Soil Water

Question 1

what is the difference between micropores and macropores?

Answer 1

• micropores: pores in clay particles
• macropores: pores between sand particles or soil aggregates

Question 2

what is bulk density of soil?

Answer 2

mass of a unit of volume of dry soil (includes solids and pores)

Question 3

what is the relationship between bulk density and pore space?

Answer 3

the lower the bulk density, the more pore space

Question 4

what is the chemical definition of cohesion and adhesion?

Answer 4

ahesion: positively charged part of H2O sticks to the negatively charged surface of soil particles
cohesion: negatively charged part of water binded with positively charged part of water

Question 5

how do you explain the behavior of water on hydrophobic surfaces?

Answer 5

formation of water droplets because surface of water is trying to be as small as possible

Question 6

what is the relationship between capillary rise and pore space?

Answer 6

the finer the texture, the smaller the pores, the higher the capillary rise

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

what is gravitational potential?

Answer 8

due to differences in elevation of soil water relative to reference pool

Question 9

what is hydrostatic potential?

Answer 9

due to weight of overlying water in saturated soils (positive)

Question 10

what is matric potential?

Answer 10

measure of bonding strength between soil particles and soil water (negative)

Question 11

what is osmotic potential?

Answer 11

associated with solutes in soil water, important in reducing effective availability of water to plants in saline soils

Question 12

what is the relationship between matric potential and pore size?

Answer 12

the smaller the pore size, the larger capillary rise and larger matric potential

Question 13

go through 4 points along continuum from saturated to dry soils for matric soil water potential

Answer 13

1. saturated - matric potential is 0
2. field capacity - removal from soil by drainage is very slow (equilibrium between gravitational forces and bonding strength of solids for soil water) 0.1-0.3
3. wilting point - point at which plant uptake of water for transpiration is severely reduced, 15
4. hydroscopic coefficient - no further drying in air (equilibrium between evaporative forces and bonding strength of solids-water), 31

Question 14

why do clay particles have high wilting point?

Answer 14

high surface area, so lots of water retained by soil and unavailable to plants

Question 15

what is the relationship between SOM and field capacity?

Answer 15

SOM increases FC which means it helps to provide available water for plant uptake

Question 16

what are 5 factors that influence saturated hydraulic conductivity?

Answer 16

• pore size
• packing of particles
• soil structure (aggregation)
• biopores
• preferential flow paths