Soil Water

Section 1 The types of soil water and measuring soil moisture content

The importance of soil water： It is the major constituent of plant protoplasm. It is essential for photosynthesis and conversion of starches to sugar . It is the solvent in which nutrients move into and through plant parts.

What are the main components of soil? Mineral Matter Air Water Organic Matter

Classification of soil fractions Classification System USDA International (ISSS) DIN, BSI, MIT Sand 2.0-0.05 mm 2.0-0.02 mm 2.0-0.063 mm Silt 0.05-0.002 mm 0.02-0.002 mm 0.063-0.002 mm Clay < 0.002 mm USDA U.S. Department of Agriculture ISSS International Soil Science Society DIN German Standards BSI British Standards Institute MIT Massachusetts Institute of Technology

Spaces for Gas and Water Partical size effects spaces for gases and water. Water movement is dependent on the spaces

Adhesion and cohesive forces This is called capillary water

1、Classification of soil water 一、The types and available of soil moisture 1、Classification of soil water Adsorbed water Capillary water Gravitational water

（1） Soil adsorbed water ： held by strong electrical forces - low energy little movement- held tight by soil exists as a film unavailable to plants removed from soil by drying in an oven

Soil Water Adhesion Water- water attracted to solid surfaces held by strong electrical forces - low energy little movement- held tight by soil exists as a film unavailable to plants removed from soil by drying in an oven

Sketch map of adsorbed water

（3） Soil capillary water ： Capillary water-The water held in the “capillary” or small pores of a soil,

Water is drawn up into the capillary tube Capillarity： 0.1-1mm Capillarity obvious 0.05-0.1mm Capillarity strong 0.05-0.005mm Capillarity very strong 〈0.001mm Capillarity disappears Water is drawn up into the capillary tube

Capillary water sketch map Soil particle

（4） Gravitational water Gravitational water -Water which moves into, through, or out of the soil under the influence of gravity.

Water adheres to soil particles Water held in large pores Hydroscopic Water Capillary Water Gravitational Water Water adheres to soil particles Water held in large pores Available for crop use Water drains through soil profile Wilting Point 15 bars Field Capacity 1/3 bar

What is Field Capacity? when the soil contains the maximum amount of available water, the greatest amount of water it can hold against gravity

Section 4、Control of soil water 一、Availability of soil water Available soil water-The amount of water released between in situ field capacity and the permanent wilting point (usually estimated by water content at soil matric potential of -1.5 MPa).

Estimating water contents Gravimetric method: The soil sample is dried in an oven at 105°C and the mass of dry soil recorded. Water potential Neutron scattering method

Calculating Soil Moisture Gravimetric Pw = (weight of wet soil – weight of oven dry soil) X 100 weight of wet soil

Soil Water Potential Description Measure of the energy status of the soil water Important because it reflects how hard plants must work to extract water Units of measure are normally bars or atmospheres Soil water potentials are negative pressures (tension or suction) Water flows from a higher (less negative) potential to a lower (more negative) potential

Components of Water Potential Pressure potential: pushing (positive pressure, like the hose) or sucking (negative pressure, like a straw) Major factor moving water through plants Osmotic, or Solute potential: reduction in water potential due to the presence of dissolved solutes salty water has lower water potential (lower concentration) than pure water Matric potential: reduction in water potential due to the presence of matric forces (tendency for water to adhere to surfaces) pressure potential and Matric potential dominates soil water

Soil water potential Total soil water potential = Matric potential + gravitational potential + Osmotic (salts) As the soil dries the water potential decreases or a larger negative number 00 -5 -8 -10 -15 -55 -100 sat. wet - -------- dry------- > very dry

Tensiometer for Measuring Soil Water Potential Water Reservoir Variable Tube Length (12 in- 48 in) Based on Root Zone Depth Vacuum Gauge Porous Ceramic Tip

Units of soil water potential: bars MPa -0.01 -0.001 -0.1 -0.33 -0.033 -1 -10 -15 -1.5 ~FC Note vapor pressure in atmosphere is ~2 kPa ~PWP

Soil Water Classification- a way to quantitatively describe the water in the soil. -0.3 = Field Capacity -15 bar = wilt point Between -0.3 & -15 is plant available water (AWC) AWC 0 bar -0.33 -15 Saturated Field Cap Wilt point

Water Moves through soil by bulk flow The rate of water flow depends on: Size of the pressure gradient Soil hydraulic conductivity (SHC) Measure of the ease in which water moves through soil SHC varies with water content and type of soil Sandy soil high SHC Large spaces between particles Clay soil low SHC Very small spaces between particles

Water moves from areas of high potential (wet soil : -2 or -4) to areas of low potential (dry soil -8) -.4 -3 -7 -8 -2 Root Soil Soil

The End

Water secretion

Water equilibration method

Pressure Chamber

Root and root hairs

Root hair

(b) A close-up of the stele of the buttercup root. Cortex cells filled with amyloplasts Endodermis cell Pericycle cell Phloem cell Xylem vessel elements Figure 6.4: Structure of an herbaceous eudicot root. Intercellular space (b) A close-up of the stele of the buttercup root. Note the solid core of vascular tissues. Fig. 6-4b, p. 116

Casparian strip

Root hairs increase surface area and make intimate contact with components of the soil. Soil Structure

Root Absorption of Water and Solutes \figures\ch04\pp04030.jpg

Symplast and Apoplast

Apoplast vs Symplastic transport

Apoplast vs. Symplast

Ranunculus root tip: undeveloped

Ranunculus root tip: functional

Casparian Strip

Root Hairs – increase absorption develop in region of maturation extensions of epidermal cells