lecture 6

Question 1

What is the difference between micropores and macropores?

Answer 1

Macro pores are usually the pores within the coarse sand grains but can also be the pores between the soil aggregates.

Micropores are within the aggregates.

Question 2

What are the 2 ways for measuring soil water content?

Answer 2

1. Gravimetric: weigh fresh amount of soil before and after drying at 105C. the difference is the amount that has evaporated. Limited utility, it does not give information about the capacity of the soil to store water.
2. Volumetric: requires knowledge of bulk density, and gives more information about water holding and storing capacity. It compares it with point measurements such as precipitation, evapotranspiration and water storage capacity.

Question 3

What is bulk density? What are the characteristics of low and high bulk density?

Answer 3

Bulk density is the mass of a unit volume of dry soil. The volume includes both solids and pores.

Low bulk density = lower weight, more pore spaces

Higher bulk density = higher weight, less pore spaces

Question 4

What are the 3 types of packing and pore space?

Answer 4

1. Well sorted, loose packing
2. Well graded, loose packing
3. Well sorted, tight packing

A soil that is well-sorted and loosely packed can transform into a tightly packed soil (road or pathway that has been compacted, forestry or agriculture with heavy machinery, cattle grazing and trampling)

Question 5

What is the difference between cohesion and adhesion?

Answer 5

Cohesion = light binding of the water molecule between its positive and negative charges, through H bonds

Adhesion = adhesion of the positively charged molecule via H bonds to negatively charged surfaces of soil particles

Question 6

Explain the relationship of the height of the capillary rise to a hydrophilic surface:

Answer 6

The height of capillary rise is inversely proportional to the tube radius. The small the tube, the higher the water will rise.

The same principle is related to pore sizes in a soil.

Therefore, the finer the soil texture, the smaller the soil pores, and the higher the capillary rise. The larger the soil pores, the smaller the capillary rise.

The capillary rise in the sand is much smaller than in clayey loam (which has the smallest particle size).

Question 7

What are the four types of soil water potential?

Answer 7

1. Gravitational potential
2. Hydrostatic potential
3. Matric potential
4. Osmotic potential

Question 8

Why does gravitational potential occur?

Answer 8

Occurs due to the differences in elevation of soil water relative to the reference pool. Used to calculate movement in saturated soil through hydraulic conductivity and head.

The more gravitational potential a soil has, the more it will move from a higher point to a lower point.

Question 9

Why does hydrostatic potential occur?

Answer 9

Occurs due to the weight of overlying water in saturated soils (positive pressure).

Question 10

What is matric potential?

Answer 10

Measure of bonding strength between soil particles and soil water. Difference in potential due to attractive forces between soil water and solids and pure water (negative pressure).

It represents the ability of water to move within or out of the soil. In simpler terms, it indicates how tightly water is held in the soil and how easy or difficult it is for plants to extract water from the soil. A higher matric soil water potential means that the water is more tightly held by the soil particles, making it harder for plants to access. Conversely, a lower matric soil water potential indicates that water is more freely available for plant uptake.

The matric potential has the potential to draw water against the gravitational potential, it is pulling the water. This relates to the capillary rise: the smaller the pore size, the larger the rise, the larger the matric potential.

Question 11

What is osmotic potential?

Answer 11

Associated with solutes in soil water (e.g. NaCl). Important in reducing effective availability of water to plants in saline soils.

Osmotic potential is the gradient between two sides of a gradient: strongly relates to salt in arid regions of soils.

Question 12

When considering the matric soil water potential, what are the 4 points along continuum from saturated to dry soils?

Answer 12

1. Saturated
2. Field capacity
3. Wilting point
4. Hygroscopic coefficient

Question 13

Why is soil moisture content higher in silt compared to sand even as the same matric potential? Explain the general trend of this in soils.

Answer 13

Because of the larger surface area. The sand particles are larger and have more pore spaces, so the total amount of water retained will be less than smaller particles like silt or clay.

At the same soil moisture potential, different soils have different moisture contents. This relates to the wilting point and field capacity of soils.

Question 14

What is the effect of organic matter content on field capacity and permanent wilting point?

Answer 14

The higher the soil organic matter content by weight, the larger the volume of the PWP and the higher the FC.

When soil is at field capacity, organic matter has a higher water-holding capacity than a similar volume of mineral soil. While the water held by organic matter at the PWP is also higher, overall an increase in organic matter increases a soil’s ability to store water available for plant use.

This increases the water availability to plants. The soil organic matter helps with the gravitational pull and helps the water in the soil. So, SOM increases, the field capacity.

Question 15

In terms of soil water movement, define saturated conditions and unsaturated conditions:

Answer 15

Saturated conditions: fast movement in large pore spaces (sands), slow in small pore spaces (clays)

Unsaturated conditions: affected by volumetric water content and connectedness between water films along pore walls

Question 16

What do we use to calculate the saturated flow through soils?

Answer 16

Darcy’s law

Question 17

What for is the saturated hydraulic conductivity (Ks) important for performance?

Answer 17

• Irrigation
• Sanitary landfill
• Cover material
• Waste water storage
• Septic tank drainage

Question 18

What are factors influencing saturated hydraulic conductivity (Ks)?

Answer 18

• Pore size (sandy soils > clayey soils)
• Packing of particles
• Soil structures (aggregation)
• Biopores (root channels, earthworm burrows)
• Preferential flow paths of water

Question 19

What influences the moisture movement in unsaturated soils?

Answer 19

Water content, porosity, connectedness, and frictional drag from pore walls.

Question 20

What is usually the distribution of content within the soil water? How does it fluctuate?

Answer 20

Usually 25% water, 25% air, mostly mineral matter and some organic matter.

There are fluctuations between the soil water and the soil air. During dry periods where there is evapotranspiration, water will be replaced by air. If it is raining, then the air is replaced with water.

Question 21

How does the bulk density of organic matter compare to that of minerals?

Answer 21

Rich and organic matter have low bulk densities because the density of organic matter is much lower than the density of minerals.

Quartz has the highest bulk density.

Question 22

Why can capillary rise occur?

Answer 22

Capillary action occurs because water is sticky, thanks to the forces of cohesion (water molecules like to stay close together) and adhesion (water molecules are attracted and stick to other substances).

Question 23

What is hygroscopic water?

Answer 23

Hygroscopic water refers to the water that is tightly held by the surfaces of soil particles due to their ability to attract and retain moisture from the surrounding air. This water is often bound to the soil particles by physical and chemical forces, such as adsorption and capillary action. Hygroscopic water is not easily available to plants because it is held too tightly by the soil particles.

Question 24

What is the order of water in the soil from saturated to dry conditions?

Answer 24

1. Gravitational water (superfluous water)
2. Capillary water (available water for plants)
3. Hygroscopic water (unavailable water for plants)

These are a function of increasing pore size. The smaller the pore size, the more hygroscopic and capillary water is present. The larger the pore sizes, the more gravitational water dominates.

Question 25

What is the field capacity and wilting like in sandy soils compared to clay soils?

Answer 25

Sandy soils: larger particles and pore spaces
- Low field capacity because water is not well retained, it dries out quickly
- Low wilting point because sand grains have a low surface area

Clay soils: smaller particles and pore spaces
- High field capacity: their small size and charged surfaces allow them to retain significant amounts of water in their fine pores, making clay soils capable of holding a substantial volume of water that is available to plants after excess water has drained away.
- High wilting point because the clay particles are flat and will have a larger surface area –> more surface = more water can be retained by the soil. Even if the soil is moist, the water might not be available for plants.

Question 26

What is saturation capacity?

Answer 26

The amount of water required to fill all the pore spaces between soil particles by replacing all air held in pore spaces. This is the maximum water-holding capacity of soils.

At this point, gravitational forces dominate and the matric potential is 0.

Question 27

What is field capacity?

Answer 27

At field capacity, plants can easily take up water. This usually occurs 2-3 days after rainfall/irrigation when all the excess water is drained out.

At this point, matric potential = gravitational potential, and the water stays within the soil content. This is ideal for plants: enough air in the soil and water is loosely held by soils.

Question 28

What is permanent wilting point?

Answer 28

The water content at which plants can no longer extract sufficient water from the soil for its growth.

At PWP, films of water are held very tightly and hence plant roots are not able to extract sufficient water for their growth. This results in the wilting of plants.

Question 29

What is the hygroscopic coefficient?

Answer 29

No further drying in air, representing a balance between evaporative forces and bonding strength of solids- water

Question 30

What does it mean for a soil to have high or low field capacity?

Answer 30

High field capacity = soil can retain a significant amount of water after saturation. Soils with high field capacity are generally able to provide a consistent water supply to plants over a more extended period, making them suitable for supporting plant growth in areas with sporadic or infrequent rainfall.

Low field capacity = soil holds less water after saturation, and the soil may dry out more quickly. Soils with low field capacity may be less effective in retaining water, and plants may experience water stress sooner, especially in environments with limited water availability.

Question 31

What is the relationship between field capacity and wilting point? What is the relationship between different types of soils and this dynamic?

Answer 31

Wilting point and field capacity increase simultaneously according to the type of soil.

Sand –> sandy loam –> loam –. silt loam –> clay loam –> clay

This is related to the size of the pores in the soil. Sand has the larges pore spaces, clay has the smallest.

Question 32

How do we calculate available water capacity?

Answer 32

Field Capacity - Permanent Wilting Point

Question 33

Explain the difference in wilting point between sand and clay?

Answer 33

Sand: The wilting point in sandy soils occurs at a relatively lower soil moisture level because water drains away more easily.

Clay: The wilting point in clayey soils occurs at a higher soil moisture level because the water is held tightly by the clay particles and is less readily available to plants.