will determine the
management practices you need to use

23. Why is soil texture important?
 The texture will determine how much air is in
soil and how well water flows through the soil
 Determines how easily the soil is worked

24. How might you manage a clay soil different than a
sandy soil?
 Irrigation – sandy soils will hold less water
 Types of crops – some crops do better in a
lighter soil (root crops, squashes)
 Fertility – may need to do in-season
applications in sandy soil
 Cultivation/cover cropping –
 Beware of erosion!

25. Pause for Questions....

26. Soil Organic Matter
 Many soil properties impact soil quality, but
organic matter deserves special attention
 Affects several critical soil functions, can be manipulated
by land management practices, and is important in most
agricultural settings across the country
 Because organic matter enhances water and nutrient
holding capacity and improves soil structure, managing
for soil carbon can enhance productivity and
environmental quality, and can reduce the severity and
costs of natural phenomena, such as drought, flood, and
disease

27. Soil Organic Matter
 Living
 Roots
 Earthworms and insects
 Microorganisms
 Dead
 Fresh plant residues
 Releases nutrients as it decomposes – allows for slow, steady release
 Recently deceased soil organisms
 Active organic matter
 Very Dead
 Well decomposed organic materials
 Humus
 Slow to decompose – may last 100’s of years
 Hay, cornstalks
 Not well understood

28. Characterization of Organic Matter:
Humus

29. Organic Matter
 Generally 0-6%
 Reservoir for nutrients, improves soil
aeration, drainage, water holding capacity
 Very complex compounds
 Difficult to characterize chemically
 Biologically controlled
 Very dynamic- differs depending on moisture,
temperature

30. Humus
 Very well decomposed
organic matter
 refers to any organic matter that has reached a point of
stability, where it will break down no further and
might, if conditions do not change, remain as it is for
centuries, if not millennia
 Mineralization of organic matter to soil humus
releases some amounts of nitrogen, phosphorous
and sulfur from organic forms, and can
dramatically influence the availability of
micronutrients

31. Relative amounts of OM throughout US

32. Pause for Questions...

33. 16 essential elements:
 C, H, and O: not consider minerals
 Macronutrients:
 Primary macronutrients: N, P, and K

Needed in relatively large amounts
 Secondary macronutrients: Ca, Mg, S
 Micronutrients: Cl, Fe, B, Mn, Zn, Cu, Mo
 Required in small amounts
 Supplied through:
 Compost, cover cropping, fertilizers (both organic and
synthetic)
 If any one is missing or low, plant productivity is
limited

34. How do plants take up nutrients?
 By roots
 Must be plant available

35. Soil solution and nutrient uptake

36. Ions
 Cation
 Calcium, magnesium
 Anion
 Certain forms of nitrogen

37. Nutrients in Ionic phase
 Magnets: opposites attract

38. Molecules: opposites also attract

39. Soil acts as a magnet
 Attracts and retains positively charged ions in
soil solution
 Prevents them from moving downward with
water

40. Clay and Cation Exchange Capacity
Brady & Weil, 2004. Elements of the Nature and Properties of Soils

41. Cation Exchange Capacity (CEC)
 It is the ability of clay, organic matter, and humus to
attract positive ions
 Clay, organic matter, and humus all have negative charges
on their surfaces
 Most nutrients (P, K, Ca, etc) are cations or have positive
charges
 “Opposites attract”
 The higher the cation exchange capacity, the more
closely the nutrients are held and the less likely they
are to be lost to leaching

42. Soils with high CEC
 Hold more nutrients
 Avoid rapid changes in levels of nutrients in
soil solution

43. High vs. Low CEC

44. Soil organic matter and CEC
 Structure of soil organic matter is extremely
complex, with that can interact with cations (plant
required nutrients)
 Often contributes from 30 to 90% of the total charge
present in the soil surface, even when present in
relatively low amounts.

45. pH Dependent Charge on OM

46. What else has positive charge and
affected by CEC?
 Water

47. OM vs. Clay
 Because of it complex and open structure, the
ability of humus to pull water from the
surrounding saturated atmosphere of the soils
approaches 80 to 90% of its weight, as
compared to 15 or 20% for soil clays.
 Humus does not readily fix exchangeable
cations, as do some clays, but maintains these
ions in an easily exchangeable form

48. Management
 High CEC soils (high clay content, moderate to
high organic matter content)
 high water holding capacity – be careful not to over-
irrigate
 low leaching potential for cationic nutrients - less
frequent need for lime and fertilizers (except N)
 their physical properties may make it difficult for a
farmer to cultivate or maintain good aeration

49. Management
 Low CEC soils (high sand, low organic
matter)
 need for more frequent irrigation
 may need to apply fertilizer throughout season to
meet plant demand
 may be more prone to erosion
 can be easier to work

50. Questions?