Chapter 7

1. Chapter 7 Soil Water

2. Objectives • After completing this chapter, you should be able to: • Identify the role of water in plant growth • Define the forces that act on soil water • Classify types of soil water • Discuss how water moves in and over the soil • Explain how plant roots remove water from the soil • Describe how to measure soil-water content

3. How Plants Use Water • Functions • Tissue composition: 50-90% water • Tissue turgidity • Cell expansion and growth • Photosynthesis • Transpiration • Nutrient availability • Nutrient and material transport within plants • Chemical reactions • Root and microbe growth

4. Effect of Water Stress and Excess Water • Water stress • Caused by water shortage • Causes plant growth inhibition and wilting • Temporary wilting point -Point of plant water content at which plant wilts but at which plant will recover if watered, cooled, or placed in humid air. • Permanent wilting point -Water content of soil when a plant wilts and does not recover when placed in a humid chamber. Also called the permanent wilting percentage. • Excess water • Displaces air from soil pores • Causes oxygen and nutrient deficiencies, susceptibility to fungi, toxin build-up, and root damage e.g. root rot

5. Forces on Soil Water • A number of forces influence the way water behaves in the soil • Gravity –pulls water down through the soil • Adhesion –the attraction of soil water to soil particles • Cohesion –the attraction of water molecules to other water molecules

6. Forces on Soil Water (cont’d) • Together, adhesion and cohesion create a film of water around soil particles. The film has two parts. • A thin inner film is held tightly by adhesion. Adhesion water is held so tightly that it cannot move. • A thicker outer film of water is held in place by cohesion to the inner film. Cohesion water, or capillary water, is held more loosely, can move in the soil, and some can be absorbed by plants. • Thus, plants use cohesion water that is clinging loosely to soil particles.

7. Forces on Soil Water (cont’d) • Capillarity • Soil water exists in small spaces in soil as a film around soil particles • The small pores can act as capillaries • Capillary is a very thin tube in which a liquid can move against the force of gravity • Soil-water potential • Plants obtain moisture by drawing off water from films surrounding soil particles • Soil-water potential is the concept used to measure soil-water forces • It is defined as the work water can do when it moves from its present state to a pool of pure water in a defined reference state, which is assigned a potential value of zero • In the past this force was measured by the soil-moisture tension (SMT)

8. Forces on Soil Water (cont’d) • The lower the soil-water potential, the more tightly water is attracted to soil particles and the less freely it can move.

9. Forces on Soil Water (cont’d) • Soil-water potential consists of the sum of several separate forces • In most soils the main force is the matric potential • Matric potential results from the attraction of water to soil particles. • Matric potential is always a negative value because of the definition of potential. • Adsorbed water has less ability to do work than free water in a pool, which is defined as zero potential. • Rather than being able to do work, work must be applied to adsorbed water to move it.

10. Forces on Soil Water (cont’d) • A second force is the gravitational potential • Soil water is elevated above the water table and so carries potential energy from gravity. • To achieve a lower energy state, water simply percolates through the soil to a lower elevation. • Gravitational potential is usually a positive value. • The third force is Osmotic potential • Osmotic potential is important in soils of high salt content • Because water molecules are polar, they are attracted to charged salt ions •  In soils of low salt content, osmotic potential makes a minor contribution to total soil-water potential. • Osmotic potential is always a negative number, because the reference state is pure water without dissolved ions.

11. Forces on Soil Water (cont’d) • Total water potential can be expressed by the formula: • Ψsoil = Ψg + Ψm + Ψo • This says that total water potential is the sum of the gravitational, matric, and osmotic potentials.  • The official unit of water potential, acceptable for scientific publications, is the megaPascal (MPa). • Still in common usage is the older term bar, which is equivalent to 0.1 MPa and is slightly less than one atmosphere pressure (14.7 pounds per square inch).

12. A water film showing bars of potential. A plant root must work to overcome that potential.

13. Types of Soil Water • Include: • Gravitational water • Water that moves through the soil under the influence of gravity. • Hygroscopic water • Water held tightly by adhesion to soil particles. Cannot be used by plants and remains in soil after air-drying. Can be driven off by heating. • Available water • that part of soil water that can be absorbed by plant roots • Mostly cohesion water, defined as lying between the field capacity and the wilting point (−1/3 to −15 bars, or −0.03 to − 1.5 MPa soil matric potential).

14. Types of soil water • Important concepts • Saturation • At saturation, matric potential is essentially zero, and gravitational potential dominates total water potential. • Field capacity • The percentage of water remaining in the soil after drainage has just stopped.

16. Water Retention and Movement • Water retention • How much water soil can hold affects how much water is available for plants to use • Water-holding capacity • Water movement • How water moves through the soil affects how well a plant can access it • Gravitational flow • Gravitational flow occurs only under saturated conditions, when matric forces cannot hold water against the force of gravity • Saturated flow • Unsaturated flow • In unsaturated flow, the largest pores are full of air, so water has to move around these voids

18. Water-holding capacity of soils at field capacity (upper curve) and wilting point (lower curve). Available water lies between the two curves. Silt loam holds the most available water. While clay holds the most total water, most is unavailable.

19. Water Retention and Movement (cont’d.) The shaded area shows penetration after 24 hours. The bars show how deep 2 inches of water penetrates for each soil texture. The figure shows that water percolates more slowly and less deeply in the clay loam but moves further laterally. • The wetting front • Water advances into the soil with a definite wetting front: wet behind a distinct line, dry ahead of it

20. Capillary rise • Water moves upward in the soil as surface layers dry, moving from areas of high potential to areas of low potential

21. Effect of soil horizons • Water flows differently in soils of different textures Water infiltrates clayey surface soil. Wetting front strikes layer of sand and spreads out along boundary. When the clay is saturated, water begins to be released into the sand.

22. Preferential flow • Water may encounter soil channels (e.g. biopores) and quickly enter the channel and flow downward under the force of gravity deeper into the soil profile • Can greatly increase infiltration and percolation, reducing runoff and allowing deeper penetration of water • Can result in deeper penetration of pollutants

23. How Roots Gather Water • Soil-water potential • Governs uptake of water by plant roots • Roots absorb water if: soil > plant

24. Pattern of water removal Water withdrawal from soil. When the whole rooting zone is moist, the plant draws mainly from soil near the surface. As the surface dries, the plants begins to draw more heavily from deeper soil.

25. As roots grow at the tip, the most actively absorbing zone, including root hairs just behind the tip, also advances further into the soil. • Older parts left behind lose their root hairs and absorb less readily. • If older root tissue is not replaced by younger, uptake of water and nutrients become less efficient.

26. Measuring Soil Water • People who design or use irrigation systems need to be able to measure the amount of water in a soil. • Four methods are common: gravimetric measurements, potentiometers, resistance blocks, and other devices or procedures. • Gravimetric measurements • Gravimetric methods directly measure soil water content by weight. • Weight basis, • water content = moist weight − dry weight dry weight

27. Suppose one needs to measure water content of a soil at field capacity. A sample is taken two days after a heavy rain. If the sample weight were 150 grams when wet and 127 grams when dry, the moisture content would be • water content = 150 grams − 127 grams = 0.18 • 127 grams • Example: • The gravimetric water content can then be converted to percent water by weight simply by multiplying by 100, so • Percent water by weight = 0.18 × 100 = 18%

28. Volume basis • It is often more useful to calculate water content on a volume basis. • However, it is impractical to measure a volume of water in the soil. • This problem can be solved by making a weight determination and converting it to volumetric water content using soil and water densities  • volumetric water content = gravimetric water content × soil bulk density • water density • Example: • The density of water is 1.0 gram per cubic centimeter. In the previous example, if the bulk density of the soil sample were 1.5 grams per cubic centimeter, water content by volume would be: • volumetric water content = 0.18 × 1.5 gm/cc = 0.27 • 1.0 gm/cc

29. Soil Depth Basis Let us say one could take 1 cubic foot of soil and squeeze all the water out of it into a 1-square-foot cake pan. How many inches of water would be in the pan? This can be calculated simply by the equation inches water per foot soil = 12 inches × volumetric water content In the previous sample, then inches water per foot = 12 inches × 0.27 = 3.24 In the sample, each foot of soil depth contains 3.24 inches of water. If a soil profile were 3 feet deep, and each foot was the same, then the total of the entire profile would be 9.72 inches of total water.

30. Potentiometer • Acts like an artificial root • Measures soil-moisture potential and so gives a “root’s eye view” of how much water is available • As dry soil pulls water out of the porous tip, a partial vacuum is created inside the tube that is measured by the gauge. The vacuum pressure measures soil-water matric potential

31. Resistance block (or Bouyoucos block) • Measures soil moisture • When using a resistance block, the meter reads resistance to electrical flow between two electrodes buried in the block. • The drier the soil, the greater the resistance.

32. A number of other more expensive devices are also available for measuring soil moisture. They include: • Neutron probes • Time domain reflectometers (TDRs)

33. Where Does Water Go? • Water movement • Water moves in response to differentials in gravitational, matric, and osmotic potential • Ponding • Water that lands on the soil but fails to enter the soil collects on the surface to form puddles • Physics • Water will flow down the steepest gradient available • Under certain circumstances, water also flows downslope in the soil

34. Summary • This chapter reviewed several topics • Soil-water potential • Types of soil water • Water movement and penetration • Soil types • Water absorption