Soil water content in soils

1. Soil water content in soils Rafael Muñoz-Carpena

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

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

4. SOIL Volume Mass Air Water Solids 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 Water is held in the soil pores

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

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

8. 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…

9. 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

10. Air water soil root 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.)

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

12. Wet soil Dry soil 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.)

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

14. Clay Sand Water content (in3H2O/in3Soil) Suction, 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. • Water content in the soil is related to suction (energy)

15. Yes!, moisture is related to suction

16. Clay Sand Water content (in3H2O/in3Soil) Suction, Moisture holding is related to texture • Coarse soil releases moisture rapidly with less energy required. • Fine soils hold moisture longer, even at high energy (suction) • Water content in the soil is related to texture

17. Clay Sand Water content (cm3H2O/cm3Soil) Structure Suction, Texture vs. Structure Texture

18. 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.

19. 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?

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

23. Questions?