1. Soil and Plant Relationships

2. Characteristics: Water is the universal solvent
One of nature's most stable compounds,
Water molecule is not symmetrical (creates a dipole)
This phenomenon of polarity creates an attraction between water molecules. Water molecules can also attract or be attracted by
Cations, such as Na+, K+, and Ca++, or
Anions or clay colloids in the soil.

3. SOIL WATER Availability
Even though water is present in the soil, it sometimes is not available to the plant.
The pore spaces are always filled with water, air, or a mixture of both.
When the pore spaces are filled with water, the soil is said to be saturated. Saturation is an unhealthy condition for plants if it lasts too long because the oxygen needed for respiration is missing.
When the pore spaces are filled mostly with air, the soil is too dry → -ve effect on plant growth.

4. The number and size of the soil pores vary with the soil's texture and structure.
Clay soils have smaller but more numerous pores than sandy soils. Thus, an equal volume of clay soil holds more water than a sandy soil when the pores are filled (Fig. 12-2).

5. The ability of the soil to retain water is called its water-holding capacity. Fig. 12-3.

6. Water Movement and Retention in Soil
Three forces are responsible for water movement within the soil.
1- Gravity causes water to move downward and is the principal force when a soil is saturated = percolation.
2- Adhesion is the force of attraction between unlike molecules (soil particles and water).
3- Cohesion is the force of attraction between like molecules (water and water).
2+ 3 = capillary motion
The latter two forces can cause water to move by capillarity in any direction—upward, downward, or laterally—and are the principal forces that move water in an unsaturated soil.

7. Water Uptake by Plants be

8. The upward movement of water, called capillary rise, is responsible for the loss of water from the soil surface by evaporation (Fig. 12-4).
As soil dries, the water film surrounding each soil particle thins. Consequently, the adhesive and cohesive forces of attraction increase rapidly, making it more difficult for the plant to extract water that is held tightly in soil particles.

9. Salts are present in soil water
Salts are present in soil water. The salts create osmotic energy, and if the salts are present in a sufficiently high concentration, the osmotic energy prevents water movement into the plant (Fig. 12-6) = Plasmolysis.
Remember that fertilizer is a salt.

10. After a prolonged rain or irrigation, the air in the soil pores is displaced with water. In this condition, the soil is saturated,
When no more water is added, losses continue, first from the larger macropores=percolation and then from the smaller micropores=infiltration. Loss of water continues until the adhesive and cohesive forces equal gravity.
At this moisture content, the soil is said to be at field capacity. The water has drained from the macropores but the micropores still contain water.

11. Transpiration

12. Soils support urban life/Soils and Climate
Shttps:// view February video and November video

13. Homework Finish pages 383 #1-5 and 388 #1-6 questions
Finish Soil, Drainage and Runoff using your own telephone

14. Soil is essential for life – even fish depend on nutrients released from soil into streams and oceans.
Images: Martin Miller
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15. Soils store water for plants and improve water quality as it passes through them in the hydrologic cycle.
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16. Soils recycle nutrients and carbon.
Image: T. Hiett
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17. Soils help recycle people’s waste.
Image: T. Hiett
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18. Soils influence how we build our homes, cities, and transportation systems.
Image: Digital Stock
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19. Soils Are a Very Important Earth Resource.
Image: T. Loynachan
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