1. SOIL FERTILITY

2. Ability of soil to supply nutrients for plant growth 

3. Soil is storehouse of nutrients Some available, some not
   

4. Soil fertility: quantity of nutrients soil contains
how well protected from leaching
how available to plant
how easily roots can take them up

5. Plant nutrients  
Essential elements needed for plant growth

6. Plant nutrients Plants absorb 90 elements Only a few needed for growth
Some not needed by plants but by animals that eat plants (cobalt)
Others not needed, can be toxic (lead)

8. How to determine which are essential?

9. Essential plant elements
1. lack of element stops plant from growth or reproduction
2. element is directly involved in plant processes
3. shortage of element can only be corrected by supplying that element  

10. 17 essential elements 3 account for 95% plant needs
carbon, oxygen and hydrogen
come from water, air

11. Non-minerals
Macronutrients
Air and Water
Carbon (C)
Hydrogen (H)
Oxygen (O)

12. 14 come from soil 6 used in large amounts: macronutrients
Nitrogen Calcium
Phosphorus Magnesium
Potassium Sulfur

13. Primary Macronutrients
Nitrogen Phosphorus Potassium
Not always available in large enough quantities
Add by fertilizing

14. Secondary Macronutrients
Calcium Magnesium Sulfur
Soil usually has plenty

15. Micronutrients 8 nutrients left Used in small amounts
Plant won't grow normally without them

16.  NUTRIENT IONS
Plants absorb some of the nutrients as ions instead of elements
Ion has positive or negative charge

17. Ion has:
positive charge cation
negative charge anion

18. Ions form in soil when compounds dissolve in water Example:
Ions form in soil when compounds dissolve in water   Example:
Potassium nitrate (fertilizer) dissolves in water,
molecule breaks down into potassium ion and nitrate ion

19. Plant roots absorb ions - soak like a sponge
Soil particles adsorb ions - hold on to them - stick to it

21. SOURCES OF ELEMENTS IN SOIL
Nutrient elements present in soil in four places (pools)

22. SOURCES OF ELEMENTS IN SOIL
1. soil minerals
major source
released slowly by weathering
not source of nitrogen

23. SOURCES OF ELEMENTS IN SOIL
2. organic matter
large amounts of nitrogen
nutrient anions
released by decay   

24. SOURCES OF ELEMENTS IN SOIL
3. adsorbed nutrients
held by clay and humus particles
relatively available to plants

25. SOURCES OF ELEMENTS IN SOIL
4. dissolved ions
ions in soil solution
plants absorb directly
may be leached

27. Soil colloids   
Tiny particles of clay and humus with slight electrical charge
This charge attracts plant nutrient ions

28. CATION EXCHANGE Negative charge on soil colloids:
attracts positively charge ions
repels negatively charged ions

29. adsorption  
Negatively charged colloid attracts swarm of cations from soil solution

32. Cation Exchange
When one ion taken up by plant (pulled off soil particle), replaced by another.
Replacement of one cation for another

33. Ability of soil to hold nutrients –
Ability of soil to hold nutrients –
directly related to the number of cations it can attract to soil colloids
Determined by the amount of clay and humus in soil mix

34. Displacement of cations depends on:
Relative concentration
high concentration displaces low
The number of charges on a cation

35. high CHARGE displaces low Al>Ca>Mg>K>Na

37. Plant roots have negatively charged surfaces
positively charged hydrogen ions attached
Cation exchange takes place when plant roots exchange positive hydrogen ions for cations on soil colloids or in solution

38. Cation Exchange Capacity (CEC)
The ability of soil to hold exchangable ions
CEC expressed in milligram equivalents per 100 grams of soil (mEq/100g)

40. Bonding strength If two cations are present in soil in equal numbers
one that bonds most strongly will be adsorbed
others will be leached out

41. Mass action 
more ions in soil,
more exchange sites it will occupy

42. Weakly held cations are more available for plant uptake

43. A clay particle is covered with negative charges
Opposites attract, ions with positive charge(s) stick all over surface of clay
root hairs secrete
H+ into water around clay particles
Smaller H cations
replace larger cations

44. Several nutrients available to plants as negatively charged ions – anion exchange
Negative charge means it is repelled from cation exchange site
Anion exchange greatest in acid soils

45. Implications for Growing
High CEC soils have more clay
Low CEC soils more sand

47. Herbicide CEC determines how much herbicide should be used.
Colloids absorb pesticides also, tie them up.
High CEC, clay soils usually need more to get effect you want

48. Fertilization  
High CEC soils have greater ability to hold nutrients - larger amounts, less frequently
Low CEC - smaller amounts more frequently - leach out

49. Golf courses - all sand - low CEC - fertilize lightly and often
Golf courses - all sand - low CEC - fertilize lightly and often
Greenhouses - soilless - low CEC - fertilize lightly and often

50. Improve CEC by adding organic matter
Improve CEC by adding organic matter
Clay soils need less organic matter except to aerate soil

51. NUTRIENT UPTAKE  

52. Nutrient absorption  
Nutrient ions cross cell membranes of root cells and move into vascular system

53. Some uptake is passive
Most uptake is active - takes energy to pull nutrients into high concentration already in plant  

54. Roots produce energy by respiration
Roots produce energy by respiration
Waterlogged soil limits respiration - limits nutrient uptake

55. Root hairs get ions from soil solution by their own form of cation and anion exchange

56. As root tips grow, move through solution, constantly finding more nutrients

57. Capillary action moves nutrients through solution toward plant roots

58. Diffusion moves ions through soil solution
Diffusion
moves ions through soil solution
– higher concentration to lower concentration

59. Factors affecting uptake
Anything interfering with photosynthesis - slows growth, slows uptake   --low light --poor drainage --soil compaction --dry soils --soil temperature

61. Luxury Consumption
Plants can sometimes store nutrients for when growth may be slowed

62. Plants with deep roots, healthy roots need less fertilization