1. How Water Moves THROUGH SOIL

2. When Rainfall Hits the Earth It can either:

3. 1) Runoff the Land

4. 2) Move through the Soil OR

5. Infiltration – the downward entry of water into the surface of soil or other materials.
When discussing soil health and rainfall management, you are going to hear a lot about INFILTRATION of the water into the soil.
So, it is important to define infiltration.

6. How does water infiltrate into the soil?
Through the pore space in the soils.
Soil Particle
Demonstrational tool – clear bottle of golf balls or ping pong balls. Hold it up and point out spaces (representing pore space) between balls (representing sand grains).
To help understand pore space, visualize a bottle of golf balls. The golf balls are sand particles. If you look, you will see spaces (voids) between the sand particles. This space is not empty it is filled with something. In this case, these spaces are filled with air but they can also be filled with water.
The pores space within the soil is that portion of the soil not occupied by mineral matter (rock, sand, silt, clay) or organic matter and that allows the passage or absorption of fluids
Pore Space

7. About ½ of the soil volume is solid particles
About ½ of the soil volume is pore space
And, this pore space makes up about ½ of the soil volume.

8. The amount and size of pore space affected by:
Texture
Structure
Activity of Beneficial Soil Organisms

9. TEXTURE
Demonstrational tool – bottle of golf or ping pong balls and a bottle of table salt (glass salt shaker).
Texture – size of the soil particles – affects the size of the pore space.
Let’s go back to that bottle of golf balls (representing sand grains). Now, let’s compare it to a bottle of table salt (representing the much smaller clay particles). The pore space between the golf balls is large compared to the pore space between the salt grains.
The golf balls or the sand particles have the largest pores – water can quickly move through a sandy soil as a result. Conversely, clayey soils have very small pores – water moves through at a slower rate.
So, texture (percent sand, silt, clay) affects the percent (quantity) of small vs large pore space. The more sand – the more large pore space you will have, etc.

10. The quantities of large and small spaces directly affect plant growth.
Related to the quantities:
Large spaces important for infiltration and movement through soil. Small spaces important for water holding ability (storage of water in soil).
On clayey soils, a lack of large pore spaces restricts water and air infiltration and movement, thus limiting root growth and activity of beneficial soil organisms.
On sandy soil, while water moves easily through, the lack of small pore pace limits the soil’s ability to hold water and nutrients.
So, textures like loam with a good balance of soil particle sizes is the best for the soil’s ability to both move water into and yet hold on to some of that water for future use by plants.

11. STRUCTURE
Soil structure describes the arrangement of the individual soil particles, and therefore, the arrangement of soil pores between them.
So, soil structure has a major influence on water and air movement.

12. Using Pointer
Highlight how water moves through the various structures comparing how easily it moves through granular compared to platy. It takes much longer to move through platy.
Paths of water flow through soils with granular, prismatic, subangular blocky, and platy structure, respectively

13. Compacted Soils Lost pore volume
The use of machinery and implements and even the trampling of animals can compact a soil. Large, continuous soil pores are lost or are reduced in size, leading to poor water infiltration, slow drainage and reduced aeration for healthy root growth and nutrient uptake for maximum crop yield.
In short, compacted soil do not provide adequate space for the storage or movement of air and water in the soil.
Use Pointer – Note how water does not infiltrate where soil is compacted under the tire tracks or where cattle trampled site.

14. BIOLOGICAL ORGANISMS
The burrowing activity of earthworms and other invertebrates creates channels (large spaces) that allow rapid flow of water into or through soil.
Earthworms and other critters create channels (large spaces) that water can move through. And, they produce waste and and assist in decomposition of organic matter.

15. Organic matter acts as the “glue” to hold the frame work of soil particles and pores together in place, allowing the infiltration of air and water, and may hold as much as twice its weight in water.
Fungi and bacteria also produce substances that help bind soil particles together; keeping them in place.
Organic matter is important in creating large pore space.
It acts as a “glue” holding soil particles in place.
If the soil particles are not held in place, soil particles can move into the pore space – clogging them. Think of a clogged up sink – the water is not moving through.
Soil organic matter is also the storehouse for the energy and nutrients used by plants and other organisms.

16. Vegetative Cover
On bare soil, raindrop impact breaks large soil clumps into smaller particles. These particles then clog soil surface pores (forming a crust) reducing the movement of water into the soil.
On the other hand, vegetative cover (living or dead) absorbs most of the energy of the raindrops that fall on it and by the time this rainwater reaches the soil below, its ability to dislodge fine particles is greatly reduced. Consequently, there is little or no clogging of surface soil pores by detached particles, and little deposition of soil particles that would form a crust on the surface.
Vegetative cover slows down any runoff, giving more time for infiltration.

17. Once in the soil, the water movement is affected by one of two forces:
Gravity
Capillary Action

18. Gravity Pulls the water downwards.
Largely occurring in the large pore spaces.
Primary force once the soil becomes saturated (all the pore spaces full of water).
Relatively rapid
Gravity - “Water always flows downhill” (even in soil)

19. Capillary Action Refers to the attraction of water to:
Itself (cohesion)
Other solid surfaces (adhesion)
Can move in all directions
Occurs in the small pores
Primary force when soil unsaturated (all the pore spaces are not full of water).
Relatively Slow
Attraction – example of cohesion – water beading on new waxed truck; example of adhesion - paper towels absorb fluids through capillary flow – stick the tip of a paper towel in spilled milk and the milk will immediately start moving into the paper towel;
Capillary may be in all directions i.e. it may be downward, lateral or upwards – defying gravity
Water is held tighter in the small pores
Primary force in unsaturated soil - However, as the soil dries out, the pore size increases and gravity starts to turn capillary water into gravitational water and it moves down.
Capillary water is the primary water that is available to plants as it is trapped in the soil solution right next to the roots of the plant.

20. Questions?