Soil water content in soils Rafael Muñoz-Carpena

Outline Soil Hydrology A soil and water “refresher” Capillarity theory Field capacity

Hydrological Methods This refers to water balance methods. RO: Runoff P: Precipitation F: Soil Infiltration D: Deep percolation  = Soil moisture If RO=0 and all but ET are measured we can Estimate ET Atmosphere Soil Aquifer

Soil is made of three components Pore space=Va+Vg Porosity=Pore space/total volume= (Va+Vg)/V Water content (in volume) = Volume of water/total volume =Va/V (in weight)=Ma/Ms Bulk density= Mass of solids/total volume = Ms/V Air Water Solids SOIL MassVolume Water is held in the soil pores

Soil particle size have an effect on soil water holding capacity Clay Silt Sand Texture is made out by the the relative content of each of the soil particles Pores are spaces between particles

…as does soil structure… Block Prismatic  Structure is the association of particles in larger lumps.

A paradox? The coarser the soil the less water it contains - The coarser the particles the larger the pores but the total amount of pores is small On the other hand… -The finer the particles the smaller the pores but the total amount of pores is large. Also water flows slower in fine soils…

Does love make the world go around? Energy, or rather differences in energy do… The universe tends spontaneously to lower energy stages: “chaos” or “disorder” Soil water movement follows the same pattern

Water in soil is related to energy Water does not move freely as it does above the surface, but is held in the grasp of the soil which determines how it will move and how much energy (work) the plant roots have to invest to withdraw it. (Drawing source: SoilMoisture, Inc.) Air water soil root

Potential: Energy in the soil  t =  g +  p +  o t: total g: gravitational p: pressure o: osmotic

As soil dries more energy is needed Increasing work is required to remove the water from the small sized pores compared to the large pores, as the soil dries out. Because of this, plants find it increasingly difficult to get adequate water as the soil dries. When remaining water is held only in extremely small pore spaces, the plants cannot exert enough force to withdraw it, and the plants wilt and die (even when there is still water in the soil). (Drawing: SoilMoisture, Inc.) Wet soil Dry soil

Pressure (capillary) potential  P=  g |h c | Weight-unit volume units  p = h c |h c | = 2  cos  / (  gr)

Moisture is related to suction “Soil Suction”(negative pressure potential) is the work that plants have to do to get needed water, and the energy that determines which way moisture will move in the soil. Clay Sand Suction, Water content (in 3 H 2 O/in 3 Soil) Water content in the soil is related to suction (energy)

Yes!, moisture is related to suction

Moisture holding is related to texture Coarse soil releases moisture rapidly with less energy required. Clay Sand Suction, Water content (in 3 H 2 O/in 3 Soil) Fine soils hold moisture longer, even at high energy (suction) Water content in the soil is related to texture

Texture vs. Structure Clay Sand Suction, Water content (cm 3 H 2 O/cm 3 Soil) Texture Structure

Field capacity: Hydrology or Agronomy? In 1949 Veihmeyer and Hendrickson “in 2-3 days after rain or irrigation in soils of uniform texture and structure soils” When gravitational and capillary forces equilibrate after a water application event, the soil stops draining freely.

It is a static concept, while the system is dynamic (redistribution does not stop after FC). In sandy soils the concept is closer to reality (why?) Ways to estimate it: 1/3 bar with Richards plate, centrifugue at 1000 rpm Factors affecting FC?

Wilting point Field capacity Water content Clay (w/ organic matter) (w/o organic matter) (w/ organic matter) (w/o organic matter)

Questions?